Thursday, March 22, 2018

SWR and the Contester

An antenna with a low SWR is good; we all know that. What is not always understood is why a low SWR is good. I would hope that by 2018 most hams would no longer suffer from the lore of myths of long ago and have a technically accurate understanding of SWR. Even so there are bound to be gaps in our knowledge and a refresh can be helpful.

The headline benefits can be summarized as follows:
  • Minimize transmission line loss
  • Transmitter (and amplifier) linearity and efficiency
  • Reduce risk of equipment and component failure and, where protection circuitry is present, ensure full power output from transmitters and amplifiers
Contesters gain additional benefits when all antennas have low SWR across all the spectrum of interest. All hams see the same benefits but for contesters the benefits are more acutely felt. This is what I want to cover in this article, which I will come to after reviewing the impact of high SWR.

How low is low?

HF transmitters and amplifiers are most happy when the SWR is no more than 1.5. However this is a broad generalization. The importance of low SWR tends to be proportional to power. The culprit, as we'll see, is heat. When I contested with my KX3 at 5 watts I never worried about SWR; the KX3 rarely balked at an SWR as high as 3 or even 4. I didn't bother purchasing the ATU option.

Most 100 watt transmitters will not roll back the power until the SWR is 2.With the FTdx5000 at 200 watts it appears to reduce output when the SWR exceeds 1.5. This seems to be a sensible choice. Rare is the broadband kilowatt amplifier that will not turn down the power when the SWR is 1.5, and may go so far as to shut down until the problem is resolved. You don't want that happening in a contest.

Even if your equipment will comfortably handle higher SWR, with or without a tuner, there are still benefits to be had. High SWR affects more than just the transmitter.

SWR and impedance

I have always found that talking about reflections, reflected power and return loss not terribly effective when it comes to gaining a basic understanding of RF networks. For me it is far easier to approach the problem as about impedance: Z = R + jX.

Now we assume a generator that is optimized to transfer power to a load of 50 Ω, with an optional network in between. The network can be as simple as a transmission line with a characteristic impedance of 50 Ω. If we further assume the transmission line is lossless it essentially disappears. In fact for the purpose of this discussion I'll assume that we have a transmission line of this type. At HF with large diameter hard line, such as the Andrew Heliax I use, this is pretty close to reality.

For a load impedance that is not 50 + j0 Ω there is a mismatch. That is, power is reflected back toward the generator where, in our steady state case, appears as a complex impedance as determined by the transmission line length. This is easiest to see with a Smith chart.

Smith Chart with circles drawn for SWR 1.5, 2 and 3
(original figure from Wikipedia, by Wdwd)
At the centre is an SWR of 1, relative to a selected resistance, which although it can be any value we'll stick with 50 Ω. SWR circles (constant radius) are shown. For an SWR of 1.5 you can see that R ranges between 33.3 Ω and 75 Ω, and varies between -20 Ω and +20 Ω. At higher SWR the range of both increases. There is an infinity of points (R, X) for any given SWR.

Once you know the load impedance (and therefore SWR) you walk around the Smith chart to find the impedance at the generator end of the transmission line, with each completed circle representing ½λ of electrical length. Thus you can estimate the impedance seen by the transmitter.

Why broadband transmitters demand low SWR

An RF active circuit is a finicky creature. Departures from the designed load impedance (output port) change the behaviour of the circuit. Its efficiency will decline and it may have reduced linearity resulting in additional distortion products. Misbehaviour can increase for maximum excursions of R and X for a constant SWR. In years past it was not unusual for hams to change the transmission line length (values of R and X) to tame transmitter difficulties when faced with a high SWR.

If the load is 50 Ω all is well. The generator (our transmitter or amplifier) is happy, transmission line loss is as low as it can be and the load (antenna) accepts all the power. In real life it is never that simple. 

Consider transmitter efficiency. Assume that our 1,000 watt transmitter operating in AB class has an efficiency of 60%, therefore generating 670 watts of heat. This heat which is produced in the active devices (tubes or transistors) and other elements of the circuit must be removed to prevent component destruction. This should come as no revelation to anyone who is not new to the hobby.

The challenge grows as the impedance at the output port moves from the optimum value. This is primarily seen as a reduction in its efficiency, meaning more heat produced for a constant output power, which is how most of our equipment is designed to perform. For example, if efficiency drops to 50% the heat produced rises to 1,000 watts, or 330 watts more than in the optimum case. In addition, high SWR (large R and X variation) can place high current or voltage at critical locations that can lead to component failure.

You can see how fragile our high power equipment can become when faced with a mismatch. Equipment size and cost (active devices, transformers, coils, etc.) can be increased to handle more heat, current and voltage, or we can lower the SWR threshold at which power is rolled back. Although efficiency depends on the specific values or R and X it is easier and more reliable to measure and trigger on SWR.

With that overview I have covered enough about mismatch behaviour to continue onward without exhausting my limited knowledge of the subject. I will leave the gory details to the engineering literature where you can indulge yourself if you want to learn more.


Any contester will know that operating agility is a competitive edge. When all antennas have low, flat SWR you can achieve the following:
  • Switch among antennas on a band without the need to adjust amplifiers or tuners
  • With a broadband amplifier, switch bands without the need to adjust amplifiers or tuners
The benefits of this level of station automation are legion:
  • No time wasted during band and antenna changes, and therefore more potential QSOs
  • Quickly move multipliers to another band without losing your run frequency or losing the multiplier due to your delay in setting up on the other band
  • Avoid mistakes that weaken your signal during band and antenna changes, either due to inadvertent high SWR, tuning error or amplifier dropping offline
It should be no surprise that the big guns frequently adopt the following strategies when building their antenna farms:
  • Broadband amplifiers with automatic band switching, or at least automated tuners that react to frequency changes
  • OWA yagis, even at the expense of serious mechanical challenges on 40 meters
  • 4-squares on 80 and 160 meters, which excel at broadband SWR and performance in comparison to alternatives
These come at a price, and that price can be very high indeed. It is only a small percentage of contesters that reach the ultimate goal of flat, low SWR on all antennas and the automation to exploit it. Even among the big guns the percentage is not terribly high. Multi-multi stations can avoid some of the expense and challenge by dedicating operating positions to one band apiece.

The rest of us can pick our spots to improve our agility since we cannot do it all. There exist inexpensive shortcuts such as sticking labels on manually tuned amplifiers to mark the settings for each band, band segment and antenna. For my own station plan I have a lot to think about.

There are a variety of other benefits to contesters of low SWR antennas. I believe the time spent delving into SWR will show its value in the following sections.

Transmission line loss

I'll keep this brief since every ham ought to know that a mismatch increases transmission line loss. For contesters with long runs to reach antennas on high towers the impact can be worse. When the SWR is kept low the design loss objective can be met with less expensive transmission line.

Use software such as TLW (comes with the ARRL Antenna Book) and online calculators to quantify the loss of transmission line alternatives and the impact of SWR. On the higher HF bands and at VHF and UHF the results can be enlightening.

Power division for stacking

Equal power division for effective stacking can only be accomplished when the impedances of the antennas is equal. A power divider, whether done transformer or phasing lines, is essentially just loads connected in parallel, and we know from Ohm's Law that the power in each load element depends on the impedance of each.

The most common solution is to use identical antennas in the stack. It is certainly possible to use dissimilar yagis if the impedances are near equal, in both R and X components. This is typically only practical when the SWR is low for all the antennas. An alternative is to insert a network at the feed point of at least one antenna that is carefully designed to match the impedance curves of the other(s).

Failure to closely match antenna impedances will send most of the power to one antenna, undercutting the objective of stacking and may present a high SWR to the transmitter. When a transformer is used as a power divider the mismatch due to a poor SWR on one or more antennas in the stack can alter the transformation ratio and increase heating in the ferrite core on which the transformer is wound. When running high power that heat can permanently damage the ferrite core.

Filters and stubs

The band pass filters used by many contesters protect receivers and reception quality by removing harmonics and other out-of-band emissions from a second transmitter, whether SO2R or multi-op. Most often they are placed between transceiver and amplifier. Sometimes (and more expensively) they are placed between the amplifier and antenna. In the former case stubs may be used to notch harmonics since they are a low cost alternative to high power band pass filters. Triplexers for sharing tri-band yagis utilize both band pass and band stop filters.

A filter is a network element like any other; look at the picture up above and imagine that the filter is the network between generator and load. Some manufacturers provide excellent technical resources on how their filters work and should be used. Here's a quick quiz: where does the out of band energy go?

Even if you don't know the answer there are just a few possibilities: dissipated as heat; grounded; or reflected. In the latter case this would be a high SWR outside the pass band, while in the former cases the impedance would have to be near 50 + j0 Ω, which is that of a pure resistance. It should be self evident that within the pass band the impedance would also be near 50 + j0 Ω. As you can see, impedance is a useful way to think about filters. Provided that harmonic energy is a small percentage of the total transmitter power (as it should be!) how that energy is handled by the filter will not noticably affect the transmitter.

Be aware that the source and load are integral components of the filter design. The filter only does what you expect if these are close is those impedances are close to 50 + j0 as well. Depending on the filter design a high SWR at the antenna port can degrade filter performance both within and outside the pass band. If you want your filter to work at its best antennas with low SWR are desirable. A few decibels can be enough to cause grief on other bands.

Performance of transmission line stubs to reject (reflect) the harmonic energy they are tuned for is sensitive to placement on the transmission line. Although the SWR may be low at the fundamental that is typically not the case for the harmonics. For best performance a predictable impedance is required and that will depend on the generator, load and stub placement. Since K9YC covers this topic so well I'll point you there for the details.

Common mode chokes

Preventing antenna currents on the outer surface of transmission lines protects the pattern of directive arrays, ensure power goes where it is most effective and reduce noise on reception. Common mode chokes at all antenna feed points are needed. Chokes typically take the form of 1:1 current baluns or coiling the coax on a ferrite or air form.

In the case of a 1:1 balun there is less core heating when the SWR is low; as with stacking power dividers (see above) efficiency suffers for high SWR and reactive loads. Coax coils wound on ferrite toroids are less susceptible to high SWR. Air core coax coils are sensitive to frequency and therefore less effective, which is a shame since they can handle higher power even when the SWR is high.


The typical contest station has dozens of relays to manage antenna switching and sharing among operator positions. If an antenna has a high SWR the current or voltage at the electrical distance of a relay from the antenna dictates whether the relay contact see abnormally high voltage or current. These can affect relay reliability and lifetime by arcing or heating when using high power.

Although relays with wider contact spacing and current capacity can reduce the impact they are often not used in commercial antenna switches. You can build your own switches if you insist on operating with high SWR antennas. Keep in mind these relays are larger, a little more expensive, and the coils require 100 ma or more at 12 VDC versus ~40 ma for the ones typically found in these products.

Conclusions and suggestions

Non-contesters can get by using antennas with high SWR over parts of the band, for the most part not suffering from many of the problems discussed. Contesters should reconsider if they have not dealt with these issues. It is something that I am striving for in my antenna farm, although at this time I have progressed very far.

All my yagis are either tri-banders or have loaded elements, which results in high Q behaviour and therefore higher than desirable SWR over significant parts of every band. My wire antennas for 80 and 160 meters fare a little better. Since these antennas are temporary, power is limited to 200 watts and I don't yet do SO2R or multi-op I am able to get by without too much trouble. For the antenna with the highest SWR on a band I train the rig's ATU for it and then have to remember to switch the ATU in and out depending on the antenna in use. Of course I sometimes forget.

I propose the following suggestions for the contester with a growing antenna farm to achieve the lowest possible SWR:
  • Avoid multi-band yagis, and especially those with traps. Multi-band yagis with interlaced elements or, better yet, mono-band yagis should be used.
  • Consider OWA yagis despite the complication of an additional element for a given boom length. Low SWR across the band can be readily achieved from 40 meters on up.
  • An alternative to OWA yagis when boom lengths are long are the optimized yagis documented in the ARRL Antenna Book and elsewhere. These yagis can achieve low SWR across the band from 20 meters on up.
  • If a 40 meter yagi is to have only 2 elements a Moxon is a good choice. Convert an XM240 or build a W6NL Moxon from scratch.
  • On 80 meters you best choice for low SWR from 3.5 to 3.8 MHz is a 4-square. Many contesters use 4-squares on 40 meters with good results, and is a far less challenging project than a large yagi.
  • Use good quality coax and test it periodically. Old coax, even Andrew Heliax, will wander away from 50 Ω as time passes. From the testing I've done a lot of this old coax tends to develop a lower characteristic impedance, reaching as far as 45 Ω. That's an SWR of 1.1 to a perfect dummy load.
  • Matching networks, where required, should be placed at the antenna feed point, not in the shack. If necessary make the network switchable to change the low SWR range between band segments depending on the contest mode (CW, SSB, RTTY).
Admittedly this can entail a lot of work and expense. It's up to you how far to go in pursuit of the ultimate in contest agility and performance.

Wednesday, March 14, 2018

More Pile-up Techniques

After a period of relative quiet this month numerous DXpeditions are on the air. It's been a lot of fun. While only one new country made it onto my list (9M0W, Spratly Islands) quite a few new band slots have been filled. Particularly challenging has been the highest bands and the low bands, and in the case of 9M0W finding just the right combination and band and time to find effective propagation in this time of zero sunspots.

More so than in the past I am able to rely on brute force to get through the pile-ups. Antennas up high is a great help even without the aid of high power. About half the time I must still rely on agility and technique to get through the pile ups quickly. This is not strictly necessary since all these DXpeditions are one to two weeks long and eventually the depth of callers will thin and even those with small stations, and QRP, are likely to get through.

Jumping in early is more about the fun of the chase and honing my pile up skills. There is always more to learn. Some pile up skills apply equally to chasing contest multipliers, so this is good practice. In contests the technique must adjust for pile ups on the DX's transmit frequency; that is, no split.

This is an opportune time to review a few advanced techniques applicable to DXpeditions and contests. For many veterans there will be nothing new to read here. It can still prove helpful by reminding ourselves of what lurks in the bottoms of our toolboxes, digging them out and blowing off the dust. For some readers the information will be new and therefore of greater interest. Search this blog and you'll find similar articles covering a variety of pile up techniques.

Brute force

When propagation, antennas and power favour you it is best to rely on brute force to get through. Learn the DX operator's pattern -- listening frequency change between contacts -- then find the current lucky DXer and transmit where the DX is most likely to listen next. Don't worry about the presence of many others doing the same since you count on your superior signal to stand above them all. Or at least enough of them that you'll get through within a minute or two.

You may be thinking that this is hardly a pile up technique! Yet it is. Even if you have a modest station there will be times when propagation favours you and brute force works. When it does it is the most reliable and predictable way to get through pile ups fast. Spending time jumping around will only slow you down.

Leave the more advanced techniques for when you really need them. On the other hand, the big guns have to be wily when propagation is unfavourable and brute force doesn't work. For us in North America this is common when propagation favours Europe for DXpeditions such as Spratly Island. The reverse is true for Pacific Ocean DXpeditions.

Going below

Many DXers hamper themselves by collaring themselves the same as dogs restrained by invisible fences. On CW the spectrum within 1 kHz of the DX transmit frequency is avoided since it is a guard zone to prevent QRM on the DX who is operating split. There are times when the prohibition can be ignored to increase success if it is done respectfully and carefully.

Pay attention to whether the DX operator ever works stations below the edge of the 1 kHz guard zone -- listen and you'll notice that some use larger guard zones, such as 2 kHz, and on SSB it is typically 5 kHz or more. Should you probe below without that indication try to keep the split to no less than 800 Hz, and 900 Hz is a better limit if you want to be heard. I strongly recommend you never transmit closer to the transmit frequency even when the DX operator shows a willingness to listen there or bedlam can ensue.

This technique works because the majority strictly respect the guard zone regardless of the DX operator's observed behaviour. As I said, be careful when you do it. Don't QRM the DX!


A typical listening pattern for the DX operator is to shift the listening frequency in small steps. When he judges that the split is great enough he'll do one of two things: reverse direction or jump back to the edge of the guard zone. You usually notice this has occurred when your natural inclination to QSY up results in your inability to hear the station being worked.

When you determine that the listening frequency has been "reset" to the edge of the guard zone you have learned an important datum. The next time that invisible line is approached that is your signal to QSY to the edge of the guard zone and wait for the reset to occur. Should there be others doing the same it can help to move up slightly from the edge (say, from +1 kHz to +1.1 kHz). Also consider dropping into the guard zone slightly (+0.95 kHz) per the previous section.

There is another time when a reset can occur and catch everyone sleeping. This can be your opportunity if you stay nimble. The DX operator may show evidence of frustration when copying become difficult because the pattern is well understood by many and they then keep calling despite the attempt to focus on one identified caller. They may QSY randomly or they may do a reset.

Other indications of an impending reset include when they stop to identify several times or seem to be responding to no one for a while. In the latter case they may be changing operators. It is well worth the gambit of QSYing to the edge of the guard zone and calling there. I did this with one of the African DXpeditions currently active (I believe it was TN5R) and put them in the log with just one call precisely 1 kHz above the transmit frequency.

Calling in the hole

I discussed this item in the context of CW DXing. It can also be applied to good effect when encountering SSB pile ups. The technique also happens to be easier to apply.

A typical SSB split operation will announce a range of where the DX station will be listening. For example. 5 kHz to 10 kHz above the transmit frequency. On 40 meters and sometimes 80 meters the range will be similar but with a greater separation to take advantage of the frequency allocations available in different ITU regions.

What you will soon discover is that the bulk of the callers will transmit exactly at the boundaries of the range; that is, 5 kHz and 10 kHz up. In wider ranges, every 5 kHz. There is some sense to this behaviour because SSB requires greater signal separation to avoid an impenetrable wall of QRM. Many DX operators tend to jump in 5 kHz steps and we respond in kind, and so we all become accustomed to it.

Of course many of the pursuers are listening to learn the split of the last successful caller which results in deep QRM centred on these discrete frequencies. As often happens the DX station will work numerous stations at one frequency before listening elsewhere in the range, making the QRM worse for a while.

When brute force is not an option there is a way, obvious perhaps, to get through. That is to call in between those discrete steps. For example if the range is 14.195 MHz to 14.200 MHz, most of the callers are exactly at those two frequencies and you should call in the hole at 14.1975 MHz. Since this is 2.5 kHz from both those frequencies it is relatively free of QRM. Keep calling there. When the DX operator finally spins the VFO you'll be right there.

It's as simple as that. Even dogs have figured this one out. Ever notice how it is when they want attention that they're underfoot when you turn around? That's no accident!

The secret

Calling this next one the secret is a bit of a joke. It isn't much of a secret despite so many hams being unfamiliar with it. I don't mind sharing it since, even worst case, my readership is modest and, frankly, most won't use it anyway. Thus there will be little impact -- perhaps I'm being too cynical.

It is most useful in CW contests where even the most wanted multipliers operate simplex. When the pile up is on the DX station's transmit frequency the DX station has difficulty isolating one station (often it's the loudest one) and few in the pile up can hear the DX station underneath the QRM. Everyone's rate suffers. Discipline is usually good since otherwise no QSOs take place.

Unless you're that loudest of stations you are faced with a challenge. Since you aren't the loudest what you want to do is sound different. You do this by offsetting your transmit frequency. The shift must be enough to be distinctive yet still within the DX station's receiver pass band. Many DX stations are looking for that difference as well and will use their RIT feature to allow a wider shift.

In the former case a shift of at least 50 Hz is necessary but probably no more than 100 Hz. Aim for the high side (positive offset) since most receivers place the BFO at the bottom of the pass band (USB reception) and lower pitched tones are not as easily noticed. Don't be afraid use a negative offset since they may be using LSB reception (e.g. Elecraft transceivers).

Try it. You'll be surprised at how much of a difference it makes. If you've ever been on the receiving end of a pile up you'll know that all those zero beat callers are nearly impossible to separate, yet a shifted tone that's weaker than the others will be copied at least well enough to capture part of the call sign. It is for this reason that N1MM Logger contest software introduced jitter by default when clicking on a spot to minimize the risk of zero beating. Most rigs are so frequency precise that otherwise everyone clicking on a spot will end up 10 Hz of each other!

When the DX station is in on the game and you are paying attention you'll notice how he will split further and further, and then suddenly shift to the opposite offset. The vast majority of callers never seem to catch on. Get in the game and in moments you'll be trawling the bands for the next multipliers and leaving the messy pile up behind.

That's all there is to the secret. Have fun with it.

Tuesday, March 6, 2018

SSB Low Power: The Muddy Middle

I operated part time in this past weekend's ARRL DX SSB contest. This was a decision made before the contest since I knew that 150 watts, even with good antennas, would be a struggle in a SSB DX contest of this magnitude at this point in the solar cycle. However I left enough latitude in my schedule so that I could operate more if I changed my mind.

Of the several technical problems that appeared soon after the contest started most aggravating was a (still unexplained) distortion of the computer-based digital voice memories, making them unusable and CQing unpleasant. I ended up operating a little over 11 hours until I pulled the plug late Saturday evening. Although my alarm was set to allow me to catch the pre-sunrise low band openings I had no desire to continue and went back to sleep.

To explain my attitude let me start with an anecdote. Soon after Saturday sunrise 20 meters opened strongly to Europe. I positioned myself a little below 14.150 MHz, the edge of the US phone band, to call CQ to see what would happen. After a few CQs a German station replied. When we were done he said "I will spot you". I continued to CQ for another minute with no replies. Then bedlam descended. For the next 15 minutes I had a deep pile up of European callers at a rate of 4 to 5 QSOs per minute. It didn't last.

The rate abruptly dropped to no better than 1 QSO per minute and then died completely. It seemed I was done despite the good conditions. Never again was I able to effectively run to Europe, or to anywhere else for that matter. Further running attempts on any band rarely netted more than one QSO.

This is not necessarily a terrible thing and few would sympathize with my inability to run DX stations. Indeed from what I heard on the bands even the big guns in the US and Canada were not achieving high rates. For me and the majority of participants, even those with QRO, this was a primarily S & P (search and pounce) contest. Often a frustrating one at that.

It is an indication of the challenges anyone will encounter in SSB contests at this stage of the solar cycle. With 10 meters almost dead and 15 meters little better the main stage is 20 meters. The band is intensely crowded and the QRM overwhelming. The lower bands fare worse. On 40 meters the available SSB band segment is narrow, the noise higher and the MUF drops low a few hours past sunset. The lower bands are challenging for SSB DX at any time due to the higher noise level and the relatively poor antennas that are typical.

We must get used to this for the next few years in SSB contests. More stations are crammed into smaller spaces resulting in massive QRM as stations squeeze far closer together than 2.5 kHz. As I tuned across the bands it was difficult to hear any but the biggest guns. There are smaller stations to be found, if you can hear them underneath the tightly packed big guns. Highly directive antennas can sometimes help but on the low bands at least they are not practical for most.

As a result you can't run with low power and S & P turns only turns up the stations you've already worked. Smaller stations can't easily find each other. That is, unless you're the only one on from a country and can attract a constant stream of multiplier hunters. Entering an 'assisted' category alleviate much of the drudgery, which I may do.

Therefore QSO totals are low and hunting for multipliers becomes the primary pursuit. Those running QRO fare relatively better. Even for those with modest antennas the addition of an amplifier can deliver a large dividend. Low power combined with big antennas cannot do the same since it is quite difficult (and expensive) to deliver an additional 10+ db that an amplifier offers with the flip of a switch. Unfortunately when everyone is tempted to run QRO we get a tragedy of the commons where the sum of sensible individual decisions can ruin the bands for everyone.

Well, that's contesting. You play the hand you're dealt or exit the game. I choose a middle position, playing where I can excel or altering my objectives. This contest ultimately bored me since with a better antenna farm I want to do better than scrimp for QSOs the way I did when I ran QRP. Back then the high bands were great and I could run with 5 watts on SSB. Now I can't run with 150 watts and better antennas. SSB low power stations are in the "muddy middle" where they cannot effectively run yet must not stick with S & P since that will not bring success.

Propagation is determined by the sun and ionosphere, not our antennas and transmitters. We build what we can and adjust our objectives to the prevailing conditions. When it stops being fun it is better to step away from the radio, or at least change perspective and objectives.  One cure that works for many is to join a multi-op. That way you share the burden of poor rates and you get to take frequent breaks without affecting the final score.

Long term burn out is a symptom of forcing yourself to stay when it isn't enjoyable. This weekend I chose to step away from the radio. Had I stayed for the duration perhaps I would have placed well, but I simply didn't care to do so.

Thursday, March 1, 2018

Big Station, Big Maintenance

For those of you prone to vertigo I apologize for the picture. It isn't gratuitous; I have a point to make. This picture was in the second last slide in a talk I gave on station building earlier this winter. The topic was maintenance.

The more towers and antennas you put up the more maintenance is required. Although also true for the quantity of equipment you have inside the shack there is an added component of danger, difficulty and expense with the former. Never forget that. There are substantial benefits to building your station with a eye on minimizing maintenance. It can never be eliminated so be prepared.

I was reminded of that this week when a problem occurred on top of my 150' tower. This was a potentially disastrous problem. On Monday I noticed that the top yagis were pointing towards an unexpected direction. As I gazed upward I noticed that the action of the wind was slowly turning them, first in one direction and then the other. Obviously something bad had happened.

Binoculars showed nothing amiss. I could not climb just then since it was immediately before sunset. The next day, with the wind howling but with unseasonably warm weather, I went up there to check it out. Before doing so I tried to arm myself with information by corresponding with another ham with extensive prop pitch motor experience and with the style of drive system I'm using.

Once up there I quickly saw what had gone wrong. The six sets of bolts, lock washers and nuts securing the motor flange to the drive platform had all unscrewed. The bolts fell out the bottom with the lock washers and nuts stranded up top. The loose motor allowed the mast to spin freely and yanked the motor wires out of the splices to the main cable run back to the shack.

The two bolts that landed on the drive shaft bearing were enough to temporarily secure the motor. The next day I climbed up with new hardware and electrical tools and fixed everything. Tower time was 2 hours, plus 90 minutes the previous day. The wind was howling which made the job unpleasant and difficult since I had to fight that wind to turn the mast and antennas to a better position for attaching the wiring. You can be sure that this time I made certain to properly torque the grade 5 galvanized fasteners.

This was also an opportunity to fix the coupling to the direction pot which will save me some grief. The fewer times I have to climb the tower the better.

This fiasco was, of course, entirely my fault. I am not ashamed to say it. We are all human and we make mistakes. How I made this mistake I don't know other than recalling the urgency to complete the project in December as the weather deteriorated day after day.

Mistakes at the top of 150' tower in winter are not like a mistake on a 50' tower or inside the shack. All mistakes and failures are aggravating but most do not involve dangerous repairs. On the bright side the weather was warm and the yagis -- TH6 and XM240 -- are pretty well torque balanced and so did not turn hard in the wind. All coaxial cables were undamaged.

The bigger your station the bigger your problems. If you seriously want a big station prepare yourself. There is lesson in this: build it to last. That was the message of the slide in my talk with the vertiginous picture. The picture shows me climbing the mast of the big tower to release the coiled up coax from the XM240 feed point. Although impressive in a way not even I want to do it often!

It is well worth the expense to buy or build the very best up front to reduce maintenance events. Use the best parts and methods and you can increase MTBF (mean time before failure). The initial expense will be recouped many times over the coming years. You do not want to be climbing towers and making repairs every week or two. It not only puts you at risk it is expensive and, perhaps most important, it can put you off the air just as a major DXpedition or contest occurs.

If, like most hams, you do not climb towers you must factor that into your calculations. Educate yourself about materials and engineering needed to build survivable antenna systems. Otherwise you are at the mercy of others and their opinions, and you will not know enough to distinguish good advice and workmanship from bad. What you don't know will hurt you. Money spent in no guarantee of quality. It helps a great deal if you are able to climb although it is not mandatory.

Do it right and be careful out there. Spring antenna season is nearly here.

Wednesday, February 21, 2018

ARRL DX CW: Antenna Lessons

I had what I consider a successful operation in the ARRL DX CW contest this past weekend. From early reports my competitive position is good despite not likely to win my category of single operator, all band (SOAB LP). Since my station is new and incomplete and my experience with putting it to best effect is a work in progress it was an ideal opportunity to learn and adjust my station building plans. Lessons were learned, several of which I'll share in this article.

There were of course a series of problems which seem to crop up during contests and not at other times. A few of the important ones I'll briefly mention to get them out of the way; I don't want to dwell on them unnecessarily.
  • Prop pitch rotator: Sometime during the first night a fault occurred and I didn't realize it until I have overturned the rotator by 100°. Since the indicator pot decoupled back in January and it's too cold to climb I got into the habit of counting seconds to set the position. This is more difficult at night since confirmation of direction is more difficult. Luckily the problem was in the shack and I got it fixed Saturday morning, at the cost of 30 minutes of prime time operation.
  • Noise: That power line QRN that was cured a month ago? It came back Sunday. Obviously something out there is still amiss. I'll try to better localize the source before calling the utility to deal with it.
Now I'll move on the what I believe are the important lessons that I learned. I am restricting this article to antennas rather than contest operating techniques. I may get those in a subsequent article. Contest operating with big antennas is not the same as it is with small or modest antennas.

Europe on the high bands

For contests there is no question that for us in North America it is Europe that provides the bulk of the QSOs and multipliers. They are numerous, active in contests and not so far as to be difficult to work. A log analysis shows over 75% of my QSOs were Europeans. It should be obvious that Europe is a priority in my antenna plans.

As I previously noted the lower antenna (Explorer 14 at 34 meters, fixed toward Europe) always outperformed the higher yagi (TH6 at 43 meters) on 20 meters. This has more to do with height (elevation angle) than gain since the lower antenna has lower gain on 20. For the same reason the higher antenna does better to Europe on 15 meters. I am now revising my opinion.

The optimum elevation angle for a particular path varies widely depending on MUF (solar flux, time of day and time of year), absorption at D and E layers (geomagnetic storms, etc.) and other factors. This weekend, especially on Sunday morning, the antennas were more equally matched to Europe on 20 meters. Indeed when I tried the high yagi the run rate increased; this did not happen on Saturday morning. While insufficient as proof it is very suggestive the greater height does have value on 20 meters towards Europe even when the antennas seem to be the same on receive.

A few decibels difference is often not obvious when receiving due to the heavy QSB typical at HF. However on the other end it can draw in more callers.

As expected the high antenna always outperformed on 15 meters to Europe for the limited openings we had during the contest. There was no 10 meter opening other than working CR3W and hearing but not working CU4DX. Unsurprisingly they were favoured by the higher yagi.


On 20 and 15 meters the TH7 at 21 meters always outperformed higher yagis to the Caribbean and Central America. Often the difference was 2 to 3 S-units. This weekend the TH6 at 43 meters was always better on 10 meters, no doubt because the MUF barely reached 28 MHz on this short DX path.

Although there are relatively few stations active from these areas it is important in contests to quickly bust the inevitable pile ups and move on to make other QSOs. I quickly learned this lesson and would keep the TH6 turned south or south-southeast to search for and work those multipliers. Switching from the high yagi to the low one often resulting in just one call to cut through the pile up despite running only 150 watts.

Compared to most of the US we have a better shot at the Caribbean on 20 and 15 meters when the solar flux (MUF) is low; many in the US are in the skip zone on this short path. For the longer southerly path, even as close as the north coast of South America the higher yagi is better on all the high bands. The optimum range for a low yagi is narrow but important.

Longer paths

As ought to be expected the high yagis were the go-to antennas for longer DX paths on the bands from 40 through 10 meters. Whether CE, LU, VK, ZL, JA, DU, UA0, ZS and others the difference was never less than 2 S-units and could be in excess of 5 or 6 S-units. These antennas were a big help in putting many Japanese stations in the log and distant multipliers I would otherwise be unable to work or not be able to work so quickly and easily.

40 meters

As my antennas improve I learn more and more about this band. It can be interesting. It is also important to a contester at this stage of the solar cycle since it is the second most productive band for QSOs and multipliers.

First off let's look at the 80/40 inverted vee (apex at 32 meters). Put bluntly it was almost totally useless. On all DX paths the XM240 at 46 meters was always the superior choice except when the direction was directly off the ends of the elements, including when off the back of the yagi. Two element yagis (other than the Moxon) have poor F/B, yet even with 10 to 15 db rejection it still outperformed the inverted vee.

I only using the inverted vee if it was inconvenient to move the XM240 a few degrees to catch a multiplier. The inverted vee remains valuable for working short paths within eastern North America.

The path to Europe and other points eastward opens in mid-afternoon. I was able to begin working (and running) Europe around 2100Z, which is 90 minutes before sunset. This was not possible when the yagi was at half its current height. Stations running a kilowatt could work Europe a full hour before I could. Height and gain can overcome the pre-sunset path loss and higher received noise in Europe. This isn't possible with the inverted vee.

Speaking of noise, another way I fought QRM and atmospheric QRN on 40 meters after sunset was to use the northeast Beverage. The SNR on received signals was better and was a definite help with the weaker signals. The beam width is too narrow for working anything other than Europe since on 40 meters the 175 meter long Beverage is 4λ.

80 and 160 meters

As everyone knows conditions were poor to middling for most of the contest. Although disappointing it is a situation everyone experiences and so does not mean a great deal. For me the big impact on the low bands was that the extra decibels of path attenuation put my 150 watts below the noise for far too many stations.

Not only were my QSOs and multipliers low on these bands I only improved my 160 meter DXCC count by two. Although both my 80 meter (temporary inverted vee) antenna and 160 meter antenna offer decent performance it is obvious that I can do better.

Another challenge I faced on 80 and 160 meters was skew path propagation to Europe and perhaps other directions. This is reportedly not unusual during a geomagnetic disturbance. With just the one Beverage (northeast) all I could say for sure during the contest was that at times it did no better than the transmitting vertical antenna on receive. I learned about the presence of skew path after the contest from the reports of others.

While running low power the occasional ineffectiveness of a receive antenna is not a disaster. It will prove problematic when I return to QRO operating since my signal will attract weaker callers if I don't have receive antennas for other directions.

Station automation

This is a work in progress. At the moment I have little more a manually controlled remote antenna switch (2 x 8) and N1MM Logger with CAT control of one rig. I still do not have all the equipment to do SO2R and automatic antenna selection.

It is very easy to choose the wrong antenna when you're tired or in a hurry, both of which are common in a 48 hour contest. Since the SWR protection kicks in this is only a time waster rather than a potential disaster. When I go QRO the same event can become more serious. Manual switching is time consuming and deflects my attention from focussing on operating.

SO2R requires more work. The SCU17 interface for the FTdx5000 died so the one CAT cable I have goes to it rather than the (idle) FT950, as I had intended. I do not have a headphone mixer to listen to both radios at once or band pass filters to protect the receivers. All this and more are still to be done. Up until now station automation has been low priority in comparison to antennas. I need to practice doing SO2R, which is a skill I do not yet have.

Once I have SO2R working I will also be in a position to invite others to do a multi-op contest. Without SO2R I was unable to capitalize on many opportunities this past weekend to run on 20 or 40 meters and concurrently hunt for stations on other bands. That put me at a competitive disadvantage.

Antenna conflicts

With a limited number of antennas it is perhaps not unexpected that I would encounter conflicts. For example, in late afternoon I want the XM240 (40 meters) pointed to Europe and the TH6 (20 meters) pointed to Japan and the east Asia. My current choices are to lose time rotating the yagis back and forth or use sub-optimal antennas. Either way QSOs and multipliers are negatively affected.

If I had a second tower such conflicts can be avoided. There is the added benefit of increased isolation between antennas. It is common that serious contesters have at least two tall towers. With tow of them one is typically dedicated to 40 and 10 meters and the other to 20 and 15 meters. Although there are still conflicts they are less serious. A rotatable multi-band yagi or a few fixed yagis at an intermediate height can resolve almost all the remaining conflicts.

Impact on 2018 antenna plans

Many problems can be addressed with a second tall tower. That is in my plans although I am undecided whether to do it this year. My concern is that the effort required to put up a tower of between 120' and 140' will mean little time left to design, build and test antennas. Further, the choice of rotatable, fixed and stacked yagis for 40 through 10 meters depends on whether I have one tower or two.

I definitely plan to stack yagis on 20 and 15 meters for additional gain towards Europe and to match elevation angle to the prevailing propagation as it changes. Now that I know for certain that more height can be beneficial to Europe on 20 meters I am rethinking my plans for side mounted yagis and the rotatable antenna at the top. Depending on whether I go with a second tower the yagis will either be mono-band or multi-band.

On 40 meters I would like a fixed, reversible northeast-southwest (Europe-USA) 3-element yagi up ~25 meters. Regardless of whether I can fit in a rotatable yagi better than the XM240 up top into this year's schedule the added performance and flexibility provided by the fixed yagi will be a help during contests. My preference is for a tubing antenna rather than wire (inverted vee) to reduce interactions, improve performance and avoid additional anchors in the hay field. I am currently investigating designs and material choices.

80 meters is an easy decision: build the vertical yagi. In contrast I remain uncertain how to deal with 160 meters. The vertical I have up at the moment will have to come down by May at the latest due to the arrival of haying season and because it will interfere with work on yagis on the 150' tower. If nothing better comes along in my plans it will go back up in September or October, deferring a decision at least one more year. My expectation is that this is what will happen.

Reversible Beverages remain my preferred choice for low band receive antennas. Paths for two of these have been surveyed and most of the material purchased. On the critical path is the design and construction of a remote switching system. Until I have that ready putting up the Beverages is low priority.

Other than antenna plans I have been getting into the specifics of station automation equipment design. I am closely reviewing commercial products and public designs while also injecting my own ideas on what will work best for me. The final product will combine commercial and custom hardware and software. I don't know how far along I'll get by the fall contest season.

I have a challenging year ahead of me. No matter how much I accomplish I'll be in a better competitive position for next season's contests.

Thursday, February 15, 2018

Twisted Inverted Vee

Several weeks ago a problem showed up with one of my 40/80 meter inverted vee antenna. I first noticed it when the SWR on 40 meters was unusually high. At first I assumed it was due to weather since moisture or ice on the wires will affect resonance, and we get lots of both in the winter. I let the rig's ATU calibrate to the new impedance and continued operating, expecting the impedance to return to normal later.

When the problem persisted after two days of fair weather I decided to look into it. Or, if you like, I looked up at it. What had happened was immediately evident. On one leg of the vee the wires for the 40 and 80 meter antennas twisted. A closer inspection with binoculars revealed that the twist was comprised of 3 or 4 rotations. The picture is annotated due to the poor resolution; I had no intention of climbing up there for a better view.

In an article last fall I explained how this was to be a temporary antenna until I could put up permanent antennas for 80 meters and a lower one for 40 meters. I built it very quickly, using existing 80 meter and 40 meter inverted vees from my my stockpile, tying them together and constructing a set of spacers made from small PVC pipe (3 per leg). Small ropes extend from the bottom spacers, positioned at the ends of the 40 meter wires to the ends of the 80 meter wires. As I said, it's very simple.

Unfortunately I made a bad decision on the ropes used to tie the antenna to anchors on the ground. I went with expediency rather than good sense since I was so pressed for time. I had hundreds of feet of ¼" polypropylene twist rope for which I had no other use. It has been in storage for many years. Why I originally bought it I no longer recall.

I cut 75' lengths and completed the antenna. Since twist rope of this type develops a torque when put under tension we had quite a job preventing the antenna from twisting when first installing it. When it seemed stable I let it be and hoped for the best.

But hope is a 4-letter word. After a late January day with strong winds the twist reappeared in one leg of the vee. The twist is approximately 3 meters from the feed point, between the top and middle spacers. The middle spacer is ~5 meters along the antenna, near the middle of the 40 meter wire.

Although an inconvenience, and not a disaster, the result of this accidental experiment is instructive and merits a brief article. It is worth thinking about should you ever run into a similar problem with one of your antennas. So let me step back and describe where I started.

As with any fan antenna of this type the elements for the lowest frequency are almost totally unaffected by those for the higher bands. It is the higher bands that see the impact. The reason is that the ends of a dipole are most susceptible to capactive coupling to adjacent elements; that is why capacity hats must be placed far along an element to be effective. For this antenna the susceptible band is 40 meters. The resonant frequency on 80 meters was not noticably affected by the fan arrangement. In contrast the 40 meters resonance moved well below the band due to that coupling by increasing the antenna's electrical length.

I knew this would happen and that there would be no time to tune the antenna after raising it. The difficulty is overcome by use of the rig's ATU. However there is some challenge when switching between the inverted vee and XM240 on 40 meters since the SWR curves are so different. For operational simplicity I currently reserve the ATU for the inverted vee and disable the ATU when switching to the yagi. It's an acceptable inconvenience for the short time the inverted vee is expected to be in use.

When the elements twisted together the impedance impact showed up on 40 but not 80 meters. Some change on 80 must have occurred though not enough to require reprogramming the ATU. Interestingly the 40 meter resonant frequency didn't move far. Perhaps that's because the end of the 40 meter element is still properly separated from the 80 meter element. Instead the negative impact is a substantial decrease in the SWR bandwidth.

The resonant frequency moved downward at least 75 kHz and the 2:1 SWR bandwidth decreased to ~150 kHz. When undamaged the SWR at 7.0 MHz was ~2 and ~2.5 at 7.1 MHz. I have done no further investigation to determine why the impedance changed in this particular fashion since it is of limited interest to me. In any case modelling the twisted elements is almost certainly beyond the capabilities of NEC2.

While not an ideal situation it does not hobble its performance. Single element antennas can survive a lot of abuse since even drastic impedance swings due to rain, ice, tangling and environmental coupling do not affect the pattern. If the loss due to a higher SWR is managable there is no need for emergency repairs. The same cannot be said of directive arrays whose patterns and impedance are very sensitive to changes.

I never did fix the problem and frankly I can't be bothered to spend more effort on it. The antenna works and that is what matters for the few remaining months it'll be up there. The anchor for that leg is frozen to the ground (two large rocks) and there is the risk of making the problem worse by trying to untangle it from the ground. It isn't worth the trouble and risk of climbing the tower in winter weather.

Saturday, February 10, 2018

80 Meter Vertical Yagi: Revised

As I write these words it's -21 C and the wind is howling. There is no antenna work getting done. I am well behind schedule with my winter projects due to the severity of the weather. In time it will moderate and work can resume. Until then I am limited to what I can do indoors. One of those is sitting at the computer and modelling antennas.

Since I was planning to begin construction of the 80 meter vertical array in the winter I have revisited my original design. Changes have been made. Please refer to that article for design and construction details not included here, and for additional background. I won't unnecessarily repeat myself.

For the first change the tower (driven element) will be a little shorter. This means that the "stinger" at the top will need to be ~6 meters tall rather than just 1 meter to be λ/4 on 80 meters. Adding a switchable section on top to have a 160 meter vertical is physically unreasonable with the now much longer stinger. I had hoped to use the extensive radial field on 160 meters, keeping land use and cost to a minimum. This plan has also changed.

For contests I do not want to be in a position where I cannot be on 80 or 160 meters at the same time, whether for SO2R or multi-op. For the next few years over the duration of the solar minimum these may be the only two productive bands for several hours at night.

The other reason is that I have been unable to come up with a design to switch between 80 and 160 meters that does not compromise performance on 80 or 160 meters or both. Whether it be a trap or mutual impedance with an isolated pole and top hat for 160 meters there are negative impacts on the 80 meter array's pattern and bandwidth. To be clear, it can be made to work, just not to my satisfaction.

With this exclusion I am free to focus on 80 meter performance. Despite this the physical design still constrains the electrical options. When I first developed the model I did not own this property and could not predict the specific layout of the property and my site plan for towers and antennas. I am in a better position to do so now.

Elements of the redesign

A casual search for additional sections for my small tower -- as the main support and driven element -- didn't turn up anything suitable or economical. Since the tower is currently 14 meters tall (6 x 8' sections, with splice overlap) and a λ/4 on 80 meters is ~20 meters a "stinger" of ~6 meters length is required. This is straight-forward. However the original plan for a 20 meter tall tower allowed for an isolated stringer for a 160 meter vertical with a capacity top hat.

The wire parasitic elements will continue to be supported from the top of the stinger. Due to its lower height (14 meters vs. ~25 meters) the wires cannot simply be wires hanging from support catenaries. I am therefore using angular T-top verticals. The array will closely resemble the original design by K3LR, details of which can be found in ON4UN's Low-band DXing book.

This adds some uncertainty to the NEC2 model due to the odd shape of the parasitic elements. This must be compensated for during construction and testing with an antenna analyzer. I found this with the similarly shaped 160 meter antenna I recently built, which resonated ~80 kHz lower than the model. Interestingly there is a chart in ON4UN's book that  recommends dimensions that appear to be more accurate than what I can model with NEC2 (EZNEC). I expect the same for this antenna.

There are two additional changes I'm making. The first is to exclude SSB. This simplifies the design and construction without giving up too much with regard to my operating interests. I can always add it later. The array will be an omni-directional single element vertical between 3.65 and 3.8 MHz. When receive directivity is needed it can come from the Beverages (still to be built).

A further change is to radial system. Rather than busses at intersections of radials between the 5 elements I will put down 5 independent and overlapping radial systems. This is difficult to model so I can only rely on reports that performance is not compromised. My reason is solely to simplify construction since creating the bare copper busses and the multitude of soldered connections, and not with lead-tin solder, is a lot of work. The price is the amount of radial wire required. I can change to a bus system later if I wish or if necessary to optimize performance.

Developing the model

The array is designed to act as both a 3-element vertical yagi and as an omni-directional λ/4 vertical. Since the yagi performance is narrow band it is designed for CW only. However as a simple vertical it can be more broadband than that, and indeed can be made to work well from 3.5 to 3.8 MHz. Therefore the first objective is to resonate the driven element more centrally in the band; the L-network -- switched in for yagi operation -- is easily adjusted to accommodate the higher resonance of the driven element.

After some modelling work I settled on 3.6 MHz as the resonant frequency for the driven element. The match is very good, deliberately favouring CW and the DX & contest segment of 80 meters which is what matters to me. You can choose another frequency without affecting yagi performance since a matching network is required regardless. Keep in mind that a matching network may be required as the radial system is improved beyond that in this model since the feed point resistance will drop.

The impedance is dependent on the ground system since the ground loss is in series with the radiation resistance. For this model I used MININEC ground in EZNEC and inserted a 5 Ω load at the base of all 5 elements to emulate a very good radial system. This is far easier than creating a radial field for all 5 elements in the model yet gives results that are close to reality. The load resistance can be adjusted to test the antenna's predicted performance with other radial systems. There are tables of approximate equivalent resistances of radial systems (length and number) to be found in several places, including in ON4UN's book. I'll have more to say later about the radial system and its effect on the antenna.

The next step was to design the wire parasitic elements, including their vertical and T-top lengths. Spacing to the driven element in all cases is 10.5 meters, or λ/8 in the CW segment of 80 meters. Since the array is reversible different director and reflector spacing is not possible. Consequently there is some loss of performance (gain and F/B) relative to an optimized (unidirectional) yagi, though not enough to be of practical concern.

As I saw with my 160 meter antenna the model for an element of this style is not accurately modelled using NEC2. Two reasons of which I'm aware are the acute interior angle of ~45° on the low side of the T and the effect of ground.

The error can be corrected during construction by floating all the other elements including the driven element (disconnected from ground) and adjusting the wire element to self-resonance at 3.68 MHz, as a director. Symmetrical trimming of the two halves of the T is recommended. With the 2.1 μH reflector coil in line at the element base the self-resonance is 3.45 MHz. First tune the wire element as a direction and then with the coil in line adjust the coil, not the element, to tune its self resonance as a reflector.

Since the radiation resistance of a yagi is lower than a simple vertical a matching network is required. I used TLW (comes with the ARRL antenna book) to design the network based on the EZNEC reported impedance. The designed network is inserted into the EZNEC model to confirm that the antenna is now matched. In practice you'll want to measure the array's impedance once it's built and then design the L-network to transform that impedance to 50 Ω. As you can see coil Q is not critical as there are no large losses in the small transformation ratio required. I used a "low pass" L-network to help attenuate harmonics for SO2R and multi-op contest operation.

Within reason the director and reflector self resonant frequencies can be adjusted to centre the array on another band segment without going to the trouble of a complete re-modelling. The reflector coil value stays the same. A small improvement in gain and F/B can be achieved by tightening the tuning of the parasitic elements. For example, lowering the reflector coil to 1.8 μH gives several more db of F/B and ~0.2 db of gain. In this case the director self resonance should be lowered ~30 kHz.

The SWR bandwidth will be narrower. That may be a fair trade-off since the SWR bandwidth of this antenna is superior to the original. For this specific case the designed L-network still works well.

I modelled the elements with insulated AWG 14. The vertical length of the wire elements is 10.2 meters and each half of the T is 6.3 meters. The vertical length is a compromise between minimizing the length of the T (capacity hat) and minimizing the distance from the tower than the element must be anchored. I want to keep the anchors within the radial field to reduce the amount of land dedicated to the antenna which would otherwise have to be taken from the haying.

If one is careful the tuning is only required on one parasitic element. The others can then be cut to match it. Even so it is probably wise to measure and trim them all to resonance to avoid surprises. Either way it is done the reflector coils ought to be adjusted to accurately resonate the elements as reflectors.

Model performance

SWR, gain and F/B bandwidth are better in this antenna than in the original design that used straight parasitic elements. Unfortunately the gain and F/B are not as good over most of the operating range. The difference is not severe but requires consideration. Peak gain drops from 4.5 to 4.2 dbi, which is close to negligible and does not overly concern me. Peak F/B drops from well over 20 db to only 15 db. That is perhaps the only negative performance impact of note

Let's look more closely at the numbers, in particular in comparison to the original design. I kept the 5 Ω equivalent series resistance of the ground loss to ensure the comparison is valid. While not charted the SWR bandwidth is superior to the original design. The T-top elements at least achieve that much.

Although the loss of F/B is disappointing the overall performance change is neutral in my opinion. You may feel differently. While the wider bandwidth is not consequential to a pure CW operator it does matter if your interests include digital modes and SSB. With coil switching to support the SSB segment (as in the original design, and which can be added to this one) it can be a good performer from 3.65 to 3.8 MHz. Perhaps one day, but not initially in my case.

F/B performance is less of a concern where this array is primarily devoted to transmit and a separate, multi-direction receive antenna system is available. Those using a 4-square antenna on 80 meters often reporte they only occasionally use their receive antennas since the 4-square's F/B is quite good. That is one comparative disadvantage of the yagi array.

Before constructing the model I speculated that the F/B would improve. The reason is that the top of the T of all the wire elements lean towards the driven element, thus increasing capacitive coupling with the driven element. This is how the Moxon works where critical coupling serves to equalize current, a necessary condition for a high F/B (field cancellation). Of course there are 3 elements, not 2, so perhaps the better comparison is a Spiderbeam style of yagi.

Obviously this didn't happen. Looking at the element currents it is clear that the elements are nowhere close to critical coupling as the tips come no closer together than 6 meters (~0.07λ). Another hope dashed on the shores of reality.

I notice that the March QST has an article on a 3-element vertical Moxon yagi. Unfortunately I don't have it yet since it would be interesting to compare. If it looks promising I may model it and compare to what I this yagi design. Should that happen I'll write a follow up article.

Ground sensitivity

It is no surprise that the quality of the radial system and the conductivity of the ground below have a strong influence on the efficiency of vertical antennas, especially ground mounted verticals. For every antenna we have to find an acceptable trade off between cost & convenience versus performance. Directive arrays such as the vertical yagi and the 4-square are more affected since their lower radiation resistance results in greater ground loss versus a simple vertical for any given radial system.

Compared to perfect ground this vertical yagi and the 4-square have approximately the same peak gain ~6.5 dbi. The 4-square has better F/B and both F/B and have a much wider bandwidth. On the other hand the vertical yagi is simpler, cheaper, more amenable to experimentation, direction choices and the addition of more directors. For me these make the choice easy. The majority of contesters I know choose the 4-square since their primary motivation is competitiveness without undue time spent on experimentation and home brewing the control system.

When ground is imperfect, as it always is, the 4-square has greater efficiency than the vertical yagi for any given radial system. The vertical yagi's radiation resistance is lower due to the closer element spacing -- λ/8 vs. λ/4 -- and the consequent higher element currents lead to higher I²R ground loss. The better the radial system the less the difference. Providing you are committed to an extensive radial system there will be little efficiency difference between the two antennas, even accounting for the dump load (up to -0.5 db loss) in the 4-square.

As ground quality improves the vertical yagi's peak performance moves lower in frequency. As with any yagi the frequency of maximum gain is correlated with minimum radiation resistance. For this antenna that occurs below 3.5 MHz. Over a perfect ground this vertical yagi's gain peaks ~3.47 MHz and the peak F/B rises well above 20 db at 3.525 MHz. If the antenna is built with a superior radial system, one with an equivalent series resistance of 3 Ω or less it can be worthwhile to shift its tuning upward by 30 or 40 kHz to exploit that change.

With the very good but not great radial system in my model -- 5 Ω -- the modelled ground loss is -2.4 dbi, although it varies with frequency, increasing towards the bottom and top of the operating bandwidth. EZNEC reports quite high loss as the frequency increases, exceeding 300 watts at 3.65 MHz. This isn't bad. You can always add more radials over time if desired.

The above chart compares base element currents for the original and new versions of the array, both with 3 Ω equivalent series resistance for ground and 1,000 watts, to match the values I chose in the original article.  It is possible to greatly reduce ground loss by improving the radial system for the driven element. In fact the loss becomes quite low when the driven element ground resistance is lowered to 2 Ω and the parasitic elements to 5 Ω.

Review the original article and note that it is more important to lower the ground loss in the driven element since its current is always higher than in the parasitic elements, which is unlike the 4-square whose elements have no unique identity. In its omni-directional configuration the ground loss is lower.

For a fixed amount of wire the performance can be optimized by putting more of that wire into the driven element radial system than the 4 parasitic elements. This is not a 4-square where the elements should be treated equally! You'll even gain some performance benefit by use of wire thicker than 14 AWG in the parasitic elements; I'll be using 14 AWG wire since that's what I have on hand.

Radial topology

By using MININEC ground the details of the radial system can be glossed over by substituting fixed load resistances between each element and the perfect ground. That detail cannot be avoided when designing the actual radial system. Aside from the size of the radial system to achieve the target ground loss the topology is important since the radials are longer than the distance between elements. That is, they must either overlap or be connected.

I modelled connected and overlapping radials quite some time ago in an attempt to determine whether one is better than the other in phased and parasitic vertical arrays. Although there are measurable and significant differences in the radial current amplitudes and distribution in the end it seemed to be one of small differences rather than one topology being obviously superior. In both cases the currents on radials between active elements can become quite complex, and perhaps not intuitive, due to the superposition of fields of the mutually coupled elements in the return paths through the radials and ground beneath.

Overlapping radials are easier to construct but more expensive. The capacitive coupling between crossing radials is only significant off to the side where currents are lower and voltages higher at the crossing points. Radial interconnection, via busses or directly, is difficult in practice and forces return current to zigzag at the interconnection points. This is difficult to model and compare.

My present inclination is to go with overlapped radial fields for each element, based partly on my (inconclusive) models and not well quantified data (to my knowledge) from experimenters. I intend to keep it simple and create a thick radial field for the driven element where the potential loss is greatest and sparser and shorter radials for the parasitic elements. I want them shorter so that the land impact is minimized. Long radials are not a problem for the driven element since it's at the array's centre.

Should I be unhappy with the results I can revisit the decision and redo the radial field.

What is the reality of performance?

Modelling is not the final word on an antenna of this type. Ground influences, radial topology and the environment have significant impacts that are very difficult to model. NEC4 can do better than NEC2 though even that has limits. Relying solely on models to characterize performance and comparison to the 4-square (this array's nearest competitor) may be unwise. How will they perform in practice?

Perhaps the greatest problems with comparisons are propagation variability and instrumentation for measuring differences. Direct A/B comparisons are typically impossible since no one I know has both a 4-square and a 3-element vertical yagi for the same band.

F/B on both antennas is sensitive to tuning. I wonder how many 4-squares are tuned so well that F/B measures at or near what is theoretically possible. Maximizing F/B is difficult in any antenna since balancing phase and amplitude so that near perfect cancellation of fields in the reverse direction occurs. Consider than 30 db of F/B requires 99.8% field cancellation! That is a challenge even with commercial phasing and switching systems.

Claims of 25 to 30 db or more of F/B should be looked at critically. How was it measured? With an S-meter? That is a sure path to overstatement. S-meters are not linear and follow no standard. There are a few recent model SDR receivers that do better by digitally compensating for the analogue data coming from the receiver. Unless you have and can confirm calibration no S-meter should be relied upon for a dependable measurement.

We have also seen that the vertical yagi F/B is sensitive to ground loss, and therefore the quality of the radial system. More and longer radials improve gain and F/B. Even so it can never reach the performance of a well-tuned 4-square. Even then the performance bandwidth is narrow. Many owners of 4-squares find that no separate receive antenna is necessary other than in exceptional cases since the directivity is quite good. That's persuasive.

A vertical yagi will require resorting to a separate highly directive receive antenna more often than with a 4-square. I believe I can live with that. I may change my mind after building and living with this antenna for a while. Unlike in an earlier article that derided the importance of F/B the viewpoint I espoused is less supportable on the low bands where good directivity is needed to copy under the prevailing low SNR conditions.

Construction plans and testing

The construction and testing sequence is laid out above in the modelling section: tune the driven element after construction it and its radial system and then move on to the wire elements, tuning each of those with all other elements floating. Only then should the system be driven as a parasitic array and the L-network designed and built.

I will initially use 20 meter long radials for the driven element and 15 or 16 meter radials for the parasitic elements, as reasoned above. My aim is 32 radials for the driven element and 16 for the parasitic elements. Doubling the number of radials to improve performance can be accomplished later by placing a new one between each pair of existing radials.

Directions covered by the array do not have to be at 90° intervals, unlike the 4-square. The only constraint is that the director and reflector must be in a line with the driven element (tower). My choices (already surveyed and staked) are: 50° and 230°; and 160° and 340°. The first pair covers Europe and most of the US and Pacific. The second pair covers Japan and east Asia, and the Caribbean and South America. From here those are the most productive directions for contests. The beam width is wide enough that there are few coverage gaps, and one can always resort to the omni-directional mode.

As mentioned at the beginning the weather turned foul very quickly in December. I did manage to get the tower anchors installed mere hours in advance of the initial blast of frigid temperatures. During a brief January thaw I tested the anchors and found them to be inadequate. The screw (auger) anchors have to be longer and/or wider to withstand the wind load. Altering or replacing the anchors is not difficult but it cannot be done in the winter. Hence construction is delayed by a few months.

Once the tower (driven element) is up and the radials rolled out I will compare it to the high inverted vee so that I have data on how they compare. After that the inverted vee will be removed, at least for the time being. The tower has to be cleared of obstructions to raise side mount yagis, a priority this year.

Spring is coming

Sunday, February 4, 2018

CQ 160 With the New Antenna

Last weekend I entered the CQ 160 CW contest with the objective of running up my DXCC total and working whatever else I could find when I wasn't doing that. I not only had no intention of being competitive I didn't even notice the point structure until after the contest started. It seems that this is another contest in which US and Canadian scores are not comparable due to the population asymmetry. In that it is no different from CQ WW.

By not being competitive I was free to operate when I pleased, in whatever manner I pleased, and to walk away from the rig when it stopped being enjoyable. The last occurred when the availability of stations to work dropped off. This is typical of single band contests and in contests like ARRL Sweepstakes where you can only work a station once regardless of band. For that reason I kept to regular meal times knowing that the majority of stations would be there later.

DX results

If you peruse the claimed scores on 3830 you'll notice how it played out. My country total was  relatively high compared to my peers (single op, low power) while my QSO count was low. That is as it should be. Conditions to Europe were especially good the first night (as most participants noted) and I had no trouble running Europeans as the sunrise line swept across the continent. Indeed many stations were worked well after their sunrise. Out of 666 QSOs 130 of them were 10 pointers (between continents), the large majority of which were European. That's pretty good for 150 watts.

The final tally was 51 countries worked (not including VE and K) and boosted my DXCC count to 82 on top band. It has since climbed to 85 (LoTW shows 60). My goal of reaching 100 countries by the spring is well on track.

This is further confirmation that my antenna works, and that it very competitive with other stations. There is no surprise in this since most hams have great difficulty putting up an efficient low angle radiator on 160 meters. Another way of putting it is not that my antenna is great but that others' are so poor. Imagine what 160 would sound like if everyone could put up an effective antenna!

Running up the QSO total

If you peruse the results posted to 3830 you'll notice that many report having been active for only a brief period, some no more than one or two hours. To have a chance of working those casual operators you must be active for the maximum allowed under the rules -- 30 hours in this contest for single op entries. These stations are difficult to work them they do not call CQ and only work the stations they find or want. You must run to have a chance, hoping they find and call you during the brief time they are on.

After the first night when the majority of serious competitors have been worked it can make for seriously low rates and boredom. Yet it's necessary. Operating assisted can relieve much of the boredom since you can take a break from continuously CQing to QSY, work the fresh meat and then return to running. I learned to do this pretty well while operating this contest from a multi-op station.

For unassisted stations such as myself last weekend running can be as exciting as watching paint dry. That is why I kept stepping away from the shack. New stations are always showing up and running can be resumed later with decent rates, for a little while at least. Search and pounce is largely pointless since there are so few unworked stations that are running. Some operators combine running and hunting by going SO2R or SO2V. I didn't do this despite having this capability with the two receivers in my FTdx5000. I'll consider doing so in future.

The point is that running is mandatory, no matter how boring it gets, if you hope to do well. I avoided this for the most part since I was not aiming for a winning score.

The terminator is your friend

The proximity of the terminator, whether sunrise or sunset, at both ends of the path can be critical to understanding propagation on 160 meters. This is because atmospheric noise strongly determines success.

For example, I can hear Europeans on my Beverage antenna well before my sunset even though absorption is quite high in the sunlit hemisphere since noise and signals are similarly attenuated. Unfortunately the reverse is not true in Europe where night is well advanced and they are receiving atmospheric noise from all directions. Hence they cannot copy signals from North America. Even after our sunset terminator passes the inequity of noise levels continues for at least another hour, after which copy becomes equally good (or poor).

As sunrise approaches Europe from the east their noise level drops. At this time they begin to be able to copy weaker signals from North America. This most likely explains our success with working Europe after our midnight, and even after the sunrise terminator has passed for some stations in Europe. This is where directive receive antennas show their mettle by making it possible hear the Europeans who are then better able to copy us.

With my northeast Beverage antenna many of those who replied to my CQ in the contest at that time were barely above the noise level. Without the Beverage they would not have been copied and some might not have been heard.

Many of the so-called gray line contacts on the low bands can be attributed to reduced atmospheric QRN at both ends of the path rather than propagation enhancement. However the latter remains an important factor in many if not most cases.

Future work planned

Spend any amount of time on the low bands and you'll soon learn who the alligators are. Those stations everyone can hear yet they copy only the strongest signals. Sometimes that's due to man-made or tropical/summer QRN while other times they do not have directive receive antennas, even a small one such as pennant or flag that can fit in a small space. In a few case it's because they run excessive (and illegal) power.

I am well set up to receive European signals that are close to the noise level. I now need more Beverage antennas to cover more directions. This will be especially important when I make the move back to QRO operating since a bigger signal attracts more and weaker callers and I don't want to become one of those alligators. I want to be able to work them.

I have been surveying routes for the Beverages and making a list of parts to order to construct a remote switch to select among the Beverages. The next one will be a reversible Beverage made with coaxial cable. It is more complex than a unidirectional Beverage but saves a lot of effort overall. I have several resources from which I am adapting the design. When it's done I'll write an article about it. If it works well I'll build another, otherwise I may go back to unidirectional antennas.

My next article will be about antenna design rather than operating. Those appear to be the most popular of the articles on this blog and are the ones I most enjoy writing.  Spring is coming and I want to be prepared.

Friday, January 26, 2018

Phantom QSOs

I live in an RF quiet environment. Or at least I do now that the local utility has fixed an intermittent source of power line arcing. Because it's so quiet I hear things that others do not. Indeed I can hear quite a few stations that cannot here me at all, which is especially true on the low bands. For example, in the run up to CQ 160 contest I could copy many European stations on my northeast Beverage a full 2 hours before sunset.

Having decent antennas does not compensate for being -10 to -13 db from the QRO of many (most?) stations on 80 and 160 meters. I also have to contend with the fact that the vast majority of stations do have local QRN that raises their noise floor, making it difficult to impossible to hear similarly equipped stations. In severe cases they can copy no one. A good recent example was the 6O6O DXpedition from Somalia who often had such strong QRN in Mogadishu that they simply shut down for hours at a time.

Unfortunately QRN from many sources in our modern civilization has become unavoidable. For the majority in urban and suburban locales it must simply be dealt with. It is no surprise that many stations, and not just on the low bands, cannot hear many of the stations that call them. QRO helps them to be heard with the small antennas they can fit within their properties, but that helps not at all on receive. Nulling loops and other compact directive receiving antennas cannot perform miracles.

As a result they have difficulty working the DX or contest QSO they are chasing. It is perhaps no surprise that some will take shortcuts to achieve their goals. By this I do not mean the cheaters using remote receivers that are easily accessible over the internet -- that's an entirely separate discussion. What I mean are those who complete their QSOs with a wish or a prayer. Let me give you a few examples.
  • In the 160 meter Stew Perry Top Band Challenge there was one European station I called that managed at first to successfully copy my VE3 prefix and nothing else. This QSO would be a challenge regardless since I entered as QRP (5 watts). After a few more tries he started guessing. I attempted to correct his guesses, which he also couldn't copy. Eventually he settled on a particular call and stuck with it, going so far as to imagine the exchange and log it despite my repeated attempts to correct him. I didn't log the QSO.
  • Several times while tuning 160 and 80 meters CW I would come across a QSO between a North American station and a DX station where one or both has difficulty copying. Whether it's QRN, local QRM, a power difference or an antenna difference I couldn't know. I can copy both perfectly well which gives me a front row seat to what follows. One station sends the incorrect call and other can't tell it's incorrect. They proceed to complete the QSO after numerous turn overs, with one or both call signs incorrect! I've heard the same with contest exchanges.
  • In a DX pile up there will occasionally appear a caller who obviously does not hear the DX station very well for whatever reason. I hear a lot of this on split operations since I have dual receive, with one receiver to each side of the headphones. That operator proceeds to imagine that the DX has responded to him, despite (obvious to me) that it is another station that the DX is responding to. Usually there is some similarity between the call signs, which creates hope for our intrepid DXer. The phantom QSO is completed and logged without the DX operator's awareness.
As I said this is more common on the low bands although it does happen on higher bands. Hope springs eternal, I guess. Be sure that this doesn't happen to you. If you do not positively copy the other station sending your call sign and exchange don't let your hopes and imagination run wild. You will be disappointed when the QSO doesn't appear in their log.