Thursday, December 12, 2019

Performance of the 80 Meter 3-element Vertical Yagi

The 3-element, 4-direction vertical yagi I recently completed is not a perfect antenna although it does perform very well. It has its pros and cons. I learned a great deal designing and building it, which was one of my main objectives apart from putting out a competitive signal on 80 meters. The antenna is a variation of the K3LR array described in ON4UN's Low Band DX'ing book.

No antenna stands on its own merits; every antenna must be compared to alternatives. For this discussion of the yagi's performance I will use the big gun antenna standard for 80 meters, the 4-square.

This is the sensible baseline since it is important how I do relative to other serious contesters and DXers. It makes little sense to compare the yagi to an inverted vee -- of course it's better but the comparison is of little value.

This article is not a mystery novel so I will put the answer right up front: the 4-square is superior on the majority of metrics. That said the details of the comparison can be subtle and enlightening for those with a passion for antennas. A truthful comparison helps direct my future plan for 80 meter antennas. That will be briefly addressed towards the end of this article.

Let's start with the basics before delving into details.

First up is a fundamental of physics: conservation of energy. For antennas of equal efficiency a corollary is as follows:
To achieve gain in one direction requires taking energy from other directions. Conservation of energy informs us that to achieve gain the antenna must be directive, and vice versa. The two are inextricably linked. The 4-square's better directionality largely explains its gain advantage over the 3-element yagi.

However it is not quite that simple. Dropping a secondary lobe from -10 db to -20 db (assuming for the present argument there is only one lobe other than the main lobe) the main lobe energy increases from 90% to 99% of the energy. This is an almost negligible gain improvement of 0.4 db. Reception improves but not transmission effectiveness.

Further gain improvement requires narrowing the beam width of the main lobe. For a non-rotatable antenna like a 4-square or wire yagi too narrow a beam width can be detrimental since there will compass points where gain is poor.

Although the 4-square is more directive than the yagi the gain improvement is not substantial. The better gain of the 4-square mostly comes from other differences between the two antenna types. Both have sufficiently modest gain/directionality that 4 direction switching covers 360°.

With that fundamental observation made let's look at how the antennas differ. There are several factors:
  • Element spacing: On a side the 4-square element spacing is 0.25λ, and the diagonal spacing is 0.35λ. For the yagi the element spacing is 0.125λ. The closer spacing of the yagi increases the mutual coupling. This is required in a yagi but not is a 4-square.
  • Element shape: It is typical to use straight elements in a 4-square although that isn't necessary. The yagi has a straight driven element and sloped T-top loaded parasitic wire elements. Again, that is a choice not a requirement. Element shape and diameter effects both antennas and we will have to normalize them to make a fair comparison.
  • Forcing: Yagis work by mutual coupling alone. The 4-square uses phasing lines and combiners to engineer phase and amplitude of antenna currents. However mutual coupling exists in a 4-square and is a significant factor in its design and engineering.
  • Ground dependency: The antennas behave differently for the same radial system (near field). Distant ground (far field) effects are the same for both.
These are the major electrical factors. There are also other factors, such as cost, that must also be considered.

Notes on modelling

All the software modelling is done with EZNEC. Medium ground (0.005, 13) is used throughout even though the ground conductivity in my rural locale is better than that. A fair comparison depend on a standard environment.

MININEC ground is used rather than "real" ground so that the radial system can be easily modelled as a resistance load at the base of each element. MININEC assumes a perfect ground with respect to the near field. The resistance accurately represents the equivalent series resistance (ESR) of the radial system and ground beneath. But you have to know the ESR of your radial system. The model departs from reality for a small number of radials since they affect the antenna resonance. These effects must be compensated for during antenna construction and testing.

There is loss in more than just the ground. Wire elements have non-negligible loss whereas tower and tubing vertical elements have negligible loss. Coil, capacitors, phasing lines and hybrid combiners each contribute loss. In particular the 4th port of the hybrid combiner used in most 4-square antennas goes to a 50 Ω dump load, which can be as lower gain by as much as -0.5 db at the band edges, though -0.1 to -0.2 db is more typically .

Since this is comparable to the approximate -0.15 to -0.2 db resistance loss in the wire yagis elements I will treat them as equal, and leave them out of the antenna comparison. The 4-square model is adapted from one packaged with EZNEC uses lossless phasing lines and no combiner or dump load.

The azimuth pattern comparison is typical. The difference in practice has many factors, as listed above. For my current radial system the 2 db difference of the inner plot is a fair representation. In other configurations the difference can be better or worse and in the ideal can approach equality. We'll come to that later in the article.

The elevation patterns are similar for both antennas. This is primarily determined by ground quality and topography outside of the antenna's local environs.

Turn a yagi on its side

Verticals arrays -- yagis and 4-squares -- have relatively poor side lobes in comparison to horizontal arrays. Many of you know why that is but let's review it anyway.

A dipole has low radiation off its ends. An array made of dipole elements is the same since adding nothing to nothing equals nothing. Therefore the typical horizontal yagi has deep side nulls. In free space the elevation pattern has quite a lot of radiation directly up and down. For a typical 3-element yagi in free space the blue plot is the elevation pattern and black is the azimuth pattern.

Over ground the way to remove the high angle radiation is to place the yagi at a height that is an odd multiple of λ/2 so that the ground reflection is out of phase with the direct wave resulting in cancellation. At intermediate heights there can be substantial high angle radiation, and that is rarely desirable.

Rotate the boom 90° and the elevation and azimuth patterns are swapped. That is in essence what you have with a vertical array: lots of radiation off the sides and very little at high elevation angles. In a conventional vertical yagi like my 3-element 80 meter antenna radiation to the sides is worse than shown in the adjacent plot.

For a driven array such as the 4-square it is possible to reduce the side lobes. Thus a 4-square can have better directionality than a 3-element yagi. Of course the 4-square has one extra element, which may seem an unfair comparison until you consider that the two antennas are of similar size.

Element spacing

Comparing element spacing of the two antennas can be confusing since although they occupy a similar area the yagi has a fifth element in the centre -- the driven element -- and two of the elements are inactive. Further, because two elements are inactive the 0.35λ spacing between adjacent parasitic elements is irrelevant. The element spacing is 0.125λ for the yagi and 0.25λ for the 4-square, a ratio of 2.

The significance is that the mutual coupling between yagi elements is higher than the 4-square. The elements can be more widely spaced to equalize the antenna footprints. This would increase the boom length to 0.35λ (0.175λ element spacing), a length that is near optimum for a 3-element yagi. That does indeed improve the yagi's performance, as we'll see.

In addition to achievable gain the increased spacing modestly improves F/B. Of perhaps greater importance is that the mutual coupling is reduced which increases radiation resistance, and that lowers antenna currents and I²R ground loss. Driven at 1000 watts the typical 4-square element current is ~2.5 A. Currents in the yagi elements cover a wide range, from as low as 1.5 A to as high as 9 A, with more typical values between 2 A and 7 A.

The gain improvement of 0.175λ yagi element spacing is ~0.6 db (perfect ground), which is marginally significant. Reduction in ground loss results in greater efficiency for the same radial system. Gain improvement is greater with a poor radial system and less with a better one.

Element shape

The sloping T-top wire parasitic elements are convenient since it uses the driven element as the support structure. It comes at a performance cost since the element shape is not optimal. There are two problems:
  • Radiation resistance: The acute angle on the lower side of the T causes field cancellation with the monopole part of the element. Field cancellation lowers radiation resistance and this increases loss in the radial system and to a lesser amount in the element wire.
  • F/S: There is a horizontal component to the azimuth pattern due to the T which lowers overall directionality by increasing radiation to the sides and rear.
The lower acute angle (close to 45°) requires NEC4 for accurate modelling. With the more commonly used NEC2 there is a significant discrepancy so the wire elements dimensions must be determined in the field. NEC4 isn't perfect but you will get close.

Modelling with EZNEC predicts an approximate 5 Ω reduction of radiation resistance from 31 Ω to 21.5 Ω. compared to a straight wire element. With my analyzer I measured 25 Ω including an estimated ground loss no worse than 5 Ω. For a 5 Ω radial system the loss is 16% versus 14% with straight wire elements. Although that's small the loss multiplies for poorer radial systems and in a yagi where the radiation resistance is lower and the current higher.

A comparison of straight wire elements versus the T-top wire elements was discussed in a previous article. Look there for the relevant charts since I won't reproduce them here. You will see that directionality and gain are better with straight elements, especially directionality . Unfortunately straight elements are not easy to make from wire due to the need for suitable supports. A coil loaded shorter straight element is feasible except that efficiency is far worse. If you go to the trouble of rigid parasitic elements I believe it is more sensible to build a 4-square rather than a yagi.

Forcing

Yagis rely on mutual coupling alone to achieve current amplitude and phase for desired behaviour. Current forcing is a feature of driven arrays. Since there is substantial mutual coupling in a 4-square it is not a purely driven array; the elements would have to be much farther apart for that.

Forcing is simple in its basic concept. The generator always sees a single impedance. By tying all the elements to the feed point the amplitude and phase is uniquely determined. Networks between the feed point and elements set the amplitude and phase to achieve the desired behaviour.

It is quite complicated since you want a 50 Ω load for the generator and accurate power splitting and phase across 4 elements with network that must sustain complex loads (high voltage and current) at high power and with direction switching. Elements must be made as identical as possible. Not many hams design and build their own 4-square control systems!

The EZNEC model used in this article is adapted from one provided by W7EL with the software. It uses fixed phase lossless transmission lines. This is impractical for real antennas due to the direction switching challenges and the frequency sensitivity of the phasing lines. Hybrid combiners are more suitable.

More than you could ever want to know about 4-square design and hybrid combiners can be found in ON4UN's Low Band DX'ing book. For the present discussion I will only mention a couple things. First, the phasing lines experience high SWR and have attendant losses, although those are low with good quality coax at 3.5 MHz.

Second, hybrid combiners are not lossless since frequency dependent imbalances among the 4 elements present at a 50 Ω port where a dummy load dissipates the power due to the imbalance. A failure in one element or icing can cause a large increase in the the dump power. Monitoring or protective circuitry is important.

Modern 4-square controllers usually offer an omni-directional mode in addition to the 4 directions, just like I built with my 3-element yagi.

Ground dependency

Ground ESR in series with the antenna impedance is the most important factor affecting the yagi in comparison to the 4-square. For the same radial system the ground loss for the yagi is higher, and it can be substantially higher. That is due to the low radiation resistance due to the aforementioned factors: element shape and mutual coupling. The better the radial system the closer the yagi's performance to that of a 4-square.

The yagi should have a radial system ESR of less than 5 Ω and lower is highly desirable. In my antenna I have twice the number of radials on the driven element as the parasitic elements since currents are highest in that element. Current in the yagi elements can be more than 3 times higher than in the 4-square. If you are limited in how many radials you can put down go with the 4-square.

Measuring the ESR of a radial system is difficult. My estimate for those in the yagi is based on the trend line of element self-impedance as radials are added. This is a common technique and usually the only practical one. The measurements suggest that the driven element radial system is in the range 2 Ω to 3 Ω, and that of the parasitic elements 4 Ω to 5 Ω. For modelling purposes I use the values at the high end of these ranges.

I will keep it simple and state a few modelled comparisons rather than draw up a bunch of charts. As a baseline with a perfect radial system of 0 Ω the 4-square has approximately 0.5 db more gain than my style of yagi, assuming the previously described internal loss typical of the 4-square and yagi. For a 5 Ω radial system the 4-square gain declines by 0.5 db and the yagi gain declines by 2 db. Therefore with a large but not extreme radial system the 4-square gain is better by 2 db. There is frequency sensitivity in these figures for the yagi so I took the average.

That's a substantial difference. With a smaller radial system the difference will be larger. You need a lot of radials to make the yagi perform well. As I said in an earlier article that although the directionality of the yagi is lacking it is of little consequence in contests since I can work stations off the back and sides with good success and that puts more QSOs in the log. Receive performance is compromised so it is occasionally helpful to use a high directionality receive antenna.

Pros and cons vs. the 4-square

This list is a set of subjective and objective observations of the yagi versus the 4-square. You may disagree with some points or weigh their importance differently.

Pros:
  • Low cost
  • 20% less land use
  • Flexibility of direction choice, more than 4 directions, and ability to add more directors
  • No dump load or phasing harnesses: all the power is radiated or lost in the ground
Cons:
  • Lower efficiency for the same radial system
  • Must home brew: there are no commercial control systems
  • Directionality and gain
I believe most hams would, and should choose the 4-square. It is possible to build it with wires to reduce the cost with external supports or with elements similar to that used in my yagi.

The decision is not quite so straight-forward since my yagi design is not the only one. The yagi can be improved in various ways.

Alternatives

The yagi will remain as it is for some time. There are too many antenna projects for the next year to worry too much about 1 or 2 db. What I can do, now that I am indoors more often due to the weather, is to explore alternatives. Alternatives range from the highly disruptive to modest.

Taller centre support to allow straight (unloaded) elements

Straight wire elements increase gain by ~0.5 db. Side lobe radiation is reduced almost to that of the 4-square. Directivity is improved so that that it comes close to that of a 4-square though frequency dependent. The taller central tower can serve as an efficient 160 meter antenna with suitable switching to retain its performance as an 80 meter omni-directional vertical and yagi driven element.

Increase boom length so that it covers the same area as the 4-square

Element spacing increases from 0.125λ to 0.175λ, for a boom length of 0.35λ. Gain increases ~0.6 db and directionality is improved. Of course the radials for the parasitic elements must be relocated, and that job takes several days. The increased spacing permits converting the array into a 4-square. The central tower can be used as a simple support, or continue as an omni-directional vertical (if the commercial 4-square controller doesn't have this feature) or as a 160 meter vertical. For the latter case modelling confirms that a central 160 meter vertical does not affect 4-square performance on 80 meters.

Bent elements

Removing the lower half of the sloped T-top loading section on the parasitic elements has several advantages. Parasitic element efficiency because the radiation resistance rises from 21.5 Ω to 31 Ω. Gain increases 0.6 db with the existing radial system. Directionality is within a few decibels of a 4-square. There are no mechanical changes. Parasitic elements must be tuned for different self-resonant frequencies and the L-network adjusted to compensate for the higher feed point impedance.

More radials

This is perhaps the simplest alternative. By doubling the radial count the ground loss is reduced. In addition to a gain increase of 0.6 db the directionality is modestly improved. Apart from laying the wire the L-networks must be adjusted for the lower feed point impedance and the parasitic elements retuned to the desired self resonant frequencies. Wire isn't always cheap and doubling the radials will require 1600 meters of wire and many days to install them.

Paths forward

The described alternatives can be combined for further performance improvement. For example, by doubling the radials, using bent elements and a 0.35λ boom length the gain comes within 0.5 db of a 4-square and directionality is similarly close.

One or more of the alternatives will be explored in depth in future articles. This one is already long enough and I don't have the time right now. Although I may eventually go for a 4-square I am not done exploring yagi designs. Modifying the existing yagi is far easier than rebuilding.

A little more gain and directionality would be beneficial, especially on receive. It would be advantageous during contests to keep receive antennas primarily for 160 meters to reduce contention between operating positions. In contests I often use the northeast Beverage while working Europe to improve copy of the weakest callers.

In conclusion there are many ways in which the yagi can be improved and experimented with. In that light this is an ideal antenna for me and my interest in antenna design. A 4-square with a commercial switching system is not so interesting to me. Others may have different objectives.

Wednesday, December 4, 2019

1,000,000 Point Coax Connector

The weather broke for long enough for lowering the XM240 40 meter yagi from my Trylon tower. I previously noted that I lost the antenna during the CQ WW CW contest, probably due to a loose connection at or inside the balun. It has happened once before. Losing this antenna cost me a lot in the contest since I had no backup 40 meter antenna due to ongoing antenna work.

The first day I rigged a winch on the ground to handle the lowering and lifting of the 75 lb antenna. My lawn tractor could not be used because its weight is too low for traction on snow. On the second day I climbed the tower with the cable and other ropes to rig the antenna. Before doing that I disconnected the coax at the rotation loop.

It's a good thing I did that first. I had a good look at the connection once I had the weatherproofing removed. At that point I could have dropped the cable and rope to the ground since I knew I wouldn't need any of it.

The problem was the connector. In a way that was fortunate since I really didn't want to lower and lift this yagi in the cold and snow, not to mention the difficulty of getting friends out in this weather and their increasing family commitments due to the approaching holidays. It's a busy time of year.

The UHF female barrel connector is old and not the best quality. Notice that the outer edge has only 4 indentations; the best have a continuous set of indentations. There are typically 2 or 4 matching tangs on the male PL259. When engaged they resist the connection from unscrewing from twisting and vibration. Failure to engage those tangs promises future problems, perhaps sooner than you think.

On one side you can see the orange discolouration and grime from past water damage. The dielectric should be white. Despite the appearance this is the side that is working properly. We have to turn it over to discover the proximate cause of the problem.

At first glance it looks good. A closer inspection reveals discolouration surrounding the centre conductor. By eye (obscured by the camera exposure) it is clearly visible. The same discolouration is on the centre pin of the mating PL259 from the rotation loop. It comes from vapourization of the metal plating.

Sliding the two connectors together and apart a few times tells the tale. There is almost no contact between them. The springiness of the female receptacle tabs is gone. This leaves a gap and hence the intermittent continuity, and arcing when running high power.

The barrel connector is old. How old I don't know, possibly decades. I pulled it out of a bin of new and used barrel connectors I keep handy I ought to have tested it before putting it to use. It is the same barrel connector that was on the yagi when it was atop the big tower, where it also was occasionally intermittent.

With the weatherproofing on the feed point side in good condition I never inspected it as I moved the antenna from tower to tower. The intermittency followed. Each time I managed to convinced myself that other known problems such as dirty relays were responsible. Assumptions are dangerous weapons.

I picked a new (and better quality) barrel connector from the bin, tested it and installed it between the rotation loop and the coax running along the boom to the balun. Done? No, not yet. Troubles never travel alone. They enjoy company.


After testing the new barrel connector -- it was good -- I proceeded to inspect the connector on the rotation loop. It was not good. In addition to water damage there was evidence of arcing. Worse yet the body of the connector rotated. Pulling back the outer shell I saw that there was more corrosion inside the PL259 solder holes and the solder had no hold on the coax shield.

Now thoroughly irritated with myself and the chill from working on the tower in cold weather I removed the rotation loop and descended. In my workshop I pulled it apart and quickly realized I had to discard the coax and connectors. I made a replacement with better connectors and, after testing it, ascended the tower. Although it was a chilly 0 C with snow flurries it was would the best weather for close to a week. So I kept at it hoping to get it done before sunset.

After successfully testing the rejoined sections of coax with an analyzer I did a careful job of weatherproofing with my best materials, then routed and secured the new rotation loop. I descended and cleaned the site in the deepening dusk.

With some trepidation I entered the shack and moved the rig and antenna switch to 40 meters. This time the antenna worked perfectly. Assuming I did it right this time the repair should last. An hour later I made my first QSO with 4U1UN for a new one, counting from when I returned to the hobby in 2013.

I call it my 1,000,000 point coax connector (connectors actually) because that it cost me at least that much in the recent CQ WW contest. My claimed score is ~3.5M. Had I done comparably well on 40 meters the estimated additional 700 contacts and 40 multipliers would have lifted my score to about 4.8M.

According to the raw scores that would have moved me from position #55 to #37 in the single op unassisted high power category. That's a big jump though still not a great score. The higher score certainly wouldn't win me a plaque, not even for Canada. It's annoying though not a disaster. I had no illusions about winning or placing high in the standings.

In a big station there are so many parts that some mistakes and unexpected faults are inevitable. When the mistake is up in the air at the end of a long boom it can be costly. An expedient decision at a critical moment is all that it takes. It isn't easy to force yourself to slow down and test every little thing yet it saves time and pain in the long run.

I'll try to do better. We all should.

Saturday, November 30, 2019

CQ WW CW: What I Learned

Despite being at the bottom of the sunspot cycle the bands came alive this past weekend during what is arguably the biggest contest of the year: CQ Worldwide. This was the CW weekend. The contest was an opportunity to test out my station. In pursuit of that objective I turned on the amplifier. Comparison with the best stations and operators is enlightening.

I had no illusion about doing well. My station is incomplete, both inside and out, and I have just the one amplifier and so SO2R is handicapped. Worse, during the contest I lost one of my antennas due to a known intermittent that I did not yet find the time to repair. Nevertheless I soldiered on, operating for 42 hours out of 48.

In the first part of this article I'll run through the bands to allow focus on antenna performance. In the second part I'll cover everything else from propagation to station equipment and the lessons to be learned therefrom. No matter how long I've been a ham and a contester there is always something new to learn and, ideally, use the knowledge to do even better next time.

What I won't bother with is my claimed score and placing. That would only be of interest to me and so would bore readers, not to mention that it is not impressive. Whether or not you find these lessons useful I hope you will enjoy following along.

160 meters

Lately I've taken to operating on 160 more often with the amplifier. Having a close to full size vertical makes me more competitive in the pile ups. When conditions are favourable it is not difficult to generate long QSO runs on top band almost any winter night. I expected to do well on 160 during the contest and I did. Had I been better configured for SO2R my QSO total would have been higher since it would have allowed me to do more running of US stations.

My country total of 68 is excellent despite the best stations in this part of the world exceeding that by 40% or so. Americans run 2 db more power and assisted stations benefit from spotting and skimmers. The best stations have antennas with gain, utilizing 2 or 3 elements yagis and a few have full size 4-squares. Those I'll never compete with. Unlike my first forays with my 160 meter antenna in contests I am pleased to confirm that the antenna is truly competitive.

The Beverage antenna to Europe continues to pay dividends. I did not put back up my short west Beverage and I have not yet had the time to put up other receive antennas. Perhaps this winter. Having good ears is critical to working the many stations with lesser antennas that can hear me but have difficulties putting out a powerful signal due to the inefficiency of the small antennas that are typical on top band. That conditions were excellent Saturday evening certainly helped but since it everyone has the same benefit it did nothing for my competitive placement.

To do better is difficult and expensive. While I do plan on certain improvements those are low priority and are unlikely to provide more than a few decibels of gain.

80 meters

One unexpected lesson is that when running high power the 30 meter high inverted vee is useless: I do very well with the vertical yagi alone. This is despite its superiority for working nearby US stations and the comparative advantage of horizontal polarization before and just after sunset.

The inverted vee has a maximum advantage of perhaps 10 db, which is compensated for by the 10 db of the amplifier. Since these signals are already quite strong the extra 10 db isn't needed and mulling over which antenna to use is a distraction. The inverted vee remains valuable for low power and QRP contest operation and for select DX paths, just not for high power contest operation.

The 3-element vertical yagi performed very well. Although I cannot say whether my results -- 800 contacts and 76 countries -- would have been substantially lower with high power alone. With rare exceptions if I heard it I worked it. In many of the cases where I couldn't work a station neither could many of the big guns I heard calling at the same time.

A good example is the Sunday morning opening to the far east. There were many weak Japanese and a couple of RT0 stations that were being heard here yet they could hear few of us in this region. A few more decibels are needed to compensate for the difference in band noise: low here post sunrise and high there post sunset.

I will have more to say about these elusive decibels when I write my article about performance of the yagi and how it compares to alternatives. Overall it performed very well. I could easily establish long runs to Europe and its modest directivity allowed Americans to hear me when I was pointed northeast. I'm happy.

40 meters

As noted above this was my disaster band. Had I been seriously competitive I might have quit or refocused on a single band effort. Since this contest was a learning experience I persevered with this severe handicap. It is not possible to do well in this contest without 40 meters. The estimated loss was at least 700 contacts and 40 multipliers.

Unfortunately I had no backup antenna. The 80/40 fan inverted vee was converted to 80 meters only when reinstalled and the new rotatable dipole for 40 meters is not complete.

This is entirely my fault. When I had the XM240 on the ground I checked the connections and all seemed good, despite knowing that the Cushcraft balun previously experienced internal loosening of connection studs. My inspection was cursory because I had discovered a faulty relay in the antenna switch in the port used for this antenna and assumed that was the problem.

The fault was rediscovered soon after the antenna was raised to its new location. I isolated the problem to the antennas itself and knew it would have to come down. When the intermittent went away I delayed the work to focus on other projects (80 meters, new 15 and 20 meter yagis, etc.).

Now the weather has turned foul. These words are being typed just after I called my friends to cancel the repair job because of ice on the tower and a howling north wind. It will get done. After all, I originally put this antenna up in January!

20, 15 and 10 meters

At present I have two tri-band yagis for these bands: TH7 up 43 meters and a TH6 fixed approximately south up 22 meters. Unfortunately this is a poor combination for SO2R with high power and my limited filtering so I had to stick to one high band at a time. Had the 40 meter yagi worked I could have put a second radio on 40 late in the afternoon when both 20 and 40 meters were productive.

One thing I noticed is that just like on 80 meters, despite the high directivity of the yagis, with high power an enormous number of stations could be worked off the back. I could run Europe and US at the same time without having to switch antennas. When I heard a multiplier I called and worked them no matter the antenna direction.

Again, that 10 db power boost makes this possible. While any power boost is beneficial in this regard the bump up to a kilowatt is the ultimate since that is everyone's maximum power level. That is why, with a few exceptions, you can work it if you hear it.

There is really little more to say. The antennas worked well for what they are. One surprise is that the low yagi was superior on 10 meters towards the Caribbean and South America during Saturday's opening. This is unusual. Perhaps the reason is that sporadic E provided the first skip and that is typically better at higher elevation angles.

I expect improved results and operating flexibility when the new stacks are operational.

With the band by band breakdown covered I'll now move on to more general topics.

Running

It is no surprise that with high power running is easy when propagation exists. Indeed it is mandatory. From a strategic standpoint the challenge is not so much whether to run but when to run and when to hunt for multipliers and other stations. For those in an assisted class you learn to interleave calling spotted stations into the ongoing run, or runs in the case of SO2R.

Regardless of your strategy do not be so enamoured of your audience that you forget to hunt others form time to time. You may forget due to the joy of having many new mults call you when you run with a big signal.

At this stage in my education I run only one band at a time. The second station is for S & P. So far the only exception has been Sweepstakes CW where my rate was low due to running QRP. During CQ WW I almost always abandoned the S & P station when I had multiple callers to my CQ. I need more practice and better SO2R equipment.

Running is hectic since a bigger signal draws more callers. Common difficulties includes several callers zero beating each other and those who continue to call when I respond to someone else. The bedlam is a challenge even though it isn't nearly as bad as what DXpedition operators endure. Spot clickers may not call twice in a row, opting to click another spot when they fail to work you. They come back later. I've done the same when I was in an assisted category.

Although running can be fun and productive it is also a chore. The faster you can service your "customers" the longer they'll stick around to work you and the better your rate. For example, if you copy a partial call -- multiple caller QRM or distracted by the other radio -- it is faster to respond to the partial call with a full exchange, copy it in full next over and confirm the correction in the solicitation for the next QSO. Soliciting repeats to get the full call before sending the exchange should be limited to cases when only one or two characters are heard. The solicitation also tends to incite others to try again, and you don't want that.

It is good practice to send your call at the end of every QSO: "TU VE3VN". Passersby hear it and stop. I may interrupt the message after "TU" for one two QSOs when I have a few callers in the queue to work them faster and encourage them to stick around a few more seconds. You may reduce the bedlam by not sending your call as often -- passersby pass by when they don't hear it -- at the price of missing some of the S & P crowd. Try it both ways and then use your discretion.

Power allows me to hold a frequency. Other big guns are wary of getting too close or trying to steal attractive real estate at the low end of the band. Conflicts do occur and must be dealt with. I continued to do a lot of running high in the band since many small stations like to avoid the noise and crowding.

Operating crutches

In unassisted class spotting networks and skimmers are not allowed. Others do use them. No matter what frequency you call CQ the assisted operators will quickly find you. Don't be discouraged when starting a run attempt that little happens for a minute or two.

The very same tools that deliver QSOs to your frequency can occasionally be the cause of unwanted problems. Skimmer and human spotter are not perfect. A mistake in your call will begin a string of dupes. Eventually they'll realize the error and skip over you. Until then be prepared for those dupes. In this contest there were a few times during which every second QSO was a dupe. Just work them since it's quicker than trying to explain the problem.

Another crutch is the master database of call signs known to operate contests. These are collected from submitted contest logs and distributed to the contest community. Hence the Super Check Partial database (SCP). In the past I avoided using SCP since it felt mildly unethical to have the computer present alternatives calls in case of copying errors or to confirm the potential validity of a call.

I have been using SCP for the past year. Although a crutch it does save some effort and that can stave off fatigue. Unfortunately when there are many similar call signs SCP can be more confusing than helpful. It is better to correctly copy a call sign and not lean too much on SCP. On the plus side it can trigger me to ask the other station to confirm their call when they are not in the data base.

In one instance this weekend the received call sign had a single close match in the database. Since it differed by just one dit from what I copied through the QRM I sent back the call sign suggested by SCP. The other operator energetically corrected me. The database was wrong and I was right. Perhaps the log that contained his erroneously copied call was not filtered out when the master database was built. Learn to trust your ears.

Many use call history files to pre-fill the exchange. These files can be built from your own logs of previous contests and there are publicly available history and country files. My current opinion is that this is a crutch too far. I don't use this feature. In any case it is not very useful in CQ WW since the exchange, other than 599, is the zone number. The zone can is in most cases uniquely derived from the call sign. That is not true for the US and a few other countries and regions. Again, learn to trust your ears and rely on that rather than blindly accepting the pre-filled information.

SO2R

My primitive SO2R setup is fine for getting started. That will change. It will include more equipment, station automation and practice, practice, practice. I am exploring options and expect to be in better shape by the end of the current contest season. I will continue using two keyboards.

I discovered early on that the second radio was not very useful. Since I have only one amplifier the second radio is 100 watts. That's fine if you're low power and not so fine otherwise. You cannot expect to get through on the first call or even the second or third. It gets tedious with a handicapped second station. It was also not possible to effectively operate on two high bands at the same time since with just two tri-band yagis, one of which is fixed south, with only select multipliers available on 15 and 10 meters due to propagation.

When the 40 meter yagi failed the possibility of operating on 20 and 40 meters at the same time vanished. By the time 80 meters opened there was little left to pursue on 20 meters. Operating on 80 and 160 at the same time seems attractive but not with 100 watts. Low power on the low bands results in a low rate and the high frustration. Although I love low power and QRP contesting it is a poor fit when the other radio is running a kilowatt.

When the running was fast on 20 meters I found it difficult to tune and listen to the second radio. I am not yet that skilled. In the end my SO2R operation was less than 10% of the time. It wasn't a significant score booster in this contest.

When you work an SO2R operator don't be surprised at the curious delay before their responses. The best operators run on two bands almost seamlessly except that many transmissions must be slightly delayed to prevent having two transmitters on at the same time. At first it may be mystifying since you don't hear the other QSO. Rather than fret about it be amazed that they have this advanced skill and can do it for hours on end. Although talent helps we can all do it if we have the drive and put in the work.

Amplifier

Operating the amplifier for 48 hours straight in a major contest was a risk. My primary concern was the T/R relay, which is original (over 40 years old), loud and aggressively repaired using sandpaper a few months ago. There was no point in putting off the inevitable so I took the risk. It performed flawlessly.


To speed band changes I put sticky notes on the load and tune controls and marked the positions for each antenna, band and select frequencies where it mattered (mostly the low bands). You'll get close but you won't hit the perfect spot doing it this way. Once settled after the band change I would often tweak the tuning to be sure the amplifier was operating at maximum efficiency.

Before the contest I ran through the bands to make the labels. You must do this at full power or the power you intend to operate since amplifier tuning is power sensitive. I fixed the transmitter power to 65 watts since coarse tuning at low power was never necessary and the fixed input power ensured that the markings could be relied upon. It is a good idea to remove the sticky notes after the contest so that the glue doesn't mar the front panel due to heating of the glue by the amplifier.

Fixed input power is helpful for rigs that do not have a front panel power control. Going into a menu to repeatedly change the power for amplifier tuning is a tremendous inconvenience. The FTdx5000 has a power control while the FT950, my second radio, does not. The lack of a power control is not unique to Yaesu. Consider that when shopping for a rig if you have an amplifier.

Another inconvenient fact about the FTdx5000 and many other radios is that there is no front panel control to transmit a carrier. I rigged a foot switch to a rear panel plug so that I could tune the amplifier. There is a keyboard feature in N1MM Logger that will do this if you have no hardware mechanism to generate a carrier.

Propagation

No matter how good your antennas there will always be stations you cannot work or that are the limit of intelligibility. However the better your antennas more stations are workable and fewer are unworkable. The weak ones will still tease you, but without better propagation there may be no hope. The only cure is to get on a plane and operate from the tropics.

XKCD
Eastern VE3 is better than some and worse than others with regard to HF propagation. Many paths north of an east-west line traverse the auroral zone so that we are at the mercy of geomagnetic activity. At all times and especially during a solar cycle minimum stations a short distance east and south can enjoy remarkably better propagation on those northerly paths. By short I mean as little as a few hundred kilometers. Many the time I could only listen as many W1/2/3/VE1 stations work what I cannot hear or hear weakly.

This contest is no different. Many stations I am familiar with, including low power ones, could either put hundreds more European QSOs in their 20 meter logs or work many more zone and country multipliers. The difference is reduced when the sunspot count climbs and we are more competitive. This is with some decent antennas and heights. Some VE3 stations with better antenna farms do better than me but still not as well as their better located peers.

Skew propagation is well known on the low bands although I noticed little of it during the contest. Instead there was skew path on the high bands. This may be less well known yet it is common during marginal conditions. Europeans on 20 meters in our afternoon were peaking towards the east. This is usually ionospheric scatter from areas with a high enough MUF and not a true skew. A similar phenomenon occurs at sunrise when the rule is to "shoot the sun" on the high bands since that's where ionization is densest. There is also back scatter to nearby stations when we all beam to Europe, North America or elsewhere. This may be the only way to work them on the high bands when skip is long.

Unfortunately back and forward scatter is attenuated relative to a direct path. You need high power to have good results and even then you will likely only work the biggest stations. But it's better than not working them at all. Auroral zone scatter is also common on arctic paths. This was responsible for the strong Scandinavian signals and a BY in zone 23 that I worked on 20 meters deep in their night times.

Those with knowledge of these probable though not certain propagation phenomena can boost to their multiplier count. You can do it too by paying attention during your everyday DXing and using that experience during the contest.

Taking breaks and comfort

With high power there's always something to work no matter the band or time even when propagation is poor. If you can operate for the full 48 hours your score will show it. In the extreme some forgo food and drink to avoid the inevitable. For the humans among us it is necessary to take an occasional break. To keep your butt in the chair (BIC) you need every comfort you can manage. I am not that fanatical although I do pay attention to aids that keeps me in the chair.

One recent change was a new headset. After some consultation I purchased the Yamaha CM500. Although I was most interested in a robust cord I found them so comfortable that I could keep them on my head for long periods. This was not true of my previous headset the Koss 45 which would make my head ache after a few hours from the pressure against my glasses and skull.

I need a new operating chair. The old wooden office chair I've used for many years was comfortable when I was younger but no more. I need one with better ergonomics. Speaking of ergonomics I suggest paying close attention to the desktop. You need the table top (or your chair) to be at the optimum height to avoid fatigue from typing, sending CW and operating the rig. The less you have to swivel your head or chair to reach something the longer you can remain comfortable.

When you really need a break take it. Do not torture yourself. A few minutes walking around the house and talking to family members will refresh you. There is no need to take an official 30 minute break; just accept that you'll miss a few QSOs. Pour yourself a glass of water and head back to the shack.

I am fortunate that I can get by with little sleep. Each nights I only slept 2 hours, approximately from 4 to 6 AM (09Z to 11Z) when there is little to work after the low bands close to Europe and before the morning grey line opening. Speaking of sleep be sure you have an alarm clock that reliably wakes you up. Contests have been lost this way.

Wednesday, November 20, 2019

80 Meter Vertical Yagi: Completed

From first models to final design to final antenna took three years. It isn't that I was so slow than it was relatively low priority to putting up towers and other antennas. Construction began almost 2 years ago, then abandoned over the winter, and continued in earnest in 2018. When that was done I had a manually switched 3-element vertical yagi. For me that was an accomplishment, and I enjoyed the fruits of my labour over the winter.

What I didn't do last winter was to complete the array. The missing piece was the switching system. Its components include:
  • Switchable L-networks for the yagi and omni-directional modes
  • Tuned, parasitic element switching system: off-line; director; reflector
  • Switching matrix: diode array to select relays for the selected mode and direction
  • Control unit: manual switch in the shack to select mode and direction
All that work is now complete and the antenna is fully operational. Barring any repairs due to construction errors the antenna is complete. It works and it's a lot of fun to use. With this project out of the way I can concentrate on raising yagis, weather permitting.

In this article I'll step through the construction, testing and tuning. After reading this you'll likely wonder why I didn't build a 4-square with a commercial control system. Good question. For now I will only say that I enjoy designing and building antennas -- as should be evident from this blog. This antenna is an interesting and economical way to an 80 meter array with gain. I've made progress since the day it was little more than a giant pile of parts.

Parasitic element switch boxes

When I first tuned the parasitic elements and successfully rigged the antenna as a yagi I gathered sufficient data on the ground and on the air to ensure performance would be in accord with the design. Over the winter I gathered parts and refined the design of the switch boxes. Only then did I proceed to build the switch boxes.

The units are almost perfect clones. That is important to achieve near identical behaviour in all 4 directions. The coils were designed by software and a prototype was built. It was modified until it exhibited the exact required frequency shift -- from 3680 kHz (director) to 3450 kHz (reflector) -- when installed at the monopole base. With one working to my satisfaction I built the other 3 to the same specifications and confirmed that they behaved identically.


Box details are shown in the two above pictures. A few points about the design are worth mentioning:
  • Coil Q is not critical since the loss is low. Even at a kilowatt the estimated dissipation is less than 10 watts, and perhaps a little more due to dielectric heating of the PVC form. Insulated solid AWG 14 THHN wire is used. The insulation prevent winding shorts and ensures consistent winding pitch.
  • Wire routing and gauge is roughly identical to ensure all conduction path lengths are equal, and therefore frequency shifting is identical. AWG 18 wire is used for RF, which is easy to work with and more than sufficient in these short lengths to handle high power.
  • Stainless steel fasteners provide the studs for the monopole wire and radials. A solder lug is placed under the screw head inside the box.
  • The relays are sealed SPST-NO 12A in a PCB mount package. They are inexpensive when purchased in quantity. The pins are directly soldered, with the wire providing mechanical support. These relays are perfectly good when placed in a low impedance (hence low voltage) antenna point, just as they are in antenna switching products. When the element is floated the voltage across the contacts of the bottom relay is very low since the element is non-resonant. This was confirmed using EZNEC.
  • There is a hole in the bottom for the Cat5 cable and a pair more for the cable tie to hold it in place. These holes double as weep holes for moisture. The coloured wire pairs connect to a small terminal strip. If corrosion is a concern the screws can be coated with dielectric grease. Don't use silicone or other heavy duty gunk or you'll regret it when it comes to maintenance.
  • The boxes and coil forms are PVC. They are UV resistant and the box has a rubber gasket. The small 4" × 4" × 2" size is perfect for this application.
The boxes are mounted on a short length of scrap pressure-treated lumber which is turn is clamped to ~1 meter of rebar that is pounded into the ground inside the round PVC pipe that serves as the radial hub. You can study the physical construction in the picture below.

Holes are drilled into the wood to serve as a strain relief for the wire element. This is more reliable than wiring it directly to the box.

A length of wire beyond the connection point allows the element to be lengthened when more radials are added, which will raise the resonant frequency. The wire stub does not affect element resonance or performance if it's short with respect to wavelength; I tested this with up to 40 cm of wire stub with no problem. The end is pushed through another hole in the wood to keep it from coupling with the radials (and lawn mower blades).

Element tuning

While testing an element all the other elements, including the driven element, are floated; that is, disconnected from its radials and, in the case of the driven element, the transmission line.

A 9 volt battery powers the relays. Modern sealed relays with a 12 VDC coil reliably close with as little as 8 volts. The battery is small and portable, ideal for this testing in the field. Check the voltage periodically since it will wear out powering many relays.

Clip lead length does not affect tuning since DC common is isolated from antenna ground. The extra wire needed to insert the antenna analyzer between the radial hub and the box does matter. On 80 meters the frequency shift is ~10 kHz for every 6 cm of wire. The effect of the extra wire is therefore predictable and can be compensated for during tuning. For the best impedance measurements the analyzer should be connected on the radial side of the box.

All the coils were individually tested on one parasitic element to ensure they shifted resonance from 3680 kHz (director, with the coil shorted or out of circuit) to 3450 kHz (reflector). The same coil location and wiring topology in all boxes (see above) ensures identical behaviour of the elements.

With predictable coil performance all I had to do was adjust the monopole wire length to set the resonant frequency. The bottom relay connects the radials to the monopole through the coil to make the element a reflector, and floats the element when not energized. Energizing both relays -- ground the element and short the coil -- the element is a director.

The tarp provides a clean surface for tools, equipment and small parts. It further reduces the risk of ticks which are quite common here from May to July.

Tuning proceeded surprisingly well. All the elements achieved the 230 kHz resonance shift within measurement error. Using the 6 cm per 10 kHz rule mentioned earlier each element took only one or two adjustments. For example, to raise resonance from 3420 to 3450 kHz a knife is used to remove insulation 18 cm up the wire and then reconnected to the box.

The driven element resonated substantially lower than before due to the new longer stinger. Without a matching network the driven element has an SWR below 2 from 3.5 to 3.8 MHz. The resistance ranges from 29 Ω to 34 Ω and the resonant frequency (X = 0 Ω) is 3680 kHz. That this frequency is exactly that of a reflector element is purely coincidental is irrelevant to antenna behaviour.

Switching matrix

Direction is set from a control unit in the shack by placing +12 VDC on one wire and common. The control cable is Cat5 rated for UV and direct burial, in the same trench as the LDF4 Heliax transmission line. The power supply is 13.8 VDC which will be lower at the antenna. Voltage drop is due to the switching diodes, RF chokes and AWG 25 conductor resistance.

In some instances it may be helpful to draw up a voltage budget to ensure that switching systems work as expected. You can often get away with smaller gauge wire -- more economical -- than may be specified for commercial products.

Although I have yet to measure the voltage at the relays they are rated for full closure down to ~8 volts and there is no problem. Testing and tuning with a 9 volt battery in the field worked well, even as the battery aged and sagged down to 8.5 volts. 


The prototype board on which it is built is sitting on the circuit diagram. The 4 wires at the bottom are for direction selection: NE, SW, SE NW. Each side of the board is for a pair of opposite elements, with the 2 wires destined for the parasitic element switch boxes, one for grounding the element (make it active) and the other to short the reflector coil (make it a director). The other 2 elements are floated to be non-resonant and therefore inactive (no induced current).

The small components are 1N4148 switching diodes. The larger ones are RF chokes, one on each line to the shack and one on each line to the parasitic element switch boxes, so that all lines in and out, including the common, are choked; there are more chokes elsewhere.

The chokes reduce the risk of RF on the control lines which could affect the array and control software and EMI due to rectification of RF by the diodes. EMI protection is desirable on the shack end of the control lines although I do not have that as yet since there is no ill effect during use.

The diodes double as back EMF protectors when the relay coil voltage is interrupted. The common lines -- one to the shack and one to each parasitic element -- are also protected by RF chokes but are not on the PCB. The top diodes on the board select the L-network configuration. Antenna impedance is different for yagi and omni-directional modes. More about that in the next section.

The switching matrix is installed in a large PVC box installed at the base of the driven element. Content and wiring of the box is discussed below.

L-networks

Designing L-networks is easy using TLW (comes with the ARRL Antenna Book). All you need is the impedance of the antenna, which is best determined by measurement not the software model. As much as I and others heavily use computer modelling the impedance calculation for verticals and their radials over real ground with its variable composition, the model isn't sufficiently accurate.

Modern antenna analyzers come to the rescue. Use one of suitable quality and accuracy and the job of L-network design and tuning will be easier. My weapon of choice is the RigExpert AA54.


With the parasitic elements tuned and their switch boxes installed and operating the entire array can be configured from the control lines at the base of the driven element. Using a battery and clip leads I manually run through the parasitic element modes. When the array is unpowered it is in omni-directional mode and tuned for the CW end of the band.

Impedance was measured and recorded in 25 kHz steps from 3.5 to 3.65 MHz in all 4 yagi directions and every 50 kHz from 3.5 to 3.8 MHz in omni-directional mode. Recall that the yagi functionality is, at present, only available for the bottom 150 kHz of 80 meters. I wrote all R and X values on paper, which I find is more convenient than pulling the data from the analyzer onto a computer.

The impedance is not the same for all yagi directions despite the care taken in tuning and radial layout. There is a small amount of asymmetry due to construction and (very likely) lack of homogeneity within the mass of soil and rock across the 1 acre of land the antenna encompasses.

The reactance among the 4 directions is very close but the resistance at any one frequency varies a few ohms. That seems small but when the R component of the impedance is 15 Ω a difference of 2 or 3 Ω is proportionately large. It impacts the impedance curve among the 4 directions. In practice the SWR deviations aren't problematic (measurements further below).

With a full set of measurements in hand I played with TLW. My objective was a set of C and L values that would be convenient for switching among modes.


The basic L-network for the yagi modes is as calculated above. The impedance is an average among the 4 directions. The result is a low SWR across the CW segment and acceptable for the digital modes up to 3650 kHz. Yagi performance is degraded but still effective up to 3650 kHz.

As it happens I have a coil already wound, once I trim it down to size. Using K6STI's most recent Coil program my coil with 1" diameter and 6 turns per inch needs to be 1.9" long and will have a Q in line with TLW's estimate for loss. The coil is tapped approximately halfway along for an inductance of 0.75 μH which is needed to improve the SWR in omni-directional mode for SSB.

For the shunt C I use two 1200 pf capacitors in series to give 600 pf for omni-directional mode, one of which is shorted for yagi mode. Each capacitor is a vintage 1000 pf mica transmitting capacitor in parallel with 100 pf high voltage, low RF loss and zero temperature coefficient disk ceramic capacitors. Due to their relative values when in parallel the mica capacitor carries the bulk of the current, for which it is better suited.

The carefully engineered compromise of L-network components results in low SWR in all modes, including omni-directional SSB. Interestingly the SWR in omni-directional mode dropped to well below 2 without an L-network after I rebuilt the stinger. The reason is that the resonant frequency dropped to 3650 kHz due to its longer length. Of course the R value is low enough that impedance transformation remains worthwhile.

During final tuning I found that I could not achieve an SWR below 1.5 for the yagi modes. After making a few measurements and testing alternatives with TLW I added two 100 pf capacitors to the shunt capacitor (the other series capacitor is shorted in the yagi modes). Only one was needed to drop the SWR to 1 at the design frequency of 3550 kHz.

Frequency of minimum SWR differs among the yagi directions due to impedance differences among the elements. Since the SWR is between 1 and 1.2 at 3550 kHz it is inconsequential. The SWR at the edges of the design range -- 3.500 and 3.650 kHz -- is about the same since it is dominated by the large resistance and reactance change. This is typical of antenna matching networks.

The SWR curves are displayed further below. They were measured after the L-network adjustment complete.

A negative consequence of the L-networks is that the antenna no longer works on 30 meters. The topology acts as a low pass filter that has a high SWR above 80 meters. I chose the network topology to reduce inter-station interference during contests. The 80 meter inverted vee through the rig's ATU works well enough on 30 meters to be an interim solution.

Control box

I purchased a 6" × 6" × 4" PVC electrical box to house the switching matrix, L-networks and cable terminations. It sits at the base of the driven element (tower). External connections are for the coax, driven element RF connections, control cable to the shack and control cables to the parasitic elements.

The 5 Cat5 (8 conductor) cables pierce the wall of the box and are routed to the barrier style connector strips. On balance I deemed this approach better than connectors with respect to waterproofing, convenience, cost and labour. It requires some care in the layout to make it work well. The picture is of the completed box before being installed and the cables attached.


Yes, it does look messy! Despite that it works quite well. The relays are very lightweight and can be supported (suspended) by the solid connecting wires for RF. Control wires are separated from the RF section (bottom and lower left) to minimize interaction even though coupling with short wires at 3.5 MHz isn't a serious problem.

The control wires are made as long as necessary and RF wires are kept as short as possible. The photograph makes the depth look shorter than it is; there is more vertical separation than is apparent.

Layout detail:
  • Switch matrix is on the right wall. One screw with an insulated spacer supports it.
  • Connector strip for the control cable is on the bottom. Every terminal is labelled. The blank line carries +13.8 VDC to allow testing on site without need for a battery.
  • Labelled connector strips for the control lines to the 4 parasitic elements are at top centre. Chokes for the common lines and reverse EMF protective diodes for the 3 internal relays are between the strips.
  • Holes for the control cables (5 of them) are at the lower right. There are barely visible due to the camera angle. There are additional  holes for tie wraps to hold the cables in place.
  • There are 3 stainless studs at the bottom. From left to right they are for the driven element monopole, radials and 160 meters. Again, these are not easily seen due to the camera angle.
  • Coax connector (N) is at the lower centre of the left wall.
  • L-network series coil with taps for yagi and omni-directional modes. The relay just to its bottom right shorts the lower coil section for omni-directional SSB use.
  • The relay above it shorts one of the series L-network capacitors for yagi use. The capacitors are large transmitting mica with a few 100 pf 1 kV parallel high-Q ceramic capacitors. These are partially hidden by the coil and relay.
  • The DPDT relay at centre bottom directs output of the L-network to either the driven element or the 160 meter stud. The 160 meter option will be described in a future article after it is added to the antenna.
High value carbon composition resistors will be added later to drain precipitation static to the radials and ground from the driven element. It is a minor concern in our cold winter so it can wait for spring. The lightning protection system will be added at the same time.


The picture shows the completed box attached to the driven element and with all cables connected. Despite being even messier with the cables attached it was straight-forward to wire and test. The bracket that holds the box to the tower needs to be replaced with a piece of lumber for improved mechanical support.

Colour coding is standardized with colours written on labels inside the box and documented in my files. Make sure you do this since you won't remember (trust me on this). There are 24 control lines: 8 to the shack and 4 to each parasitic element. After considering options I opted to install short cables to the internal terminal strips, which could be down indoors in comfort. Crimps on the outside of the box to attach them to the 5 cables.

It took a couple of hours in the field to get it all connected, tested and temporarily weatherproofed. For maintenance that requires box removal it is inexpensive and quick to simply cut the lines and reattach them later. I will use connectors for the external connections if maintenance and antenna improvements occur more often than currently anticipated.

Impedance tuning of the antenna

The antenna has 7 operating modes: 2 omni-directional modes optimized for CW and SSB band segments; 4 yagi directions; and 160 meters. The last is not yet implemented. That leaves 6 modes to be matched to the 50 Ω transmission line.


With the L-network installed the initial tuning could proceed. First to be done were the CW and SSB omni-directional modes. These did not require manipulating the control lines to the parasitic element, which were left disconnected and not yet routed into the control box. As before a 9 volt battery was used. The taps for the full coil (CW) and SSB were adjusted until the SWR curves were optimized.

The coil is at least 50% longer than needed. It was pulled out of my junk box and I didn't bother shortening it to give me more tuning room should I ever need it. The coil values calculated with TLW are approximately 1.5 μH for CW and 0.75 μH for SSB. Based on the coil dimensions the taps ended up not far from these values.

The shunt capacitor should be approximately 725 pf for CW and 500 pf for SSB. Instead I fixed it at about 600 pf for both band segments; that is, with the two 1200 pf capacitors in series. One of them is shorted for yagi modes since its shunt capacitor is calculated to be in the range 1150 pf to 1300 pf. The range is due to the impedance differences among the 4 directions as previously noted.

The component choice allows for a minimum of alterations (by relay) among modes. The design was intentional in this regard to keep the switching as simple as feasible. This is an interesting topic on its own which I may cover in a future article.


The SWR curves for the omni-directional modes are very good. They cannot both be perfect since it is only the value of the series inductor that changes, not the shunt capacitor. Adjusting the capacitor value would help little since although it would bring the minimum SWR to 1 the tails of the SWR curve would be about the same. This is typical since as you move away from the centre frequency the R and X departures from the ideal dominate the impedance.

With that done I connected the battery to the parasitic element control lines to adjust the L-network for the yagi modes. There is variation among the 4 directions due to the aforementioned parasitic element impedance differences. Rather than show all of them here is just the one for northeast. The others are similar or better.


Per the design the SWR soars at the 3650 kHz upper end of the design range. This is unimportant since the gain and directivity decline rapidly above 3625 kHz.

With the L-network adjustment complete the box was brought indoors to solder the coil taps and install the parasitic element cable harness (see below). After reinstalling the box there was a problem. A clip lead to the radial hub was used during L-network adjustment. When I replaced this with a permanent wire the SWR for all modes increased since it was shorter. It was far earsier to increase the wire length than adjust the networks. Wire length matters.

The SWR plots were done in the shack not at the antenna. Out in the field I forgot to save the analyzer plots and I didn't want to brave the cold and redo the weatherproofing just for this article. The SWR curves are more nominal at the other end of the 300' transmission line. The difference is probably due to a deviation from 50 Ω in a section of old RG213. Attenuation is very low at 3.5 MHz and thus contributes less to the impedance difference at the shack end of the transmission line.

Component choices

There are not many types of components in the switching system and networks. Judicious choices are necessary to ensure proper operation and reliability. Some of what I needed comes from my extensive junk box. Other components were purchased. The price is low when you buy in bulk from the major electronic component outlets. Most of these were purchased from Mouser and there are many other sources with good reputations and prices that also make it easy to do business with them in Canada.


  • The 1N4148 switching diode is used throughout the switching matrix. They are cheap, small and effective. They double as back EMF protection for the relays they power. While not ideal in this latter application I was willing to take the easy route. Others use them in similar antenna systems with good results.
  • For relays not powered by the switching matrix I used 1N4007 rectifier diodes for back EMF protection. They are more robust than the 1N4148 and indeed are overkill with their 1 kV reverse voltage rating. I had them on hand so I used them.
  • Old 1000 pf mica transmitting capacitors in combination with parallel 100 pf disc ceramic capacitors make up the two 1200 pf series capacitors in the L-network. These are stable and can handle a kilowatt of power. The 100 pf capacitors are 1 kV, low temperature coefficient and the ceramic material is low loss at RF. They each carry about 10% of the antenna current since they are in parallel with 1000 pf mica capacitors.
  • The RF chokes (lower right) are rated for 600 ma and are resonant at 6 MHz. The current capacity is well above the draw of the one to three relay coils they each support. The series resistance of ~1.5 Ω has negligible voltage drop in my control system. It is important that the self-resonance of the chokes be outside of amateur bands to ensure they perform properly.
  • The TE Systems 12A SPST-NO relays do the bulk of the switching chores in the L-network and parasitic element switch boxes. Both sides of the Omron G2RL DPDT relay switch between 80 and 160 meters at the output of the L-network to double the contact rating. All the relays are small, sealed and reliable up to a kilowatt. This type of relay should only be used for low impedance switching since with high impedance the RF voltage is too high. For those applications use vacuum relays or relays with a large contact gap.
  • PVC pipe as a coil form is acceptable at 80 meters but not for QRO above 20 meters or so. Loss (heat) in most PVC formulations rises with frequency and becomes a problem. I use PVC forms and insulated (THHN) wire in the parasitic element coils to achieve dimensional uniformity. The L-network has an air core bare wire coil from my junk box. Bare wire is handy for tapping the coil to adjust the impedance match.
  • Internal RF wiring in the 5 boxes is AWG 14 and 18. Smaller 18 gauge wire is acceptable despite the high RF currents since the lengths are short and ohmic loss is negligible. The dead bug construction method favours small gauge wire connections to the relays.
Control unit

The cutouts for the 80 meter array were included in the manual antenna control unit when it was built. A 6-position rotary switch and toggle switch do all that is required in the present antenna configuration. The only design question is which direction and mode to assign to each position.

When unpowered the antenna is in omni-directional mode with SWR optimized for CW (see L-network discussion above). For switching convenience I placed the CW omni-directional mode between the 160 and yagi position. Since the most common yagi directions are northeast (Europe) and southwest (bulk of the continental US) they are the adjacent positions. The less common directions are in the furthest clockwise positions: southeast (southeast US, Caribbean and South America) and northwest (Japan, east Asia).


During CW contests antenna mode and direction requires one one click most of the time. This is usually between northeast and southwest, with occasional forays to southeast, and rarely to northwest except grey line openings to Asia.

For SSB contests there are only two choices, both omni-directional modes with SWR optimized over different band segments. Most often the SSB switch is the only one needed; set once and forget unless you want to "rest" the L-network relay when 80 meters isn't in use.

As currently configured the SSB switch position disables the rotary switch. This works since the only SSB mode is omni-directional. Should I add SSB to the yagi modes the wiring must change so that the mode selection would select (short) coils in the 4 parasitic elements to move the centre frequency from 3550 kHz to 3700 kHz or thereabouts.

A wiring error among all the cable harnesses reversed the SE and NW directions. I'll have to track this down. I could reverse the wires in the control unit except that would violate my Cat5 colour coding and invite future confusion. There is no rush and there is no great consequence during use. Apart from this one error the control unit works perfectly.

Grounding

With the driven element, parasites, radials and control system isolated from physical ground there are a few challenges implementing lightning protection. Although this region has a lower risk of lightning strikes than many other protection is still required. After all, an 80 meter full size vertical sitting in the middle of a hay field can be a very attractive place for a lightning channel to form.

There are two problems to be addressed: lightning strikes and precipitation static. For a direct or secondary strike I want an easy path to ground for the lightning current despite the antenna's isolation from ground in normal operation. The electronics are unlikely to survive and are easy to replace, but I do want little of the strike current reaching the shack ~100 meters distant. Precipitation static needs to be continuously bled to ground to avoid excess receiver noise when it rains or snows.

For both problems ground rods are needed. To drain static it is enough to place a high value resistor between the antenna elements (including the radials) and ground rods. The typical method to deal with direct and secondary lightning strikes is with two copper balls separated by a spark gap, one on the driven element and one on the ground rod.

I hope to implement both solutions next spring before summer storms arrive.

Performance

Does it work? The short answer is yes. There is close agreement between what the computer model predict and how it performs on the air.

This is not a "perfect" antenna and it was not designed to be. Its performance pros and cons are quite interesting. This article is already long enough and I would like more experience using it before writing it up for the blog. Perhaps in December, after CQ WW CW and when the weather forces an end to tower and antenna projects this year and I spend more time indoors.

Summing up

For someone who enjoys playing with antennas this has been a fascinating project. I've learned a great deal and I've gained an effective directional antenna on 80 meters. I will continue to monitor its performance over the winter in contests and daily DX chasing. The future of the antenna will then be decided.

I definitely plan to add 160 meters to it, per the design and construction. It won't be difficult and will be an interesting project. I may in time want to add SSB to the yagi modes, something that isn't possible now due to the narrow bandwidth (less than 150 kHz) of the yagi. It will require changes to the parasitic elements and control architecture; the control lines are already installed.

What is particularly interesting is how the yagi's performance compares to the popular 4-square that is used in many big gun stations. The major differences I knew before starting this antenna project. Now I'd like to make the comparison quantitative, for my benefit and for readers. It will be enlightening.

Thursday, November 14, 2019

Abrupt Change of Season

The best laid plans are no match for Mother Nature. November has been less than kind in this part of the world. November began cold and windy, then came the snow. So far we've had 25 cm of snowfall. It isn't melting. Temperatures have gone as low as -17 C with wind chills even lower. Records have fallen. One wind storm brought gusts well above 100 kph that had the house shaking. After the sun rose I saw that all the towers and antennas survived.

Some moderation is in the forecast though not what it should be. Normal highs in early November range from 8 C to 6 C. This is comfortable for antenna and tower work. My plans relied on weather that did not vary too far from the expected. Now I'm faced with this:


In the foreground is one of my new 20 meter long boom yagis. In the background is one of the 15 meter yagis. A second 15 meter yagi is buried in the snow and can't be seen. These are 3 of the 4 big yagis for my 15 and 20 meter stacks for the newly completed 140' (40 meter) high tower in the background.

The frustrating thing is that the antennas work. They've been tuned and tested. All that's needed is the permanent gamma match assemblies, mounting hardware and boom trusses. Despite progress of all of those it is unlikely that these antenna will be raised in time for CQ WW CW which is rapidly approaching.

However it is not all bleak. With a moderation in the weather (likely) the side mount yagis will be completed and raised. The tower brackets are ready to go and the tram line is in position. Most of the truss hardware is ready to be assembled. Heliax feed lines are in the midst of assembly and testing. All of it looks good.

With these pointed at Europe I will have more capability in upcoming contests. If I get lucky the mast can be raised and the rotatable yagis can be lifted onto the mast. These operations require less snow on the ground to allow a power lift: they're too heavy to be trammed by muscle alone.

My 40 meter rotatable dipole project is similarly on hold. In fact I have not assembled it although all the material is on hand. This is an impediment for chasing distant multipliers on 40 meters since I moved the XM240 from its previous 46 meter height down to 21 meters. On the positive side this antenna should not be too difficult to raise. All that will be needed is to build and tune the gamma match. The transmission line and rotation loop are ready for use.

For now I do what I can on the ground and hope for better weather. Failing that I am forced to wait until spring. Around here that means at least 5 months (April) and possibly May. June through mid-August is haying season when little work on the big towers is possible.