Wednesday, November 27, 2013

Contest Impact on DXCC Totals

I entered the CQ WW CW contest this past weekend in the single-operator, unassisted QRP category. The only change I had to make to comply with the rules was to dial down the power output of my KX3 from 10 watts to 5 watts. The loss of 3 db for an already small station worried me before the contest. I am already at a competitive disadvantage against other QRP participants due to the lack of antenna height and gain.

As it turned out my worries were for naught. I did quite well, even surprisingly well considering I only operated part time, not close to the permitted 48 hours. Since this article is about DX rather than contests I want to show just how much impact operating in a contest can have on DXCC objectives. It can be large even if one doesn't work the rare ones.

I summarized my results in the following table. The first column contains my achievements up to but not including the contest. The second column is just the contest. The third is the sum of both. The process of transferring QSOs from N1MM to Ham Radio Deluxe was easier than expected. However I had to fix up some of the entries since my version of HRD made some errors in country determinations.

Nov. 22
Nov. 25

WARC bands (30, 17 and 12 meters) are off limits for the contest, which explains the zeroes in the contest column. I have also never operated on 12 meters so those are also zero across the board. I once made a few 160 meters contacts with my eaves trough antenna then never again.

I used a tuner on 80 meters during the contest to snag a few multipliers to boost my score. Nothing more was possible since the efficiency of both the 40 meters delta loop and multi-band inverted vee are very poor on 80, though the latter is less awful.

During the contest alone I nearly reached the bottom rung of DXCC with 95 countries worked. I did not expect this much success with 5 watts and fairly low single-element wire antennas. It just goes to show what is possible with even small stations. Of course a lot of the heavy lifting for these contacts came from the other end where the majority of contesters have yagis, power and height.

QRP DXing is clearly a viable pursuit, with even substantial totals possible with little investment of time and money. It also shows that when it comes to results that CW is very effective. SSB with the same station is substantially more difficult.

Notice in particular the success I had on 40 meters. I think it is fair to conclude that the delta loop is an effective antenna even with QRP. It was remarkable how many stations responded to me on my first call, and copied me well enough to not need me to repeat my call sign or exchange. My 40 meters contacts extended as far away as central Asia (Kazakhstan/UN), South America (Argentina/LU) and west Africa.

Post-contest I have now exceeded 100 countries worked on three bands: 20, 15 and 10. Although the total countries worked only went up by 3 -- FK8, 4U1ITU and V5 -- I can only be very pleased with my results. In retrospect I wish I'd taken the contest more seriously and operated more hours. I am especially regretful of missing the bulk of the excellent 10 meters opening Saturday morning.

Contester or not, QRP or not, if you are passionate about DX you are missing opportunities when you avoid contests.

Monday, November 25, 2013

Pursuing Rare DX During a Contest

Even the most contest-averse DXer knows that contests are an opportunity to work new countries, band-countries and band-mode-countries. This got its own chapter in the venerable book The Complete DX'er by John Locher, W9KNI.

This comes to mind now that the CQ WW CW weekend is in the books. I had originally planned to use both the above strategies to do some DXing this weekend. However I also was once a contester. As the weekend and propagation forecasts looked favourable I got more interested in actually competing. QRP contesting can be even more frustrating than QRP DXing so I did not get overly enthusiastic about entering my first contest in nearly 25 years. I may say more in future about my contest weekend. But now I want to talk about the finer points of using DX contests for DXing, rather than the strategy of avoiding contests.

Let me start with a true and entertaining story from this weekend that I will use as a launching point for this subject.

Sunday afternoon I was working my way down the 10 meters band looking for new contacts and multipliers. The DX I heard was mostly the Caribbean and South American. When I came across a PY (Brazil) station calling CQ I entered the call on the computer. It wasn't a dupe (duplicate) so I reached for the paddles. Although this took just a few seconds it was enough time for the PY to send "agn" in reply to a signal he heard. I could not hear the caller. I patiently waited on frequency and listened.

Much to the PY operator's astonishment (and mine) the caller was XZ1J, the DXpedition to Myanmar that is currently roiling the bands. I could not hear XZ1J, which is not surprising for that time of day, band and solar conditions. Stations in the tropics have more openings all the time which those of us in higher latitudes can only envy.

The PY operator lost the ability to send legible CW for at least 10 seconds. Eventually he completed the QSO, ending with profuse thanks to the XZ1J operator. I expect he'll be telling that story for years to come. As a perfect anti-climax I became the next QSO in his contest log.

While extreme versions of this story are rare they are actually not uncommon. The best in my own memory was during a multi-operator SSB contest (either CQ WW or WPX) about 30 years ago when one of the other operators was called by JY1, the late king of Jordan. His reaction was similar to that of the PY above. Less rare but truly noteworthy DX entering the logs of CQing contesters is routine, and fun.

But that's not only a fun and interesting story. There is a message that should mattes to any DXer. The question to ask is yourself is this: why is a rare DX station calling me rather than sitting on a frequency with a massive pile-up? For the answer we need to look at the contest from the other side, the perspective of the holder of that rare call sign.

Turn on your receiver during any major contest. What you will hear is a wall of noise from the bottom to the top of the mode segment of the band. At least that's what it sounds like to a non-contester. It is simply the focussed but frenetic activity of thousands of hams trying to log as many QSOs and multipliers as possible. You typically multiply them together to get the score.

Points = Multipliers * QSO-points

Contesters are competitive: they strive to win. QSOs are mostly straightforward so let's look at the multipliers term.

In CQ WW the multipliers are zones and countries. XZ1J will typically count as two multipliers for most participants, one for the zone and one for the country. However the same is true of, say, XP2I who I worked on 15 meters this weekend: Greenland and zone 40. Not very rare but rare enough that he attracts attention. The attractiveness is not just the same as XZ, it is far easier to work by being a short hope from NA and Europe.

A contester will not stick around in a pile-up for more than a few minutes to pick up needed multipliers. In that same span of time an easier multiplier or several ordinary QSOs of equivalent points can be worked. The pile-up I heard on Z81X was no deeper than the one on XP2I, or even GJ2A.

DX stations that are far outside Europe and NA (the bulk of contest participants) have trouble running stations. This is equally true of rare ones .Let's run through some of the reasons.
  • QRM is fierce! I worked 3DA0ET on two bands during the contest and each time he was calling CQ with no takers. Swaziland is far from NA and Europe and is not easily heard through the din of super-stations.
  • Interest in the rare DX is weak for the reasons I described above.
  • Multi-operator stations don't have an interest in working rare ones. You don't get DXCC credit for contacts made at another station under a call sign not your own. Although these are the stations best able to work the DX they, for the most part, merely contribute to fierce QRM for everyone else (see the first point). This makes it hard for the DXer using the contest to prowl for new countries.
  • The majority of contesters have (no surprise) directional antennas for at least the high bands. When the band is open to Europe the majority of yagi in NA are pointed northeast. The southern half of Africa and east Asia are in the deep side nulls while the Pacific Ocean is off the back. Stations such as 3DA0ET, ZD8M (which I worked on 2 bands), XZ1J and TX8B (see below) often go unheard. Even when they call other stations they can encounter difficulties.
  • There is an "unassisted" category in most major contests which precludes the use of spotting clusters and other more direct tip-offs from others. So they don't know where the rare DX is on the band unless they find them by themselves. They typically don't spot what they find.
  • Sometimes the DX wears a disguise. For example, when I ran across TX8B on 20 meters the usually reliable N1MM logging software I was using could not assign a country. I knew that his zone was 32 (South Pacific) and the prefix is allocated to France. I knew the zone because he sent it as part of the contest exchange. He had no pile-up and I worked him on one call. Later I discovered this was a small DXpedition to New Caledonia (FK8). FK is a new one for me in my 2013 QRP DX pursuit.
  • Use the "anti-contest" method of DXing I described in a recent article.
If your sole interest in contests is to work new or interesting DX you should keep those points in mind. Some strategies you should consider follow. It helps if you have a station bigger than mine but don't let yourself be intimidated by the big guns and competition. Just be sure your receiver has excellent RF filters and a bulletproof front end.
  • Use the cluster to find spots for the stations you want. If you're not competing you are under no restriction against their use.
  • The pile-ups are thin but ever-renewing. If you have a small signal you might have to wait for an opportunity to make the one call that finally gets through. The pile-up has this characteristic even although serious contesters don't stick around for long if they fail to work the DX. They are quickly replaced by others tripping across them as they meticulously scan the band for QSOs and multipliers.
  • Exploit the fact that contesters are often pointing their big yagis in directions other than towards the DX you are pursuing. Point your antenna that way as you scan the bands. For example, on 20 meters in the late afternoon this time of year from this part of NA the path to the Pacific and Southeast Asia opens up while the path to west Europe and north Africa is still hot. So point your antenna north, northwest or west and see what shows up. When you find a tasty morsel you might have it to yourself. The contest QRM will also be lessened in those (to the contester) less productive directions.
  • Call "CQ contest" even if you are not in the contest. If the rare DX is answering the CQs of others you will need to do the same. Combine this tactic with that of the preceding point for greater success. Think of it as panning for gold. You must discard a lot of silt and gravel to find the rare glittering gem. Even without finding many gems you will log lots of DX, which I assume you would enjoy. The points you provide will also make many contesters happy. So jump in. This week I got called by a P3 (Cyprus/5B) by calling CQ on 20 meters. You could easily do better, perhaps even XZ1J.
Whatever path to QRP or QRO DX happiness you take always remember to enjoy the journey.

Thursday, November 21, 2013

SWR - How High is Too High?

To repeat something I've said before, I am more interested in an antenna's pattern than its match. That does not mean that the match is unimportant. What I am looking for in an antenna is one with the pattern that suits my operating interests (primarily DX) and a match that does not get in the way of the pattern being attainable in practice.

The match -- and therefore SWR -- has two major criteria:
  • Transmission line and conductor loss -- Additional loss due to mismatch.
  • Rig compatibility -- Tolerance to mismatch, which may result in folded-back power.
Let's do this with an actual example from my station. My multi-band inverted vee is cut for 30, 20, 17 and 15 meters. Its SWR is low on all these bands. The antenna also works on 10 and 6 meters, but with higher SWR.

With the developing double peak in the current sunspot cycle there are daily openings on 10 meters. I deliberately didn't bother to add 10 meters to the inverted vee because I hadn't really expected the openings on 10 to be so good. My assumption was that the other multi-band dipole -- TH1vn -- would be sufficient, even though it has sharp side nulls. With these great openings I really want to fill in those nulls with the inverted vee since it is oriented nearly perpendicular to the TH1vn. Plus it is higher (14.2 meters apex).

The inverted vee hears well. On transmit the SWR is 2.5 at 28.000 MHz. That is generally considered high for a modern, no-tune transmitter. My KX3 QRP rig has no problem putting out 10 watts (full power on this band) into that high-SWR load.

Since the rig doesn't complain I have no reason to complain. That's all there is to that. I trust Elecraft to properly engineer their rigs. The transmitter doesn't fold back with such a load, and it hasn't yet "fried" after many QSOs with this antenna on 10 meters.

That's one question answered: the rig doesn't complain so the SWR, though fairly high, is acceptable.

Now on to the transmission line loss due to the SWR. The run of coax is about 25 meters of RG-213/U. The common-mode choke is made from RG-58 let's round up to 30 meters (100') equivalent of RG-213/U.

The matched loss of this transmission line is approximately 1 db/100'. Plugging the numbers into an online calculator I get the following results:
  • Feed point SWR is 3.4
  • Total loss is 1.7 db
The additional transmission line loss due to the SWR is less than 1 db. That's a small sacrifice when you consider that the side null of the other antenna is well over 10 db.

The analysis is just as simple as shown above. Rig happy? Check! Acceptable transmission line loss? Check! We're good to go. If all works as it should any SWR isn't too high.

Monday, November 18, 2013

QRP DXing is Painful, and Interesting

The end of antenna season has arrived. All I am now doing outside is sealing joints, securing ropes and all the other little things to help my antennas and supports survive the winter months in good order. My winter activities will turn to the indoors, including making my bare-bones shack more comfortable and modelling antenna designs for 2014 and beyond.

QRP DXing is often painful so I may not stay with a small station forever. I am thinking of ways to get to 100 watts, either by refurbishing my ancient FT-102 or buying a new rig. For 2014 I am contemplating new antennas, ones with at least some gain over a dipole. For the winter I will continue with my small QRP station and single-element antennas.

Since, as they say, necessity is the mother of invention, I have had to find new ways get the most out of QRP and small antennas. No secret weapons, just a conscious recognition of needed that extra edge to compete for the increasingly difficult DX. It is increasingly difficult since as you add new countries to the log the next ones become harder to accumulate. It's the nature of the game that the easy DX comes first, leaving the difficult and rare ones for later.

Despite the difficulty I am quite pleased with my results so far. Since returning to the hobby at the start of 2013 with 10 watts from my KX3 and an eaves trough for an antenna I exceeded 100 countries worked. With dipoles, inverted vees and now a 40 meters delta loop my total has gradually inched upward, now exceeding 170.

It's all QRP and all CW. I have only hit the 100 countries mark on one band -- 20 meters -- but I am closing in on that mark on other bands. Some DXpeditions I failed to work (such as K9W on Wake Island) while others I've worked on multiple bands (such as T33A and XR0ZR). My next goal is 200 countries. If I'm very lucky I might get there by year end. That will be far from easy. Thus the pain I mentioned.

There are many common techniques for navigating pile-ups to snag a rare DX station. I know them and use them. Mostly these consist of  finding the station the DX is working, and then the next and the next after that, establish a pattern and predict ahead to where he'll be listening next. I often get it right. But with 10 watts and wire antennas it often does me no good at all.

While most DXers in a pile-up are either unpracticed in advanced pile-up techniques or are not agile enough to persistently and effectively apply them, there are many who are very good at it. When I successfully pick the correct frequency there are many others already there, especially for the rare ones. When that happens I lose.

Even in cases where I am the only one who got it right, many DX operators ignore my weak signal and simply slide by to a stronger station. That I can't fight. All I can do is wait for improved propagation and try again. In the QRP DXing game patience is a virtue.

This has motivated me to try new DXing techniques. There is a joy in trying and succeeding with new DXing strategies. I can honestly say that this experiment in QRP DXing has been very interesting. I've learned new techniques that add to my success, things that I might not have otherwise learned. That's a good thing. Let me list a few. You may already be doing some or all of them.
  • The rarer the DXpedition the longer I wait to jump in. A DXpedition that lasts a week or more tends to clear out all the pile-ups and leave opportunities for the little guys. Although it's very tempting to jump in early the effort is almost certainly wasted. Wait a while, pick a band & mode with few callers and go for it.
  • Work them on 30 meters. The majority of DXers on 30 meters respect the power restrictions. That means the pile-up is 10 db weaker than on other bands. This is equivalent to the little guy with 10 watts moving up to 100 watts on other bands. The other advantage is that many DXers do not put up yagis for this band. A wire antenna is therefore more competitive.
  • When the DXpedition says "QRX", you really should wait. The longer they are absent the more stations drift away. They probably click through to other pile-up spots with the intention of checking back later. Many times the DXpedition actually does leave for good when this happens. Other times it is only a short break and they're back in 5 minutes. When they first return there will be less competition. This is how I worked XR0ZR on 17 meters.
  • If the DX station is hopping around in frequency it can sometimes pay to ignore the pattern. It can sometimes be better to instead find a frequency within the listening range (that you determined by listening) that is free of other callers. These voids do occur, but be aware that what you hear as a void might not be at the DX end. It can be a bit boring but it can work since you can avoid the worst of the competition from bigger stations. I worked T33A on one band this way. Check your transmit frequency from time to time to ensure it is still quiet.
  • Plan for times of enhanced propagation. As the terminator sweeps across your location there is often a period of enhanced propagation. Since most callers at that time are either in full daylight or night you have a momentary advantage. Best times tend to be just after sunset and just after sunrise. This works because as the MUF (maximum usable frequency) passes your frequency (whether moving in the upward or downward direction) path loss can be substantially reduced. You'll know it when you hear it.
  • When contesters turn left, you turn right. Not all DX stations operate in contests so they tend to avoid contest modes and bands when they occur. If the contest is on SSB, operate CW or RTTY. Even better, the competition for the DX will be less, which benefits the small station. The reason is that there is a large overlap between contesters and DXers, and they are also the ones with the biggest stations. Plan ahead for those special weekends.
When all is said and done, is it worth the effort and frustration? That depends. One of the things that struck me many years ago was that I got bored with the relative ease of logging many rare ones with my yagis and kilowatt. The challenge was more one of information, not getting through the pile-up. Unlike today, back then there was no convenient way to know where the DX stations were. A lot of listening was involved. And by "a lot" I mean a lot.

With today's worldwide DX clusters and computer-enabled rigs there is, perhaps, less need to listen. Rather than spending one's time listening one is likely to spend that same amount of time sitting in pile-ups. This is the inevitable result of everyone having access to the same tools and information. Years ago it took time for pile-ups to form, as DXers ran across the DX station and the keener ones then started phoning their friends or getting on FM to announce on the local DXers' repeater or packet cluster.

Having a big signal today may be more important than ever. If every DXpedition has a 24x7 pile-up a big signal can ensure you spend less time in each. However that is not my station in 2013. Whatever skills I may have they are often useless since I cannot compete against other stations, even those with the typical 100 watts rig, no matter what their antennas may be.

Tuesday, November 12, 2013

Comparison of 40 Meters Wire Antennas versus Height

As I concluded in the earlier comparison of my delta loop and a reference inverted vee at the same apex height of 15 meters, the delta loop is the overall winner. This is fair even though the inverted vee would outperform the delta loop in the broadside direction by a small amount -- a little more than 1 db -- because in all other directions the delta loop wins. If you can only put up one 40 meters antenna at this height, you should expect the delta loop to win. Just be sure to feed it for vertical polarization.

If you able and willing to put up two inverted vees orthogonal to each other or a turnstile inverted vee at the same apex the question can be reconsidered. The only significant disadvantage is the longer run of transmission line compared to the delta loop, since the feed point for the inverted vee is at the top of the support versus close to the ground for the delta loop. The matched loss of even a long run of RG-213/U at 7 MHz is quite low so the only real issue is the cost of the additional cable.

Having established that the length of transmission line is a minor concern I now want to consider height. In particular, if the apex of the inverted vee is raised at what point does it become the victor of the performance competition? That's the question I want to deal with in this article.

No, I am not going to actually put up any of these antennas! It is sufficient to do some modelling, here in the shack where the cold wind doesn't blow. The only antenna work I am doing in November is sealing joints and tensioning cables and wires.

I ran the models through EZNEC and produced the following graph. The broadside gain was calculated for each model in 1 meter height increments from 10 to 25 meters.

I ran 3 wire antenna models through EZNEC. A dipole requires more than one support but I threw it in as a reference with which everyone should be familiar.
  • The delta loop is fed λ/4 from the apex. The azimuth pattern is similar at all show heights, with the gain off the ends approximately -3 db from the broadside direction.
  • I chose 10° elevation angle to compare gain since this approximately the median angle for longer, DX paths. My interest is DX so I don't care about higher angles except to lower the gain there, if possible, to reduce QRM from W/VE. Below 20 meters height the dipole and inverted vee have a substantial amount of gain at higher angles, whereas the delta loop attenuates those angles.
  • The choice of ground has less impact on the horizontal antennas. The delta loop, being vertically polarized, is sensitive to ground quality in both the near and far fields.
  • The interior angle I selected for the inverted vee is 120°. That seems a reasonable median point between the delta loop (60°) and the dipole (180°). More acute angles lower the gain, mainly due to the lower average height. As designed, the inverted vee tracks about -1 db gain versus the dipole, until lower heights where the differential increases.
  • The feed point impedance and resonant frequency for all of the antennas is sensitive to height. Each should be cut (by modelling or otherwise) for its installation height.
You can see where the delta loop shines when mounted at 15 meters, which is how I've built it. However it only wins in a tight range of heights. Below 14 meters it cannot be installed since the lower wire would be below ground. Some people choose to distort the loop shape or slant it to make it fit but that is not optimum. If that is a real problem a vertical dipole of some sort would be a better choice.

Although any 1 meter step up or down results in a small change in gain, these add up. For the dipole and inverted vee the gain change is approximately 0.6 db/meter through most of the modelled height range. For example, go up 5 meters and the gain increases ~3 db. The similar figure for the delta loop is only ~0.1 db/meter.

While I do not have a dipole or inverted vee for 40 meters in my current station, I did have one in the late 1980s. Its apex was about 17 meters, about 2 meters below the top of the tower and a TH6DXX. My recollection is that it slightly outperformed the delta loop, however I never had them both up at the same time.

Soon after I turned the dipole into a 2-element inverted vee yagi, switchable to either broadside direction. That antenna worked very well. Perhaps sometime this winter I'll rebuild the model of that antenna and write an article about it. The original model was in done with MiniNEC-based ELNEC (the precursor to EZNEC) and so would benefit from a redesign.

Dipoles and inverted vees are good choices for 40 meters yagi elements. The same can be done with delta loops although they are "bulkier" and tend to have bigger side lobes. Of course if you can have only one antenna for this band that might not be so bad.

Thursday, November 7, 2013

40 Meters Delta Loop on Other Bands

Dipoles and loops resonate at harmonics of the fundamental frequency. However they do so differently. A dipole resonates on odd multiples and loops resonate on even multiples. We should therefore expect a 40 meters delta loop to resonate (zero reactance) on or near 20, 15 and 10 meters.

It isn't quite as easy as described above. Consider the following points:
  • The loop is close to the ground. That interaction will be quite different over the range of 7 to 28 MHz. That is, the pattern and match could be very different than what it is on the fundamental frequency.
  • While the loop is vertically polarized on 40 meters that is not necessarily so on its harmonics.
  • A loop that is larger than 1λ, as with a dipole that is longer than λ/2, has minor lobes in its far-field pattern, and deep nulls between those lobes. The number of lobes increases with each harmonic.
  • The loop has an impedance over 100Ω, which the ¼-wave transformer converts to 50Ω. The transformer is cut to be λ/4 only on the design (fundamental) frequency. It behaves as we want on odd harmonics but not even harmonics. For this antenna the transformer works on 21 MHz (¾-wave). On 20 or 10 meters the RG-11/U adds a reactance to the load that worsens the match to 50Ω coax.
Since the match is nominally good on 15 meters let's look at that band more closely.

The first thing I did was measure the SWR on 20, 15 and 10 meters. As expected the SWR was over 3 on 20 and 10, most likely due to the RG-11/U section acting nothing like a ¼-wave transformer. On 15 meters the antenna resonates a little below the band, at about 20.950 MHz. At 21 MHz the SWR is 1.6.

But as the title of this blog declares, it is not sufficient to have a match. We also need to look at the pattern. The pattern is a mess since each leg of the delta loop is now λ rather than λ/3, and the feed point is not well positioned.

The affect is obvious on the adjacent EZNEC plot of antenna current. Notice also that the tower strongly interacts with the loop on 15 meters which does not occur on 40 meters. This is due to the tower height being more compatible with the shorter 15 meters wavelength.

The tower model that I built in EZNEC only approximates the reality, especially when we consider the fact that the tower is (obviously) ground mounted. The SWR predicted by EZNEC looks nothing like what is measured, calculated at over 3 across the band.

Those ground interactions should also affect the pattern. The degree of interaction is hard to model due to that long and low horizontal leg of the delta loop plus the mutual inductance with the tower. Even so it is worth looking at how EZNEC predicts the antenna pattern.

The pattern is quite poor, as the plot demonstrates. Most of the radiation, though vertically polarized, is lost to high angles. At low angles, which are key to DX, the gain is atrocious. It gets worse in other azimuth headings (not shown).

Of course the real test is putting the antenna on the air. Tuning in to any DX station on 15 meters and switching from the inverted vee or dipole to the loop drops the signal by a significant amount. For long paths it is worse. Switching to the loop while listening to K9W (Wake I. DXpedition) caused the S5 signal to vanish into the noise

On 20 and 10 meters, in addition to the poor match, the antenna is horizontally polarized. A horizontally-polarized antenna at  such a low height is certainly a poor DX antenna. I won't even bother to show those patterns.

It is always possible to match an antenna and (usually) get a reasonably radiation resistance on frequencies higher than its fundamental frequency. However that says little about performance, and especially DX performance. Beware antenna advertising that promises multi-band performance by means of a sophisticated matching network! It'll work better than a dummy load though perhaps not by much.

Before we leave this topic let's go in the other direction: 80 meters. On 80 meters a 40 meters loop is λ/2 long. A dipole is that long and it works, so perhaps the loop can play on that band. This idea was attractive to me since I have no antenna for 80 meters at present.

The pattern looks not too bad, considering that the antenna apex is up less than λ/4 at 3.5 MHz. The ground losses are high, though not too different than at 7 MHz, and the polarization is vertical.

There is however a serious problem with this antenna. That is the radiation resistance. Across the 80 meters band the antenna's radiation resistance is less than 0.2Ω. It doesn't rise to anything near reasonable below 6 MHz.

A matching network to bring the feed impedance to 50Ω is difficult, and very difficult to do so without excessive loss. It will also have a high Q and therefore a narrow bandwidth. This is not a challenge I am willing to tackle.

An alternative is to use a relay to break the loop on 80 meters. The antenna would be very unbalanced but with care could be made to behave as an end-fed antenna. Again, I doubt this is worth the effort since the performance would remain poor and it adds a lot of complexity in design and construction. The modelling I've done for this configuration has so far been inconclusive. I prefer to just not operate on 80 meters for the next while.

Wednesday, November 6, 2013

Strengthening the 40 Meters Delta Loop

Extending masts far above the top of a tower raises questions about safety. Simplified tower ratings for wind load make strict assumptions on how the tower is supported and antennas are placed on the tower. Depart from those assumptions and you can rip up the manufacturer's headline loading specifications. This may then require the assistance of a structural engineer.

I did not call upon an engineer when I designed and installed the extended mast on my tower to support both the multi-band dipole (TH1vn) and 40 meters delta loop. Which should raise the question: is it safe? I will deal with that question in this article as the major part of the follow-on to the article on the mechanical design of the 40 meters delta loop.

The extended mast rises 6 meters above the tower so that the peak is at 14.8 meters above grade, and therefore just within the Industry Canada's policies regarding the "duty to consult". The tower is 8.8 meters tall, the TH1vn is at approximately 11 meters and the delta loop apex is at 14.7 meters. Let's look at the wind loads.

I'll stick to English units since tubing and most wind load equations and specs are shown in those units. Calculating the projected wind area is not difficult, being mostly a matter of multiplying width and length of tubes and pipes. The approximate projected areas of the major components above the tower are annotated on the attached picture.

The total wind area is 5.3 ft². Golden Nugget tower is rated for an antenna load of 3 ft² when mounted just above the tower top, with the tower bracketed (or guyed) one section down from the top and a maximum of 3 sections below the bracket. My tower is guyed at the top so we have to adjust for that.

First off we need to know the wind area of a 10' tower section. This is tricky since there is shadowing of tower components leeward of components facing the wind. I won't go through the details, and just report that my rough estimate is 3 ft² of cylindrical wind area for one Golden Nugget section.

Extrapolating from this calculation and the manufacturer's headline specification the tower is rated for good for 3 ft² of horizontal antenna load mounted above 3 ft² of vertical load, with the vertical load extending 10' above the tower.

From the picture above you can see that (roughly speaking) there is a 2.5 ft² horizontal load above about 10' of steel mast plus a bit of tower from where the guys are attached. That is well within spec. In an earlier article I described the response of this approximate configuration to a 100 kph wind. That was without the extended mast and the wind area of the TH1vn was just under 2 ft² since it had not yet been enhanced to support 17 meters. It passed that test with flying colours.

However there is now a further 1.8 ft² of fibreglass mast above that. While 5.3 ft² is less than 6 ft² the load is not configured per the manufacturer's spec. For a maximum wind of 140 kph (North America wind zone A) the wind pressure on the fibreglass extended mast is 36 lb (16 kg). At 80 kph, which is the wind it recently survived, the wind pressure is a more modest 7 lb (3 kg).

As I mentioned before that the extended mast bounced around a lot in those high winds, while the lower mast and TH1vn were far more steady. The guyed tower itself showed no stress or movement. The bouncing itself is a concern even if the wind load alone is not since oscillations can multiply the effective wind load. Although I don't have a reference handy I have in the past seen published reports of instantaneous loads due to oscillation of up to 3x and 6x the static wind load. The wind load equations typically only account for 30% additional load due to gusts and turbulence.

It is this concern regarding turbulence plus that of the unknown breaking strength of the fibreglass mast that motivated me to guy the extended mast. This ameliorates most of the residual concern of the load height of the extended mast by reducing the majority of mast motion in the wind and the bending moment on the structure below.

The guying of the extended mast is not perfect since the 3 guys are not close to being separated by 120°. This is because two of the guys are the vertical legs of the delta loop itself. These are angled out from the tower so that the delta loop plane is not vertical, resulting in an interior angle of approximately 160°. There is a back stay now added, with the help of horizontal bar, to act as the third guy. It is secured further down the tower. There is approximately 100° between it and each of the delta loop legs.

You can see this arrangement in the adjacent picture. The perspective is distorted since I took the picture while on the tower, which I did in this way to show more detail.

Balancing the tensions in the loop legs and the third guy took several attempts. The residual curve you can see in the extended mast is quite small and not a concern. The modest amount of bending load on the fibreglass mast in the direction of the rope guy helps to steady it by compensating for the less-than-ideal guying behaviour of the loop legs.

Since completing the construction to strengthen the entire system we have had winds of up to about 50 kph. This is not a proper test but all I can do is deal with what nature provides. In that wind the motion at the top of the extended mast was no more than 2 or 3 cm. So far so good.

In the electrical design article I described the remote placement of the coax choke for the 40 meters antenna. Now that I've finished that part of the antenna I can show what it looks like. The deflection of the loop due to the weight of the coax is less than it was at first since with the completion of the extended mast guying I was able to add substantially more tension to the delta loop. Supporting the coax choke on the steel guy wire substantially reduces the stress and deflection of the loop wire. The red circle on the picture is the feed point, which is ¼λ down from the antenna apex.

The choke consists of 12 turns of RG-213/U with a diameter of 7" (18 cm). This should supply over 1,000Ω of (mostly reactive) impedance at 7 MHz.

As a final step in the construction I moved the bottom corner anchor points on the loop to better account for the splicing in of 3.4 meters of wire in the bottom horizontal leg. With the increases tension on the loop this improved loop symmetry and keeps the bottom leg out of hand's reach. Unfortunately these changes also shifted the resonance of the antenna about 100 kHz higher. Now the SWR at the bottom of the band is 1.4 rather than 1.1. This is still perfectly acceptable so I am leaving it alone at this point.

I will have more to say about antenna performance in future. Although the antenna works there remains the tough challenge of using QRP on 40 meters.

Sunday, November 3, 2013

40 Meters Delta Loop Electrical Design

For those interested in why I chose to go with a delta loop for 40 meters please refer back to this earlier article on vertically-polarized antennas. That article and the earlier ones it links explain it, so I won't bother to repeat all of that here. All that I'll repeat here is the elevation pattern of the antenna at 7.050 MHz. This is in both broadside directions. In the plane the gain the antenna it is -3 db from the peak. The pattern is almost perfectly symmetrical, only slightly distorted by the presence of the tower and steel mast.

From deep inside my expansive junk box I retrieved a 40 meters delta loop and ¼-wave transformer. Like many hams I tend to not throw out stuff so almost all of my previous antennas are packed away somewhere. I chose to reuse it rather than spend the time making a new one since it's certainly good enough for my present needs.

This particular antenna was the first at this QTH, and went up in the air soon after arrival in October 1984. First it was supported by a temporary guyed mast made of storm-salvaged aluminum boom and element tubing. The next year it went onto the 20 meters tall tower I put up. It came down around 1990 when I replaced it with a switchable 2-element wire yagi. Looking back, the delta loop and 100 watts did a great job putting lots of great world-wide DX into the logbook.

This is an old antenna, and its age shows. The insulation on the 12 AWG stranded copper wire is not UV-resistant. Originally beige its colour ranges from off-white to dark brown. It is also cracked and there is some copper corrosion visible when the insulation is stripped. Despite these shortcomings, well, copper is copper. It doesn't have to look pretty and if it corrodes further that is not a problem for the next season or even a few years. The important point is that I didn't have to build a new delta loop from scratch.

The length of the antenna puzzled me. It measured 40.1 meters, which is substantially less than the 43.3 meters that my EZNEC model requires for resonance at 7.050 MHz. Since it's easy enough to add wire to it later I just shrugged away the mystery and put it up as is. The only modification I made was to moved the tie points so that the feed was per the model for vertical polarization and omnidirectional pattern. This is 25% up the vertical leg closest to the house, or ¼-wave down from the apex. The original antenna was fed at one of the bottom corners.

I tried to make the model reasonably comprehensive so I included the tower and metal part of the mast. I skipped adding in the TH1vn since earlier modelling showed that the interaction was negligible. That made the length disagreement more of a mystery.

Much to my chagrin the antenna resonated at 7.6 MHz. I can only surmise that when originally tuned there was interaction between the delta loop and the all metal mast. On that mast the antenna apex was secured directly to the mast top, with only the wire insulation separating them. The apex, even with corner feed, is still at a high impedance point on the loop so the coupling may have lowered the frequency of resonance, thus requiring a shorter loop length. In the present configuration the apex is tied to the top of 3.4 meters of fibreglass mast so that coupling is no longer present.

I made the adjustment in two stages, finally reaching resonance at 7.050 MHz with the addition of 3.4 meters of wire. The tie points were shifted in proportion so that the loop remains roughly equilateral. This makes the final loop length 43.5 meters. This is spookily close to the 43.3 meters found with EZNEC. It gets better. Take a look at the following SWR curves.

Either this is a great model (and modelling software) or it's all just a happy coincidence. It is rare to have such excellent agreement between a model and the real antenna. There are just too many sources of error that can creep in, such as insulation dielectric constant and thickness, ground conductance, nearby metal, accuracy of the ¼-wave-transformer, tower interaction, etc. The SWR/Power bridge is a pretty good one -- Daiwa CN-620B, almost 30 years young -- so I made the measurements with it rather than the comparatively inaccurate one in my KX3 rig (it often measures lower than reality).

If you read the previous article on the mechanical design you may have noticed the lack of a common-mode choke on the coax. This is deliberate. The weight of a coax choke made of RG-11/U (¼-wave transformer) is quite heavy and the loop already deflects quite a bit as it is. Instead my plan is to place the coax choke at the other end of this section, in the main run of RG-213/U. This can be done because the ~7 meters long run of RG-11/U appears as a high impedance wire on 7 MHz. This assumes the choke has a high impedance so that the electrical length is approximately the same as the physical length.

To test this out I modelled the RG-11/U as a thick, insulated copper wire in the EZNEC model. I tried it with the coax running orthogonal to the antenna plane and at more realistic drooping angles.

The current plot shown here is with the coax running in a straight line downward toward where it would meet the actual choke. The real coax curves (as does any suspended cable). The current on the coax exterior remain very small even at greater angles. The modelled affect on the pattern is no more than about 0.1 db. However, if you try this don't push your luck and eliminate the choke entirely. That will certainly mess up the pattern, bring RF into the shack and increase the risk of RFI.

There really isn't much more to say about the antenna design and model. It's pretty simple, and it works. I don't have a suitable comparison antenna on 40 meters so all I can do is put it on the air and try it. I have done that now for a few evenings. Considering that I am running QRP (10 watts) I am pleased with its performance. It appears to work equally well in all directions and I was competitive in several pile-ups. Examples include ZD8O, ZB2FK. As always there are some stations that I cannot work, even when they have no callers. On the other hand I notice that many other callers find themselves in the same predicament. Either those DX stations have a lot of QRM or QRN or, perhaps more likely, are alligators with a "mouth" much larger than their ears.

There are some interesting DXpeditions on at present, so I have some opportunities during November to put the loop to the test. My calls to K9W (Wake I.) and others on 40 meters have not yet resulted in contacts. With QRP it is generally better to wait a few days until the big guns make their contacts and thus reduce the competition for the rest of us.

Although I don't have a comparison antenna what I can do is model alternatives. Since I excluded verticals both short and long for performance shortcomings in my earlier modelling experiments I instead modelled an inverted vee. I chose the same support configuration as used for the delta loop, so that its apex is also up 15 meters and it goes to the same tie points. Doing so results in an interior angle of about 130°, which is quite good. I could do even better by raising one end to the chimney (up 8 meters), although the difference is minor. This also reduces the little omnidirectionality an inverted vee offers.

In the broadside elevation pattern I show the gain at the same angle as where the delta loop peaks. It is only a little more than 1 db better. Although the ground losses in the inverted vee are lower, at this height most of the radiation is wasted at high angles. This is not only useless for DX it can also increase the QRM from domestic stations, which can be particularly troubling during pile-ups and contests. Off the ends, as one should expect, the inverted vee performs poorly in comparison to the delta loop, down 5 to 10 db or worse.

An inverted vee is not quite as broadband as a loop but it still adequately covers the 40 meters band when cut for CW. With a 7.050 MHz resonance the SWR rises to 2.5 at 7.300 MHz.

I will likely follow up in the future as I get more time in with this antenna, and especially with the CQ WW weekend coming up at month's end. One thing I want to try is some evening gray-line openings to Asia and Australia, particularly Japan, which are coming up very soon on the calendar.

I expect that my next article will follow up on the mechanical design. Those in the Ottawa area will know that we just had a day-long bout of high winds, with gusts up to 90 kph. I wasn't expecting that, so I thought I had the luxury of time to finish strengthening the extended mast on the tower. Everything survived the wind but that is no excuse to put off this work any longer.