Wednesday, March 25, 2020

The Final Courtesy

Hams have passionate opinions on QSLing contacts. I am no different. Some readers will not like what I have to say about the matter. I'll introduce my take with a short history of my ham life.

Before I became a ham I did a little broadcast SWLing around 1970. It was a thrill to borrow the family's kitchen 5-tube AM radio in the evening and listen to stations from all over North American. On occasion I could copy Spanish stations from further south and a few from Europe. The idea and the dynamics of radio propagation and its possibilities were mesmerizing.

I sent out some QSL requests and received cards back from several broadcast stations. I proudly propped them up on my bedroom desk. For a while. The thrill of SWLing died quickly once I became obsessed with becoming a ham. That occurred in 1972, when I was still quite young.

Soon I was making contacts and receiving QSL cards. I had very little money so these sat around until I could manage to get a few hundred cards printed cheaply. The backlog was cleared and I sent envelopes and money to the local incoming bureau. More cards arrived and I happily answered them. Postage to the outgoing bureau was another monetary burden. Work part time I was able to afford this and other shack equipment while saving for university.

Contests increased the flow of cards. I was being inundated. Many hams wanted a VE4 confirmation for awards or just because it was relatively uncommon. As my school and social activity filled my time I began to resent every envelope from the bureau. QSLing had become a burden. Despite this, and as my budget allowed, I sent money and SAE to rare DX to get their cards. More often I rolled the dice by hoping for success via the QSL bureaus.

In short I did not enjoy QSLing, either the sending or receiving of cards. I continued because I felt a duty to do so. As it was said and still is: QSLing is the final courtesy of a QSO. So I persisted, and grumbled quietly. Although receiving rare DX cards was nice I discovered that after admiring them for a few minutes they went into a box and were rarely looked at again.

My reluctant QSLing followed with my move to VE3 and the building of a new station. I would stack received cards until they toppled over then spend a snowy winter day grinding through the lot. When I went QRT in 1992 outstanding QSL requests were ignored. Within two years the influx slowed to a trickle and then nothing. In any case my supply of cards was exhausted. I was gone for the hobby for good. Or so it seemed.

When I returned to the amateur radio in 2013 (and started this blog) the idea of QSL cards was far from uppermost in my mind. I dabbled with small antennas and QRP and pursued DX. Soon I was receiving nag messages from the incoming bureau. I sent them money and cards trickled in. But I did not print cards. Indeed I haven't bought stamps for about as long and I have mostly eliminated printed material from my home, be it books, magazines or QSL cards.

Once my remaining supply of about a dozen cards was exhausted replying to direct requests I stopped QSLing entirely. Instead I joined LoTW (Logbook of  the World) and regularly uploaded my logs. As my station and contest activity grew my count of uploaded QSO count grew to tens of thousands. I could watch my awards credits accumulate. Very few of the countries I worked don't confirm via LoTW so it works well. For those that don't I merely shrug and carry on; it does not bother me in the least.

Yet a small but steady stream of direct cards continued as did a larger number via the bureau. I regret to say these cards are never answered and I will not apologize for that. They are opened, perused, closed and filed out of sight. Enclosed money and postage remains untouched.

As you can see, other than a brief period of youthful zeal I have never enjoyed QSLing and I derive little or no enjoyment from those I receive. Neither do I need them for awards since I never apply for them, not even DXCC. Many hams are passionate about awards and pursue QSL cards to that end. The ones I know seem to enjoy the challenge of extracting cards from reluctant hams.

Confirmation of my QSOs is solely via LoTW. There are other electronic QSL services that may be of use for non-ARRL awards. I've looked at a few and I have yet to see much value in uploading my logs to those services. Were I ever to do so it would be to help others since I gain nothing by it.

Those who do send me cards especially those who send them direct a further explanation may be due. I'll do so as directly as I can. Most of my QSOs last no longer than 30 seconds. The vast bulk of those are contest QSOs which may take 10 seconds.

Responding to a QSL request takes far longer. I must open the envelope, find the log, search it for the QSO, check the details, fill in a card then mail it or collect them for the outgoing bureau. Unlike when I was young the money involved is not a concern. Time is the problem. Responding to a QSL request can take 10× as long as the QSO and it is tedious and unwelcome work that keeps me from doing things I enjoy.

It isn't courteous to expect that. For me the final courtesy of a QSO is to request confirmation without placing a burden on others who neither want nor deserve that burden. I extend the courtesy of QSO confirmation by uploading my logs to LoTW after every contest and monthly for my non-contest logs. Others can extend the courtesy to me by doing the same.

There are many who feel differently and may take offense at my approach to QSLing. I am not antagonistic or doctrinaire about it, just very very possessive about using my time to do what I like and making it possible for others to confirm our QSOs with the minimum burden on both parties.

That is my definition of the final courtesy of a QSO.

Wednesday, March 18, 2020

160 Meters on the 3-element 80 Meter Vertical Yagi

My 160 meter vertical wire antenna must be removed between May and August so that the field can be hayed. Radials are not compatible with harvesters and balers! Each May I roll up the radials and coax and tie off the antenna to the nearest guy anchor. After the harvest is done I am free to put it back up. I don't do it immediately since I want to avoid trip hazards for my friends who assist me with tower and antenna work.

This process is not onerous: it takes a little more than 2 hours. For an excellent antenna on top band it is worth the small effort twice each year. Until I come up with a better idea the ritual will continue. It will take longer starting this year since I intend to double or quadruple the set of 8 30 meter long radials. Other changes to improve performance are also planned.

Although summer is not peak season on top band there are contests and DX to be chased. A second antenna removes that hole in my calendar and provides a fall back in case of main antenna failure. Ice storms, wind and other periodic calamities can destroy a quarter-wave of wire in the air.

From the start I planned for a 160 meter option on my 3-element vertical yagi for 80 meters. The idea was to turn the driven element into a short (loaded) 160 meter vertical. The radial field for the five 80 meter elements is extensive and therefore low loss; the 4 parasitic elements are disconnected from their radials for 160 meter operation. Loading lowers efficiency but if not excessive it is a price well worth paying. After all, a few decibels of loss is better than -∞ db!

As constructed the 80 meter yagi switching system and shack control head support 160 meters. A relay in the switching system selects an alternative path to the driven element. See the previously linked article for the design and construction of the 80 meter yagi control box. I won't repeat any of that information this article.

My short term solution is to install a base coil to the driven element. In future I would like to move the coil farther up the tower, a project that will reduce loss.

I made several mistakes right from the start. Since these are educational I will tell you about them. Too often errors are excised from articles (and far too many science papers). The presented results are "sterile" when the all important process and experiment errors are absent.

If you build this or a similar antenna you may make the similar mistakes. By documenting those mistakes my hope is keep others from repeating them.

Mistake #1

The first was to estimate the coil inductance from the EZNEC model rather than by antenna measurement. The model said the antenna impedance at 1.83 MHz was approximately 20 - j320 Ω. This requires an inductance of 31.5 μH to cancel the capacitive reactance. There is still the low R value to deal with but until I knew the measured coil ESR (equivalent series resistance) and radial system equivalent resistance the true feed point resistance.

I built a coil with an inductance of 31 μH and a Q of 400 using K6STI's Coil program. The modern version of this software appears to work well. Alternatively there is an online coil calculator by ON4AA that includes Q, reactance and other data that works well but has a few quirks.

The coil form is a PVC coupler for 3.5" pipe which, including insulation thickness gives a coil diameter of 3.5". Form loss is negligible at 1.8 MHz. A "fat" coil like this has a high Q and therefore a low ESR which reduces the typically high I²R loss of a base loaded short vertical.

After winding it using scrap AWG 14 THHN wire I measured the inductance with my ancient LCR meter. Stray inductance of the leads is 6 to 7 μH which is subtracted from the measurement. Accuracy is further affected by the meter making the measurement at AF rather than RF. Adjustment of the inductance must be done in the field.

The coil is connected between the 160 meter stud on the control box and the driven element (tower). For the initial test I didn't do this since it requires opening the box and manually powering the switching circuit. Instead I disconnected the 80 meter stud and connected the coil to it.

Since the unpowered (default) state is omni-directional 80 meter mode and the DPDT relay to select 80 or 160 meter stud is on the output of the 80 meter switchable L-network this temporary connection is the same as normal use. More on this design curiosity later.

The antenna did not behave as expected. Resonance was near 1300 kHz, very wide of the mark. Bypassing the control box I measured the impedance of the driven element at 1.83 MHz without the coil as 17.5 - j167 Ω. R is in close agreement with the model but X is twice the magnitude.

The coil is obviously far too large. It was a mistake to rely on the model and skip measuring the impedance before designing and building the coil.

Mistake #2

When I wired the control box I attached the 160 meter stud (via relay) to the output of the L-network. This was expedient because the L-network input was not very accessible. Since the network used a low pass topology and modelling confirmed that the low pass L-network had a low impedance at 160 meters I believed this would work. Wrong!

There are two reasons why this was a mistake. The first is that although the L-network is low pass it is not transparent. Here are the measured impedances of the driven element on 1.83 MHz:
  • Transmission line port: 3.2 - j66 Ω
  • Isolated driven element: 17.5 - j167 Ω
At first glance the difference is profound. On closer inspection they are not all that different. Within the measurement error of the AA-54 antenna analyzer for high mismatches the SWR is the same: in the range 31.5 to 33.5. Therefore the network, although essentially pass through, introduces a phase shift; that is, all we've done is travel around the SWR=32 circle on a Smith chart because at a lower frequency R is low and X is high.

This is not a disaster. If the 160 meter base coil brings the match reasonably close to 50 Ω at 1.83 MHz the SWR will be low regardless of the L-network phase shift. Unfortunately it's not that simple. First, the base coil lifts the antenna R very little, so the SWR can be no better than 2.5. Were that deemed acceptable it still wouldn't suffice.

Although the 80 meter L-network is pass through at 1.8 MHz the addition of the 160 meter base coil changes that. The L-network and added series coil are not separable: we have instead made a completely different L-network, one that is not transparent on 160 meters. The shunt capacitor is the 600 pf used for the 80 meter omni-directional mode and the series inductance is the sum of the ~1.5 μH of the L-network coil and that of the 160 meter base coil.

There is no escape. The 160 meter mode of the 80 meter yagi requires its own L-network. Designing one with TLW is pretty simple.

The new L-network is in series with the 80 meter L-network. With the 160 meter network in place the 80 meter L-network is once again pass through since its output port is near 50 Ω on 160 meters. They do not otherwise interact.

Network loss is low. The shown C and L loss is for component Q of 1000 and 200, respectively, at 1500 watts. The Q of my coil is calculated at 400 so the loss is half that shown. I also run less than 1000 watts (our legal limit is lower than the US) so the loss is proportionally lower. Loss in 80 meter L-network should be very low and has not been calculated. This can be done in EZNEC with careful adjustment of component ESR per the calculated Q.

As shown further below I initially installed a 2000 pf shunt capacitor and the coil was adjusted in the field. The low pass L-network design works.

Mistake #3

Let's see if you can follow along without a detailed schematic. There are two L-networks in series, the first for 80 meters and the second for 160 meters. The 80 meter L-network is always in line. Its antenna side port is switched by relay between the driven element and the 160 meter L-network. Each of these has a stud.

The relay is only activated for the 160 meter mode. When 160 meters is selected the L-networks ports are connected. However, at all times the 160 meter L-network series coil is connected to the driven element and the shunt capacitor is connected to the radial hub. Therefore when an 80 meter mode is selected there is a series LC circuit formed by those components across the antenna port of the 80 meter L-network.

This could be a problem. The series LC circuit resonant frequency is F = 1 / (2π sqrt(LC)). That's the frequency at which impedance is minimum and current maximum because XL = -XC. In this case the frequency is in the vicinity of 950 kHz. The problem is that the series LC circuit has a low enough impedance on 80 meters to take appreciable current and therefore upset tuning of the L-network and add loss on 80 meters.

The calculated impedance at 3.5 MHz is ZL + ZC = 330 Ω. This is simply a voltage divider. Current in the series LC circuit is less than 10% of the total due to an 80 meter impedance of 30 Ω (omni modes) or less than 20 Ω (yagi modes). This isn't large but neither is it negligible. Performance on 80 meters may be affected. Correcting that problem requires a relay to disconnect the 160 meter L-network from the radials when operating on 80 meters.

Calculation can do wonders here however I simply measured the SWR. There is negligible effect on all 6 of the 80 meter modes. On air it the 80 meter array continues to perform as it should. I decided to leave it alone, at least for now, since there does not appear to be a problem in practice.

Tuning it up

With all the mistakes fixed or accounted for I proceeded to tune the antenna on 160 meters. The coil was adjusted until the minimum SWR was approximately where I wanted it, at 1830 kHz. A shunt 2000 pf mica capacitor was added between the radials (ground) and the 160 meter stud.

It should be obvious that this is a temporary installation. The capacitor will be put inside the control box and adjusted to improve the matched SWR. I have small ceramic capacitors for this task. The coil is large to maximize Q so it will remain on the exterior. It will be given mechanical support and a rain cover to preserve Q in wet weather.

Despite the high Q coil and capacitor the SWR curve is pretty good. By decreasing coil inductance a small amount the match will move a little higher and achieve a low SWR from 1800 to 1900 kHz. Although my interest is CW during major contests activity does extend that high.

The series LC circuit formed by the added coil and capacitor has not changed the 80 meter SWR for all of its 6 modes. I didn't expect to get that lucky. There is no need to use a relay for the capacitor.

Tuning was facilitated by the +12 VDC on the spare control line. This was my first opportunity putting it to use. With some squeezing of the fingers past the various cables I used an alligator clip to switch among the 7 modes: 6 for 80 meters and 1 for 160 meters.

Performance: model

A short vertical has poor performance relative to a full size vertical. This antenna cannot do as well as my primary antenna. That isn't its purpose. It's intended to be my year round antenna that is always available when my primary antenna is removed during the haying season.

Relative performance is limited by the following:
  • Short antennas have "fat" patterns. For a short vertical the elevation beam width is larger so there is less gain at low angles. DX suffers although there is a boost for shorter paths.
  • Base coil loss converts power to heat. This can be mitigated with a high Q coil and its lower ESR, which I did (see calculation above).
  • Short antennas have a low radiation resistance. Since the radiation resistance is in series with the ground loss more power is dissipated in the ground as in a larger antenna. This can be mitigated with a more extensive radial system.
I used EZNEC to compare this antenna to my primary 160 meter antenna. The model predicts from 2 to 4 db worse performance. The range depends on direction because the 150' only 20 meters from the vertical segment weakly acts as a reflector.

Performance: on air

In practice the difference is difficult to discern. QSB cycles are out of phase due to the different locations. The actual losses in the short vertical may be less than predicted. The equivalent resistance of the 80 meter radial system cannot be easily calculated since there is a complex combination of relatively short radials (20 m) on the driven element interconnected in numerous places to the 4 parasite 15 meter radials systems. However it is likely quite favourable based on its measured resistance (sum of radiation resistance and ground loss) and the extent of the combined radial system.

The first test is noise. When noise is equal in all directions two antennas of equal efficiency will produce the same noise power in the receiver. The new antenna is quieter by a little less than 1 S-unit. However from the pattern shown above the primary antenna is not omni-directional: it has relative gain away from the 150' tower at a heading of approximately 200°. Noise is always stronger in that direction since weather is more intense towards the south. I see the same noise level difference on the 80 meter yagi, with northeast and northwest directions quieter. The difference disappears during daylight when the band is largely closed, so the noise is not local QRN. The directional effect due to the new 140' tower is not included (yet) and it is 40 meters southwest of the primary antenna.

When evening arrived I listened at intervals. Activity was light. US stations were consistently stronger on the primary antenna by 1 to 2 S-units, depending on direction and measurement uncertainty due to QSB. European stations later at night were typically no more than 1 S-unit stronger on the primary antenna. So far this is consistent with the models, understanding that on the FTdx5000 the S-meter scaling is non-linear and is no more than 4 db per S-unit. It also gives me a nudge to do something about that gain loss towards Europe due to tower interaction with the primary antenna.

I have made a couple of contacts with the antenna and it does get out okay. I have not asked anyone to participate in an A-B test but may do so later. Since it takes a kilowatt without blowing up the coil loss is not extreme. I have not run out to the hay field to check its temperature!

I will continue to compare the antennas until early May. That's when the primary antenna will be rolled up for the season and I only have the new antenna for top band. For the present I am pleased with it even though I have to give up a few decibels of gain.

80 meter checkup

Because this was the first time I had the control circuitry for the 80 meter yagi open since it was completed last fall I did a brief maintenance checkup. The inside of the box was dry and there was no discolouration of metal surfaces and wire insulation. In fact it looked pristine.

The one problem that had developed was severe attenuation on receive, for all operating modes. When it first appeared as an intermittent problem I thought it might be a relay in the 8×2 antenna switch.Swapping the coax with another port made no difference. Transmit performance was unaffected. Corroded and abused relay contacts behave in this fashion but it wasn't the relays.

The cause was dissimilar metal galvanic corrosion. In my haste last fall I attached the wire to the driven element (tower X-brace) with nothing more than a stainless hose clamp. I had intended to solder on a tinned lug so that galvanic corrosion would be slow. Copper to zinc are far apart on the anodic index.

Sanding off the corrosion and reattaching the wire restored receive performance. I'll make the necessary modification in the coming days or weeks. The same goes for the temporary joint between the 160 meter coil and tower.

Sunday, March 15, 2020

Amateur Radio During a Pandemic

Hams are not in world of their own. As the COVID-19 pandemic unfolds we are affected like everyone else. Unlike other social events there is no lessening of on-air activity. Amateur radio is a social activity yet one that does not require personal contact. You could say it's the original social distancing app.

On the bands this weekend there were contests, DXpeditions and all the other kinds of operating that we do. There was no dearth of activity and there appears to be more than usual as people take to staying home with family. There is only so much television you can watch. Families themselves may see increased (friendly) friction from being cooped up together. An occasional trip to the shack can benefit all.

For myself it's a time to concentrate on antenna projects that I might otherwise leave until later. Most of the parts I need arrive by courier. Antenna design requires just a computer and spending time by myself out in the field or bush with tools and test equipment. As the weather warms I have made a few forays up towers to do inspections and to install rigging for lifting antennas, masts and other items.

There are bigger projects on my plate that require assistance. Those I am pondering while we all decide how much isolation is truly needed. That is, I am hesitating to invite people to visit. So far it hasn't stopped me from visiting others. In this region we have been fortunate with few cases of the disease: none in rural areas and a handful in Ottawa nearby.

Soon the weather in this part of the world will be warm and the virus will subside. Viruses don't like the heat and this one is reported to lose viability above 26° C. It's the same reason why influenza and other viral diseases are relatively uncommon during summer. All the more reason to slow the propagation of the virus for a few months until the weather turns in our favour. Next winter will remain a hazard but beyond that we will likely have a vaccine to halt the disease.
Spring hamfests and flea markets are being cancelled. That's unfortunate even though it is sensible. With the high average age of hams too many of us are in the high risk age bracket. Summer events should be safe except for cancellations made out of an abundance of caution.

Once our homes, finances and families are taken care of it can be anodyne to use our new found free time to get on the air and be active. We can be eminently social while maintaining a safe social distance. The virus doesn't travel by radio waves.

The risk of catching and succumbing to the disease is low for most people and I have not been too worried. Yet as the economy is shuttered and frightened people hunker down (after raiding grocery store shelves) it is nice that we have the comfort of amateur radio.

Of course everyone has the internet and social networking app but they are not the same. Amateur radio is more like mixing in public since we communicate with strangers: other hams. Social networking apps are more inward looking and we mostly talk to people we already know.

Get on the air and talk to people. Maybe I'll see you there.

73 and take care. Technical articles on the blog will resume shortly.

Thursday, March 12, 2020

Battling Noise - Theirs, Not Yours

You can't work 'em if they can't hear you.

This turns the old cliche on its head yet it is as true as its original. This can be particularly true on the lowest bands where the ambient noise is high and our antennas are often less than full size. Even for those of us with QRO and good antennas on 80 and 160 meters noise is a challenge.

I can control noise on my end and I am. Right now I am in the midst of installing a reversible Beverage antenna that, if it works as it should, will be replicated and then the three antennas brought together to a remote switch. That will give me 5 compass directions and more will be added later. I may add another receive antenna for the second operating position (multi-op contesting).

What I can't control is the other station's noise. My only weapons are power, antenna efficiency, antenna gain and good timing so that I operate when there is propagation enhancement. The latter includes sunrise (theirs or mine) and difficult to predict periods of low absorption. Beyond that I can only hope that the other station has a low noise receive antenna and that their local noise environment permits them to copy me.

Which brings me to the impetus for writing this article now: 9J2LA.

Those who have been calling them on the low bands are well aware that they are not hearing well. The local man-made noise has severely their ability to copy callers. It is particularly dire on 160 meters. Despite their many attempts to lessen the problem with directional receive antennas working them continued to be a challenge.

They are to be commended for their efforts to ameliorate the problem and to persist nightly with making the attempt to work others on top band. Listening through strong QRN is very fatiguing and not much fun. I suspect quite a few readers who are urban dwellers can commiserate.

I tried and failed to work them on top band on a few nights. Few in North America were being heard and from the size of the pile up on this end many, like me, wanted a top band QSO with Zambia. I stopped trying if their signal was of average strength since I knew I wouldn't be heard. I did work them on 80 meters and even there they had trouble copying my kilowatt and 3-element vertical yagi.

After coming home from a pizza night with fellow QRP'ers I turned on the rig and heard 9J2LA stronger than ever. I decided to give it another try even though, again, few were getting through. But first I listened. My objective was to better understand their operating style and to see what the successful callers were doing. Too many hams jump in too fast and fail to survey the field and develop a strategy. The investment of a few minutes of observation can pay dividends.

The first thing I noticed was that of the few stations they were able to copy all but a couple were Europeans. Luckily they were strong enough that I could copy 9J2LA on the northeast Beverage and copy most of the Europeans calling and working them. There was little VFO movement on their part between QSOs so the callers clustered in a spectrum +1 kHz to +2.2 kHz up.

The downside was the QRM from overlapping signals. Many in the pile up picked up on the pattern and set their transmit frequencies accordingly. It is likely that the QRM and their QRN were of similar strength which would make it difficult to focus on one signal. That many stations continued to call whether or not they were being called (or having a call sign containing the partial call being sent) didn't help. Perhaps they should have QSY'd more between QSOs to avoid the clustering of callers.

Then they worked a US station. That signal was +2.1 kHz where I could hear no other signals close by. Clearly the operator did finally spin the VFO a little higher. Of course several North Americans then called a little up or down from that frequency. However the next QSO was from the pile up at +1.75 kHz and everyone moved back to where they were before.

The diagram gives a rough idea of what I was hearing. Most callers were between +1.1 kHz and +1.9 kHz. Signals overlapped such that no one of them was alone inside a 100 Hz filter. Callers were either not listening on their transmit frequency to see if it was clear or they were big guns who saw no need believing they could out-muscle the competition.

A few callers straddled the +1 kHz minimum offset hoping to be heard eventually. That is not a bad strategy and I've used it to good effect many times. In this case the pattern didn't fit so it didn't work for them.

Numerous stations zero beat the last successful caller and called there, sometimes while the QSO was still in progress. There's nothing like a DX pile up to bring out the worst in amateur radio operators. I won't mention call signs other than to note at least one is on the DXCC Honor Roll and some are regular denizens of "the Gentleman's Band". A few others are well known DXers and contesters. Trust me, people are listening and will remember.

Predicting where the DX will listen next is an art and a science. I decided to only call where I was alone within a 100 Hz bandwidth (at least 50 Hz on either side of my transmit frequency). In an earlier article I dubbed this technique "calling in the hole". My reasoning was that with their high noise level two or more signals in their receiver pass band could not be disentangled except by a very rare and talented operator or unless one of them was exceptionally strong. Odds were against both.

By listening I knew he was focussing in a narrow range between +1.5 kHz and +1.9 kHz with occasional forays beyond +2.0 kHz. So I transmitted around +2 kHz, moving up or down as necessary to avoid other callers. That kept me out of the QRM but close enough that I stood a good chance of being stumbled upon. A few minutes later I heard the welcome sound of "VE3?".

At this point you might think I'd bang on the keyer button to send my call a couple of times. When SNR is very low this is usually a poor approach. From long experience I know that copying "VE3" is rarely a problem since it's a common prefix and it has a distinctive swing on CW. The greater challenge is getting the "VN" suffix across successfully!

I resorted to the paddles to alter the character cadence. I kept my speed where it was, set to closely match their speed. Several unsuccessful callers persisted in sending at a significantly higher speed. The operator at 9J2LA would keep missing dits or confusing the prefix and suffix since they ran together when the call sign was sent two or more times in quick success.

Working DX is not a high speed CW contest or brag-fest so think very carefully. When the DX station slows down it is for a reason. The reason may be that that's the speed they're comfortable with. In this case it was almost certainly because they needed longer dits and dahs for their ears to integrate the signals across the rapid and repetitious man-made impulse noise.

I used the paddles to send VE3 with normal character spacing and then send V and N with larger spaces around each character. I put a larger pause before repeating my call sign in the same fashion.

Either I was lucky or my technique worked since my full call sign was repeated back to me along with a signal report. A repeated 599 in reply and I was in the log. Further luck was no one jumping onto my frequency during the brief QSO. With a clear frequency and carefully articulated characters the QSO was not drawn out like many others so impatience had little opportunity to overrule manners and sense.

The same technique can be useful in everyday DXing for non-rare stations. Indeed it also works in contests to work many of the slower speed casual participants. Sending fast and carelessly slows you down since you won't be copied the first time, and maybe not the second or third time if you persist.

Before you call give some thought to the other station's operating conditions and adjust your technique accordingly. Sitting in front of a radio all night long trying to pull eager callers out of loud and grating noise is difficult. Don't make it any more difficult for them. In the end you, too, will benefit.

Tuesday, March 3, 2020

40 Meter Dilemma: Interactions & Size

For almost as long as this blog has existed 40 meters has been a recurring theme. In the beginning when my station was small my focus was on how to achieve good DX performance from a small antenna. Peruse the 2013 archives and see all of my failed experiments! Later that grew into how to achieve good DX and contest performance with less than full size yagis. I have not yet answered the question to my satisfaction.

It seems I am not alone. Among readers of the blog it is the articles on 40 meter wire yagis that are by far the most popular. The most popular of all is for the design of a 3-element inverted vee reversible yagi. It has many good features, including: low cost, good gain, F/B and SWR bandwidth, electrically reversible and it can be erected with a couple of moderately high supports.

Much to the surprise of some readers I've corresponded with I've never built that 3-element wire yagi. However I did build the reversible 2-element version a long time ago and it worked very well. It is only more recently that I've returned to the idea of building a wire yagi for 40 meters fixed on Europe, primarily for contests. I arranged the tower that way.

Of the larger, rotatable small yagi designs I've played with over the years, and ultimately rejected, are:
  • Coil loaded elements: too much loss for reasonable component selection and tuning difficulty.
  • W6NL style Moxon: modest gain and narrow gain bandwidth but excellent F/B and SWR.
  • NW3Z "V" yagi: low gain for its size and tuning difficulty but excellent F/B and SWR.
Currently my only 40 meter antenna is a Cushcraft XM240 up 22 meters because I ran out of time in 2019 to build 40 meter yagis for the 150' tower. I want a mid-height, high gain fixed yagi on Europe and a rotatable full size yagi at the top. This is a very productive contest band that demands the best antennas I can manage.

The dilemma

The south facing panorama below provides context. The 150' tower on the left will have the rotatable 40 meter yagi on top with a 10 meter yagi above. The TH7 will be relocated to ~40' and probably stacked with the TH6 at 75' for shorter paths in the south through west quadrant. A fixed 40 meter yagi of some type will be placed ~100' if I can resolve interaction issues with the top yagi. One or two 10 meter yagis on the upper 50' will be used for stacking.

The 140' tower further away is for the 15 and 20 meter stacks. The lower yagis of the stacks can be seen through the trees. The 40/10 tower is roughly in the direction of Europe from the 15/20 tower, and they are 60 meters apart. The 22 meter tower with the XM240 and A50-6 is on the right. It is 65 meters from the 15/20 tower and 50 meters from the 40/10 tower. The 80 meter vertical yagi is in the foreground. Low band receive antennas are in the bush well off to the left (east).

It was the limitations of candidate designs that gave me the impetus to work out the logistics of a full size 3-element yagi. The challenge is so great that I reconsidered. The difficulties of a full size 40 meter yagi include the following:
  • Interactions with 15: Although 60 meters (4λ) away the 15 meter stack will point at the 40/10 tower when working Europe. As we'll see this can be a problem. I already have minor guy wire interactions on the first 15 meter yagi and I don't want to add to the difficulty. The TH6 and TH7 on the 40/10 tower will suffer from interactions on 15 meters.
  • Size and weight is a construction and maintenance hazard: No robust 3-element full size 40 meter yagi weighs less than 250 lb (75 kg) and it can weigh up to 400 lb (120 kg). There are ways to build, lift and service it without a crane but these involve significant risk. I can do it but I'd rather not.
  • Cost: In addition to the metal and fasteners there is the lifetime cost of equipment and services to raise and maintain the antenna. As I and my helpers get older I'll have to hire riggers.
Modelling of interactions with the full size 40 meter yagi shows that they are significant. I modelled a wire yagi with sloping elements to minimize interactions with a top yagi and the 15 meter stack however its performance is impacted by ground absorption of the large vertically polarized energy -- unless you are pointing it over salt water and I'm not. Horizontal HF antennas are usually best except on the lowest bands where horizontal antennas suffer from ground loss and high elevation angles.

To keep the 40 meter antennas horizontal and reduce interaction with the 15 meter antennas requires moving the 3λ/2 resonance well outside the 15 meter band. As it turns out there are ways to accomplish this. These designs also offer a path to reducing cost and size.

Loaded elements, done with coils, capacity hats or both, shift the resonance of the 3rd harmonic of a 40 meter dipole. Yagis behave differently but have a similar shift. For example, the XM240 with its coils and capacity hats resonates well below the 15 meter band. For the time being I use it on 17 meters since it performs moderately well as a long dipole.

Unfortunately it is not quite so simple. There are many ways to load elements and many ways to combine them into yagis. Candidates designs must be tested for interactions on 15 meters and other bands. Sometimes pushing a resonance out of one band puts it onto another. It's time to do some computer modelling!

Interaction with resonant 40 meter antennas

Let's start with a dipole. The wire dipole shown here is resonant at 7.1 MHz and its 3λ/2 resonance is 21.65 MHz. It is typical that the harmonic resonance is a little higher than 3× the fundamental.

My 5-element 15 meter yagi will be used to test for interaction. First I will place it above the 40 meter dipole at several heights: 10, 5 and 2.5 meters. These separation distances are typical of antennas sharing a mast or tower. The elements are parallel (as depicted) to represent the worst case.

Pattern distortion is modest, with the shape of the main lobe altered by a fraction of a decibel. Minor (rear) lobes are more distorted and vertically asymmetric with magnitudes up to 10 db worse. Gain is identical while rejection of signals off the back suffers but is not catastrophic. The SWR change (not shown) is negligible.

The more interesting case is when the yagi is pointed directly at the 40 meter yagi. For this I put them at the same height and vary the distance between them -- 60, 30 and 15 meters -- as measured from the front of the yagi.

This analysis is in free space so by "height" I mean that the antennas are coplanar. I have removed the ground so that we can deal with one thing at a time. When ground is added later the pattern will show differences but won't change the key findings.

At 60 meters distance the pattern distortion is notable with the appearance of many minor lobes. As the distance decreases there are fewer but stronger minor lobes. Rejection of unwanted directions is impaired though not to an extreme. As the antennas get closer the main lobe widens, the minor lobes grow and the gain declines slightly. The SWR is barely changed, with variations of only 0.1 to 0.2 at the closest distance of 15 meters.

Of course this is dipole and not a yagi. A 40 meter yagi is more pertinent for my station so we'll do that test next. The differences that may affect the interaction are several: each yagi element has a different resonant frequency; array impedance is different from that of a single element; and a 40 meter yagi covers a large area.

I'll use a full-size 3-element 40 meter yagi developed for an earlier article. Because of the sizes involved and the use of towers the experimental setup is modified. For vertical stacking the distances are doubled and the yagi centres vertically aligned. When pointing the 15 meter yagi at the 40 meter yagi the distances are centre to centre.

It is no surprise that the interaction is worse than with the 15 meter yagi above the 40 meter dipole even at the greater distances being: 20, 10 and 5 meters. Gain and directionality suffer quite a lot at the closest vertical spacing of 5 meters. It is a bad idea to put two antennas of this type on the same tower!

As before the SWR hardly changes at all. The lesson is not to rely on the SWR (feed impedance) to judge whether interactions are present.

The patterns are far worse when the 15 meter yagi is behind (pointing at) the 40 meter yagi. I skipped the 15 meter centre-to-centre distance since it is unrealistic and the interaction extreme. Even when the heights of the two antennas are offset you should expect the interaction to be a problem though different since the main lobe is an ellipse with a large area.

The SWR does exhibit noticable change in the 30 meter separation scenario yet remains low across the band. Perhaps surprisingly it simply isn't a problem.

Of course these configurations are worst case. When the 40 meter yagi is well off the 15 meter yagi's main lobe or the 40 meter yagi is pointed elsewhere the interaction will be far less and usually not too concerning. I will be in a worst case scenario when both the 15 meter stack and 40 meter yagi point to Europe (northeast). Actually it's worse because pointing the 40 meter yagi in the opposite direction, southwest to Central America or the south Pacific, gives the same severe interaction.

This is the crux of my dilemma and thus my pursuit of alternatives.

40 meter yagis with shortened elements

As discussed earlier, elements with loads typically do not resonate on harmonic frequencies. The reactance of loads in short elements, L or C, is frequency dependent. By carefully engineering the element it is possible to avoid resonance on higher amateur bands and to reduce the size and weight of the antenna with only a small reduction of performance compared to a full size antenna, single element or yagi.

That's a challenge I am willing to tackle. There are several considerations:
  • Coils must be high Q (typically > 500) to avoid excess loss in a yagi. They are also vulnerable to weather, especially precipitation that can drastically lower Q.
  • Large capacity hats are vulnerable to wind and ice, putting stress on the element. For example the large horizontal capacity hats of the W6NL Moxon often require that the elements be guyed. Large capacity hats are fragile and complicate antenna lifting and maintenance.
  • Array (feed) impedance of a yagi with shortened elements obscures the behaviour of the individual elements. Don't expect to find the harmonic resonances from the feed point. The XM240 with its dipole feed is better in this respect (see SWR measurement above) but a yagi with a matching network can obscure harmonic behaviour. Modelling or a special measurement mechanism may be required.
  • NEC2 does not accurately calculate the resonant frequency (primarily reactance) of elements with loads. Some error is to be expected. EZNEC version 6 includes a correction for simple loads however I don't know the reliability of the algorithm. Simple coils should be fine but probably not capacity hats.
To begin let's look at the SWR sweeps of two short 40 meter dipoles. Both use 25 mm (1") constant diameter tubes since SDC (stepped diameter correction) is incompatible with capacity hats.

The coil loaded dipole is 75% of full size and the one with capacity hats is 65% of full size. Neither has a 15 meter resonance. The coil loaded dipole harmonic resonance is at 24.6 MHz. The capacity hat dipole has its harmonic resonance within the 10 meter band and an unexpected resonance at 10.5 MHz which is close to the 30 meter band. These calculations were done in free space with dipole feeds.

Although the dipole with capacity hats must be modified to avoid resonance within an amateur band it does meet the primary criterion of avoiding the 15 meter band. The resonance of the coil loaded dipole near the 12 meter band is of little concern to me due to my contest focus. It is in fact ideal since it is midway between 15 and 10 meters.

A different capacity hat design studied but not shown here is better suited to wire yagis. Its harmonic resonance is at 23.9 MHz which is far enough to minimize 15 meter interactions.

For the interaction test I'll use the coil loaded dipole and the corresponding 3-element coil-loaded yagi design from a previous article. The precise design detail will be skipped over in this initial study since it is not needed to gain an initial insight into the interaction problem.

Now that's a big improvement. The effects of interaction are negligible even when the 40 meter dipole is just 15 meters in front of the yagi. Moving the harmonic resonance 3.5 MHz above the 15 meter band is obviously beneficial.

That's a good first step so we can proceed to do the same interaction test with a 40 meter 3-element yagi using a similar short element design.

The interaction has reappeared despite the 40 meter yagi using elements that are not resonant within the 15 meter band. It isn't as bad as with the full size 40 meter dipole and 3-element yagi but bad enough to be of concern. As before the SWR is negligibly affected. EZNEC can tell us what appears to be the problem.

There is a resonance at 25 MHz and there is evidence of mutual coupling within the 15 meter band. The resonances in a yagi are not the same as in the individual elements. We can inspect the element currents for further insight.

This is becoming interesting. Notice that it is only the driven element that is exhibiting a resonance and only between the coils. The induced current is quite large so this must be the culprit. The other elements carry almost negligible current.

The only unique property of the driven element is a beta match. This is a shorted transmission line stub inductor and capacitance from shortening the driven element. It transforms the low yagi impedance to a 50 Ω match at 7.075 MHz. Somehow the combination of the beta match and the coils is creating a broad resonance where we don't want it.

Many of the better antenna switches used by contesters place a 50 Ω load on unused ports rather than shorting them to avoid similar transmission line induced resonances in disconnected antennas. The problem we have here could be a phantom or it could be real depending on how the 40 meter yagi transmission line is terminated.


I am not going to solve the discovered interaction problems here and now. The study has generated food for thought as I consider 40 meter yagi design. Additional work and real life testing is required. For readers with similar concerns and objectives you should pay attention to these interaction issues. They not easily solved.

One can put up multiple towers in strategic relative placement to minimize interactions. Few are the hams with the ability to do this. As we've seen the distances at which interactions occur is large and not all yagi directions can be accommodated during contests. On balance I don't get overly anxious until the gain takes a significant hit.

The degree to which interactions can be tolerated depends on many operating factors including inter-station isolation, poor directionality can be a bonus when you're called by stations off the side and back and more. Several large stations I've visited must have interaction issues yet they do exceptionally well in contests. No matter how you arrange the towers and antennas you will have problematic interactions at least some of the time.

I have twice at this station stacked tri-banders, small and large, on the same mast as an XM240, which (see above) has no 15 meter resonance. I noticed only modest interaction on 15 meters but honestly this is really difficult to assess. Mostly it was hearing (and working!) more off the back than ought to be there. SWR was unaffected, as we should expect from models studied in this article. Gain was too difficult to assess but I suspect it remained good. Maybe I'd have had a different opinion with more sunspots and a crowded 15 meter band.

There are other interaction issues that I have not addressed in this study and may be significant:
  • Guy wires: Despite chopping the steel guys of my towers into segments not resonant on any contest band there will be interactions. Non-resonant mutual coupling tends to increase as frequency is increased. On the lower yagis of my 15 and 20 meter stacks the 20 meter SWR is unaffected but on 15 meters it is slightly different from when it was tuned.
  • Full wave loops: These antennas have a resonance on every harmonic and can wreak havoc if not employed with due care. I have none in my current station so I won't soon look at this issue.
  • Boom resonance: When a yagi is turned 90° the primary interaction with the elements is eliminated and a secondary one can appear. For example, a full size 3-element 40 meter yagi is approximately resonant on 80 meters when fed at the centre of the boom -- the boom is the element and the elements are capacity hats. Some hams exploit these serendipitous resonances with gamma and omega matches on the boom. These resonances depend a great deal on yagi construction so there is no general rule to determining their frequencies. Models can help.
With the lower yagi of my 15 meter stack installed 100' high and pointing at the 40/10 tower there may be interactions with the two tri-band yagis currently on it. There is no coax (yet) to the 15 meter antenna so that assessment will have to wait.

Perfection is never attainable and we should not go overboard trying to get there. This is a hobby so we do the best we can with our resources. Ultimately we all have to do the best we can with what we have, whether the station is small or large. However don't see this as an excuse to be sloppy. Understanding can be educational and illuminating even if remedial action is impractical or not desired.

Next steps

Although I used coil loaded 40 meter elements for this study that was merely expedience: I had the models in my files. I prefer shortening with capacity hats for improved efficiency despite the mechanical challenges. I plan to explore 40 meter yagi design using elements of this type and see whether I can achieve my performance, interaction and mechanical objectives.

Once a design is in hand I will construct a 40 meter dipole and put it on the tower. There I will adjust the tuning to compensate for NEC2 inaccuracies with this type of antenna. It will use a dipole feed (split element centre) to best measure reactance right across the HF spectrum. Matching networks make measurement difficult so I'll deal with that afterwards in whatever yagi design I settle upon.

If all goes very well this year I'll have a 40 meter yagi (or two!) on the 150' tower by next winter with, I hope, minimal disruption of the 15 meter stack.