Tuners Do Not Solve All Ills

I do not have antennas for 80 and 160 meters.

That may not sound like a profound statement yet, for me, it is. As far as my DXing goes it simply means I don't go there. Until I am in a position to raise effective DX antennas for those bands I stick to 40 meters and higher bands. The dilemma comes from contest operating. If I don't operate on those bands I miss out on some easily-acquired multipliers, whether it be CQ/ITU zones, countries (VE & W), ARRL sections, or states/provinces.

For my proximate need there is no need for a great antenna on those bands. It is minimally sufficient to work only a few stations to give the contest score a big boost. For example, in my report on the CQ WW CW contest in 2013 I mentioned that I used a tuner to put enough of a (poor) signal on 80 meters to work a couple of stations and add 4 multipliers (VE, W, zones 4 and 5). Yes, that really does make a difference to one's contest score and is well worth several minutes of effort to re-cable and fiddle with the tuner.

As it turns out it isn't as easy as it sounds. Certainly I have a tuner, several in fact, including one that is big enough to have only modest losses at high SWR. It has no trouble at all getting a 1.0 SWR match on 80 meters for my currently largest antennas: multi-band inverted vee (30 through 10) and delta loop (40). However the results were not at all equivalent, or even what I expected.

Neither antenna is even close to resonance on 80 meters. The SWR is high, very high. It is so high that EZNEC gives up on the calculation, only indicating that it is above 100. That is the feed point SWR, not what you see in the shack. At such high mismatches there is considerable loss due to transmission line attenuation. It may seem odd but this can be viewed as an advantage since the resulting high (not extremely high) SWR means that the tuner can achieve a match without risk of excessive loss in the tuner.

To give you some idea of how high the SWR is on 80 meters with those two antennas I used EZNEC to quantify the antenna feed point impedance at 3.525 MHz.

• Delta loop: Z = 0.5 - j80 Ω. For 50 Ω transmission line this gives an SWR of ~6700!
• Multi-band inverted vee: Z = 3 - j1000 Ω, for an SWR of ~350.

Since the true SWR is very sensitive to environment factors at these extreme impedances these are at best only rough estimates. The point is that the SWR is pretty much off the charts, and it has consequences.

The drawing of the setup gives us an idea of where to look for trouble spots.

• Tuner loss: Regardless of whether the feed point SWR is 350 or 6700 the SWR at the shack end of the coax will be much the same, since the dominant factor is transmission line loss. Since the actual value is not extreme (measured to be, very roughly, 10 or somewhat less) and is similar for both antennas I will ignore this factor in the analysis. If you like, assume a loss of -2 db, which is typical of a mid-sized tuner matching a high SWR at 3.5 MHz.
• Transmission line loss: There are two components to the loss. The first is the matched loss, when the SWR is 1 (load impedance is 50 Ω). The second is mismatch loss due to the signal reflecting back and forth between source and load, where attenuation is suffered on each reflection.
• Antenna I²R loss: The radiation resistance of an antenna rapidly declines below its resonant frequency. Since the conductor resistance is in series with the radiation resistance, as the latter gets very low more of the source power is dissipated in the conductor.
• Ground loss: As you go lower in frequency the antenna is closer to ground when measured in wavelengths. Near-field losses increase due to interaction with (typically) lossy ground.
• Pattern loss: The radiation pattern can tilt upward due to the lower height in wavelengths, especially for horizontally-polarized antennas. With more power radiating at higher angles there is less going toward low angles. If your objective is DX this factor can be viewed as "loss" even if the antenna is perfectly efficient.

Having dispensed above with the loss due to the tuner I will first turn to the radiation and ground loss. After all, the title of this blog is Pattern and Match and I often emphasize putting the priority on pattern.

In the above patterns I have normalized the gain to 25° elevation. It should be no surprise that the horizontally-polarized inverted vee directs most of its radiation straight up since it is very low to the ground on 80 meters. The pattern suffers less when using the 40 meters delta loop, remaining primarily low angle and vertically-polarized. Both antennas are close to omnidirectional on 80 meters since they are quite small in terms of wavelength.

Even with its excessive high-angle radiation the low-angle radiation (25°) is 1 db better on the inverted vee. The reason for this is due to ground loss: the modelled loss over medium ground is -1.5 db for the inverted vee versus -6 db for the delta loop.

The antenna I²R loss is low in both cases despite the very low radiation resistance. It is no more than about -0.2 db with 12 AWG insulated wire. I expected worse, so that is one positive outcome.

Now we must deal with transmission line loss. This can be difficult to model since many of the more common equations in use become increasingly inaccurate at very high SWR, and the SWR of these antennas is very high indeed.

Unfortunately the VK1OD transmission line loss calculator I've used in the past has been taken down by the author. There are other calculators on the internet but may suffer from the inaccuracy cited above. Nevertheless that is what I went ahead and did with a couple of online calculators. The true loss could be higher than the figures I am going to report.

• Multi-band inverted vee: -11 db
• 40 meters delta loop: -23 db

Both figures include the sum of matched loss and mismatch loss for ~25 meters of  RG-213 coax between the tuner and antenna feed point. I did not compensate for the λ/4 of RG-11 matching section on the delta loop since with these levels of mismatch the difference on the results is unlikely to be significant.

Gaze at those loss figures for a moment and think what they mean. If you transmit 1,000 watts on 3.525 MHz on the delta loop more than 990 watts is dissipated in the transmission line. In a way that's a good thing since at these levels of mismatch the high voltage points along the coax could otherwise punch through the dielectric and destroy the cable.

Summing all the losses, at 25° elevation the gain at 3.525 MHz on the inverted vee is -11 dbi and on the delta loop is -24 dbi, minus any tuner loss you wish to include. Most of the loss components are already included in the EZNEC model. I did not use EZNEC to model the transmission line loss. These loss calculations explain why, while both antennas performed poorly on 80 meters with a tuner, I could make a few marginal contest contacts with the inverted vee but not at all with the delta loop.

Lessons learned

Using a shack-based antenna tuner can produce far worse results than you might imagine since the SWR can be extraordinarily high. Do not be deceived by the facility with which a non-resonant antenna can be matched in this way. Tuner and other losses pale in comparison to transmission line losses for all but the shortest runs.

In retrospect this is why I was able to do so well with my eaves trough antenna, including making contacts on 80 and 160: there was no transmission line between the tuner and antenna. Although I cannot easily do an A/B comparison, I believe that the eaves trough antenna performed better on 80 meters than either the delta loop or inverted vee. This is despite its low height (6 meters), bends, corners and attachment to a conductor- and dielectric-ridden house.

The options to deal with this problem should be clear. I will list them anyway.

• Put up a resonant antenna (duh!). Even if you have to use loading coils or a matching network at the feed point it will almost always do better than a non-resonant antenna with a long run of coax and matched in the shack with a tuner. A large, efficient tuner makes almost no difference.
• Only use a shack-based tuner for small excursions from resonance for coax-fed antennas. Don't rely on the SWR you measure from the shack since transmission line losses reduce the maximum SWR you'll measure in the shack, telling you nothing about the severity of mismatch at the antenna feed point.
• You can use open-wire line to tame the transmission line losses, but is a lot of trouble to do right. Plastic-encased ladder line is not low loss; you must use true open-wire line, and you'll probably have to make it yourself. Open-wire line is fraught with difficulties, including getting through walls, preserving the differential phase between wires, corrosion, precipitation and coupling to metal obstructions.

There are other conceivable solutions to specific circumstances. For example, a switch or relay can be used to break the 40 meters delta loop into two separate arms, resulting in an asymmetric λ/2 doublet. Unfortunately it does not resonate at half the 40 meters resonance, and for anything other than QRP the switch (relay) will very likely flash over when transmitting since the voltage at the ends of a doublet can be very high.

Another possibility (one that many have tried, including myself) is to unscrew the coax connector so that only the center conductor of the coax is connected. Then ground the tuner (or transmitter) side of the outer conductor. While there is the risk of RFI, hot grounds and higher ground losses, the antenna can become much more efficient when matched by a tuner. The transmission line becomes part of the antenna. When it does work it usually only works on 160, and not so much on 80.

Final thoughts

Non-resonant antennas in typical use are poor performers, as measured by system efficiency and pattern. Unless designed for a specific purpose, a non-resonant antenna that is opportunistically tuned to make QSOs may suit in a pinch but is otherwise a bad idea. It will net a few multipliers in select contests, but nothing more.

If you want an effective signal for DX, contests or other interests, design and build an antenna that will achieve your goal. Opportunistic use of a tuner is rarely effective. You may be better off locking up or selling your tuner so that you never succumb to temptation.

Late-winter Doldrums

Too cold to work on antennas but so close to warmer weather that I can't bring myself to only plan but not do anything outside. I have pretty much convinced myself that I need to take a step up with antennas and possibly with power. QRP with small antennas has been great fun this past year, it just isn't what I want to do forever.

The spring equinox is as good an opportunity as any to look back at my DX accomplishments over the past 14 months with QRP (10 watts maximum), CW only, and simple, low wire antennas, since ending my 20-years hiatus from amateur radio. That will be a good base from which to look forward to the rest of 2014 and beyond.

DXCC

I now have over 100 countries on each of 40, 30, 20, 17, 15 and 10 meters. The last band on which I achieved this mark was 17 meters, where I now have 102 worked. I was surprised at my low country count on 17, so I had to catch up. This is most likely due to it not being a contest band. I have mostly worked 17 meters to catch some rare DXpeditions (FT5ZM, VU7AG, etc.). I have yet to work F and G on that band. Go figure.

Unsurprisingly 20 meters is my best band with 153 worked. All other bands are in between. I don't have antennas up for 6, 12, 80 and 160 meters so apart from a smattering of contacts using a tuner my efforts there are approximately nil.

My overall total countries is 193 worked. It's slow going at this point. I have heard lots of workable stations in perhaps 50 more countries but my puny signal was not heard. My objective of reaching 200 countries with my current station may not happen before I begin antenna work this year.

LoTW

Logbook of the World has proved to be a great way to confirm countries for DXCC credit. As of my last upload at the end of February I have 145 countries confirmed through LoTW. A confirmation rate of 75% is quite good.

On a per-band basis I have noticed an interesting trend. On the bands where contests are held (80, 40, 20, 15, 10) my confirmation rate is ~65%. On the other bands (30, 17) the rate is ~50%. I suspect the reason is that contesters are more likely than others to upload their logs to LoTW. Many DXpeditions delay uploads or do not use LoTW.

Contests

My DX totals were enhanced by participation in contests, both semi-serious and serious. However on deeper reflection my results mask an unpleasant truth: I am mostly working only big-gun contesters, especially on 40 meters.

It is by working the big guns on every band that my QSO totals get as high as they do. I can't run stations (sit on a frequency and call CQ or QRZ, and get answers) and I don't work many of the stations that have similarly puny signals or even those with average signals. I remember one very weak European on 15 meters that answered my CQ, and later discovering he's a regular contester in the same QRP category as myself. That's how I must sound to most stations that I call. That's why I have to call the big guns.

This is easily noticed in the logging software by the number of big guns that I've worked on every band. All it takes is working 100 of these stations across the bands to reach 500 QSOs. In DX contests where VE can work W/K there are even more of these stations to work.

Whether for purely contesting objectives or as a path to DX success I need better (bigger) antennas.

***

Looking forward to 2014 I will briefly outline the topics that are of interest to me in my pursuit of better antennas. If you follow the blog you will likely see one or several articles on each this year.

Tower

With some reservations in advance of a post-winter inspection, I believe that my opportunistically-guyed small tower passed the weather challenge quite well. This included not only cold and ice, but also some strong winds. If it checks out I plan to install a more substantial tower, though still one that would be considered light duty.

My objective is simple enough: a 3-element yagi at 14 or 15 meters height for 3 or more of the high bands will provide 10 db or more of gain over the multi-band dipole and inverted vee I currently use. Part of the improvement is antenna gain and part is greater height. If I decide to stay with QRP this change alone will make me significantly more competitive in DX pile-ups and contests. Jumping up to 100 watts would add a further 10 db gain.

My plan, if I come across something cheap and used, is a Delhi (now Wade) DMX-52. I can mount this in the same location and manner as the current Golden Nugget tower (Site C). I prefer this approach so that no concrete base is required and I stay under the municipal/federal "duty to notify" regulatory requirements that are in effect for structures higher than 15 meters above grade.

If that goes well (and I don't again lose interest in the hobby) I will consider a more permanent, stronger and higher tower in 2015. Any such tower must go to Site D in order to be clear of the septic system tile bed, yet keep a decent distance from the rear property line.

High-performance yagis for the high bands

A light-duty tower requires an antenna (or antennas) that don't stress the tower plus guying. The TH6DXX I have in storage is heavy and has more wind area that I am comfortable putting on a tower of this class. It also doesn't include 17 and 12 meters.

A rotatable wire yagi is more suitable. I am beginning to seriously look at the 5-band Spiderbeam. From people I've talked to it appears to be up to surviving our local weather and its performance claims appear to be legitimate. I found an EZNEC model of the 3-band version (20, 15, 10) and have started experimenting with it. A Hexbeam is also a possibility, except that it has a large vertical height that would easily put it over my 15 meters height limit when placed on a 14 meters high tower.

If I get ambitious I'll also put up a short yagi on 6 meters. If I don't get around to it by July I will probably not do so at all this year since sporadic-E season will have already come and gone.

Low-bands antennas

For 40 meters I may replace the delta loop with a 2-element switchable array, probably the diamond loop array I have already designed. I will need to supplement this with a dipole, possibly on the house-bracketed mast, to fill the side nodes of the array and to effectively work the northeast US in contests.

Getting an effective DX antenna on 80 meters will be difficult. Something like an inductor-loaded half-sloper might work out. However there is the potential to interfere with the performance of the 40 meters array, and there is an unknown capacitive loading due to the wire high-bands yagi. I don't need a great antenna for 80 meters, just one that will collect multipliers in contests and allow me to do some DXing.

I have no plan for 160 meters in 2014.

I have been idly playing with EZNEC to model a variety of potential 40 and 80 meters antennas for 2015, in the case that I put up a proper tower. The main challenge is getting a high-performance antenna to fit the 15 meters (50') width of my property. For instance, a rotatable 40 meters yagi cannot have elements longer than 13 meters. Managing loss in short antennas is the objective, and therefore I have started to explore in that direction.

When these models reach a suitable level of maturity I will write about them.

Antenna interactions

My immediate interests with respect to antenna interactions fall into two categories:

• Interactions among many antennas sharing one tower
• Interactions among stacked, rotatable yagis

In 2014 the first category is of most interest to me. That is why I spent some effort investigating how a vertically-polarized low-bands antenna interacts with a tower, and how to resolve problems.

The second category is more of a future concerns, but an important one. There are specific ideas I want to dig into that may shed more light on this question. Most hams go by rough, and often unverified rules-of-thumb, while other elect to ignore the issue or go to unfortunate extremes. For example, loss of structural integrity by using masts that extend far above the tower top.

Wind load

Because of my choice of tower and guying arrangement I have a renewed interest in acquiring a better understanding of wind load. In particular, the quantified wind load of antennas and other tower loads, and the behaviour and real carrying capacity of towers. This will also be useful should I erect a larger tower in 2015.

The big problems with wind load is that the marketing of antennas and, to a lesser extent, towers does not provide reliable figures. It is quite easy to find quoted square footage of popular antennas that cover an almost 2-to-1 range of values. Towers manufacturers are typically better at providing good data, if you know how to interpret and apply the data. These data are critical not only to build a robust installation but also to pass the requirements for a building permit.

This is not an unfamiliar area to me since I have put up countless towers and antennas over the decades. I have seen many towers and antennas fail as well. There is good information out there, which I have begun to collect.

Moving Into the Shack

As you might be aware the winter in central and eastern North America has been long, cold and snowy. This makes it difficult to put antenna designs into effect. At this point I am getting tired of modelling antennas. Unfortunately that's all I can do. Well, not quite. Surprising as it sometimes seems to me there is more to this hobby than antennas.

Which brings me to this article's topic -- moving into the new shack -- just for a change of pace.

I have been gradually working towards finishing my basement shack over this winter. Progress had to be gradual since I've been busy at many things, not least of which is the antenna articles I've written over the preceding months. Now that the end is near I was able to finally move back into the shack and set it up as a more permanent area for radio operation. The critical finishing pieces were the door, trim and (very important) the floor.

Once that was done all I had to do was put in an operating desk and reinstall all the equipment. The room was designed as a shack when the house was constructed in 1993 so it has all the necessary infrastructure, including dedicated electrical circuits and two 240 VAC outlets for amplifiers. It sat mostly neglected when I decided to not continue with the hobby. Now, 21 years later, I've finally moved in.

Rather than go back to a simple desk I have restored the custom operating desk that I built 30 years ago. It was used in my first station (1984 to 1992) since moving to Ottawa from VE4. I supplied the basic design parameters to my old friend and excellent amateur woodworker VE3NVM, from which he came up with a construction template. With his help and workshop the desk quickly came together.

Here it is in my new shack, already celebrated with some contacts, including one new QRP country: 5H.

The tabletop measures 84"x30", so it is an imposing presence in the modest-sized 120 ft² room. Made from plywood and seasoned maple it can not only support a lot of equipment, it is perfectly safe to stand on. The only equipment it didn't support was my old Collins 30S1 amplifier, which was meant to stand on the floor. Notice how the KX3 is dwarfed by space meant to hold an older generation of transceivers and accessories.

Let me take you through the design parameters I came up with all those decades ago so that you can get a sense of what I was attempting to accomplish with this desk. The effort I expended is more than most hams would bother with, yet the concepts are equally applicable to the selection and assembly of "off-the-shelf" products.

• Surface height is measured to fit my body. When seated in a chair, with its height set so my thighs (femur) are parallel to the ground, the desk height is such that grabbing the paddles and sending CW is almost effortless. Almost every commercial desk has a higher surface. This can lead to fatigue, especially during a weekend-long contest. One reason the surface is only ¾" thick is to provide sufficient leg clearance despite the comparatively low height. Maple is used to brace the surface due to this choice, yet still support a lot of heavy equipment.
• The lower shelf is for power supplies and other equipment which do not require operator interaction other than being turned on and off. The power bar (bottom right) is used to power them all on with one switch. Right now there is just the 4 ampere DC supply to power the KX3, and the AC power supply for the laptop. Back in the day that shelf was crowded. Its height and placement is designed to not get in the way of your feet. For SSB I had a foot switch on the floor beneath the power supply shelf.
• The rigs I used most often went into the lower bays of the upper shelf unit. I chose an antenna switch that permitted the coax cables to exit straight back. This saves space and has a clean appearance, but at the cost of some difficulty in attaching and removing those cables. The B&W switch is, regrettably, intermittent. This is a design flaw and not due to ordinary wear and tear. Worse, the unit is sealed and difficult to repair.
• The middle deck was used to hold VHF transceivers, pre-amps and amplifiers, plus an assortment of measuring devices. All I have there now is an SWR/watt meter. In the centre is a slot that held the logbook, countries list and other paper resources. All of that is now done with software.
• The upper shelf was for everything else, such as a world globe and spotlight lamp.
• On the right side is a longer open area which I used to work on equipment. It had its own power bar and lamp. When I bought my first PC in 1991 (a speedy 16 MHz) it went in this space. Thus began my obsession with antenna modelling, starting with the DOS-based of ELNEC. Even simple models could take many minutes to run on that PC.

There is one terrible lack in this otherwise functional operating desk. Do you see it? There is no place to install a flat screen PC monitor. This ought to be easy to remedy. Some of the upper shelf space will be covered by the monitor, but since today's equipment is smaller that shouldn't be a problem. Then I'll be able to put a keyboard up front and make it easy to arrange things as in any modern PC-centred ham shack.

Since this desk (less the upper shelf unit) was the centrepiece of my upstairs home office for the past 20 years I had to replace it, and fast. My new office desk is a bizarre hybrid of an old, small Ikea desk and odds and ends from Home Depot. I worked quickly this weekend to both rebuild the shack operating desk and construct and install a new office desk. Now I am not only on the air with my old and trusty operating desk but also ready to get down to work Monday morning.

Detuning a Tower from a Vertically-polarized Antenna

In my previous article on a 2-element loop array for 40 meters I issued a caution regarding the potential harm from a tower that is resonant, or just near-resonant, on the band of interest. This type of interaction can destroy the performance of a directive low-bands antenna that is vertically-polarized. Although even a single-element vertically-polarized antenna will excite a tower resonance the impact is typically not a major concern other than the effect on impedance matching. I have a particular concern with tower resonance since that may be only way I can achieve higher performance with a low-height antenna for 40 meters.

As W8JI describes it is possible to detune the tower so that, at least on one band, the tower can be made to effectively disappear. That is, become non-resonant on the band of interest. This allows the vertically-polarized low-bands antenna to meet its potential.

Of course the tower (plus ground and loading due to yagis mounted above the tower) might not be resonant on the target band and therefore there is no cause for concern. But you can only know for certain by exciting the tower with a nearby vertically-polarized antenna for that band. It would be a shame to go to all the effort of designing and building a high-performance antenna that isn't going to work out.

A better strategy is to remove the tower resonance once it is found, and ensure the antenna fulfills its potential. This is not only a one-time concern since, after all, the tower's resonant frequency will not stay fixed for all time: changes to other antennas on or near the tower will shift the resonant frequency, and we all add and remove antennas on an often yearly basis.

Since the weather is continuing to stay cold, windy and generally miserable here in Ottawa I decided to do a little more computer modelling to test methods for detuning the tower. It would be good to know this since if I erect a larger tower this year I will want to build a directive antenna for 40 meters, and due to its inevitable low height I prefer to go with a loop array rather than a yagi.

W8JI provides some general guidelines for designing and tuning what is effectively a trap on the tower. What is missing are specifics. This is understandable since there are many variables that are installation specific. However I don't want to just wing it. This is where EZNEC comes in handy, allowing us to parameterize the design so that we can succeed faster when we spring into implementation.

For the following discussion I will stick with 40 meters and the switchable 2-element narrow diamond loop array from the previous article. The lessons learned should be applicable to other bands and antenna configurations.

The basics of the trap design model are shown at right. Tom suggests a trap length of no more than 3/16-wavelength so I made my trap 5 meters long (A, wires 10 & 14), 0.5 meters wide (B, wires 12 & 13), and centred on an 18-meters tall tower (wires 9-11, with 10 as part of the trap). The 3 wires are ¼" aluminum rod, which make the trap rigid and adjustable. Heavy-gauge copper wire can be substituted for wire 14. The 18 meters tower height was selected since it is the worst case for 40 meters resonance that I previously discovered.

Currents are shown on the EZNEC plot for the case of trap resonance. Notice that the while the current in the trap is high the current in the tower segments above and below the trap are low (they decline to 0 at each tower end). In this view you cannot see the currents on the elements since EZNEC plots those at a right angle to those shown here.

A series capacitor is positioned at the lower-right corner of the trap, where it is most accessible for tuning. I assumed that the capacitor is a fixed, transmitting "door knob" capacitor or an air variable that is both low loss (small equivalent series resistance, which I inserted into the model) and can withstand the voltages present with high power. The capacitor should be protected from the weather with a cover or enclosure, and protected from mechanical strain by, for example, placing it in parallel with an antenna wire insulator.

Safety note: Place the trap high enough that it is out of arm's reach from the ground since there can be high voltages present on the trap when transmitting high power on the test antenna for which the trap is tuned.

The model was developed without doing any calculations. I simply made an educated guess at the inductor dimensions (length and width) and then adjusted the capacitor value (using an RLC load in EZNEC) until the current was maximum at the initial test frequency. This was easier than determining the trap's resonant frequency, even though in practice the latter is usually easier to measure -- I have a dip meter but not a suitable RF ammeter. Either ought to work since maximum current in the trap should coincide with the resonant frequency.

It only took about two minutes of value substitution to find the capacitor value for the test frequency: 49 pf. That's a useful value since I have several 50 pf transmitting ceramic door knobs in my junk box.

However this, as it turns out, is inadequate. I chose a test frequency of 7.02 MHz since that is the frequency at which the antenna gain is maximum. When I checked across the band I found that the F/B and higher-frequency performance were degraded. The F/B is the most valuable metric since any extraneous current will disturb the fine balance of current phase and amplitude between the antenna elements: a F/B of -20 db requires a power subtraction of 99%. Gain is less sensitive to minor phase and amplitude deviations.

Following further experimentation I found that it is possible to adjust array performance by changing the resonant frequency of the trap:

• Frequency of maximum gain (7.02 MHz, 49 pf): The result is as described above. The gain went up by a small amount, about 0.1 db, which is negligible.
• Frequency of maximum F/B (7.08 MHz, 45 pf): This gave the closest match to the gain and F/B curves for the model that has no tower, as was done in the previous article on this antenna. There is some degradation of gain and F/B at the top end of the frequency range (7.2 MHz).
• Frequency higher than maximum F/B (7.14 MHz, 35 pf): Maximum gain dropped -0.1 db (which is negligible) and the frequency of maximum F/B rose to almost 7.1 MHz. Gain and F/B improved a small amount at 7.2 MHz.
• 7.2 MHz (~30 pf): Maximum gain dropped a more significant -0.3 db and the frequency of maximum F/B rose a bit further than the preceding case. Gain and F/B made further improvements at 7.2 MHz, though not by much.

From this I conclude that precise tuning of the trap is not necessary. If you do want to eke out every decibel per my original antenna design objectives the trap should be tuned for resonance between 7.05 and 7.1 MHz. Matching is not a concern since the SWR curve shifts a managable amount as the trap is tuned across the band.

Tuning the Trap

Although this is a purely software model, one I have yet to build and use, the design must be amenable to tuning. Since it is difficult to directly measure the degree of interaction between the tower and the antenna it is best to focus efforts on the trap. Luckily this should work well, as W8JI said and as my modelling seems to demonstrate.

NOTE: If you see evidence of tower interaction during the initial setup and tuning of the loop array you must put that aside until the tower trap is installed and properly tuned. You should only continue tuning the antenna (per the procedure in the loop array article) when the trap is tuned.

Construction and configuration of the trap are the foundation of the tuning system. The horizontal arms (B) are modelled as solid aluminum rods not only for strength but to allow the inductance to be varied. I kept the arms short enough that the entire trap can be reached from the tower.

The vertical arm, A (parallel to the tower), can be wire, just take care that the copper to aluminum junction is solid and protected from corrosion. Solder lugs are a good choice, much better than clamping the wire directly to the aluminum.

The capacitor is placed at the bottom of the trap for a reason: to allow adjustment with the minimum possibility of coupling between the trap inductor and your body. It should be solidly attached to the bottom arm so that it is robust against abuse and tuning (if it is a variable capacitor). Tune it with your head below the level of the bottom arm and good insulation between your hand and the body of the variable capacitor.

If you choose to use a dip meter to tune the trap a small pickup loop should be inserted at the bottom of A. This is easier to do when vertical arm A is wire.

C vs. L -- The trap can be tuned by varying either the C or L component, although until now I've only discussed C. The L value, although not directly measured and difficult to measure in practice, increases as the A or B dimension (see above plot) increases, and vice versa. For example, if the B dimension is shortened from 50 cm to 30 cm (1 ft.) the required C value must be raised from 45 to 50 pf to counteract the reduced inductance and keep the trap resonant at 7.08 MHz.

It may be preferable for trap robustness to use a fixed C and a variable L. To vary L you slide the rods in and out of the tower or slide the taps for the A arm along B rods. Adjust both ends of A at each step so that the A wire is parallel to the tower.

If a variable capacitor is used you can opt to remove it from the trap after tuning is complete, measure its value with a capacitance meter and substitute a suitable transmitting capacitor of that value. Getting an exact match will be difficult so you should adjust the trap inductance afterwards to compensate. Whether a fixed or variable capacitor is used you must protect it from the weather to prevent damage and so that precipitation does not alter its value or breakdown voltage.

After tuning you must test the antenna from the shack. Confirm that the F/B has a sharp peak and that the SWR curve and resonance are as per the design. You can then complete tuning the of antenna, confident that the tower is no longer interfering with its performance. The trap should not need further adjustment after the antenna is tuned.

Variations on a Theme

Trap orientation -- As modelled the plane containing the trap inductor is orthogonal to the loop elements. While this is largely immaterial to the design and tuning of the trap there is a small affect on the antenna pattern. When the plane of the trap inductor is parallel to the loop elements the pattern becomes asymmetric near the pattern's nulls and rear direction. The effect isn't large so it can be ignored. However it does demonstrate how fine a balance between element currents is required to achieve a large F/B.

Vertical trap placement -- The trap can be moved up and down the tower, but does it make it difference? In my model I centred the trap on the tower, so that there is 6.5 meters of tower both below and above the 5 meters high trap section. I tested this by first moving the trap so that its bottom is 3 meters off the ground. This might be preferred to make it accessible from a step ladder.

I again tuned the trap to 7.08 MHz, and it turns out that the value of C is unchanged. I had suspected that by proximity to ground would alter the inductor value. Unfortunately the frequency of both maximum gain and F/B shifted downward by ~25 kHz and the F/B curve across the band degraded by several db. This isn't a large problem, but still. On the plus side the SWR curve improved! There is now another dip to 1.0 at 7.2 MHz (see chart). However the antenna performs almost no better than a single loop above 7.2 MHz.

Next I moved the trap higher on the tower, so that the top of the trap is 3 meters below the tower top. Interestingly the pattern and match behaviour was almost identical to that with the trap low on the tower. The significant differences are that the SWR is even better, staying below 2 even at 7.3 MHz. This time, to my surprise, it was necessary to raise the capacitor value from 50 to 63 pf.

In both cases the worse performance is visible in the currents plot. The longer section of tower outside the trap develops a higher current which interferes with the desired performance of the array. Of course if you're willing to sacrifice pattern performance (primarily F/B) for full-band matching you now have an option. However keep in mind that this is only a simple model, and once you add in the capacitive action of high-band yagis above the tower what you are likely to achieve in reality will differ, no matter which height you place the trap. My guess (which I won't bother modelling right now) is that with yagis in play the trap ought to go above the half-way point so that the electrical length of the sections above and below the trap are approximately equal.

Trap versus No Trap

Building, tuning and maintaining a tower trap requires effort, some expense and ongoing maintenance which, I believe, should only be undertaken if proven necessary. Since the resonance of the tower (plus ground and other attached antennas) is typically too difficult to predict it is best to try the vertical array (or just a single test element) first and determine whether the tower can degrade to the loop array performance. Do this by looking for anomalous SWR -- where the impedance curve and resonant frequency significantly departs from the model. Anomalies in F/B and gain are more difficult to discern.

If a trap is warranted don't hesitate to do it. When you go to all the trouble of building a large antenna to improve low-band performance it is unwise to ignore the signs that the antenna cannot perform as intended. This is too often easy to overlook (and convince yourself otherwise!) when a second antenna for the band is not available for comparison.

Even after you do build the trap you might not yet be done: solving one problem can introduce others. The placement of a trap for 40 meters in the tower could alter the performance of nearby 80 and 160 meters antennas by introducing a tower system resonance on those bands. As with any trap, on lower frequencies it acts as an inductive load which could lower the antenna system resonance to one of those bands, creating a destructive resonance that was not present before installing the 40 meters trap.

Of greater concern is any antenna for 80 or 160 meters that incorporates the tower as part of the antenna. Examples include half-slopers and shunted towers. At the very least those antennas will require retuning once the 40 meters tower trap is installed and tuned.