Faraday Rotation and Comparing Inverted Vees

In my previous article I mentioned that I barely got the second 40 meter inverted vee up installed in time for the CQ WW contest this past weekend. Since inverted vees are not particularly interesting antennas to discuss I will only briefly describe the process.

First I modelled the vee with EZNEC to ensure I cut the legs to the right length. The resonant frequency of an inverted vee is very sensitive to the angle between the legs when that angle drops below 90°. In my case that angle had to be 70° to be compatible with the available tie points for the chosen orientation: fence post and a tree limb. For 12 AWG insulated wire the modelled length was 10.35 meters per leg.

I could have opened up the vee to 90° but at the cost of a substantially more complex installation. A pole of 5 or 6 meters height attached to the fence post would be needed, and climbing up into the tree while threading the wire through the branches. The small benefit in radiation angle and feed point impedance wasn't worth the effort. Nor did I have the time if the antenna was to be ready for the contest.

I reused the 40 meter loaded sloper antenna -- a failed experiment from last year -- since it had most of what I needed: centre insulator with SO-239 and most of the needed wire already attached. Splicing brought the length to 10.5 meters per leg, adding 15 cm to allow trimming to resonance. It was quick work to run up the tower to install and connect the feed point, run back down and tie down the legs, then off to the shack to check it out. Trimming 10 cm per leg brought it to resonance approximately where I wanted it.

The SWR dipped almost exactly to 1 even though the model showed an impedance of 35 Ω. This may be due to ground and environmental loss or perhaps interaction with other antennas. When perfectly symmetrical the non-resonant support tower should "disappear" since the vee's legs induce equal currents of opposite phase, thus cancelling. Perfection is often unattainable so there may be a small net current.

Another thing worth mentioning is that I designed this antenna to withstand more tension than is typical. Slack reduces the interior angle of the vee, degrading its performance. I didn't want the angle to be any less than the already small 70°.

Modelling, testing and directivity

I describe my inverted vees as oriented east-west and north-south even though that is not quite the case. Broadside for my existing mult-band inverted vee is roughly 100°/280° true bearing, and 30°/210° for the new vee. Although not ideal I am constrained by the availability of supports and lot width.

The pattern shown is for the new vee. The one for the existing, multi-band vee can be seen in a previous article. Two things to notice are that the multi-band vee shows more directivity than the new one and that, due to asymmetry, the former has a deep notch to the south.

The interior angle affects the directivity. The smaller the angle the more omni-directional it becomes. The multi-band inverted vee has an interior angle of about 120°, which is approaching that of a dipole, and therefore has a pattern not too different from a dipole.

The difference in directivity is small in most cases. The new vee does much better to the south, making it easier to work W4, the Caribbean and South America. The multi-band vee performs better for W6 and the Pacific. Towards Europe the new vee does better. From Ottawa, the bearing to central Europe from is 50°.

This came in very handy during the CQ WW contest. The difference to zone 8 (Caribbean) was as much as 2 S-units on the KX3. The differences to Europe and the Pacific were more subtle, never exceeding 1 S-unit. For many hams this might seem inconsequential, yet it makes a noticable difference. In marginal cases switching from one antenna to the other with my QRP SSB signal allowed a QSO to be made compared to getting no response to my calling them. With QRP every decibel counts. That's no exaggeration.

The inverted vee works well on 15 meters. Since it is broadside to the Caribbean it came in handy to work several multipliers and US stations during the day when the yagi was aimed at Europe. The other inverted vee isn't as favourably oriented and so was less useful for the diversity I require during contests.

Faraday rotation

Now I want to back up the Thursday evening before the contest when I was comparing antennas on 40. I was briefly baffled by the relative performance of the antennas which made for confusing comparisons. Eventually I figured out what was going on. That it took me 30 minutes to work it out is a little embarrassing since it should have been immediately obvious. But first some background.

Faraday rotation in the ionosphere is most often responsible for the cycle of QSB on HF signals. HF antennas are almost always linearly polarized, whether vertical, horizontal or, less often, an angle in between. As Faraday rotation alters the polarization of the incoming signal the induced power on our antennas changes. Signal strength is maximum when the signal and antenna polarities align and can dip 20 db or more when they are opposite (90° apart).

Many hams use both vertically and horizontally polarized antennas on the low HF bands to pull through weak signals (like mine!). To do this you switch between antennas during polarization dips, use a signal combiner or simultaneously listen to two receivers.

The phenomenon I encountered was that on many stations the dip due to Faraday rotation on one antenna seemed to exactly coincide with the peak on the other, and vice versa. At first I put it down to bad luck and I kept trying.

I soon realized this behaviour was no accident. Have another look at the azimuth plot up above. There are 3 field strengths plotted: horizontal, vertical and total. Notice that the orientation of the horizontal and vertical polarization pattern lobes and nulls are at right angles to each other. This can be explained by imagining yourself looking at the antenna from a distance.

When you are broadside to the antenna it appears to have a horizontal orientation. This is the direction in which the horizontally polarized field is maximum. Looking at the antenna from the side and its horizontal width is zero. It now appears to be a vertical, with one leg behind the other. This is the direction in which the vertically polarized field is maximum. The opposite is true for the nulls.

Here then is the answer to the puzzle. When the incoming signal has horizontal polarization its signal strength is maximum on the inverted vee that is broadside to the signal direction. The other inverted vee sees minimum signal strength since it favours vertically polarized signals in that same direction. The situation reverses when the signal is vertically polarized. When the polarization is neither vertical nor horizontal both antennas show some attenuation, and it can be difficult to say which receives best. The ambiguity also occurs for signals that are neither broadside nor off the end.

Doing a proper comparison

A proper comparison of antennas therefore requires more effort than instantaneous A-B switching. On 40 meters it can take 5 or 10 seconds of listening on one antenna to find the maximum signal strength. This must then be repeated on the other antenna. Or two receivers can be used. Comparisons are more difficult and can be muddied by other propagation effects that also affect short-term signal strength variation.

Despite this it is recommended that the inverted vee broadside to the desired direction be selected, and left there. Even though the new inverted vee with its acute interior angle is close to omnidirectional, in practice this rule still seems to apply. For example, toward KH6 (west) the east-west favouring vee has better than a one S-unit advantage during our sunrise opening. Part of the reason may be that the vee with an acute angle has poorer low elevation angle gain due to the lower height of the average current.

I'll be keeping this antenna. It works and it is not an encumbrance or eyesore.

Interactions

As already mentioned, the orientation of the new inverted vee keeps interactions quite low. It is still likely that some coupling is affecting the antenna, though with a magnitude that is not material in any way I can see. The impedance is not quite as predicted by the model, yet any affect on its resonant frequency is small in comparison to the wire length itself.

The acute interior angle also ensures minimal coupling with the yagi. In general I've found in models and in practice that wires that have an angle with the tower under 45° are free of interactions with yagis up above.

Where I did encounter a problem was with the 80 meter vertical, which is comprised of the tower, radials and yagi (capacity hat). The effect is not large and did not stop it from being used in the contest. The coupling, from the model, appears to be non-resonant, perhaps acting more as a coupled resonator due to their proximity and acute angle of 35°. I will address this in a future article, along with other items pertaining to the vertical.

CQ WW SSB - QRP SOAB

This past weekend was the 2015 edition of the CQ WW SSB contest. Like last year I entered in the QRP category (5 watts), single op, no assistance. My score was about the same as last year, though with fewer QSOs and more multipliers (zones and countries). You can compare my claimed score to other entrants at the 3830 web site. Last year I managed to be #1 worldwide in this category. But then very few competitive operators use QRP.

Operating QRP SSB is only fun for a little while, until you get past the initial joy of all the amazing DX you can work with such low power. For me the frustration of the hard work required on most contacts and the inability to work most that can be heard becomes overwhelming. I'll come back to the topic of QRP SSB towards the end of this article. First I want to review a few of the things that diverted me from that frustration during the weekend. It wasn't all misery!

Amusement

There were quite a few things in the contest that made me smile, and sometimes laugh. Amateur radio, and contesters, are a slice of humanity that shine a light on our curious behaviour. That is, people watching is fun and you can do it during a contest as well as on a street corner.

• Some operators are a little unclear on the concept of integrating computers for SSB contests. In one case a big station with a high run rate had the computer "speak" the caller's call sign and the contest report. This voice was clearly artificial and distinctly different from the recorded CQ message, and from the human operator who frequently interceded. It was weird and confusing. For the few minutes I was in the pile-up I heard numerous stations sounding bemused or perplexed. I suppose it's a technological marvel but not one that is conducive to a high score.
• Similar to the above, at many stations the messages were recorded by someone other than the human operator. It becomes confusing when the operator responds to your call. A couple of times I thought it was QRM (another station on the same frequency, as often happens in a contest) rather than a response to my call. It must hurt their rate. There's no reason for this since it is possible with available contest logging software for each operator in a multi-op to record and select messages in their own voices.
• Amid the super-stations and the many more little guns there are some little guns in rare countries and zones. Like me they often find it difficult to be heard on a crowded band. I worked some good DX by carefully scanning the band. Each time it took only one call, with just 5 watts. They badly needed to be spotted. I couldn't help since I was unassisted and disconnected (see next section below).
• As often happens there are a surprising number of US stations that slip below 21.200 or 14.150 MHz without noticing they are outside their SSB band segment. Since I wasn't running I couldn't tell them, and many foreign stations don't know that US callers shouldn't be there. They are in for an unpleasant surprise now that the CQ Contest Committee is becoming very strict about citing entrants who operate out of band.
• I also noticed a few European stations operating split on 40 meters who reversed their transmit and receive frequencies such that they were inadvertently transmitting above 7.2 MHz. They are transmitting outside their 40 meter band. Not surprisingly no one called them, on either frequency.
• Several of the US big guns operating split on 40 and 80 meters made the mistake of never listening on their own frequency. They missed out on many QSOs with Canadians, even as they avoided contending with zero-point QSOs with other US stations. In one bizarre instance the station in question was just below 7.2 MHz, rather than more typically transmitted higher in the band. He got no calls on his listening frequency below 7.1 MHz but had a small pile-up of Europeans on his transmit frequency. He heard none of them and just kept looping CQ's.
• Several US multi-multi stations operate the low bands through much of the daylight hours. They strive to scrape every conceivable QSO from these bands. Typically it's their greenest operators who do this duty, where they can learn the ropes while contributing to the team effort. But it must get very boring with the poor rates they achieve. Midday about all they can work is a few Canadian stations for points. They try to entice as many as they can to give them a contest exchange. I run into this a few times during contests when I briefly slide down to the low bands in the afternoon to pick up a few US contacts. This weekend one of those multi-multi operators launched into a rag chew, telling me about the station (which is already well known) and his brief experience in the hobby and contests. After a couple of minutes I was going crazy trying to find a polite way to get out of the QSO and back to the high bands. Sympathy for his boredom was damped by the QSOs I was missing on 15 meters.

Antennas and equipment

I did manage to get my second inverted vee for 40 meters raised about 30 hours before the contest. That didn't leave much time to test it out beforehand. All I could do was trim it to resonance and confirm that it worked to fill in the gaps of the existing inverted vee's azimuth pattern. During the contest it came in helpful, though it did behave in ways that, at first, seemed odd. Since this is an interesting topic on its own I will follow up with an article dedicated to that topic.

This contest was a poor test of the 80 meter vertical since I can work very little with 5 watts on SSB. It garnered the few QSOs and multipliers I intended to acquire, but that's all. November's contests will provide a better venue for testing its effectiveness.

Inside the shack I tried a rearrangement of equipment on the desk. Unfortunately I designed and built my operating desk 30 years ago when computers and contest logging were not a serious consideration. With two transceivers (KX3 and FT-1000 MP), a laptop and other operating accessories it was difficult to arrange them for effective daily use and for contests. So I pushed the FT-1000 MP off to one side to allow unfettered access to the KX3 with my left hand.

For the first time I made use of the KX3's DVR (digital voice recorder). In the two message memories it supports I recorded CQ in one and my call sign in the other. Considering how few times I had an opportunity to call CQ (< 1% of contacts made this way) it would have been better to use it for my exchange (59 04).

When my laptop crashed early in the contest I decided to flip the Wi-Fi off using the hardware switch. A few months ago I determined that the periodic crashes I was experiencing were due to a hardware fault associated with Wi-Fi. This was no inconvenience since I was operating in the "no assistance" QRP category. Eventually I'll have to replace my now ancient laptop. It also doesn't have the power for too many concurrent N1MM+ features and windows.

Propagation

The QRP operator is more to the vagaries of propagation than those with higher power. When you run, say, 100 watts and you're being received S7 at the other end of the QSO you are likely to be perfectly readable. When you subtract 13 db (5 watts) the situation can be radically different.

This is because communication is all about SNR -- signal to noise ratio -- not absolute signal strength. If the prevailing noise level is S3 you have 24 db of SNR on average (assuming 6 db/S-unit). QSB and propagation changes may be tolerated without loss of readability. Remove 13 db and SNR drops to only 11 db. You are now very vulnerable to conditions.

There are several consequences:

• Openings are of shorter duration and less reliable. Marginal conditions at the start and end of an opening for higher power stations are unusable with QRP because the SNR will be negative. If ionospheric absorption is higher than normal or flare reduces ionization levels the opening may be entirely unusable because of lower signal levels, and therefore no positive SNR.
• Atmospheric noise is progressively stronger at lower frequencies. QRP has a higher LUF (lowest usable frequency, where the SNR goes negative).
• QRN and QRM at the other side of the QSO similarly reduces the effective SNR, making QRP less likely to complete the contact. You have to hope for a break in the noise to slip in the report and exchange.
• What goes for CW goes even more so for SSB. With the wider bandwidth the SNR for the same power is far worse on SSB. It's no surprise that QRPers make more contacts in CW contests than in SSB contests. The opposite is the case for QRO.
• DXpeditions are much harder to work. Everyone else in your part of the world isn't running QRP, so your signal will be covered up most of the time. Good tactics may get you on a clear frequency where the DX can copy you, but you must still rely on luck since there are other good operators who know the same tactics.

The 10 meter openings in this contest were very good, though not as excellent as last year. In 2014 I managed to run Europe on the Sunday morning. This year my attempts to run 10 meters netted exactly 2 contacts. Signal strengths were not as strong and the opening was less extensive in both duration and coverage. The modest reduction in SNR had a severe impact. Others noted the poorer openings on 10, yet were still able to run. QRP suffers disproportionately.

Where I go from here

QRP SSB is tough. It's especially tough if, like me, you're interested in DX and contests. Before going QRT in 1992 I never operated QRP. The closest I came was when I was first licensed in the early 1970s and had to make do with an 807 tube rig, which was all I could afford. For most of those first 20 years I operated with 100 watts or 1,000 watts. I mostly avoided QRO at times and on bands where I might disturb the neighbourhood peace. QRO contesting was solely conducted in multi-ops at others' stations.

I first tried QRP SSB contesting in early 2014, just to see what it would be like. My choice was CQ WPX. I wrote about my experience with that in this blog. At this point I am ready to give it up for good.

Although I am able to place well in the QRP category it is just too painful. I am going to move up to low power in future SSB contests at my present station. Although I won't place well I prefer to refresh my contest skills, such as running, multiplier hunting and so forth.

But why have I stuck with QRP contesting for the past couple of years? I had reasons to do so.

• Even at 100 watts I am a bit shy about EMI events in the neighbourhood. I have (mostly) great neighbours and I like to keep the peace. On the other hand I have had no incidents in the few contests in which I ran 100 watts. Perhaps I am being overly sensitive.
• I have noise, sometimes lots of noise. With QRP I can be certain that when I call CQ few of the stations that reply will be loud enough to be heard. This is not only a problem on the low bands. At times this past weekend the QRN would be S9 on 10 and 15 meters. In the ARRL DX contest earlier this year it became a frequent problem.
• There is real joy in working the world with 5 watts. It was something new for me when I opted to go with the Elecraft KX3 when I rejoined the hobby in early 2013. Eventually I worked up to well over 200 DXCC countries with QRP, including over 100 countries on bands from 40 through 10 meters (except for 12 meters, which I don't use).

Time moves on and so have I. There is no reason for my continued use of QRP. I have nothing to prove. I may continue with QRP in select CW contests for the next while since there at least I can achieve better results. Apart from that I intend to move back into the mainstream. I am almost driven to do so by the declining sunspot count rendering the highest HF bands unusable for several years.

Boxing the 80 Meter Vertical L Network

The L network at the feed point of my new 80 meter vertical is now complete. The rat's nest of components and wires lying at the base of the tower have are gone, replaced by a box of components with (mostly) fixed components. You can look back at that earlier article for details on the L network design and tuning process.

Construction

The photo at right shows how I put the network together and how it will be attached to the antenna. The box is an ordinary 4"x4"x2" plastic junction box for use outdoors. These are inexpensive items founds in every hardware and electrical supply store. Notice the seal on the cover.

The SO-239 is attached with 4-40 screws and nuts, with a ⅝" hole for the main body. For the latter I used a wood bit at a slow drill speed to avoid melting or burning the plastic. The centre pin supports one end of the series inductor.

Aluminum flashing was hand cut, holes punched for terminals and deburred with sandpaper. Not only do these serve to mechanically stabilize the parts, they are quicker to make than wires with lugs. The only wire lugs are for the coil tap and the external connection to the radials.

The shunt capacitor is overkill in this application, even for a kilowatt. I chose this 500 pf doorknob because it measured close to the 440 pf value measured for the variable capacitor after tuning the network. Also, it has been lying neglected in my junk box for many years.

The long stainless steel screw at the top secures the network to the tower leg, while also providing the electrical connection to the tower as the vertical monopole. The shorter stainless steel screw at the bottom is for the wire connection to the radials. Nuts secure both screws to the box walls. One more nut per screw is used for their external attachments. Washers protect the tower leg and box walls from being abraded when the nuts are tightened.

Tuning woes

To my disappointment the L network did not offer the same match at the test setup. I underestimated the difference between the rat's nest wiring and the finished product. All those dangling wire affected the match. The precision needed in component values is tight enough that modest differences can have a large effect.

Ultimately I needed to increase the shunt capacitance by 250 pf to about 700 pf total, with a final coil tap to give 2.6 μH series inductance. This is quite close to the tuner design I got from TLW and verified with EZNEC. My rationalization in the previous article was off base. The design was fine as it was. Classify this among the cases where theory trumps practice!

While I have smaller doorknobs to make up the additional capacitance they are a pain to work since they have screw terminals. Instead I used 3 tubular 82 pf capacitors in parallel (not pictured). These are inadequate for a kilowatt though good enough to handle 100 watts.

The result is a vertical with an SWR of 1.1 at 3.5 MHz and rising to 1.9 at 3.8 MHz. To better centre resonance I would have had to substitute capacitors until I had the perfect combination. Since I primarily operate CW and it is adequate for a few SSB contest contacts I decided I had spent enough time on this antenna. I boxed it up and moved on.

Perhaps the lesson here is to always allow a means for tuning in a matching network. Even if only to tweak it. Going to the other extreme of making every network fully adjustable is a waste of money (high-power variable capacitors and coils) and time.

Installation

The final photo show the box on the tower and ready for use. The only things not done at that point were soldering the wire to the radial nexus and sealing the boot on the coax connector.

The installation is quite clean looking and is almost ready to winter. However I did encounter one significant problem that had to be remedied before I could turn to other projects.

The box was on the tower for 2 days and nights while I dealt with the tuning problem and tested it on the air, including the Worked All Germany contest. The cover was in place but not screwed down. It rained. After the contest on a frosty Sunday afternoon I lifted the cover and found a small pool of water in the box, and a coil partially encased in ice! More waterproofing was necessary.

I put the box in the sink to melt and drain the ice then returned to my workshop. Coax-Seal was used to line the outside of the screws and coax connector to discourage water leaking in. The cover, when screwed down, is watertight by design. Yet I know that water is pernicious, not to mention that by late winter the snow line might reach the box. I drilled holes in the 4 bottom corner of the box to drain any moisture that does get in either by leakage or condensation.

Performance update

As mentioned above, I used the antenna in the Worked All Germany contest. I made 2 contacts on 80 CW (both Germans, per contest rules). While that seems pathetically low it is not bad for a QRP (5 watt) effort under conditions that were not good. I made a few other DX contacts (Europe) outside the contest though not enough yet to get a strong sense of its performance. Certainly it's no worse than I did with the loaded half sloper. Neighbourhood QRN is equally as loud on receive, unfortunately.

Perhaps I'll know better after CQ WW SSB this weekend. I don't expect to make any DX contacts on 80 SSB, but you never know. But right now I need to put up the supplementary 40 meter inverted vee before the weekend.

80 Meter Tower Vertical with Short Radials

Now that some other tasks are out of the way and the weather is sufficiently cool that traffic has declined in my backyard, it is time to get to work on my new antenna for 80 meters. If you read my plans for this band you'll know that I am unimpressed with the performance of my loaded half sloper, and I want to do better in time for this season's major contests.

80 meter load half-sloper in storage with other antennas

In this article I'll walk through the process, starting with theory, going on to modelling and then to construction, tuning and an early performance assessment. The last is made difficult since it was necessary to remove the half sloper, and conditions for DX have been poor.

Model expectations

In that article I referenced a formula in the first edition of ON4UN's book Low-Band DXing to estimate the electrical length of a tower with a yagi up top. At 3.65 MHz the formula worked out to 105° for my 14 meter tall tower with a Hy-Gain Explorer 14 at 15 meters. Extrapolating to 90° the estimated resonant frequency would be 3.13 MHz.

I took my interaction model, added a set of 8 radials, each 8 meters long, and fed it at the bottom. Since the tower is mostly isolated from ground this is allowable, at least in theory. The model confirmed that the structure should be resonant (X = 0) at 3.25 MHz. So far so good, or at least the ON4UN formula is roughly consistent with the NEC2 model.

The modelled resistance component of the feed point impedance at 3.65 MHz is 80 Ω. It is high due to ground loss and being well off resonance. I chose to simply place a capacitor load in series with tower (monopole), chosen to have a reactance of equal magnitude to the inductive reactance of 44 Ω at 3.65 MHz. This requires a capacitor of 1000 pf. However, it isn't quite that simple since the capacitor transforms the antenna feed point impedance and antenna behaviour. Instead a value of 700 pf series capacitor inserted into the model is needed. The resulting excellent SWR curve is below 1.6 between 3.5 and 3.8 MHz. The resistance at the new resonant frequency of 3.65 MHz dropped to 65 Ω, which further tamed the SWR.

Notice that the resistance is still high for a λ/4 vertical (typically 37 Ω). The largest component of the additional resistance comes from the modelled ground resistance (loss), which is in series with the radiation resistance. For 8 short verticals that isn't too bad. NEC2 often underestimates ground loss, so this should be kept in mind.

Radial configuration

Since writing my planning article I experimented in the model with a variety of radial systems that could be built in my narrow and long backyard. I increased the radial count to 8 to minimize ground loss for what will be short radials, and to minimize the resonance/tuning effect of the radials on the antenna system. More radials would be better, but the added nuisance is undesirable and in any case would benefit performance very little.

What I did resolve was to make the radials of equal length. Having a mix of long and short radials (to maximize use of the available space) always resulted in low current in the short radials, making them redundant. The final configuration of 8 radials of 8 meters length best equalizes radial current and still fits nicely within the 15 meter width of my yard. Modelling indications of this arrangement were promising with respect to performance.

Preparing for construction

Continuity insurance

Before building the antenna I did some preparatory work. First, as I said above, the 80 meter half sloper was removed. Modelling showed that it would alter the behaviour of the vertical. In any case I wanted to free that coax for a planned inverted vee on 40 meters.

I next unrolled a 130' length of RG-213 that I had in storage and ran it from the shack to the tower base. Although it would be best to bury this coax for best common mode results this is undesirable for reasons that have nothing to do with performance. Instead it was run above ground along the same messenger cable used for the other cables going to the tower, then dropped from a 3 meter height to the base. I will return to this point later since this does have a performance impact.

In one trip up the tower I removed the half sloper, installed a short aluminum tube to support the planned 40 meter inverted vee, and then installed a wire from the tower to the rotating mast.

Since the mast and yagi are to be part of the 80 meter vertical it is vital to ensure electrical continuity from tower to mast, and then to boom and the yagi elements. The yagi is fine as is since all but the driven element are bonded to the boom, and the boom to the mast. The risk shows up between tower and mast.

The jumper wire shown in the photo is insurance against lack on intrinsic continuity due to the rotator bearings and mast bearing balls. These are coated in grease and could interrupt continuity since those many bearings may be the sole electrical path through these devices. The added wire is our insurance. In the photo the rotator is in its centre position, with the wire gently rotating with the mast. When you do this be careful to ensure the bonding is secure and that the wire won't snag any bolts or other protuberances when it turns.

Installing the radials

I prepared the radials early on, once I made the final decision the number and length. I have a large role of hookup wire which I had bought about 25 years ago for this very purpose, yet until now it was not used for radials. The 64 meters of wire I used made only a small dent in the roll's diameter. While the gauge is bit low for robustness it should serve well for this winter season. They can always be reused for another project.

With the lawn well trimmed I unrolled each radial and tied it to a central point on the wooden tower base. The first 4 were easily placed at 90° to each other. The remaining 4 bisected each pair so that the 8 radials were spaced 45°. These were connected to the coax outer conductor. They were stapled to the edge of the base to that they were held close to ground, in case I would need to mow the lawn before the snow flies.

The ends of the radials are anchored into the sod, and in one case into an old tree root. The anchors were chosen for expediency: 3" galvanized framing nails. These worked well enough, though I would not count on the tension being maintained for an extended length of time. As you can see in the photo the radial is stripped at the end to facilitate tying it off, but is not electrically bonded to the nail.

Feed point construction and initial on-air testing

For an initial test I wanted to compare the antenna against the ON4UN equation (3.13 MHz) and the EZNEC model (3.25 MHz). The measured value was in the vicinity of 3.15 MHz. with an SWR of 1.1. This is close enough to both that we can consider them sufficiently reliable. Or perhaps I am just lucky. Though short, the alligator clips I used to connect the coax to the radials and tower would influence the measurement. I was not concerned about that at this stage of the process.

The EZNEC model also predicted a high SWR but decent radiation resistance at 7.1 MHz and 10.9 MHz. The measured values were further off -- 6.7 MHz and 10.5 MHz -- but the SWR dipped to below 1.2. I took this as a hopeful sign that the ground loss is lower than in the model. It is also possible that something more serious is going on. I put that concern aside until I was able to test the antenna's performance.

The directly fed vertical seems hear well on 80 and 30 meters, but not on 40. Since it was daytime I did not attempt any QSOs. Instead I proceeded to installing a simple matching network

Series capacitor

The value of a series capacitor acts in an inverse manner in comparison to a series inductor. As inductance increases from 0 μH the resonant frequency declines. For a capacitor it is a value of ∞ pF which does not shift the resonant frequency of the antenna, and as the capacitance drops from infinity the resonant frequency rises. Of course no capacitor has ∞ capacitance, you can only approach it in the limit, where it is equivalent to a short; that is, no series capacitor.

In practice getting to 0 pf is easy while high values at RF are difficult. Or at least it is difficult when we want low loss (equivalent series resistance) and high power handling. These are increasingly important in a matching network where the Q and the mismatch are high. But for tuning purposes we can take a simpler approach.

At a recent hamfest I purchased a few variable capacitors with a large maximum value, just for this application. The capacitor pictured at the feedpoint measures 2,200 pf with all sections ganged, which is perfect for lots of variability around the modelled requirement of 880 pf to bring resonance up to 3.65 MHz. The plate spacing is fine for QRP but can handle 100 watts in this low impedance calculation. This permits full power on-air testing. If tuning is acceptable a fixed capacitor with low ESR and high power handling can be substituted.

Tuning did not achieve what I wanted. As the model shows, the resistance component of the impedance rises as the inductive reactance is cancelled by the capacitor. This is expected since the capacitor changes the electrical topology of the antenna. Unfortunately it rose more than predicted, resulting in a minimum SWR of 2 at the low end of the band. Resonance also moved closer in-band on 40 and 80, but with an SWR in the range of 2 to 3. I suspect, but cannot prove, that stray capacitance among the many cables and coupling to house pipes and wires (15 meters away) contributed to the measured difference.

This would be the perfect time to test it on the air. To my dismay the geomagnetic activity spiked to a K index of 7 (October 7-8 UTC). DX on 80 was hard to come by. Performance on 40 and 80 seemed poor in comparison to the inverted vee, which while disappointing was not a design objective. It was also possible that low angle paths were strongly attenuated due to the geomagnetic activity this far north.

L network

I proceeded to design an L network to better match the antenna while I waited for conditions to improve. Nothing more elaborate is needed. Had the coupling of the tower to ground been greater the impedance profile would be further from the modelled value so that a gamma or omega match might be needed.

Designing an L network is quite easy with TLW. Plug in the complex impedance and frequency, select a network topology, and it designs the network. For convenience I chose the topology that was easiest for me to build using available components.

That L network can then be added to the EZNEC model to complete the model. I did that, and the model predicts an SWR below 1.5 from 3.5 to 3.8 MHz.

The L network is easy to build. I used the same variable capacitor as the shunt element. The series coil is bare copper wire wound over a ¾" form. When released from the form it expanded to the 1" diameter I wanted. Inductance can be adjusted by squeezing or stretching the coil, but I tapped the coil with an alligator clip. Coil dimensions were calculated with a standard formula for an air core. While these may have some inaccuracy it is sufficient to get within 10% or 15%, which is near enough to allow adjustment for best match. Don't simply guess at the coil dimensions or you'll fail to get a good match.

A bit of tweaking brought the SWR in line with the model: below 1.5 from 3.5 to 3.8 MHz.. I left the L network exposed as shown for the Thanksgiving long weekend since no rain was forecast. No curious cats, skunks or squirrels disturbed the setup. My run of good luck continued when I finally worked TX3X minutes after taking the above photo. Despite the ugly wiring job it withstood 100 watts without any sparking.

Unlike with the series capacitor I tried first, the L network did not permit operation on 40 or 30 meters. I chose the "low pass" configuration in TLW since it gave the easy inductance to implement quickly and efficiently.

When tuned for best match the L network components were an inductance of 2.15 μH and a capacitance of 430 pf. The coil value was calculated from its tapped dimensions and the capacitor was measured. The stray inductance and capacitance of the messy wiring job are close to negligible on 80 meters.

At 3.7 MHz where the SWR is 1.0 the calculated antenna (load) impedance would therefore be 100+j0 Ω, not the modelled 72+j55 Ω. The resistance component of the impedance is at least in reasonably close agreement. The reactance is a puzzler, probably having to do with much stray capacitance among the many cables running on and to the tower.

Performance assessment

Propagation was quite poor when I was ready to try out the antenna. A long period with disturbed geomagnetic conditions made for difficult testing of DX paths (my main objective). While I did have some luck a true picture of antenna performance will take longer. As I write this the antenna is out of action because I removed the temporary L network and sealed the coax to protect against the rain.

For now I can say that it works but is not a miracle worker. That's as expected. I worked some DX in these horrid conditions, but only a few stations that seemed to run high power and so made it through. At this point I cannot say how it compares to the loaded half sloper. It should be better, but I have to be honest and say that, so far, I don't know. Without anything truly quantitative about its performance it will have to suffice for now to review some general points on this vertical's pros and cons versus my (dismantled) half sloper.

Pros:

• Wide bandwidth: I have an excellent SWR from 3.5 to 3.8 MHz, and still reasonable towards 4 MHz. This is typical of ground-mounted verticals. Resonance, as presently tuned, is around 3.7 MHz. 
• Lower radiation angle: At least that's the theory. The reality should become clearer over time.
• Common mode: Less RF is getting back into the shack than with the half sloper. That may be nothing more than blind luck (see below for further discussion).
• 160: The antenna could work on 160 meters with a suitable L network. It would not be efficient with those short radials and would require a remote switch between 80 and 160 matching networks.

Cons:

• Near-field coupling: There are 2 or 3 houses within the antenna's near field, and all the long wires and pipes that will couple to the antenna. Vertical polarization is typically more susceptible to this, partly due to maximum current at ground level.
• Radials: These are a hazard in a suburban backyard. I will likely have to remove them in the spring. They are also half the minimum length they ought to be to capture most of the return currents, therefore allowing too much current flow in the lossy, poor ground I have at this location.

Analyzer

To put it simply, I didn't use one. These are wonderful tools, but not always necessary. Don't let the lack of one stop you from experimenting with antennas. If you understand the fundamentals and the design constraints it is often quite easy to do without one for most antenna work. This is not meant to endorse the idea you shouldn't use one. It you do, take care not to be mesmerized by the numbers displayed by a pretty toy. Test equipment cannot substitute for knowledge and insight.

With EZNEC, TLW and a few simple measurements I was able to get within striking distance of my goal. When the resonance measurement differed from the model I used the same model to estimate the true impedance at 3.65 MHz. Since the model resonance was 100 kHz higher than the measurement I took EZNEC's impedance calculation at 3.75 MHz as the baseline for the matching network. This was close enough to the reality to enable a rapid tuning procedure.

Actually I would have liked to have one handy. I could have better assessed where the theory and the reality clashed, and tuning would have taken less time. Had the model been inadequate, as does happen with low band antennas in a messy suburban environment, an analyzer would be useful.

I have been shopping around for analyzers suitable to my needs. A two-port VNA (vector network analyzer) may work best for what I have planned for designing and maintaining my future antenna farm. Reasonably simple antennas such as the one in this article can be designed and built without one.

About common mode

As much as I (and many others) like to harp on the subject of preventing common mode currents on the outside of coax and other cables, this antenna makes for a good study of futility. Suppressing common mode current from a yagi or dipole is easy in comparison to this antenna. Let's look at that now.

The tower is the antenna. Cables running up the tower are capacitively coupled to the tower along its full length. Antenna currents can directly enter the inside of coax via the other antennas, especially where yagis use feed systems such as beta and gamma matches. For the same reason it would be foolhardy to operate SO2R in a contest with one rig on 80 and another on of those yagis, at least not without some exceptionally good filters.

An exercise in futility?

My overhead cable runs, from tower to house, allow coupling between radials and these horizontal runs. Burying cables beneath the radial system would help, but that is not an option in my present installation. Neither is it practical to place common mode chokes that are effective at 3.5 MHz on every cable where it approaches the tower and the house.

The upshot of all this is that I cannot properly control common mode. The coax to the vertical and the horizontal runs of the other cables will in some measure act as supplementary counterpoises, and couple some energy back into the shack (and allow local noise pickup on receive). Neither is wanted, but neither is going to be completely eliminated. The scramble wound choke I made from the surplus length of cable in the 130' run is poor at best at 3.5 MHz, but I did it anyway since the cable had to somewhere after all.

All of this is to say that I understand the implications of what I'm doing. It is not recommended and it is never a virtue (i.e. don't make excuses), but often it is unavoidable in a small station.

Finishing

The L network will be moved into a weatherproof box that will include an SO-239 connector for the coax. The box will clamp to the tower for mechanical support, to keep it out of the snow and for the electrical connection for the tower. I haven't yet decided on whether to use a variable capacitor or a fixed capacitor.

With that done the vertical should be able to survive the winter and give me reliable performance in the busy season ahead. I'll report back on this antenna when I have a clearer idea of how it's doing.

For Future Consideration: Tower

Once again I am adding another chapter onto this series about actions I have taken towards my next, and bigger, station. All that Heliax now has a tower to attached to.

This past weekend was a busy one for me. It involved renting a large truck, driving over 1,100 km, picking up loads in two distant locales, then unloading and storing the truck contents on my property. In all, the weight of everything was somewhat over 2,500 lb (1,200 kg). Now I have a large tower, and much more.

The tower you see in the photo has 15 sections, each 10' high, including one base section. With overlap for section splices this tower would stand 144' (44 meters), plus around 15' (4 meters) of mast topped with a yagi.

The assembly resting atop the base section is a prop pitch motor fitted to a platform that mounts to the side of the tower. More about that in future. Just outside the picture frame is a large quantity of guying material, pipe and tubing. Most of the guys will be stripped for the usable parts, with most of the guy wire to be discarded; due to its past service it is necessary to purchase new guy wire for this tower.

Background

This tower was manufactured by Leblanc & Royle (L&R), probably in the 1980s. L&R was in the business of designing, manufacturing, installing and maintaining tower systems for broadcast, military and other applications in Canada for many years. Many commercial towers across the country were once L&R, and many remain in service. While I don't know the details they were likely the victim of a changing industry, failing to successfully transition from previous tower applications to cellular communications. The tower requirements are quite different.

Used and surplus L&R tower can be found in many of the larger amateur radio stations in Canada. The tower I purchased comes from two other VE3 stations. The tower is used though appears to be in excellent condition. Most sections are painted in the familiar white and red colours for aircraft safet, while a few are bare galvanized.

One strong attraction of this tower to me is the many attachments that had been customized for amateur use. These include brackets for the (above) prop pitch motor for rotation of a heavy duty mast and side-mount yagis. These should not be overlooked since they will save me a substantial amount of time and money for design and custom manufacture. However, having seen long service, some maintenance is needed for the custom bearing assemblies and other components. This is something I will look at this winter, now that I have it close at hand.

LR20 specifications

Per the specification at right, my tower is "light duty" LR20 (the numerals are the face width in inches). It is conservatively rated and can comfortably handle ice and wind load when 150' high and holding several long boom HF yagis. Strict adherence to engineering guidelines for guying and loading is mandatory.

One thing that distinguishes this tower from a tower with round legs and diagonals is wind load. A tower has a substantial surface on which the wind acts, a load that the tower must support even without the additional load due to antennas. The engineering specs take this into account. Always always always follow the manufacturer's instructions for tower assembly, guys, anchors, base and load placement.

There are two girts per section, used for guy attachment, bearing plates, side mounted antennas and other loads. Guy yokes (custom U-bolts) are already attached to the girts on my tower that were previously used as guy stations. Depending on my final design I may need to move these.

This tower has a pier pin base, where the base plate is positioned over a 1" steel pin protruding from the reinforced concrete base. This is a common design element in large guyed tower to mitigate the stress on the tower caused by the torque of wind load on large antennas and bending in high winds. By allowing the tower base a limited amount of motion, the guys can absorb these dynamic loads. You can see an example for Rohn tower on W8JI's web site.

Also of note for those of us who do climb is that there are horizontal struts at ~18" intervals on one side of each section. That is a great convenience for climbing and working at height. The girts alone are inadequate since they are 5' apart. Ask anyone who has worked on, say, a Trylon Titan tower what they think of diagonal struts. It is of course important that the tower be built so that the horizontals are all on the same side. I prefer mine on the side that faces approximately southwest to keep the sun out of my eyes and the prevailing wind at my back.

A tower is not a toy

Do not be misled by any sense I might have unintentionally given that this tower is a plaything. Hams use towers to get the best from our antennas, just as we might use a kilowatt amplifier. Both can easily kill you. The only difference between falling off a 150' tower versus a 50' tower is the amount of time you have to think about it on the way down.

Safety is mandatory in the air and on the ground when working on any tower, and especially one of this size. Things always go wrong. It is our job to ensure accidents that do happen are not fatal to people or property.

The weight of the tower sections requires mechanical assistance. Do not rely on the muscles of two or more people. The risk is too high. Better to have one expert on site than a large crew of eager but inexperienced hams. This is not the place to trade safety to save money. Accept the fact that a tower this size is going to incur a substantial expense.

There are several options to put up a large tower. Use the one that works best for you, and that can be done safely and within budget.

• Gin pole - This is the cheapest solution but expensive in time and potential safety events. The pole and attachments must be up to the task of lifting and manoeuvering heavy awkward weights. A tractor or similar device can provide the muscle with a suitable pulley system to minimize gin pole stress and permit horizontal pulling. Winches have been used but are not ideal due to the amount of cable required and the risk of slippage. Steel temporary guys are required before reaching the first permanent guy station, and even higher to reduce sway during construction.
• Crane - The tower can be built on the ground and lifted all at once. A large crew is required on the ground to place the tower onto the base, plumb the tower and especially to secure and tension 12 or more guy wires. If any splice bolts slip the tower sections (typically near the centre of the span) will need to be redone once lifted but before the guys are fully tensioned. The crane will not be cheap, and you want a crane operator and ground crew who are sensible, experienced and safety conscious. If the crane boom is tall enough you can also include mast, rotator and even an antenna or two. The latter requires a second crane or a boom to mast clamp that permits 90° rotation.
• Helicopter - This is similar to erection with a crane but far more expensive and should only be performed by professionals.
• Hybrid - Use a small crane to lift part of the tower, to at least just above the first guy station. Splice slippage is kept to a minimum and only 3 or 6 guys need to be handled. The rest is done with a gin pole.

I have been involved in tower erections using the first and fourth methods. One of those was for an L&R tower of similar height to mine. While this gives me valuable experience it does not qualify me as an expert.

Before it goes up...

There is much to be done before my newly acquired tower goes up. One obvious need is the purchase of a large plot of land; this tower requires almost 1 acre of land, and more with mandatory setback from property lines. I must also perform ordinary maintenance on the tower sections to repair any minor flaws. All tower sections and guy yokes have passed my preliminary inspection. Always inspect tower sections, whether new or used. Inspection is easier on a tower with open steel members than it is on tower with tubular steel legs such as Rohn.

Mechanical work includes bearing repair and replacement, of which there are several: top plate and thrust bearings for the mast and beneath the drive bearing. The prop pitch motor may require maintenance and does require some work on the chain drive system. The motor itself works fine. The homebrew control electronics and positioning system may require an overhaul.

Guying hardware must be inspected and replaced where necessary. The tower came with a large quantity of turnbuckles, thimbles, guy grips, insulators and guy wire, mostly used but some new, in both ¼" and 5/16" sizes. I expect to discard most of the guy wire since its remaining service life is inadequate to my needs. The old guy wire that is in good condition may be used in other projects with less critical requirements.

From here on this is a winter project which I can temporarily set aside. I will now return to my fall antenna work. The 80 meter vertical is next on my schedule. CQ WW is fast approaching.