Tuesday, December 31, 2019

Ice Storm

This is not the article I wanted to finish the year with. On December 30 we had an ice storm that deposited from ¼" to ½" ice on all my towers and antennas. Unfortunately there has been damage. With all the tree limbs and chunks of ice falling it's still too hazardous to do a full inspection, not to mention the treacherous surface ice.

Freezing rain requires a fine balance of atmospheric conditions. There was little of it 100 km north in Ottawa and 200 km to the west. It's very possible that I am the only ham with a large station that has been affected.

The worst was to my new 80 meter vertical yagi. The stinger at the top of the tower folded from the weight of the ice on the parasitic wire element support ropes. Yet the weaker PVC pipe at the very top, to which the catenary ropes are attached, survived.

Shortly before the failure I tried to shake the ice loose because the stinger had a pronounced bend. Ice tumbled off the smooth surface of the insulated wire elements but was hooked deep into the rope fibres. An hour later the upper 1.5" × 0.095" wall tube collapsed at the joint with the larger pipe below.

This is disappointing since I thought it would withstand ice as well as it has 100+ kph winds. In this region ice is a greater menace than wind. The weight of ice on those long ropes is substantial. The towers themselves are fine since the ice is a modest addition to the tower weight and 1000 lb guy tension. Self supporting towers are more at risk should the wind blow hard while ice is present.

Luckily the yagis held up to the abuse. The elements bent quite a lot under the weight of the ice but bounced back afterward. The tips on the Hy-Gain yagis worried me since they're thin and low tensile strength aluminum alloy. They will break off with severe ice loads.

The XM240 elements curled downward quite a lot then bounced back as the ice broke off. Notice the condition of the trees in the photograph below. The boom of the 6 meter yagi above it was also sagging. The foreground guys are twice their normal thickness.

With most of the ice now fallen or melted all antennas other than the 80 meter array test fine. The SWR of the 80 meter vertical is low enough at 1.7 to at least be usable in its omni-directional mode. Wire antennas have stretched from the high load and will have to be tightened. Going by the SWR the stretch is in the ropes and not the soft drawn copper wires.

If repairs to the 80 meter yagi have to wait until spring I can fall back to the high inverted vee. Hopefully there will be enough mild weather to permit repairs to be done. A thorough upgrade will have to be scheduled later in the years. Antenna repairs will inevitably slow the pace of work on new antennas.

It's a somewhat sombre Happy New Year at VE3VN. See you on the bands in 2020.

Tuesday, December 24, 2019

Tuning Big Yagis

Among the many projects simultaneously underway as 2019 draws to a close is the completion of my 15 meter and 20 meter stacked yagis. Design and construction of these home brew antennas took longer than expected so here I am working away into the coldest time of the year.

Progress was quite literally put on ice for over a month when winter arrived early and fierce. Although it's Christmastime I have been creative with my schedule to take advantage of a period of mild weather. I can even work outside without gloves, which is pretty good for our chilly climate.

Rough tuning of one each of the 20 meter and 15 meter yagis was done in unusually warm October weather with the help of friends. I rigged a temporary tram line and several ropes to manipulate the yagis to get them off the ground and relatively easy to access the feed points. Gamma matches were rough made to allow easy tuning using a variable capacitor.

For these monsters I found it easier to raise the yagis above ground in a horizontal orientation rather than attempt to point them vertically upward. This appears to be the preferred method of the friends I canvassed who have big antenna farms. You'll understand the challenge with these big antennas in the picture below taken in October when the weather was warm and pleasant.

This is my side mount 5-element 20 meter yagi with a 40' (12 meter) boom approximately 20' (6 meters) above ground. It takes four strong arms to haul this heavy antenna up the tram line for tuning. My ever dependable assistant Don VE3DQN (left) and Janek VA3XAR demonstrate how the ropes are used to swing the antenna. The feed point is reachable from the ladder when the boom is pulled downward. A short run of coax and an analyzer are attached.

Surrounded by guys and the tower the antenna must be carefully oriented for accurate impedance measurements. Best results were with the yagi pointed slightly upward and away from the guys, as shown above. Assembling the guys with non-resonant segments in any HF band is not enough to completely prevent deleterious interaction.

In this article I will discuss how high a horizontally oriented yagi needs to be raised for reliable impedance matching, and then how to adjust the physical antenna so that it performs according to the computer design. For this exercise I'll focus on the 5-element 15 meter yagis since this is the one I first ran through the full process to prepare it for use.

The side mount 5-element 20 meter yagi has been successfully rough tuned. It needs a permanent gamma match and further adjustment before being raised. I will gloss over the details of the gamma match designs and tuning process since it is a topic well worth its own article. Had I known what I was getting into I might very well have opted for a different feed system!

How high?

As we saw with pointing a yagi up there is little advantage going higher than the reflector being λ/4 above the ground. This works since field cancellation off the rear is typically high so that all we need is a modest reduction of mutual coupling with the non-resonant ground to achieve an impedance close to that in free space or high up a tower. A horizontal yagi is different since there is substantial radiation downward and therefore interaction with the ground reflection.

There is no general rule since yagis of unequal size and configuration have different elevation patterns. Luckily it turns out that you don't have to go too high for reliable impedance measurements. Performance metrics of gain and pattern need a little more height. The height of the 20 meter yagi shown above is sufficient for impedance matching.

Let's take the 15 meter 5-element yagi and model it at several heights. Comparison of the SWR curves is compelling. You can reference the linked article for further detail about the antenna design. The current model includes the actual tubing schedule. Although a beta match is used in the model there is negligible difference from the gamma network used in the physical antenna.

It is perhaps surprising that you need only go up 15' (4.5 meters) to have an impedance curve similar to that in free space. At 20' (6 meters) the difference is negligible. It is possible to rough tune the impedance even lower and do the fine tuning a little higher up if that is helpful. For the 20 meter antenna simply scale these heights by the wavelength ratio: ~1.5×.

I took measurements at both 15' and 20' with the gamma match adjusted close to its final setting. Pictures of the actual setups for the measurements are included.

There is a 9 meter length of new LMR400 hanging from near the boom centre. The AA54 analyzer is on an empty cable reel. The reels keep the antenna off the ground and protect the fragile gamma match. Ropes at both ends of the boom are used to orient the yagi.

Adjusting the SWR

As a general rule do not adjust a yagi for minimum SWR at the centre of the band or, on the low bands, the centre of the design frequency range. The R and X impedance components rarely change symmetrically on each side of centre: the SWR curve is not the perfect parabola often depicted.

My 15 meter yagi is an example of a wide band high performance design that exhibits two SWR minima. This is not unusual for optimized yagis with 5 or more elements. Adjusting the matching network for minimum SWR at band centre results in an inferior outcome.

Assuming the antenna matches the model (see next section) you should adjust for an SWR of 1 at the frequency where the model shows its lowest minimum. For my 15 meter antenna that frequency is 21.100 MHz. When adjusted that way and with the physical antenna matching the model the SWR curve across the band should match the model. For commercial antennas proceed as the manufacturer recommends.

Once you have the matching network at the sweet spot raise the antenna higher and confirm that the SWR curve across the band remains where it should be. Lower and adjust as necessary, then repeat. Make sure the components of the network cannot move around during and after adjustment. Yagis are finicky beasts and it takes very little motion of the network components and antenna elements to spoil perfection.

That said, getting to an SWR of precisely 1 is not necessary. The way antenna impedance typically varies with frequency you'll notice that although the minimum is a little high there is almost no impact on the SWR where is it normally higher. A few ohms of R or X make little difference where the deviation from 50 + j0 is greater.

More important is that the SWR across the band be below your chosen maximum, or what the design or manufacturer promises. Ideally it should be less than 1.5 everywhere, especially for a contester like me. Then you won't have to worry about tuners for your rigs and amplifiers as you change bands and frequency.

Interactions with guys, towers and other antennas will upset the SWR once you move it into position after it has been tuned. If you've planned well the change will be inconsequential. If the change is large there is no point in readjusting the impedance match since the problem lies elsewhere. Find that interaction and fix it. A deviation of the SWR often indicates that the pattern is being degraded by an interaction.

Confirming the design

For the typical amateur directly measuring and optimizing the pattern of an HF yagi is difficult and almost always avoided. I am no different. I rely on software models and careful construction for my home brew antennas. Even with NEC4 it is nigh impossible to get the physical antenna to exactly mirror the software model. With NEC2 and SDC (stepped diameter correction) the divergence can be worse when good modelling practice is not followed. NEC2 has numerous quirks.

I use EZNEC with the NEC2 engine and the supplied SDC algorithm. These antennas came surprising close to the model which was a great relief. But how do I know since I cannot do a field measurement of the pattern? There are ways to go about it so that one can be confident even without a direct measurement.

Impedance is easy to measure with accuracy using modern antenna analyzers. Fortunately impedance holds the key to an indirect though quite good method of confirming the model. Refer back to the SWR curves earlier in the article for the following discussion.

If the antenna impedance is adjusted as described earlier the SWR curve will closely match the modelled antenna for the antenna reasonably high and in the clear and a software matching network that follows the same procedure. In the EZNEC model I use a beta (hairpin) matching network since unlike a gamma match it can be reliably modelled, it closely mimics similar matching networks such as L-networks and gamma matches and doesn't preclude use of SDC on the driven element.

Although the curves appear to match there is an important difference. In the model the second dip is at 21.410 MHz and is around 21.450 MHz in the physical antenna. Assuming the yagi has been constructed per the design the divergence is most likely due to element self-resonance and not interactions and ground effects. A broader measurement spectrum is useful at this point so I raised the antenna higher and measured the SWR up to 21.600 MHz.

What we have is an impedance inflection point at 21.450 MHz. Above this frequency the radiation resistance drops sharply and the resulting SWR cannot be corrected with the matching network; no simple network can tame that slope while also matching the antenna within its design range. The software model exhibits the same behaviour.

The inflection point is a proxy for the true frequency range of the yagi. You'll find an inflection point like this in almost every yagi, perhaps two or three of them. Their presence at the correct frequency is strong evidence that the yagi is tuned for optimum gain and pattern. If not the antenna elements require adjustment.

Here we have the inflection point 0.15% higher in frequency. Considering all the small construction inaccuracies, reactance "bumps" from all the hardware, elements curving under their own weight the software did a remarkably good job predicting the yagi's real behaviour. In practice this small a difference can be ignored. I didn't ignore it.

Calculation suggests that the antenna elements should be lengthened by a little more than 1 cm (½") to bring it into agreement with the model. All the half elements tips were lengthened by ¼", except for the driven element: the DE length affects the impedance match not the gain and pattern. I adjusted the second untested 15 meter yagi at the same time so that I don't forget to do it later.

The yagi was lifted and measured. The improved match at 21.100 MHz is due to bumping the gamma match whose components at the time were not fully tightened. That was dumb luck.

I call this a tremendous success. Now I have confidence the antenna will perform as designed. Perfection like this isn't necessary but I do enjoy being presented with a measurement that so nicely mirrors the design. It makes me feel good after all the work that went into this project.

Next steps

For this tuning process the tram line was moved higher up the tower so that once the yagi is ready (choke, coax run, truss) no rigging change is needed to haul it up to the waiting side mount bracket. If the weather and my luck hold that should happen before the new year arrives.

There are two LDF5 Heliax run to the new tower ready for use. They are overground until the spring when a trench will be dug for burial, including control lines and rotator power. For now I will directly connect the side mount yagis to the transmission lines and add the stacking switches later.

I hope to have the 20 meter side mount yagi tuned and raised in January in time for late winter contests. The antenna is quite heavy and I'll need a couple of helpers, none of whom are (not surprisingly) unavailable this time of year.

Once all that is done I can tune the upper 15 meter yagi at my leisure and assemble the upper 20 meter yagi. A better and stronger boom for the upper 20 meter yagi is built and ready. As the weather allows the mast will be raised and then the upper yagis lifted. That may have to wait for warmer spring weather.

I will end here and prepare to wrap up the blog for 2019. Expect a year end review article before or shortly after January 1. Merry Christmas, Happy New Year and see you on the bands.

Wednesday, December 18, 2019

FT8 - The Universal Solvent

FT8 keeps eating away at the bands, one ham at a time. Like the mythical universal solvent it cannot be contained: FT8 dissolves every container traditionalists attempt to put it into. The digital wave inexorably marches onward.

Lately I've succumbed further. Until now I've restricted my use of FT8 to 6 meters. With the long winter nights of a deep solar cycle minimum there are only the low bands available most of the time. I enjoy the low bands yet it can get tedious outside of the excitement and intensity of contests and DXpeditions.

Every night there are same stations working each other on CW. Top band aficionados continue their vigil for propagation and welcome all comers. The regulars exchange signal reports and, this time of year, supplement that with seasons greetings, wishing MX and HNY to all. Activity briefly spikes to include a broader range of stations during sunrise and sunset enhancements.

It's all very cozy. I have good antennas for 80 and 160 so I can hold my own even without enhanced propagation, although I would benefit from more receive antennas (they're coming, eventually). With my amplifier out of service until parts arrive it is a little more difficult to work DX in the everyday challenging conditions.

Then there's FT8. I have taken to monitoring 1840 kHz many evenings just to keep an eye on top band propagation when I am busy doing other things and I'm uninterested in pursuing routine CW QSOs. Of course the inevitable happened: one day I hit the Enable button in WSJT-X. My log has begun filling up with top band FT8 QSOs.

The breadth and depth of activity is startling for anyone daring to venture beyond the traditional modes. In amongst the multitude of call signs never heard on CW there can be found familiar call signs of contesters and DXers. The DX to be found is itself quite surprising. Every night I hear UA0, Africa, South America and in the mornings there's the Pacific and Far East.

Try to find these distant stations on CW and you will be disappointed. It isn't that FT8 is so much better than CW (it isn't). You can only work what's there and what's there is on FT8. The clear implication is that many so-called difficult propagation paths on 160 meters aren't really difficult at all, there's just no one active on CW.

The transition to digital modes is less extreme than on 6 meters, so far. To escape from routine QSOs with the regulars it is necessary to spend some time on FT8. My top band FT8 log is filling up with DX QSOs and DXCC countries I rarely hear on CW outside of contests. In a way it's sad that the hobby is changing yet exciting in that digital modes are spurring activity from newcomers and old hands alike. That's a good thing.

Will CW survive? Perhaps until 2040 when most of the older generation will have passed on. There are not enough young people entering the hobby with an interest in CW although it may survive among a small minority. Obsolete technologies do attract some among the younger generations, whether it be vacuum tube appliances, vinyl records or mechanical clocks. CW will have its adherents as well for many years to come.

I will continue to spend a portion of my top band time operating FT8 although CW will remain my first choice. Two nights ago I heard A50BOC on 160 meters, barely audible and not workable and it was exciting to hear. CW signals from JA and HL are far more attractive to me than FT8 despite the difficulty of making the QSOs. However I will go where the activity is, just as I did on 6 meters.

Okay, that's enough philosophical rambling. During my short time on 160 meter FT8 I've been learning a few things. Operating there is not the same as 6 meters. Openings are longer, the atmospheric and man made QRN dominant, QSB slow and deep and the activity is far greater most of the time. The spectrogram shows a busy 1840 kHz on a weekday evening.

Reciprocity of station capability is less than on higher bands. Decoding a station does not mean they can decode you, and vice versa, when your power and antennas are comparable. This is as true for FT8 as it is for CW and SSB.
  • Local QRN can differ by 10 db or more. This varies by time of day, latitude, urban/rural locale and other factors well known to low band operators. Don't be surprised when some stations don't answer you.
  • Many top band hams are unintentional alligators since, apart from the above QRN factors, most do not have low noise (directional) receive antennas.
  • Too many stations call on the CQing station's transmit frequency, which often means none of them are successfully decoded. I don't know why this seems to happen more on 160 than 6 meters, or perhaps I am suffering from selective memory.
  • Clear frequencies don't last long in that 3 kHz FT8 window. It is commonplace to have someone start transmitting on another station's transmit frequency and time slot despite signal levels implying that they must be able to hear the other station. Everyone suffers as a result. 
  • You can see a couple of poorly adjust transmitters in the spectrogram above. It is often worse.
  • DX stations are regularly covered up by nearer stations that cannot hear them and think the frequency is clear. There is no QRL? equivalent on FT8. If the spectrogram looks clear (or not) away you go.
Some of the problem is poor operating though mostly it's just regular hams doing the best they can with what they have on a band with difficult operating conditions. It's all a part of the game so get used to it. Those with skill and superior antennas have an advantage as they do on any mode, on any band and whatever the prevalent propagation. Experience and practice make a difference.

While it's nice to try something new and work new stations I don't take FT8 operating on 160 meters too seriously. That may change if the migration from CW continues. If it does I may have to concentrate on 160 meter FT8 for real just like I now do on 6 meters. I intend to hold off on other bands for a while longer, hopefully a long while.

Change is good even when it makes us uncomfortable.

Thursday, December 12, 2019

Performance of the 80 Meter 3-element Vertical Yagi

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

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

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

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

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

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

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

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

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

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

Notes on modelling

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

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

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

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

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

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

Turn a yagi on its side

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

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

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

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

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

Element spacing

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

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

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

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

Element shape

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

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

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


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

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

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

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

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

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

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

Ground dependency

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

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

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

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

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

Pros and cons vs. the 4-square

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

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

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


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

Taller centre support to allow straight (unloaded) elements

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

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

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

Bent elements

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

More radials

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

Paths forward

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

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

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

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

Wednesday, December 4, 2019

1,000,000 Point Coax Connector

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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