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.
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