Wednesday, June 8, 2016

Adjust a Yagi by Pointing It Up

The bigger the antenna the more difficult it is to tune. When done near the ground you are at risk of seeing its performance be worse when raised to full height due to the change in ground interaction. When done in the air the tuning process is difficult and potentially dangerous. Modelling can help to compensate for the height differential, but it is no panacea since software ground is rarely similar enough to real ground. Also, there are other environmental influences at low heights.

For yagis one technique many hams have had success with is to point the yagi straight up from a low base height. The idea is to get the antenna (reflector element) just high enough off the ground that its performance is comparable to that in free space (and therefore also at its intended height), and laterally offset far enough from the tower to avoid mutual coupling that will skew its behaviour. But close enough that the feed point is within easy reach. I've used this technique several times. It can work very well indeed.

Hy-gain 155BA prepared for tuning at
VE3CRG's station circa 1985
An example of the technique is shown in the adjacent photo, which I scanned from an old 35 mm print. The yagi isn't quite in tuning position for this photo op, with one side of the antenna too close to the tower. The rope attached to the reflector was used to stabilize and orient the yagi for tuning.

A casual internet search however shows an awful lot of unsubstantiated opinion and misinformation mixed with the good advice. When you do it right the technique works, so that's what we'll explore in this article.

Yagis are not dipoles -- even though they are comprised of dipole elements. Yagi behaviour does not go through the wild swings as height is varied, unlike what we see with single element antennas. Yagis are more immune to ground effects, becoming stable at surprisingly low heights (in terms of wavelength). Why that is true is interesting and worth exploration.

What the technique can and cannot do

The only performance parameter that can be conveniently adjust by this technique is the match; that is, optimizing the SWR across the band or band segment of interest. Gain and F/B optimization require other techniques, such as accurate modelling, including stepped diameter correction (SDC), field strength test range or testing with a cooperative ham within ground wave range.

Apart from that, like most hams we will have to trust the manufacturers' published dimensions for each band segment and for gain and F/B performance. If you don't trust the manufacturer (this can be an issue) design your own yagi.

With today's accurate software models and antenna analyzers it is still difficult to test and tune large yagis at the tops of towers, even if only for purposing of achieving a good 50 Ω match. For yagis with other than 3 elements the feed point is often not even within reach from the top of the tower.

Statement of objectives

We want the best possible match to minimize transmission line loss and maximize flexibility in the shack by having antennas that work well with transmitters and amplifiers that require a low SWR. The vertical pointing method is useful for conveniently adjusting the SWR of large yagis.

Or is it? Does it work, and how well does it work? Computer modelling helps us to answer these questions. The less we have to adjust our antennas at height the less chance for an accident. The best tower safety practice is the one that doesn't require a trip up the tower at all.

Test antenna model: 3-element 20 meter yagi

Rather than explain what's going on and then verify with models or measurements I will start with the model to see what it can tell us. For this exercise I will use a 3-element 20 meter yagi on a 0.35λ boom, a model I've used before as a reference for designing and evaluating tri-band yagis. This antenna is similar in scale to a Hy-gain TH6DXX but without the performance impact due to trap loss and element shortening. The model includes a hairpin (beta) match to achieve a 50 Ω match.

The following chart is from the referenced article, reproduced here for convenience. In particular notice the variation in F/B with frequency: its importance will become clear later in this article.

First we need to model the reference antenna in free space as our basis of comparison. Next, the antenna is rotated upward and placed over real ground (EZNEC medium ground). Its height is then varied and its SWR and gain calculated and compared to the reference. For what ought to be obvious reasons the F/B cannot be compared (see the adjacent elevation pattern).
Sample elevation pattern of a vertically pointing yagi;
the pattern can be quite odd in this configuration

When the impedance across the band is comparable to the antenna in free space we can usually trust that the gain and F/B are also in accord with the free space model. So if we can achieve this with the yagi in its vertical test position it should do fine when raised to the top of the tower.

Proper test setup

The yagi must be carefully positioned on the tower for this tuning method to be reliable. First, by height I mean the height of the centre of the reflector (bottom) element. I am ignoring element droop since its effect is only noticable at very low heights and for light duty elements with significant droop. Coupling is greater for the element tips and these will be closer to ground than the rest of the element.

Yagi model pointing up
The model does not include the antenna boom, tower and guys. The boom and element clamps do affect element tuning but are not relevant to our model experiment since we are looking for a change in behaviour not the absolute performance data. I further assume that guy wires are non-conductive or well broken up so as to have near zero mutual impedance with the antenna, and that the tower is "invisible" when the boom is outboard at least 1 meter from the tower and the elements are orthogonal to a line between tower centre and antenna boom.

This can be accomplished with a length of pipe or angle iron projecting outward from the tower with a pulley at its extremity and secured with ropes so that the antenna doesn't rotate in the wind, as shown in the photograph earlier in this article. It does make a difference, as I have discovered in practice when a breeze comes up and the SWR starts climbing.

If the boom is especially long it can be difficult to avoid guy wires when lifting the yagi into position for tuning. If necessary remove those elements, lift the antenna and reattach the elements.

As with all antennas a common mode choke should be used to prevent feed line radiation from disturbing the performance. The coax should be dropped straight down, either off the bottom end of the boom or along the tower vertex nearest to the boom.

Performing the comparison

I start with the antenna at a height of 2 meters (centre of reflector element to ground). The antenna is then raised in increments and its SWR and gain recalculated. The SWR and gain are calculated every 50 kHz from 14.000 to 14.350 MHz. This is sufficient granularity to derive an insight into what is going on, as we'll see; we don't need to know the R and X values of the feed point impedance, although that can be interesting and important in other instances.

At just 1 meter height (0.05λ) the SWR is already barely distinguishable from the antenna's SWR in free space. Going higher changes little. Are you surprised? Let's do it again, this time with the antenna in its normal horizontal orientation, stepping through the same heights.

Now we see that low heights have a large impact. Only when at a height of 5 meters (0.25λ) does the SWR become close to that in free space. At 10 meters height (0.5λ) the SWR is nearly indistinguishable from what it is in free space.

This confirms the technique where many hams with big towers mount their large yagis for tuning on a short tower before placing them at their final height. But now we also know that the "yagi tuning tower" should be no less than 0.5λ, at least for this antenna. Keep this in mind should you find it easier at your station to tune yagis horizontally rather than in the vertical orientation we are focussing on.

Let's move on to the gain comparison with the antenna rotated back to its vertical orientation. The results may be less surprising now that you've seen how the SWR responded. Yet the behaviour of the gain is somewhat unusual.

The yagi's gain has a complex relationship with height; there is a cyclic component due to minor lobe ground interactions. Influences include: mutual impedance with ground; ground reflections; and, directions and magnitudes of rearward nulls and minor lobes. While not shown the patterns develops odd lobes with small changes in height. The gain variation is within about 1 db, which is quite modest. If it were bigger we'd have likely seen greater fanning out of the SWR curves.

What is clear is that frequency has a stronger influence on the gain differential. SWR variation shows a similar though less pronounced trend (weak but visible correlation). This is where we need to step back and review how yagis work. Therein lies the explanation.

Why it works: mutual impedance

A yagi is a subtractive array. Strong mutual coupling among close-spaced elements causes field cancellation in some directions and field reinforcement in other directions. The relationships can be quite complex, which is why yagi optimization was so difficult in the years before software tools such as NEC were available.

Field cancellation reduces the radiation resistance which in turn increases element currents. When those currents are approximately in phase there is gain in that direction, accentuated by the higher current. Recall that the current (and antenna aperture) determine field strength. In contrast, additive arrays -- ones where the elements are far enough apart to have low mutual impedance -- work on superposition alone since the near field effects are relatively small.

Yagi elements also interact with the ground. Ground is in essence a flat, non-resonant medium that is lossy. That loss and ground's dielectric properties in combination with the induced current cause the phase and magnitude of the re-radiated field to vary with angle and polarization. It's messy but (mostly) able to be modelled. NEC4 perhaps does it best. EZNEC does pretty well with NEC2 and its real ground ground models.

Off the back end of a yagi that coupling is reduced since the cancellation of fields leaves less to interact with the ground. As long as the magnitude of minor lobes in the back 180° is small relative to the forward lobe the effects can be quite small. We can say that the strong mutual coupling between the yagi elements dominates the mutual coupling between the yagi and ground.

The gain differential with height for the yagi pointing upward correlates with the F/B. That's why I suggested keeping the plot of F/B in mind for this discussion. Notice that the free space F/B is highest at 14 MHz and falls off as the frequency increases. There is enough rearward field at the top of the band to more strongly interact with ground and thus influence the gain. Happily the effect is modest so SWR tuning is little effected when the reflector is quite close to the ground.

Yagis with poor F/B fare less well. This is especially true of 2-element yagis which have poor F/B (except for the Moxon with its critical coupling). These antennas should be raised higher, whether oriented vertically or horizontally, for the tuning to be reliable. Optimized yagis with 4 or more elements typically have excellent F/B across the band and respond especially well to the vertical tuning technique.

Myth and lore

When I first heard about vertical tuning of HF yagis many many years ago I had no understanding of why it worked. All I knew was that reputable names with super-stations used the technique and recommended it. I tried it and it worked. So I kept using it.

A lot of ham radio is like that: we do things that we have heard about but do not understand. Unfortunately this is also a good way to propagate nonsense, of which there is a lot out there.

By gaining a deeper understanding we learn to separate the wheat from the chaff, rejecting the "myth and lore" and adopting that which is science and evidence based. My hope is that this article contributes to the latter even though it only touches on the subject with a few examples. You can run your own models for the antennas you have or are designing to confirm how best to tune them.

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