Propagation prediction for most hams is a terribly inexact science. You get on the air and tune around for stations, or you click on spots. You hear the DX or you don't; sometimes they're weak and other times they're strong. But why? Well, propagation. That's as far as many hams think about the matter.
Maybe you try another band or just turn off the radio and come back another day. And if it's a contest weekend? You understand that's the playing field for every competitor so you persevere.
The reason HF propagation is difficult is because it is a complex phenomenon, much like the weather. We understand the physics but the data to make accurate predications is sparse or not collected at all. Ionospheric propagation is rarely as simple as depicted in this diagram above. There are numerous factors to consider, some of which include:
- Polarization and Faraday rotation
- Chordal hops, within and between ionosphere layers
- Geomagnetic activity due to the sun (UV, X-rays, energetic particles and more)
- Scattering (diffuse rather than specular reflection or refraction)
- Ducting
Our responsibility as communicators is to design and build antennas that make maximum use of the propagation paths provided by the environment. If a signal is vertically polarized, we prefer a vertically polarized antenna. If the elevation angle is large, we prefer an antenna that has a high angle lobe. If we want to work over the pole we prefer an antenna that favours the north (or south) direction.
The important thing to remember is that the environment decides the propagation parameters, not the antenna. It can change quickly and has pronounced daily and seasonal cycles, and that's on top of solar cycles and solar activity.
Even simple antennas can have complex patterns. This is due to their size, orientation, height, terrain, and other factors, usually with respect to signal wavelength. One antenna is never good enough for all types of propagation. We need set operating objectives and build accordingly.
The azimuth pattern of a vertically-polarized 40 meter delta loop operated on 15 meters (its third harmonic) is shown at right. You need to install it so that pattern nulls are not placed at bearing you consider important directions, and that lobes are where you want them. Compromises may be unavoidable. Rotatable antennas are less constrained but they come at a cost.
A curious situation occurred years ago when I had a much smaller station. In a now ancient article I described my initial confusion when comparing two 40 meter inverted vees. I had two so that I could cover all compass directions. Faraday rotation and polarization dependence on the azimuth angle (bearing) made for a difficult comparison since an inverted vee is vertically polarized off the ends and horizontally polarized broadside.
They were equally effective antennas but never at the same time due to Faraday rotation. That experience highlighted how antenna comparisons can be so fraught with uncertainty. You may think you're making a fair comparison -- WSPR, A/B switching, etc. -- but in many cases all you're doing is comparing antenna patterns relative to the prevailing propagation, not performance. Assessing relative performance is never easy since propagation characteristics can make a comparison worthless or misleading.
There is a saying among many hams: you can't have too many antennas. The more antennas you have the greater the probability that one of them will best exploit the propagation to a particular station or region at any given time and band. Of course most hams don't have this option. They're left guessing how well their antennas are really performing.
My motivation to write this article came from a recent antenna selection conundrum. I was listening to the FP5KE DXpedition on 20 meters and I wanted to know which of my several 20 meter antennas was best. I was driven by curiosity, without the intention of working them, since I've worked FP countless times on 20 -- it's single-hop distance and frequently active.
I first listened on the upper 5-element yagi of the 5-over-5 stack. They were surprisingly weak, no more than S5 on the meter. This wasn't surprising since that antenna, at 40 meters height, concentrates most of its energy at low elevation angles. The lower antenna does better for high angles usually associated with shorter paths.
When I switched to the lower 5-element yagi, which is about 21 meters high, the signal was still quite weak, not much different compared to the high yagi. That seemed odd to me since it was so unexpected. I then switched the stack to BIP (both in phase) and the signal jumped to well over S9. Imagine my surprise.
I suspected that the differences were due to the elevation patterns of the various antennas. There are two or more lobes with nulls between lobes. The terrain is pretty flat so the lobes and nulls of antennas at similar heights but on different towers should be about the same. The patterns are likely to closely resemble those produced by modelling software that assume a homogeneous, flat ground.
This EZNEC plot contains the elevation patterns of the three selectable configurations of the 20 meter stack: upper, lower, BIP. It is taken from an earlier article, and annotated with a few lines.
The red line at 30° elevation highlights a null common to each configuration. That can occasionally pose a problem when the propagation path favours that elevation angle. If there is real difficulty due to that null I can switch to one of the tri-band yagis.
The dark red (brownish) lines highlight elevation angles where there are two traces with nulls. That is, there is one antenna selection that will work better than the others. The chart is quite busy so I hope that readers can successfully read the critical data it contains.
Despite the generally excellent performance of the stack, it is those many nulls that occasionally render it impotent. You can get better elevation angle coverage in a stack with 3 yagis, whether of similar size or smaller.
Assuming that the plot accurately resembles that of my stack can we determine what happened when I was listening to FP5KE? We are looking for an elevation angle where each individual yagi has a null and the combined stack does not. However, there isn't one!
A reasonable conclusion is that the elevation plot produced by the model does not accurately match the real stack. That isn't too surprising. Only a small deviation from perfect terrain can move those nulls. That's because a null requires an exact cancellation of fields from the space and ground reflected waves of one or both yagis -- the requirement is equal amplitude and opposite phase when the fields are summed. The major lobes of the antennas may be close to those in the model even when the nulls are significantly different since the sums of the fields can be less critically aligned
My wild guess is that the propagation path's elevation angle is close to 30° (red line) and that one of the minor lobes of the stack on opposite sides of that angle is really much larger. But it's just a guess. It also must have been the case that the signal was constrained to a narrow range of elevation angles. Otherwise pattern nulls would not have such a prominent effect. That is often not the case, with signals appearing at a range of elevation angles due to various scatter phenomena.
A ray tracing tools such as HFTA combined with an accurate terrain profile for my QTH might deliver better insight. Or it might not. As I said, null positions are very sensitive to small deviations. Those deviations can quite easily be larger than the accuracy of the terrain data fed into HFTA.
I'm not so curious about those nulls that I'm willing to go the trouble of using HFTA. I have enough antennas that I can "navigate" around pattern nulls to work what I need to work. I am not going to rearrange my towers and antennas for dubious improvements, nor will I take the trouble to add BOP (both out of phase) to my stack switches. There are many serious contesters that would never build a station without first consulting HFTA.
I'll say it again: you can't have too many antennas.
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