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Saturday, February 9, 2019
THE MYTH OF VHF LINE-OF-SIGHT by BOB K0NR
When we teach our Technician License class, we normally differentiate between HF and VHF propagation by saying that HF often exhibits skywave propagation but VHF is normally line-of-sight. For the beginner to ham radio, this is a reasonable model for understanding the basics of radio propagation. As George E. P. Box said, “All models are wrong, but some are useful.”
In recent years, I’ve come to realize the limitations of this model and how it causes radio hams to miss out on what’s possible on the VHF and higher bands.
Exotic Propagation Modes
First, let me acknowledge and set aside some of the more exotic propagation modes used on the VHF and higher bands. Sporadic-e propagation allows long distance communication by refracting signals off the e-layer of the ionosphere. This is very common on the 6-meter band and less so on the 2-meter band. I like to think of this as the VHF bands trying to imitate HF. Tropospheric ductingsupports long distance VHF communication when ducts form between air masses of different temperatures and humidities. Auroral propagation reflects the radio signal off the auroral ionization that sometimes occurs in the polar regions. Meteor scatter reflects signals off the ionizing trail of meteors entering the earth’s atmosphere. Earth-Moon-Earth (EME) operation bounces VHF and UHF signals off the moon to communicate with other locations on earth. These are all interesting and useful propagation mechanisms for VHF and higher but not the focus of this article.
Improved Line-of-Sight Model
Now let’s take a look at more “normal” VHF propagation that occurs on a daily basis, starting with the simple line-of-sight propagation model. The usual description of line-of-sight VHF is that the radio waves travel a bit further than the optical horizon (say 15% more) . Let’s refer to this as the Line-of-Sight (LOS) region where signals are usually direct and strong. What is often overlooked is that beyond the radio horizon, these signals continue to propagate but with reduced signal level. Let’s call this the Non-Line-of-Sight (NLOS) region. The key point is that the radio waves do not abruptly stop at the edge of the LOS region…they keep going into the NLOS region but with reduced signal strength. Now I will admit that this is still a rather simplistic model. Perhaps too simplistic. I’m sure we could use computer modeling to be more descriptive and precise, but this model will be good enough for this article. All models are wrong, but some are useful.
WORKING the LOS and NLOS Regions
Let’s apply the model for Summits On The Air (SOTA) VHF activations. If we are only interested in working the LOS region, we won’t need much of a radio. Even a handheld transceiver with a rubber duck antenna can probably make contacts in the LOS region. It’s still worth upgrading the rubber duck antenna to something that actually radiates to improve our signal (such as a half-wave antenna). We may pick up some radio contacts in the NLOS region as well but our success will be limited.
To improve our results in the NLOS zone, we need to increase our signal strength. We are working on the margin, so every additional dB can make enough difference to go from “no contact” to “in the log.” Think about another radio operator sitting in the NLOS zone but not quite able to hear your signal. Your signal is just a bit too weak and is just below the noise floor of the operator’s receiver. Now imagine that you improve your signal strength by 3 dB, which is just enough to get above the noise and be a readable signal. You’ve just gone from “not readable” to “just readable” with only a few dB improvement.
What can we do to improve our signal levels? The first thing to try is improving the antenna, which helps you on both transmit and receive. I already mentioned the need to ditch the rubber duck on your HT. My measurements indicate that a half-wave vertical is about 8 dB better than a typical rubber duck. This is only an estimate…performance of rubber duck antennas vary greatly. A small yagi (3-element Arrow II yagi) can add another 6 dB improvement over the half-wave antenna, which means a yagi has about a 14 dB advantage over a rubber duck.
On the other hand, if you believe that your VHF radio is only Line-of-Sight, then there is no reason to work on increasing its signal level. The radio wave is going to travel to anything within the radio horizon and then it will magically stop. This is the myth that we need to break.
When doing SOTA activations, I noticed that I was able to hear some stations quite well but they were having trouble hearing me. Now why would this be? Over time, I started to realize these stations were typically home or mobile stations running 40 or 50 watts of output. This created an imbalance between the radiated signal from my 5W handheld and their 50 watts. In decibels, this difference is 10 dB. Within the LOS region, this probably is not going to matter because signals are strong anyway. But when trying to make more distant radio contracts into the NLOS zone, it definitely makes a difference. So I traded my HT for a mini-mobile transceiver running 25W. See the complete story in Chapter: More Power For VHF SOTA.
Weak Signal VHF
Of course, this is nothing new for serious weak-signal VHF enthusiasts. They operate in the NLOS region all of the time, squeezing out distance QSOs using CW, SSB and the WSJT modes. They generally use large directional antennas, low noise preamps and RF power amplifiers to improve their station’s performance. They know that a dB here and a dB there adds up to bigger signals, longer distances and more radio contacts. A well-equipped weak-signal VHF station in “flatland geography” can work over 250 miles on a regular basis…no exotic propagation required.
Now you might think that FM behaves differently, because of the threshold effect. When FM signals get weak, they fade into the noise quickly…a rather steep cliff compared to SSB which fades linearly. FM has poor weak-signal performance AND it fades quickly with decreasing signal strength. This is why it is not the favored modulation for serious VHF work. But the same principle applies: if we can boost our signal strength by a few dB, it can make the difference between making the radio contact or not.
So VHF is not limited to line-of-sight propagation…the signals go much further. But they do tend to be weak in signal strength so we need to optimize everything under our control to maximize our range.