VHF Propagation — Page 146, Lesson 130

I want to walk you through VHF propagation. We're starting with the core idea that governs almost all VHF communications in aviation, and then we'll look at the rare exceptions that can break that rule. First, let's talk about the basic range formula. There's a formula that accounts for a small amount of refraction in the lower layers of the atmosphere. That refraction gives us a slightly better range than we would get if the radio wave followed a perfectly straight path with no downward bending at all. Under normal conditions, for frequencies above about 30 MHz, the space wave — that's the direct wave traveling from transmitter to receiver — provides the only propagation path. So here's the key takeaway for your daily flying: except on fairly rare occasions, communications in the VHF band and higher bands suffer from line-of-sight transmission. Your range is limited by two things: the curvature of the earth, and any high ground that interrupts that straight line between you and the ground station. Now, interestingly, that range limitation isn't always a bad thing. The fact that VHF is line-of-sight means that the same frequency can be reused by different stations that are far enough apart not to interfere with each other. So when there's a shortage of available frequencies, that line-of-sight limitation actually becomes useful. Now let's move into what the book calls "freak propagation" — also called anomalous propagation. I said that above about 30 MHz, transmission is normally line-of-sight via the space wave. But under certain conditions of freak or anomalous propagation, we can achieve ranges much greater than line of sight. There are two mechanisms for this: duct propagation and scatter propagation. Let's start with duct propagation. This effect is also called super-refraction. It's associated with a temperature inversion — that's when temperature increases with height instead of decreasing — combined with a rapid decrease in humidity with height. These meteorological conditions are most often found at the surface over land, during high-pressure conditions at night and in the early morning. A warm air mass over a cold sea can also produce the effect. And it can also occur at higher levels, not just at the surface. Take a look at Figure 7.4, which shows duct propagation. The way it works is that radio signals become "trapped" in a duct of cold air. This process sometimes permits reception of signals at the surface hundreds of miles beyond the horizon. The effect is most common in the SHF band — that's Super High Frequency — and the UHF band — Ultra High Frequency. But it is also encountered in the VHF band if the duct is sufficiently deep, say about 500 feet. For you as a pilot or controller, duct propagation is important because it can cause annoying interference between control towers using the same R/T frequency — that's radiotelephony frequency. It can also cause false range indications on ground radar screens. Now let's look at scatter propagation. The E-layer of the ionosphere sometimes contains areas of very high ionization density. These areas can produce weak sky waves in the VHF band, and these are known as Sporadic-E reflections. The effect is unpredictable — the sky waves are scattered at random in the forward direction from the transmitter. With specially designed aerials, scatter propagation can sometimes be used to provide intermittent extended-range VHF R/T, but it is not a reliable means of communication. For your operations, the practical problem is that scatter propagation can cause mutual interference between VHF radio aids that share a frequency — aids that would normally be protected from interference by line-of-sight transmission. Television programmes also suffer from interference due to this effect. So to summarise what I want you to hold onto: VHF is normally line-of-sight, limited by the earth's curvature and terrain. But under specific meteorological conditions — temperature inversions with rapid humidity decrease — duct propagation can trap signals and give you ranges hundreds of miles beyond the horizon. And Sporadic-E in the ionosphere can scatter VHF signals forward, giving weak, unreliable extended range. Both of these anomalous effects cause interference problems in aviation communications and radar.