VHF Propagation — Page 139, Lesson 125

I want to walk you through VHF propagation — how VHF radio signals actually travel between a transmitter and a receiver. This is fundamental to understanding why your VHF communications can sometimes fade or even cut out, and why we can reuse frequencies at different locations without interference. Let's start with the basic idea: the path a radio wave takes from transmitter to receiver, over many miles, is not necessarily a straight line. In fact, the signal can reach the receiver by more than one path at the same time. Because those different paths have different lengths, the waves arrive with phase differences — meaning one wave's peak might arrive slightly ahead or behind another's. These phase differences directly affect the strength of the combined signal you receive. Here's the key effect: if two waves from the same transmitter travel by different paths and arrive exactly 180° out of phase — that is, one wave's peak coincides with the other's trough — and if their amplitudes are the same, they cancel each other out completely. The resultant signal strength becomes zero, so you receive nothing. More generally, as the phase difference changes, the signal strength changes, producing the effect we call 'fading'. That's the rise and fall in signal strength you might hear on the radio. Now let's look at the specific paths. First, there's the direct wave — that's the signal that travels in a straight line between the transmitter and the receiver. But in addition to that, there's normally a signal arriving at the receiver after reflecting off the earth's surface. That's the ground-reflected wave. Together, these two waves — the direct wave and the ground-reflected wave — are jointly known as the Space Wave. In diagrams, you'll see Tx for transmitter and Rx for receiver. Since the direct and reflected waves follow different paths, they can arrive at the receiver with large phase differences. And it gets more complicated: there's also a change in phase that occurs right at the point of reflection of the ground-reflected wave. The net result is that, for example, an aircraft flying towards a ground station may suffer fading or temporary loss of VHF communications with that station. The range at which this fading occurs depends on three things: the height of the ground aerial above the surface, the aircraft's altitude, and the frequency being used. So it's not random — it's predictable based on geometry and frequency. Finally, here's a practical protection we get from the earth's curvature. With VHF R/T — that's VHF radiotelephony — except in freak conditions, the curvature of the earth gives protection from mutual interference between stations using a common frequency, provided those stations are well-separated geographically. In other words, because VHF waves travel essentially line-of-sight and the earth curves away, two stations far apart can use the same frequency without interfering with each other. That's how we can reuse frequencies across the country.