VHF Propagation — Page 139, Lesson 127

I want to walk you through the factors that affect VHF propagation. Let's start with the first major concept: attenuation. Attenuation simply means the loss in strength of a radio signal as the distance from the transmitter increases. The signal strength you receive is inversely proportional to the distance from the transmitter — so the farther you are, the weaker the signal gets. There are two main reasons for this. First, the radio energy available is spread over a greater area as it travels outward. Second, radio energy is lost to the earth, the atmosphere, and sometimes to the ionized layers above the earth. One key factor that determines the operational range of a radio emission is the transmitter power. The range you can achieve is proportional to the square root of the power. In practical terms, if you want to double the range, you must quadruple the transmitter power. That's a significant relationship to remember. Now let's move to refraction. As a general rule, radio signals travel in straight lines — they follow great circle paths over the surface of the earth. But under certain circumstances, the path of a signal may change direction. That change of direction is called refraction. The amount of refraction varies considerably depending on conditions and on frequency. However, in the VHF range of frequencies, there is negligible refraction. So for VHF, we can essentially treat the signals as traveling in straight lines. This brings us to the effective range of VHF and specifically the line of sight range. The curvature of the earth limits the use of the direct wave. If an aircraft is below the horizon from the ground station, it cannot use the direct wave for communications. The lowest direct wave that just grazes the surface is called the 'horizon ray'. To restore direct wave communications for that aircraft, you would need to either raise the height of the ground aerial or increase the aircraft's altitude. There's a formula used for calculating the maximum range of direct wave reception. It is: Range in nautical miles = 1.23 times the sum of the square root of h1 plus the square root of h2. Here, h1 is the height of the ground aerial in feet above mean sea level — that's feet AMSL. And h2 is the aircraft's altitude, also in feet AMSL. So you take the square root of each height, add them together, and multiply by 1.23 to get the range in nautical miles. That figure shows you the geometry of vertical polarization, which is relevant to how VHF signals are oriented. And the line of sight diagram I just described illustrates the earth's curvature and the horizon ray concept. So to summarise what we've covered: attenuation is signal loss with distance, and it's tied to the square root of power for range. Refraction is negligible for VHF. And the effective range of VHF is limited by line of sight, calculated using that formula with the heights of the ground aerial and the aircraft.