Radio Propagation Theory — Page 20, Lesson 23

Let's begin Radio Propagation Theory. I want to walk you through the four propagation paths that matter for aviation, and then we'll dig into the first one in detail. First, the big picture. There are four propagation paths, and all four need to be considered for aviation purposes. They split into two families. The first family is non-ionospheric propagation, which covers the surface wave and the space wave. The second family is ionospheric propagation, which covers the sky wave and SatComm — that's satellite communications, the direct wave used at UHF and SHF. Let me define the key term right away. Ionospheric propagation is propagation affected by the properties of the ionosphere — that's the layer of the atmosphere that's electrically charged. At this stage, we only need to discuss the sky wave. Satellite propagation gets treated in detail with global navigation satellite systems, GNSS, in Chapter 18. And one thing to note: knowledge of propagation below 30 kHz is not required for this course. Non-ionospheric propagation covers all the other paths. Now let's focus on the surface wave, which is the first non-ionospheric path. Surface wave propagation exists at frequencies from about 20 kHz to about 50 MHz — that spans from the upper end of VLF, very low frequency, to the lower end of VHF, very high frequency. Here's the physical mechanism: the portion of the wave in contact with the surface of the earth is retarded — slowed down — which causes the wave to bend round the surface of the earth. That bending process is called diffraction. So the wave literally follows the curvature of the earth because its lower edge is dragged back by the ground. Now, how far can this wave travel? The range achievable depends on three factors: the frequency, the surface over which the wave is travelling, and the polarization of the wave. Here's the relationship: as frequency increases, surface attenuation increases, and the surface wave range decreases. In fact, it's effectively non-existent above HF — high frequency. So higher frequency means shorter range for the surface wave. The surface matters too. Losses to attenuation by the surface of the earth are greater over land than over sea, because the sea has good electrical conductivity. That's the key contrast — good conductivity means less loss. So greater ranges are attainable over the sea than over land. Polarization is the third factor. A horizontally polarized wave will be attenuated very quickly and give very short ranges. Therefore, vertical polarization is generally used at these lower frequencies. So the rule of thumb: vertical polarization for surface wave work. Now, where is this path actually used? The surface wave is the primary propagation path in the LF frequency band — that's low frequency — and the lower part of the MF band, medium frequency. Specifically, frequencies of 30 kHz to 2 MHz. And there's a handy approximation for the usable range over sea and land for an MF transmission at a frequency of 300 kHz. Over sea, the range in nautical miles is approximately 3 times the square root of the power. Over land, it's approximately 2 times the square root of the power. Let me read those as formulas: range ≈ 3 × √Power for sea, and range ≈ 2 × √Power for land. Let me work an example so you see how this plays out. Take a 300 kHz transmitter with a power output of 10 kW. The square root of 10 is about 3.16. Over the sea, 3 times that gives roughly 300 nautical miles. Over the land, 2 times that gives roughly 200 nautical miles. So the same transmitter gives you about 300 NM over sea and 200 NM over land — a clear demonstration of why sea is the better surface for surface wave propagation. So to tie it together: the surface wave bends around the earth by diffraction, works best at lower frequencies with vertical polarization, performs better over sea than land because of the sea's good electrical conductivity, and its range shrinks as frequency rises. That's the surface wave. Next we'll look at the space wave and the sky wave.