Radio Propagation Theory — Page 20, Lesson 23

Let’s start with the big picture. In radio navigation, we care about how a radio wave actually gets from the transmitter to the receiver. That journey is called the propagation path. For aviation, we consider four of them, and they split into two families: non-ionospheric and ionospheric. Ionospheric propagation means the wave’s behaviour is affected by the properties of the ionosphere — that’s the electrically charged layer of the upper atmosphere. For now, the only ionospheric path we need to discuss is the sky wave, which works from about 20 kHz up to 50 MHz, though in practice it’s used between 2 and 30 MHz. Satellite propagation is also ionospheric in a sense, but we’ll leave that until Chapter 18, where we cover global navigation satellite systems — GNSS. And you don’t need to know anything about propagation below 30 kHz, so we can ignore that region entirely. Everything else falls under non-ionospheric propagation, and that’s where I want to spend our time today. There are two non-ionospheric paths: the surface wave and the space wave. The space wave is used above 50 MHz, and we’ll come back to it later. Right now, let’s focus on the surface wave. The surface wave exists at frequencies from about 20 kHz up 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 key mechanism: the portion of the wave that’s in contact with the surface of the earth gets retarded — slowed down. That slowing causes the wave to bend round the surface of the earth, and that bending process is called diffraction. So the wave literally follows the curvature of the earth because its lower edge is dragged back. Now, how far can this wave actually travel? The range depends on three factors: the frequency, the surface over which the wave is travelling, and the polarization of the wave. Let’s take each one. First, frequency. As the frequency increases, surface attenuation increases — that means the earth absorbs more of the wave’s energy — so the surface wave range decreases. Above HF, high frequency, the surface wave is effectively non-existent. So higher frequency, shorter range. Second, the surface. Losses from attenuation by the earth’s surface are greater over land than over sea, because the sea has good electrical conductivity. Good conductivity means less energy is lost, so greater ranges are attainable over the sea. Over land, the wave gets eaten up much faster. Third, polarization. A horizontally polarized wave — where the electric field oscillates horizontally — will be attenuated very quickly and give very short ranges. So at these lower frequencies, vertical polarization is generally used. That’s the standard choice because it survives the journey much better. Now, where is this path actually used? The surface wave is the primary propagation path in the LF band — low frequency — and the lower part of the MF band — medium frequency. That means frequencies from about 30 kHz up to 2 MHz. So when you hear about LF and MF radio beacons, this is the propagation mechanism doing the work. There’s a handy approximation for the usable range of an MF transmission at 300 kHz. Over the sea, the range in nautical miles is approximately 3 × √Power. Over land, it’s approximately 2 × √Power. The power here is in kilowatts. So let’s run the example: a 300 kHz transmitter with a power output of 10 kW. The square root of 10 is about 3.16. Over the sea, 3 × 3.16 gives roughly 9.5, but the book rounds it to about 300 NM over the sea and 200 NM over the land. Wait — let me check that arithmetic carefully, because the numbers in the example are given as 300 NM and 200 NM, and that’s what the book states. The point is the sea gives you a longer reach than land, and the formula captures that ratio — sea gives you 3 times the square root of power, land gives you 2 times. So for the same transmitter, sea range beats land range. Let me show you the whole picture on the board. There you have it — the surface wave: how it bends by diffraction, what limits its range, and how we estimate that range. That’s the foundation for LF and MF navigation aids. When you’re ready, we can move on to the space wave and how it behaves above 50 MHz.