Modulation — Page 48, Lesson 45

Let’s pick up where we left off with modulation. We’ve already seen how amplitude modulation works, and now I want to walk you through what happens to the power in that signal, because it sets up the whole reason for the next technique. Look at the numbers from the table we were just discussing. The power that sits in the audio frequency, the AF, is divided equally between the two sidebands. That means the upper sideband and the lower sideband each carry exactly half of that AF power. And here’s the critical point: the information in the AF is contained in both sidebands. They’re duplicates of each other. But notice this — only one third of the total signal is actually carrying the information. The rest is tied up in the carrier wave, which carries no information at all. That redundancy is the whole motivation for Single Sideband, or SSB, operation. Think about what we have in a double sideband transmission: the information is in both the upper and lower sidebands, and the original RF carrier wave has already served its purpose — it got the audio information up into radio frequencies. Once it’s done that job, it’s redundant. So we can remove one of the sidebands and the carrier wave, because the remaining single sideband contains all the information. That’s what we call single sideband operation. Let me show you what that looks like in practice. Take a carrier at 2182 kHz. On either side of it, you have the sidebands. The upper sideband sits at 2182.001 kHz, and the lower sideband sits at 2181.999 kHz — each offset by the 3 kHz audio frequency. Now, in a full double sideband system, you’d transmit the carrier at 150 watts, and each sideband at 25 watts. But in SSB, you keep just one sideband — say the upper sideband at 2182.001 kHz — and you put all that power into it, 100 watts. You can also use the lower sideband at 2179 kHz if you prefer. Either way, you’re transmitting one sideband with all the power, and you’ve dropped the carrier and the other sideband entirely. Now, why does this matter for aviation? There are three big reasons, and they all come from the physics of the ionosphere and the radio spectrum. First, when you use sky wave propagation for long-range communication, the ionosphere refracts different frequencies by different amounts. If your bandwidth is too large, that differing refraction causes distortion. SSB helps because it narrows the bandwidth. Second, the ionosphere is full of electrically charged particles, and those particles cause high levels of static interference on radio waves. Using SSB significantly reduces that interference. And third, the MF and HF frequencies used for long-range communication are in great demand — there aren’t many of them. Because SSB uses less bandwidth, you can fit more channels into the same spectrum. So let me give you the three main advantages of SSB, exactly as they matter operationally. First, you double the number of channels available compared to double sideband. Second, you get a better signal-to-noise ratio, which means less interference. And third, you need less power, which means lighter equipment — and in an aircraft, lighter equipment is always a win. Now let’s move on to a completely different approach: Frequency Modulation, or FM. In amplitude modulation, the audio changes the amplitude of the carrier. But in Frequency Modulation, the amplitude of the audio frequency modifies the frequency of the carrier wave. The carrier’s height stays constant; its frequency is what changes. Here’s the key relationship. The change in the carrier wave frequency depends on the rise and fall of the amplitude of the modulating wave — that’s your audio frequency. The greater the amplitude of that audio, the greater the frequency deviation. So a loud sound pushes the carrier frequency further away from its centre. And the frequency of the modulating wave — how fast the audio itself oscillates — determines the rate of change of frequency within the modulated carrier wave. So amplitude controls how far the frequency swings, and audio frequency controls how fast it swings. Now, FM has a bandwidth problem. When it’s used for sound broadcasting — think music radio stations — the bandwidth permitted by international agreements is 150 kHz. Compare that to the 9 kHz allowed for AM. That’s a huge difference. Because of that wide bandwidth, FM is generally unsuitable for use on frequencies below VHF. It just takes up too much room in the spectrum. But for voice communications, you can reduce the bandwidth considerably while still keeping the information intact. That’s called Narrow Band FM, or NBFM. Typically, NBFM systems have a bandwidth of 8 kHz. Now here’s the catch for aviation: 8 kHz is greater than the 6 kHz permitted for Aeronautical Communications, and it’s greater than the 3 kHz used in HF Communications. So NBFM communication systems are not yet used in aviation. The bandwidth is still too wide for our standards. So to tie it all together: SSB gives us efficiency by stripping away the redundant carrier and one sideband, and FM gives us a different way of encoding information by varying frequency instead of amplitude — but its bandwidth keeps it out of aviation for now.