Modulation

Part II (iii) Systems for air-ground communication — carrier, AM/FM/SSB, the Tx/Rx chain

You cannot simply connect a microphone to an antenna. First, an efficient antenna must be a fair fraction of a wavelength; a 1 kHz audio tone has a wavelength of 300 km, demanding an absurd aerial. Second, every voice would occupy the same low band, so no two stations could share the air. The solution is to put the voice onto a high-frequency carrier: the carrier needs only a short antenna, and each station can use a different carrier frequency. Modulation is the act of loading the voice onto the carrier.

The carrier wave is a high-frequency wave generated by the transmitter; on its own it carries no information. Modulation varies one property of the carrier — its amplitude or its frequency — in step with the low-frequency audio, so the information rides on the carrier and can be recovered at the receiver.

In Amplitude Modulation, the height (amplitude) of the carrier is varied in step with the voice, while its frequency stays fixed. The outline (envelope) of the wave traces your voice.

Mixing a carrier (fc) with an audio tone (fa) produces two new frequencies either side of the carrier: an upper sideband (fc + fa) and a lower sideband (fc − fa). The information lives in the sidebands; the carrier itself carries none.
The total AM bandwidth = 2 × highest audio frequency.

The depth of modulation is how strongly the carrier is varied. Too little (low %) wastes power and gives a weak, quiet signal; over 100% (over-modulation) distorts the audio and "splatters" energy into adjacent channels, causing interference.

Even at full 100% modulation, two-thirds of the transmitted power sits in the carrier, which carries no information; only one-third is shared between the two sidebands. That waste is the motivation for SSB.

At 100% modulation, of the total power: ~67% is in the carrier, ~16.7% in each sideband. The information-bearing power is only the two sidebands ≈ 33%.

SSB throws away the carrier and one sideband, putting essentially all the transmitter's power into the single information-bearing sideband — a large efficiency gain, decisive over long HF distances.

In FM the carrier's frequency is varied in step with the voice while its amplitude stays constant. The amount of frequency swing is the deviation. Because most noise affects amplitude, FM is far more noise-resistant — which is why it is used for music broadcasting.

FM bandwidth ≈ 2 × (deviation + highest audio frequency). FM generally needs more bandwidth than AM, which is part of why narrow aviation channels stayed with AM.

VHF voice = AM: if two aircraft transmit together on the same VHF frequency, AM produces an audible heterodyne squeal — everyone hears that two stations "stepped on" each other and can re-transmit. With FM's capture effect, one would simply be lost. AM's "you'll hear the clash" behaviour is a safety feature.

HF voice = SSB: over oceanic distances, efficiency and bandwidth matter most, so HF uses the power- and bandwidth-saving SSB.

SSB puts all the transmitter's power into the one sideband that carries information and uses half the bandwidth of full AM. Over thousands of miles of HF that efficiency is decisive — so HF aviation voice uses SSB. The receiver must reinsert a local carrier to demodulate it.

That brief squeal you sometimes hear after a call is two aircraft transmitting together. Because VHF is AM, you know it happened and can say "say again, two together." That awareness is exactly what AM buys aviation — and what FM would hide.

An emission is classified by three symbols: type of modulation · nature of the signal · type of information. The two you must recognise:
A3E = amplitude-modulated double-sideband telephony (aviation VHF voice)
J3E = single-sideband suppressed-carrier telephony (aviation HF voice).

Microphone → audio amplifier → oscillator (generates the carrier) → modulator (mixes audio onto the carrier) → RF power amplifier → antenna.

Antenna → RF amplifier → mixer + local oscillator (shifts the signal to a fixed intermediate frequency, IF) → IF amplifier (most gain/selectivity) → detector / demodulator (recovers the audio) → audio amplifier → speaker/headset. AGC/AVC holds the output level steady (Chapter 11).

Converting every incoming signal to one fixed IF means the gain and selectivity stages are always tuned to the same frequency — far easier to make highly selective than tuning every stage to each station. It is the architecture of virtually every aviation receiver.