01Turning audio into something that travels
A radio signal begins as sound — pressure waves in air — and ends the same way, in a listener's ear. Between those two moments, something remarkable happens: the audio is converted into an electromagnetic wave capable of crossing a city or a continent without a wire. The transmitter is the device that makes that conversion. Most listeners never think about it, and most of the time that invisibility is the point.
The starting material is an electrical signal: a fluctuating voltage that represents the audio, shaped by microphones, desks and processing further back in the chain. The transmitter's job is to impress that audio onto a carrier — a continuous radio-frequency wave generated at a precise frequency. The technique for doing this is called modulation, and it is where AM and FM part company entirely.
In amplitude modulation, the strength of the carrier rises and falls in step with the audio. Loud bass note: the carrier gets bigger. Silence: it collapses toward a fixed minimum. The frequency of the carrier stays constant; it is only the height of the wave that changes. AM's simplicity made it the first practical broadcast method, and it still carries voice well over enormous distances because the sky acts as a mirror for the medium-wave frequencies typically used — a phenomenon called skywave propagation, most pronounced at night when the absorbing D layer of the ionosphere fades away.
FM — frequency modulation — works the other way around. The carrier's amplitude stays fixed while its frequency is pushed slightly above and below the nominal centre frequency in proportion to the audio. A positive peak in the audio nudges the carrier one way; a negative peak nudges it the other. The deviation is tiny — a matter of kilohertz — but because the receiver reads frequency rather than amplitude, most electrical interference (which tends to affect signal strength, not frequency) is simply ignored. That is the core reason FM sounds cleaner than AM in most urban environments. The tradeoff is range: FM's frequencies travel as line-of-sight waves that cannot bend over the horizon the way medium-wave AM can.
Most listeners never think about it, and most of the time that invisibility is the point.
02Power and the aerial
The transmitter amplifies the modulated signal to the power level the licence permits, which varies enormously — from a few watts for a community licence to many kilowatts for a national transmitter. More power means greater range, up to a point. The geometry of the terrain and the height of the aerial matter just as much, which is why transmission masts are typically placed on hills or tall structures. Height gains coverage far more efficiently than raw power, because radio at FM frequencies travels in straight lines and the aerial simply needs to see further.
The aerial — antenna — is the final stage. Its design determines how the power is distributed in space. A basic dipole radiates equally in all directions horizontally, which suits a broadcaster that wants even coverage around a single mast. More elaborate arrays can concentrate the signal toward a particular area, reducing wasted power radiating into the sea or over unpopulated terrain. The aerial is also tuned to resonate at the transmission frequency: a mismatched aerial reflects power back into the transmitter rather than radiating it, which wastes energy and can damage the final amplifier stage.
Between the power amplifier and the aerial sits a low-pass filter, sometimes called a harmonic filter, which strips out spurious frequencies that any amplifier generates alongside the intended signal. Transmitting those harmonics would interfere with other services and violates the licence conditions that govern every broadcast transmitter.