Amplitude is the magnitude — the “height” — of a wave, the peak departure of its oscillation from the resting level.1 For a radio wave it corresponds to signal strength, which a receiver reports as a power level in dBm or, inside an SDR, in dBFS relative to the converter’s full scale. Amplitude is one of the three properties (with frequency and phase) that fully describe a wave and that a transmitter can vary to carry information.
How it works
Amplitude and power are tightly linked: the power a wave carries is proportional to the square of its amplitude. So doubling the amplitude quadruples the power, which is why power is the more natural currency once a signal reaches a receiver, and why levels are usually quoted logarithmically in decibels — a scale that turns those wide multiplicative ranges into manageable additions. A single sinusoid can be summarised by its peak amplitude, but for complex or noisy signals engineers more often use the RMS (root-mean-square) amplitude, which relates directly to average power, and the peak-to-average ratio (crest factor), which matters for amplifier headroom.
As a wave spreads out from a transmitter, its amplitude falls. In free space the field strength drops with distance (free-space path loss), and obstacles, absorption, and destructive multipath reduce it further (path loss). That steady weakening is why a distant station is faint and why link budgets exist. At the receiver the wanted amplitude competes with an ever-present noise floor; their ratio is the signal-to-noise ratio that ultimately bounds whether the message can be recovered.
In practice
- The envelope. The slowly varying outline traced by a modulated carrier’s amplitude is its envelope. Amplitude modulation writes the message directly into that envelope, which a simple diode detector can recover.
- Gain staging. Front-end amplifiers and attenuators set where a signal’s amplitude lands relative to the converter’s range. Too little and it sinks into quantisation noise; too much and it clips, generating intermodulation products. Automatic gain control keeps it in the sweet spot.
- Fading. Amplitude is not static — multipath and motion make it fluctuate over time (Rayleigh / Rician fading), which receivers must ride out.
Relevance to SDR
In an SDR the wave arrives as IQ samples, and a sample’s amplitude is its distance from the origin of the IQ plane — the magnitude √(I² + Q²). GopherTrunk reads this to judge whether a channel is active, to drive automatic gain control, and to compute the SNR estimates it uses to gate weak decodes. Varying amplitude is also the basis of amplitude modulation and one axis of QAM, so tracking it accurately is part of demodulating those schemes. Because the four-level C4FM and π/4-DQPSK signals GopherTrunk decodes are largely constant-envelope, amplitude there serves mainly as a health and gain-staging indicator rather than as the information itself.