Frequency is the number of cycles a periodic wave completes each second, measured in hertz (Hz) — one hertz being one cycle per second.1 For a radio wave it is the quantity you tune to, it is inversely related to wavelength, and it is the coordinate along which the whole electromagnetic spectrum is laid out. When you “tune to 162.550 MHz,” you are selecting a frequency.
How it works
Frequency is the inverse of period: if one cycle takes a time T seconds, the frequency is f = 1 / T. A 100 MHz FM signal repeats a hundred million times a second, so each cycle lasts just 10 nanoseconds. Radio frequencies are large numbers, so they are scaled into kilohertz (kHz, 10³ Hz), megahertz (MHz, 10⁶ Hz), and gigahertz (GHz, 10⁹ Hz). The unit honours Heinrich Hertz, who first generated and detected radio waves in the 1880s.
Because all radio waves travel at the speed of light c, frequency and wavelength are locked together by λ = c / f: doubling the frequency halves the wavelength. Frequency is also the axis of the frequency domain — the view a spectrum analyzer or FFT shows — where a pure tone appears as a single spike and a modulated signal spreads into a band of nonzero bandwidth around its centre frequency.
In practice
- Bands and allocation. The radio spectrum is divided into frequency bands (VHF, UHF, and so on), and regulators assign slices of each to specific services. A frequency is not just a number but a legal and physical context.
- Stability matters. A transmitter and receiver must agree on frequency to within a small tolerance. Real oscillators drift with temperature and age, and their short-term jitter shows up as phase noise that smears the carrier and degrades demodulation.
- Doppler. Motion between transmitter and receiver shifts the apparent frequency (Doppler shift), significant for satellites and fast vehicles.
Relevance to SDR
Tuning an SDR sets the centre frequency its local oscillator mixes down toward baseband; the chosen frequency, within a band, determines what signal lands in the captured passband. Because cheap tuners have imperfect reference oscillators, their actual frequency is offset by a few parts per million, and GopherTrunk (like any decoder) must apply a PPM correction and continuously track residual frequency error with an automatic-frequency-control or frequency-locked loop so symbols stay aligned. Getting the frequency right — and keeping it right — is the first requirement for any decode.