Radio waves
A radio wave is an electromagnetic field that oscillates and travels at the speed of light. Two quantities describe it:
- Frequency f: the number of oscillations per second, in hertz (Hz). FM radio sits around 100 megahertz (MHz, millions of Hz), WiFi around 2.4 gigahertz (GHz, billions of Hz).
- Wavelength λ: the distance travelled during one oscillation. You convert between the two with
λ = c / f(c ≈ 3×10⁸ m/s). At 100 MHz, λ ≈ 3 m; at 2.4 GHz, λ ≈ 12.5 cm.
Wavelength isn't just a curiosity: it dictates antenna size (see Antennes) and how the wave passes through or bends around obstacles. Low frequencies penetrate walls better; high frequencies carry less far but move more data.
The radio spectrum is simply the set of all these frequencies, carved into bands by regulation (FM, aviation, ISM, telephony…). A HackRF covers 1 MHz to 6 GHz: an enormous window onto that spectrum.
A "bare" wave at a single frequency (a carrier) carries no information. For that, you modulate it — see Modulations : graver l'information sur une onde.
👉 See a real carrier for the first time: First contact
The map of the spectrum
| Range | Frequencies | λ | What lives there |
|---|---|---|---|
| LF / MF | 30 kHz – 3 MHz | km | AM radio, beacons, the DCF77 clock |
| HF | 3 – 30 MHz | 100–10 m | shortwave, amateur DX |
| VHF | 30 – 300 MHz | 10–1 m | FM, aviation, the 2 m band, weather satellites |
| UHF | 300 MHz – 3 GHz | 1 m – 10 cm | 70 cm, ISM 433/868, ADS-B, GSM, GPS, 2.4 GHz WiFi |
| SHF | 3 – 30 GHz | cm | 5 GHz WiFi, radars, satellite links |
The higher you go: shorter antennas, line-of-sight range dominates, higher data rates — and walls get more opaque.
Polarisation
The electric field oscillates in a plane: vertical (most mobile use) or horizontal (much TV/DX). A receiver crossed at 90° to the transmitter loses ≈ 20 dB — if a known signal seems oddly weak, straighten your antenna first.
Your turn
λ = 300 / f(MHz). Work these out in your head: 433 MHz → ~69 cm; 1090 MHz → ~27.5 cm; 2440 MHz → ~12.3 cm. These orders of magnitude come back everywhere, from antennas to obstacles.
Next up: Les décibels (dB et dBm) to measure the power of these waves.