The airband (aviation AM)
Between 118 and 137 MHz lives one of the most rewarding bands for a beginner: the airband, aviation's voice communications. Control towers, approach, pilots, weather ATIS — all in the clear, all in AM, and entirely within reach of a plain RTL-SDR.
Why AM and not FM?
FM dominates consumer radio, but aviation stayed on AM (amplitude modulation) for a safety reason:
- If two stations transmit at once on the same channel, AM lets you hear both (with a tell-tale heterodyne whistle). FM's capture effect would erase one — an aircraft might never know it was stepped on.
- AM is simple and rugged: an AM receiver fails "gracefully".
Each channel is only ~8.33 kHz wide (modern spacing): on the spectrum, a thin, brief line, not the broad steady bump of an FM station.
How it's demodulated
AM carries the information in the carrier's amplitude. To demodulate = follow that amplitude:
- Isolate the channel (filter around the frequency).
- Compute the envelope:
√(I² + Q²)at each instant (see Les échantillons I/Q). - Remove the DC component (the carrier itself) — what remains is the voice.
- A little AGC (automatic gain) levels loud and faint transmissions.
That's exactly what OpenHertz does when you listen to an airband channel.
A few frequencies to start
| Use | Frequency | Note |
|---|---|---|
| Aeronautical emergency | 121.500 MHz | the international "guard" |
| Tower / ground | 118–122 MHz | varies by airport |
| Approach / control | 119–135 MHz | regional sectors |
| ATIS (looped weather) | airport-specific | great for practice: it talks continuously |
⚠️ Receive-only. Listening to aviation is legal in many countries but not all, and re-transmitting or exploiting these communications is strictly regulated. See Légal & sécurité.
Reception tips
- A vertical ~57 cm antenna (quarter wave at 125 MHz), well in the clear, works wonders (Régler son antenne, pas à pas).
- Communications are short and intermittent: let it run, be patient, the waterfall helps you spot an active channel.
- Too far from an airport? ATIS and cruising aircraft (high altitude, huge line-of-sight range) are often still receivable.
👉 Start listening: Listen to aviation (AM)
Related: Modulations : graver l'information sur une onde · Bandes intéressantes à explorer · Régler son antenne, pas à pas · Légal & sécurité