If you've ever tuned an SDR to 118–137 MHz and listened in on air traffic control, you've probably noticed something odd: it sounds like old-fashioned AM radio.
Scratchy, a little thin, occasionally garbled.
In a world where your car radio, your walkie-talkie, and your phone all use much cleaner audio technology, why are pilots and controllers still talking over what is, technically, 1920s-era radio tech?
The answer is a genuinely smart safety decision that's held up for a century.
The problem with the "better" technology
FM (frequency modulation) sounds better than AM (amplitude modulation) in almost every way. It's clearer, it's less prone to static, and it's what music radio switched to decades ago for exactly those reasons. If sound quality were the only thing that mattered, aviation would have moved to FM long ago.
But FM has a specific, well-known flaw: the capture effect. When two FM signals hit a receiver on the same frequency at the same time, the receiver locks onto whichever one is stronger, and the weaker signal disappears completely. You'd have no idea a second transmission ever happened.
Now picture that in a cockpit. Two pilots key their radios at the same moment — it happens more often than you'd think, especially at busy airports. With FM, the controller hears only one of them, cleanly, with zero indication that anything went wrong. The second pilot's message simply vanishes.
AM handles this completely differently. When two AM signals overlap, they don't compete — they add together. You hear both, distorted and hard to parse, often with an audible whistling tone where the two carrier frequencies beat against each other. It sounds bad. But it tells everyone involved, instantly, that something went wrong.
To hear this, the demo below runs two synthesized AM carriers through an actual envelope detector — rectification followed by a low-pass filter, the same process a diode detector performs. Play each carrier alone, then both together.
A controller who hears that garbled double-transmission knows immediately to say "aircraft, say again" — and every pilot on frequency knows a collision just happened, even if they can't make out the words. That's the entire reason aviation has never switched to FM: in a safety-critical system, an obviously broken transmission is far better than a perfectly clean one that silently erased someone's message.
If you spend enough time listening to airband on an SDR, you'll eventually catch this yourself — a sudden screeching or warbling tone where two transmissions briefly overlap. That's the system working exactly as designed.
One more reason you might not expect
There's a fun side effect of how AM works: the transmitter's power output varies with how loud you're speaking, drawing more current from the aircraft's electrical system at the moment of transmission. In smaller aircraft with limited electrical capacity, this can actually cause a faint, visible dimming of the cabin lights every time the radio keys up. It's subtle, but pilots of light aircraft genuinely notice it.
So what's actually on the airband?
The 118–137 MHz range is a whole ecosystem of aviation communication, and once you know what to listen for, it's easy to follow:
- Tower, Ground, Approach, and Departure — the different frequencies a controller uses depending on exactly where your aircraft is in the process of arriving, departing, or taxiing
- ATIS and AWOS — automated, looping weather and airport-condition broadcasts pilots check before landing or departing
- CTAF/Unicom — the shared frequency pilots use to announce their position at smaller airports with no control tower at all
- 121.5 MHz — the international emergency and guard frequency, monitored worldwide for distress calls
If you're already running an ADS-B feeder to track aircraft on a map, airband is the natural next step — you can watch the planes and listen to them at the same time.
Getting started
You don't need much: an SDR, an antenna, and ideally something to keep nearby FM broadcast stations from drowning out the much weaker aircraft signals.
The FM broadcast band sits immediately below airband at 88–108 MHz, and a strong local station can genuinely desensitize a receiver tuned just a few MHz away.
That's the entire reason an airband bandpass filter is worth adding to an airband setup rather than just pointing an antenna at the sky and hoping for the best.
Once you've got that sorted, the rest is just tuning in and listening — no license required to receive, and no shortage of things happening on frequency at any airport with real traffic.