Tutorial Airband

Tutorial: Airband (AM voice, 118 to 137 MHz)

What it is

The civil airband carries air traffic control and airline voice in AM between 118 and 137 MHz, on 25 kHz channels and, increasingly, 8.33 kHz channels. OnAir listens to many channels at once from one wide capture, plays the open ones, and logs every transmission with its time, length and level.

What you need

• Any radio that tunes 118 to 137 MHz and can sample at least 250 ksps; the mode asks for 2 Msps (a 1.6 MHz channel width in the mode table). An RTL-SDR works well. See Supported radios.
• A VHF airband antenna or a simple vertical; line of sight decides range.
• Frequencies from the project's database: 121.500 MHz (international distress, guard), 123.100 (search and rescue on-scene), 123.450 (air to air, informal), 122.800 (UNICOM, US), 122.900 (MULTICOM, US), 123.400 (gliding, Europe), 118.000 and 136.975 for the band edges. Tower, ground and approach frequencies for your airport are in the airport's published charts. The mode starts at 118.7 MHz.

Before you start

• The capture is as wide as the sample rate. A channel is heard only when it lies inside the captured band, so choose a centre frequency that puts your channels in the middle. Channels outside are not available until you retune; the table offers tune here for such a channel.
• Moderate gain: strong FM broadcast just below the band can overload a wide front end.

Step by step

1. Choose the group Aviation, entry AM voice, 118-137 MHz (mode Airband).
2. Tune the centre frequency near the channels you want.
3. In Channels, type a channel in the box that reads 118.005 and an optional label such as Tower, then press Add (or Enter). Enter the channel as dialled: 118.025 is a 25 kHz channel, while 118.030, 118.035 and 118.040 are the 8.33 kHz channels of that block. OnAir answers "not a channel: 118.000 - 136.990 on the 25 or 8.33 kHz raster" if the value is invalid, and "already in the list" if you add it twice.
4. Press Test channels to load the channels of the built-in test signal around the tuned frequency, to hear how the screen behaves without a radio.
5. Adjust Squelch (in dB above noise) and Hang (how long a channel stays open after the carrier drops, 0 to 3 s). Press Start.

The airband mode in this version has a hand-made channel list; there is no airport search yet.

What you should see

A channel turns green and shows "OPEN" while a carrier is above the squelch; the audio of open channels is mixed and played. If two stations are on one channel at once, the state reads "OPEN 2 stn" (a heterodyne whistle is likely).

Reading the screen

• Channels table: Channel (the dialled name and 8.33 or 25), Name (click to rename, right-click to remove), Level, Squelch state, Carrier offset in Hz, Heard count, and three boxes: M mute (listened to and logged, not played), S solo (while any channel is solo, only solo channels play) and P priority (one channel only).
• Scan: a checkbox that stops on the first open channel and plays only it.
• Activity: a log with Time UTC, Channel, label, Length, C/N and Level. Copy copies the log to the clipboard, one transmission per line.
• Spectrum: every channel of the list, green while open, with its filter band (8.33 kHz: plus or minus 3.4 kHz; 25 kHz: plus or minus 5 kHz).
• Receiver tab: state, channels listed, in the band and open, the tuning error, squelch, audio mode, transmissions, radio centre and input level.
• Radio off by shows the measured tuning error of your radio in kHz.

Options and settings

Noise remover (neural) with a strength slider cleans voice; it is for speech only, not for data modes, and adds about 20 ms of delay.

Troubleshooting

• A channel never opens: it may be outside the captured band (use tune here), or the squelch is too high.
• Everything open all the time: raise the squelch; an interfering carrier raises the noise.
• Wrong channel heard: check the 8.33 versus 25 kHz entry; 118.030 is not 118.025.
• Distorted audio: reduce gain.
• Nothing logged: the receiver has to be running.

Tips

Mute channels you only want logged; use priority for the channel you cannot miss.

Legal and privacy notes

Listening to air traffic control is legal in many countries but not all; some forbid recording or passing on what you hear. Check your rules; see Legal. OnAir is receive-only here.

Under the hood

AM voice with channel spacing of 25 kHz and 8.33 kHz (ICAO Annex 10 vol. III and the EU 8.33 kHz regulation). Each channel is a digital down-converter with its own squelch, hang timer and AM demodulator.

Related

Airband mode page, Tutorial: VOR, Tutorial: ACARS, Scanner notes in the FAQ.