Tutorial NDB

Tutorial: NDB (long-wave beacon idents)

What it is

A non-directional beacon (NDB) is a long-wave radio beacon, usually between 190 and 535 kHz, that sends its identification in Morse at about 7 words per minute. Pilots tune it with an ADF. OnAir reads the ident of a beacon and shows how it is keyed. Receive only.

What you need

• A radio that reaches long wave. An RTL-SDR does it only in direct-sampling mode (offered in the radio settings) and the signal may be weak or absent; SDRplay and Airspy HF+ radios reach low frequencies directly. See Supported radios.
• An antenna for LF: a long wire or a loop; ferrite loop antennas work well. Place it away from computers and switching supplies, which make strong low-frequency noise.
• Frequencies: the NDB band is 190 to 535 kHz (database entries 0.190, 0.350 and 0.530 MHz are band markers, not stations). Find real beacons from an aeronautical chart or navaid list. The mode starts at 350 kHz with 256 ksps (lowest working rate 48 ksps).

Before you start

Receive at night if you can: LF ground wave reaches further and day-time noise is lower in the evening. Do not mix up NDB with the HF propagation beacons, which are a different mode.

Step by step

1. Choose the group Aviation, entry Ident, 190-535 kHz (mode NDB).
2. Tune the beacon's frequency and press Start. The side panel is headed IDENTS.
3. Wait for the ident: beacons repeat it about once a minute.
4. Choose the tone that matches the keying: Auto follows whichever of the 400 Hz tone, the 1020 Hz tone or the keyed carrier is actually being keyed. Use the listen option to hear the keyed ident tone (a 400 Hz tone follows the carrier when the beacon sends no modulating tone).

What you should see

A carrier with an offset in Hz, keying that shows as on and off, and the ident decoded in letters with the keying speed. The status reads "in progress" while reading and "no ident yet" until one is decoded.

Reading the screen

The idents list shows the ident, level, carrier offset, tone, depth, speed and the state of the decoding ("reading", "in progress").

Troubleshooting

• Nothing: direct sampling is off, or the antenna is poor. Try a long wire away from the computer.
• Ident garbled: weak signal or noise; the speed can be off for some beacons.
• Carrier but no ident: some beacons key the carrier only (no tone); choose Keyed carrier instead of Auto.
• Many carriers: LF has many non-aeronautical stations; check the frequency.

Tips

Run with a long integration at night; compare with a published NDB list to confirm the ident.

Legal and privacy notes

NDBs are public navigation aids. Do not navigate by OnAir. See Legal.

Under the hood

Amplitude-modulated carrier keyed in Morse (A1A: carrier on/off; A2A: a 400 or 1020 Hz tone keyed), standard speed about 7 WPM, ICAO Annex 10 vol. I.

Related

NDB mode page, Tutorial: VOR, Tutorial: ILS.