Tutorial HF ALE

Tutorial: HF ALE (automatic link establishment)

What it is

ALE is the system that HF radio networks use to find each other: a radio sends short digital calls on a list of channels, the called station answers and the pair moves to a working channel. Military, government, aid organisations, aviation and some amateur networks use it. OnAir listens to the public calling channels and logs who calls whom: soundings, individual, net and group calls, responses, links and short messages.

What you need

• An HF-capable receiver. An SDRplay or an Airspy HF+ tunes the HF bands directly. An RTL-SDR reaches HF only in direct-sampling mode (Radio settings, see Radio-RTL-SDR; the RTL-SDR Blog V4 uses its built-in up-converter). A HackRF or another radio that starts above the HF bands needs an up-converter such as Ham It Up or SpyVerter, which you pick in Radio settings so that the whole app shows the true frequency (see Supported-radios). The mode asks for 2 Msps and keeps working down to 250 ksps, so even a small radio is enough: the signals sit within about 3 kHz of the dial frequency.
• An HF antenna: a long wire of 10 m or more as far from household electronics as you can get it, a dipole cut for the band, or a magnetic loop for a noisy site. Keep the feed line away from switching power supplies and LED lighting, which are the usual source of S9 noise on a city balcony.
• Frequencies (USB dial, from the OnAir frequency database; the commonly cited calling channels): 3.596, 5.3305, 7.185, 10.1465, 14.109 (the default), 18.1065, 21.096, 24.926 and 28.146 MHz. The database note says to check the local band plan before transmitting; receiving needs no licence in most countries. Real networks use many channels, so the panel's Channel and Nets settings matter (see below).

Before you start

Lower the gain until the ADC level in the status line is well below full scale: strong broadcast stations next to an amateur band overload a direct-sampling RTL-SDR easily, and the result is spurs that look like signals. Use Auto-tune or Optimise gain (see Your-first-reception) rather than guessing. Choose upper sideband for all dial frequencies in the lists below; the decoders listen from the dial frequency upwards. A DC spike in the middle of the spectrum is normal for some radios and is not a signal: leave a few kHz of offset if you can. ALE signals are short bursts; AGC pumping in the radio can hide them. Use a fixed gain.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group HF utility and choose Link establishment (the entry reads Link establishment, 14.109 MHz).
2. Check the channel: the panel names it (ALE channel, ... MHz dial (upper sideband)). The Channel list jumps to the usual ones.
3. Press Start. The tab Words shows the raw ALE words as they are decoded (no words yet before that).
4. Open Events for the calls: who called whom and the kind. Open Stations for the addresses heard.
5. Press Listen to hear the sideband (dial + 200 Hz to 3.8 kHz) on the sound card; decoding is the same either way.

What you should see

Soundings (a station announcing itself), calls and responses, with the time (UTC), the frequency, the From and To addresses, the type, the BER and the SNR. Words arrive 392 ms apart; the Sync shows in sync (392 ms words). The status line gives %+.1f dB in 3 kHz, BER ..., the tone offset and the AFC.

Reading the screen

Events: UTC, MHz, From, Type (Sounding, Response, Net response, Ack, Net, Group, AllCall, AnyCall; TIS means an answer is requested, TWS no answer), BER, AMD / commands (automatic message display text). Stations: Address, Last heard, Count. Words: the raw words with the vote and Golay outcome.

Options and settings

• Channel: the dial frequency.
• Nets: addresses of nets to recognise (a list such as HFN, ...).
• Listen, Clear.

Troubleshooting

• No words. ALE traffic is on a few channels at specific times. Try several calling channels and the evening.
• Words but no events. Words are decoded but too many are damaged. Check the Words tab for the Vote and Golay columns.
• Sync lost often. A frequency offset: ALE tolerates some, the tone offset and AFC are shown.
• Only soundings. Normal: stations send soundings regularly, calls are rarer.
• Some addresses are unreadable. They are encrypted or a different ALE generation (OnAir decodes the 2G ALE standard).

Tips

• Leave the receiver on a calling channel for a few hours; ALE networks sound every few minutes.
• Compare stations across bands: a station heard on 14.109 and 10.1465 tells you about the propagation.

Legal and privacy notes

ALE is a public protocol and its calls are sent in clear text. Receiving is allowed in most countries. Do not use what you hear to interfere with a network. If a message is encrypted it appears only as encrypted: OnAir offers no help in defeating encryption. See Legal.

Under the hood

ALE (2G) is defined in MIL-STD-188-141 and FED-STD-1045. It sends 8-FSK at 125 baud (tones in the 750 to 2500 Hz audio range) in words of 24 bits protected by a Golay (24,12) code and sent three times with majority voting; each word lasts 392 ms. Words combine to calls, soundings and AMD messages, and LQA (link quality analysis) reports.

Related

Mode-HF-ALE, Tutorial-HF-beacons, Tutorial-Time-signals, Tutorial-HF-digital-FT8-and-WSPR, Radio-RTL-SDR, FAQ, Troubleshooting