Tutorial THOR

Tutorial: THOR

What it is

THOR is a keyboard mode that sends the next tone as a step away from the last one (incremental frequency keying) and adds error correction. It copes with drift and bad conditions and is faster than Olivia for a similar robustness.

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): THOR 20 m 14.077 MHz (the app default).

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. These keyboard modes are narrow and slow; a clean, low-noise setup matters more than sensitivity.

Step by step

1. Connect the radio and antenna and start OnAir. In the mode list open the group Amateur: digital and choose THOR (the entry reads Keyboard, 14.077 MHz).
2. Check the frequency: the start frequency is the usual dial frequency in upper sideband. The Band row jumps to the usual place on other bands (band plans differ by region, so treat it as a starting point).
3. Press Start. The text pane says start the receiver until the radio runs and then searching, listening at ... Hz while it looks for a signal.
4. Set Mode (the variant) and the Audio frequency, the audio pitch of the middle of the signal (300 to 3500 Hz above the dial). Or press Find signal: it looks for the signal in the audio while nothing is decoding and moves the audio frequency for you.
5. Click the text pane and watch Level, Chars and Callsign (callsigns found in the text are highlighted). Use Copy to take the text away and Clear to start again.

What you should see

Decoded text builds up character by character once the receiver has locked. On a strong signal it is clean; on a weak one you see missing or wrong characters, and the Level bar shows how strong the tones are. Text with a steady rhythm; the signal on the waterfall is a set of steps that wander up and down the tone ladder.

Reading the screen

Tones, Bandwidth and Mode describe the variant being decoded. Audio is the centre of the signal in Hz above the dial. Level is the signal strength, Chars the number of characters decoded, Callsign the last callsign found. The status line shows the sample rate and the level in dBFS.

Options and settings

• Mode: the variant (speed and tone count). A wrong variant gives garbage or silence; start with the usual one.
• Audio: the centre frequency; Find signal automates it.
• Listen: play the sideband audio the decoder hears (dial + 200 Hz to 3.8 kHz). Decoding is the same either way.
• Everything is remembered per mode.

Troubleshooting

• Nothing decoded, signal visible. The variant is wrong. Try the neighbouring entries in Mode; read the speed from the width of the signal on the waterfall.
• Garbage text. The signal is weak, the audio frequency is off or you chose the wrong variant. Press Find signal and wait a few seconds.
• No signal anywhere. These modes are usually only a few signals at a time. Try the frequency at a busy time of day and on a second band.
• Doubled text or odd characters. Spectrum inversion from an up-converter or lower sideband: use upper sideband and correct inversion in Radio settings.
• Only noise decoded. Gain too high: strong broadcast stations overload the converter.

Tips

• Keep a recording (Tutorial-Record-IQ-then-the-Signal-workbench) of a good band opening and replay it to try settings.
• The mode is narrow: one of the decoders can follow a signal that drifts, but a radio with a good reference helps.

Legal and privacy notes

Receiving amateur signals is allowed in most countries; the text you see is public by design, but do not publish other people's conversations beyond callsigns and what the operators meant to be public. Transmit works in OnAir and has been used on a real radio by the maintainer and in loop-back tests; you need an amateur licence to transmit (see Transmit and Legal).

Under the hood

THOR combines incremental frequency keying (IFK+) with a convolutional forward-error-correction code and an MFSK-style varicode. The variants (THOR 4 to THOR 100) change the symbol rate; tones and bandwidth are shown in the panel.

Related

Mode-THOR, Tutorial-HF-digital-FT8-and-WSPR, Tutorial-PSK31, Tutorial-RTTY, Radio-RTL-SDR, FAQ, Troubleshooting