Tutorial Hellschreiber

Tutorial: Hellschreiber

What it is

Hellschreiber (Hell) sends text as a picture: each character is drawn as a column of dots, keyed on and off like a fax. You read it with your eyes, not with a decoder, which makes it surprisingly robust to errors and interference. The variants include Feld Hell, Slow Hell, Hell 80 and HellX5.

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): Hellschreiber 20 m 14.063 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 Hellschreiber (the entry reads Keyboard, 14.063 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. Instead of only text, the picture pane shows the characters as lines of dots, scrolling. Before a signal arrives the pane reads no picture yet: the tone shows here as lines of dots. Clear, strong signals give crisp letters; fading and interference show as speckle you can still read.

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.
• Characters look doubled or smeared. The audio frequency is slightly off or the speed variant is wrong; adjust Audio and try a neighbouring variant.

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

Hell is on-off keying of a single carrier: each character is sent as a pattern of dots in a grid of dots and displayed as a scrolling line. Feld Hell is the original mode; Slow Hell and HellX5 change the speed. OnAir draws the picture.

Related

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