Tutorial JS8Call

Tutorial: JS8Call

What it is

JS8Call is a chat and messaging mode built on FT8's modulation: slow, weak-signal and with free text. Stations send heartbeats and CQs, exchange messages that may be relayed by other stations, and give their grid. OnAir shows every frame in the passband, puts messages together from their frames and maps the stations.

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): JS8Call 1.842, 3.578, 7.078, 10.13, 14.078 (the default), 18.104, 21.078, 24.922, 28.078 and 50.318 MHz. The channel is 3.5 kHz wide. 14.078 is shared with JT9.
• A computer clock that is right to about a second: see Tutorial-Time-stations-and-the-clock.

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.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Amateur: digital and choose Messaging (the entry reads Messaging, 14.078 MHz).
2. Choose the speed: Normal (15 s slots, 6.25 baud, 50 Hz wide), Fast (10 s slots, 10 baud, 80 Hz wide) or Turbo (6 s slots, 20 baud, 160 Hz wide). Most traffic is Normal.
3. Press Start. The panel says start the receiver to listen before that. Frames show up as they are decoded.
4. Open Frames, Messages, Stations and Map in turn.
5. Press Listen to hear the passband.

What you should see

Frames with time, audio offset (Hz above the dial), the frame type and flags; assembled messages with the relay path; stations with grid and last time heard; the map with stations at the middle of their grid squares. The status line shows the median DT of the slot and how long decoding took.

Reading the screen

Frames: UTC, Frame and the frame details. Messages: the text built from frames (a relayed message shows its path). Stations: Call, Grid and Last. Map: stations in their grid squares, never an exact position.

Options and settings

• Speed: Normal, Fast or Turbo (check the panel for any slower submode).
• Listen, Clear, zoom/fit buttons on the map.

Troubleshooting

• No frames. Check the clock (the status line shows the DT median), the dial frequency and the band condition.
• Frames seen on the wrong speed. Switch among Normal, Fast and Turbo.
• Few stations. JS8Call activity is much lower than FT8. Evenings and weekends are better.
• Messages in pieces. Frames of one message arrive over several slots and are put together; wait for the last frame.
• Stations without a grid. Only heartbeats and CQs carry the grid.

Tips

• JS8Call stations often use 14.078 and 7.078 MHz; try both.
• Keep a recording (Tutorial-Record-IQ-then-the-Signal-workbench) of a good opening and replay it to try the settings.

Legal and privacy notes

Receiving amateur signals is allowed in most countries; the text is public by design. 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). JS8Call messages can be personal; OnAir shows only what is on the air, but do not republish private conversations.

Under the hood

JS8Call uses the FT8 8-GFSK modulation with different speeds, frames with a CRC and an LDPC code, and a flag word that marks heartbeat, CQ, directed, relay and data frames. Normal mode has a 15 s slot at 6.25 baud and the Fast and Turbo modes speed this up.

Related

Mode-JS8Call, Tutorial-FT8, Tutorial-HF-digital-FT8-and-WSPR, Tutorial-Time-signals, Radio-RTL-SDR, Live-map, FAQ