Explain FSK, WSJT-X clock timing, FT8/FT4/FST4/Q65, mode traits, bandwidth versus speed and ALE.
Syllabus: Operating methods: digital modes and procedures for HF
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 13 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Data below 30 MHz uses FSK: the signal jumps between set frequencies (two for RTTY, more for modes like FT8). Direct FSK modulates the transmitter's VFO. Audio FSK feeds two audio tones into an SSB transmitter.
RememberFSK below 30 MHz. Direct FSK = modulate the VFO.
Stations take turns in set transmit/receive periods, so what keeps them in step is synchronized computer clocks. FT8 = 15 s cycles. The number in a name hints at tones: FT4 = four-tone continuous-phase FSK. FST4 is four-tone GFSK too, with variable periods and seven tone spacings.
RememberClocks sync the cycle. FT8 = 15 s. FT4 = four tones, not four cycles.
Q65 differs from JT65 by averaging multiple receive cycles. The signal repeats each cycle and the noise does not, so stacking cycles lifts a weak signal out of the noise.
RememberQ65 = JT65 plus averaging of several receive cycles.
| Mode | Trait |
|---|---|
| RTTY, PSK31, MFSK16 | Keyboard-to-keyboard chat |
| WSPR | Beacon reports only. No keyboard-to-keyboard |
| PACTOR | Can transfer binary files (PSK31, RTTY, AMTOR cannot) |
| PSK31 | Variable-length character coding |
RememberWSPR: no chat. PACTOR: binary files. PSK31: variable-length code.
More bandwidth carries more data per second. FT8 is a very narrow weak-signal mode. PACTOR IV uses a wide channel and has the highest throughput under clear conditions.
RememberNarrowest: FT8. Fastest when conditions are clear: PACTOR IV.
Automatic Link Establishment (ALE) radios constantly scan a list of frequencies. When one hears its designated call sign, it activates and answers.
RememberALE = scan a list, wake when your call sign arrives.