Everything HF-digital is FSK underneath
Which of the following types of modulation is used for data emissions below 30 MHz?
FSK.
Frequency-shift keying, in one form or another, is the family. What varies is the number of tones, the symbol rate, and how much error correction rides along.
What is the difference between direct FSK and audio FSK?
Direct FSK modulates the transmitter VFO.
Two ways to produce the same emission:
- Direct FSK shifts the transmitter’s own oscillator. Clean, no audio chain, no risk of overdriving anything.
- Audio FSK (AFSK) feeds shifting audio tones into an SSB transmitter, which translates them to RF. Convenient — any SSB rig plus a sound card — but the audio path must be driven correctly or the result is splatter and harmonics.
Most modern software uses AFSK because it needs no rig support. That is why setting audio drive levels correctly matters so much in practice.
The WSJT-X family
Which of the following synchronizes WSJT-X digital mode transmit/receive timing?
Synchronization of computer clocks.
Every mode in this family works in fixed time slots, so both stations must agree on the time to within a fraction of a second. Run NTP. A clock that is a second off decodes nothing and no error message explains why — the single most common failure in this family.
What is the length of an FT8 transmission cycle?
15 seconds.
Fifteen seconds transmit, fifteen receive, and a complete contact in about a minute.
To what does the “4” in FT4 refer?
Four-tone continuous-phase frequency shift keying.
Not the cycle length — the tone count. FT4 is faster than FT8, designed for contesting; the number names the modulation.
Which of the following is characteristic of the FST4 mode?
All these choices are correct — it is designed for the LF and MF bands, it uses multiple tones, and it has selectable transmission lengths.
Which of these digital modes does not support keyboard-to-keyboard operation?
WSPR.
WSPR is a beacon mode, not a conversation mode. It transmits a call sign, grid, and power level, and reporting stations upload what they hear to a database. You cannot chat with it — its job is to map propagation, and it does that better than anything else because it decodes at signal levels no conversation mode reaches.
Which of these digital modes has the narrowest bandwidth?
FT8.
About 50 Hz. Narrow bandwidth is why it works at low signal-to-noise ratios: less bandwidth means less noise power in the receiver, exactly as the noise-figure work in the measurement module predicts.
Conversational and traffic modes
Which of the following HF digital modes uses variable-length character coding?
PSK31.
Varicode: frequent letters get short codes, rare ones long — Morse’s idea in a digital wrapper. It lets PSK31 keep pace with typing inside roughly 31 Hz.
Which of the following HF digital modes can be used to transfer binary files?
PACTOR.
Which of these digital modes has the highest data throughput under clear communication conditions?
PACTOR IV.
PACTOR is the traffic-handling family: it acknowledges, retries, and adapts its speed to conditions, so it can move binary files reliably rather than just text. PACTOR IV is the fastest of them on a clean path. Note the qualifier in the question — under clear conditions. On a poor path a slower, more robust mode delivers more.
How do ALE stations establish contact?
ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received
Automatic Link Establishment solves the “which band is open?” problem mechanically: each station scans a channel list, continuously sounding and listening, and builds a picture of which frequencies currently reach which stations. When you call, it picks the frequency that is actually working. Automating the band-selection judgement the propagation module teaches.
Choosing a mode
| You need to | Use | Because |
|---|---|---|
| Make a contact at the noise floor | FT8 | narrowest, most sensitive |
| Contest quickly | FT4 | shorter cycles |
| Work LF/MF | FST4 | designed for it, selectable lengths |
| Measure propagation | WSPR | beacon-only, extreme sensitivity |
| Have a conversation | PSK31 | varicode keeps up with typing |
| Send a file | PACTOR | acknowledged, adaptive, binary-capable |
| Find a working frequency | ALE | scans and sounds continuously |
Check yourself
- You install WSJT-X and decode nothing at all, though the waterfall shows plenty of signals. What is the first thing to check?
- Why can you not answer a WSPR transmission?
- FT8 and PSK31 are both narrow. Why is FT8 far more sensitive?
Answers
- The computer clock. Every WSJT-X mode depends on clock synchronisation; a second of error decodes nothing, silently.
- WSPR does not support keyboard-to-keyboard operation — it is a one-way beacon that carries only call sign, grid, and power.
- FT8 is narrower still (about 50 Hz) and adds heavy forward error correction with fixed 15-second slots, so it integrates a known signal structure out of the noise. PSK31 is designed for real-time typing, which forbids that.