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Amateur Radio Info & Exams - Operations 3 - HF Digital

Upgrading to Extra gives access to extra bandwidth on HF, for both Data, and Image modes.

Modulation for data

Many HF data modes use Frequency Shift Keying (FSK), or more usually, audio frequency shift keying (AFSK), switching between two or more frequencies. In effect, a suppressed carrier of 14010 kHz and an audio tone switching between 500 Hz and 700 Hz is the same as switching an RF signal between 14010.5 and 14010.7 kHz. Interestingly, one way to generate FSK is by commanding a DDS (Direct Digital Synthesis) module between two RF or IF frequencies rapidly.


This is one of the newer modes added to WSJT software. It is for weak signal chat and contest contacts. Stations take turns transmitting, for only 7.5 seconds each time.


Forward Error Correction, FEC, is a system where additional data is sent in the data-stream, increasing the likelihood that the message will be decoded, without needing to request retransmission.

Packet on HF

Under good conditions packet radio has good data throughput, at 300 baud it is higher than the weak signal modes such as PSK31; and than RTTY, which is often 45.45 baud. It is used for sending messages and bulletins, just as VHF packet is. Messages originated from VHF stations to distant stations may be passed through an HF gateway station.

Relevant Questions

These are the actual questions from the Extra licence exam pool, as published by the NCVEC.

Which type of modulation is common for data emissions below 30 MHz?
A. DTMF tones modulating an FM signal
C. Pulse modulation
D. Spread spectrum

Frequency shift keying (FSK) or AFSK are common, answer B.

Pulse modulation and spread spectrum are for high VHF and up only.

What do the letters FEC mean as they relate to digital operation?
A. Forward Error Correction
B. First Error Correction
C. Fatal Error Correction
D. Final Error Correction

Forward Error Correction, answer A.

How is the timing of FT4 contacts organized?
A. By exchanging ACK/NAK packets
B. Stations take turns on alternate days
C. Alternating transmissions at 7.5 second intervals
D. It depends on the lunar phase

Stations take turns transmitting at 7.5 second intervals, answer C.

What is indicated when one of the ellipses in an FSK crossed-ellipse display suddenly disappears?
A. Selective fading has occurred
B. One of the signal filters is saturated
C. The receiver has drifted 5 kHz from the desired receive frequency
D. The mark and space signal have been inverted

This is indicating selective fading, answer A.

Which type of digital mode does not support keyboard-to-keyboard operation?
A. Winlink
C. PSK31

Winlink is an email forwarding system, not a live chat systems, answer A

What is the most common data rate used for HF packet?
A. 48 baud
B. 110 baud
C. 300 baud
D. 1200 baud

300 baud, answer C.

Which of the following is a possible reason that attempts to initiate contact with a digital station on a clear frequency are unsuccessful?
A. Your transmit frequency is incorrect
B. The protocol version you are using is not the supported by the digital station
C. Another station you are unable to hear is using the frequency
D. All of these choices are correct

All of the above, answer D.

Which of the following HF digital modes can be used to transfer binary files?
A. Hellschreiber

This is PACTOR, answer B.

Hellschreiber is a graphical text messaging system; and RTTY and AMTOR are text modes.

Which of the following HF digital modes uses variable-length coding for bandwidth efficiency?
C. MT63
D. PSK31

PSK31 uses "varicode", variable length coding, for efficiency, answer D.

Which of these digital modes has the narrowest bandwidth?
B. 170 Hz shift, 45 baud RTTY
C. PSK31
D. 300-baud packet

With just 31 baud, PSK31 has narrow bandwidth, answer C.

What is the difference between direct FSK and audio FSK?
A. Direct FSK applies the data signal to the transmitter VFO, while AFSK transmits tones via phone
B. Direct FSK occupies less bandwidth
C. Direct FSK can transmit faster baud rates
D. Only direct FSK can be decoded by computer

Direct FSK varies the VFO's frequency; AFSK sends audio tones to the microphone input, answer A.

How do ALE stations establish contact?
A. ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received
B. ALE radios monitor an internet site for the frequency they are being paged on
C. ALE radios send a constant tone code to establish a frequency for future use
D. ALE radios activate when they hear their signal echoed by back scatter

This system scans the ALE frequency for each band, looking for ALE link establishment signals, which sound like a turkey gobbling, answer A.

Which of these digital modes has the fastest data throughput under clear communication conditions?
B. 170 Hz shift, 45 baud RTTY
C. PSK31
D. 300 baud packet

300 baud packet is faster than the others, answer D.

PSK 31 is 31 baud, RTTY 45.45 baud, much less than 300. AMTOR is a development of RTTY.

On to: Propagation 1 - EME, Meteors, Microwaves, TEP, Grey-line

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, May 2022.

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