Python Radio 17: FSQ Mode

Simon Quellen Field

7 min read

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Sep 4, 2024

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Fast Simple QSO

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FSQ stands for Fast Simple QSO (QSO means conversation). It is a chat mode in amateur radio that has several advantages over MFSK and RTTY.

Like MFSK, it uses a varicode, so that frequently used characters are sent faster. It uses 33 tones so that all of the lower-case characters (and a few more, such as space, period, and carriage return) can be sent with a single tone. FSQ6 (the 6 is the baud rate) can send 60 words per minute if lowercase letters are used. The baud rate (number of symbols per second) can be anything from 2 to 6. A symbol is a single letter if in lowercase, and two symbols are needed for other characters. The receiver does not need to change anything as the baud rate changes, so the sender can adjust the baud rate to the propagation conditions, slowing down when there is a lot of noise. The slower speeds are more robust to interference and to weak signals.

Instead of fixed tones, FSQ mode uses the difference from the last tone. This makes it much easier to tune and makes it virtually immune to frequency drift. It also makes it less affected by inter-symbol interference that can occur in Near Vertical Incidence Skywave (NVIS) communication, where people can converse with other nearby radio operators over the horizon by bouncing the signal straight up off of the ionosphere.