Amateur Radio in Australia (VKFAQ)

Digital modes used on VHF/UHF bands

PK232MBX multi mode controller

Typical of the multimode controllers that became popular in the 80s and 90s, this box contained its own CPU and could receive and transmit a number of digital modes suitable for HF and VHF use. Fed with speaker level audio and producing microphone level output suitable for transmitting, it could convert your ordinary hf voice transceiver into a multimode system capable of Radio teletype (RTTY using baudot code), AMTOR (Amateur standard Teleprinting over Radio), FEC (forward error correcting mode baudot), ASCII, packet radio at several speeds, Morse. In receive mode it could also decode weather fax images and more. Much of this functionality can now be performed by software on personal computers, in conjunction with the computer's sound card.

VHF/UHF Digital modes

Like the HF bands, adding a computer to the hamshack can add new modes to the operator's options.

On the VHF bands, the same modes are also feasible but there is a difference in the band conditions. Whereas on HF it is common to have interference from other stations or severe fading of signals due to unusual propagation or "skip" conditions, on VHF signals tend to be received without interference. This has led to the development of digital signal processing (DSP) programs that work in conjunction with sound cards to effectively "dig" signals out of noise. Modes such as JT65 and others have been developed by Joe Taylor K1JT to enable amateurs to have radio contacts with stations they cannot hear with the ear.

This development has made it possible for relatively modestly equipped VHF/UHF stations to make contacts via reflections off the moon, something that was previously the realm of only the best equipped stations, with huge antenna arrays or large dishes, combined with power at the legal limit .

Experiments conducted almost daily by amateurs on 144.3 MHz in Australia during 2007 onwards, have also shown that these digital modes can make possible contacts using reflections from meteor ionisation trails. Signals reflected off these trails can be quite loud but short lived.

There is more development occurring daily in this area so again the reader is recommended to conduct further searches and keep reading.


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