Wednesday, 16 September 2015

Hello Zero, this is Sunray

Ok, so maybe not quite. But I have now obtained a Clansman PRC-351 VHF FM company level manpack radio, which is the very type of radio that 'Sunray', that is, the company commander, would be using. Luckily, I never had to carry one of these too far or often. They might not be as heavy as the PRC-320, but they're not exactly light by modern standards!

As a basic FM radio, its capably of roughly 4W out, on any 25kHz step channel from 30 - 76.975MHz. Frequency is set by thumb switches on the side by the antenna. It therefore covers 6m and 4m, but because those bands use 10/20kHz steps, it doesnt match exactly. This is more awkward on 6m where the FM calling channel is 50.510MHz.

It does have many features that are not immediately useful for amateur radio though. Its two volume settings, Loud and Whisper, also vary the mic sensitivity. On Whisper, the mic is more sensitive and the volume lower, helping avoid being heard by the enemy, perhaps useful for late night ragchewing when the XYL is in bed? Likewise, a pair of terminal posts, and a handy wire stripper built in, mean that the radio can be attached to a remote handset and operated by line from up to 3km away! It can also work as an intercom between the local handset and the remote handset. Perhaps there are some amateur uses for those features!

But these radios are also designed for use as a 'Rebro', that is, a re-broadcast, or repeater, system. Two radios linked by field telephone cable (DON-10) and on different frequencies can be set to act as a repeater automatically, but, to do so require a 150Hz tone. Much like most modern amateur repeaters!

When I got it, it was pretty mucky. Now, contrary to usual thinking, a well bashed and mucky field radio usually means its been reliable in the field. A clean radio always begs the question - why was it left in the comms stores? So, ive given it a bit of a clean.

Because of the 150Hz rebro tone, these radios can get bad audio reports on amateur bands, where the tone isnt expected. In this case, the tone deviation was over 1.7kHz!!! Luckily, its not hard to turn this tone down, which I did by adjusting R9 on module 13, and made it deaf in the process!

Now, the reason for this is, that these radios dont use a normal noise gate squelch circuit, they use a tone squelch, but in order for them to be inter-operable with non-NATO/Clansman users, an internal 160Hz tone is generated that defeats the squelch on the users behalf. By turning the 150Hz Tx tone right down, I'd also turned down the 160Hz internal tone.

This can be corrected by adjusting various tone signal path presets, but to to so would require having the set powered whilst open, something I cant do yet as I dont have a battery extension cable. So, with a bit of intuition from 5yrs servicing public safety radios, I blind set the Tx tone to a low value that I hoped would work and not be annoying on air. With the radio back together, I tested it again, finding now that the squelch worked properly again, and that the 150Hz tone on Tx was now at a comfortable 500Hz deviation.

A check of receive at 70.450MHz shows a sensitivity for minimum discernible signal of about -123dBm, and the squelch break at about -118dBm, pretty much how I used to set Kenwood TK-349s for the prison service! Tx power at this frequency is about 3.3W, reaching a tad over 5W at 30MHz, and a bit over 4W at 76MHz. The only thing I really want to adjust now is the Tx deviation, which is a bit high for my liking and may cause distortion in modern narrow band amateur receivers.

Ive yet to have a QSO with it, and I fully appreciate that its low power means I need to get a good take off location to do so. As for claiming the callsign Sunray - I only manage 'Sunray Minor' in this house...

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