From: Wilbert Knol (hgpq@uiuc.edu)
Date: Mon May 05 2003 - 06:59:46 EEST
> Would electrically isolating the Tx/Rx/PTT signals like in the Rascal
> kit make a significant difference? I read that the isolation is
> important if you want to use packet on HF.
I won't go into details, but there are several reasons why HF digital modes
are so sensitive to RF feedback. Audio isolating transformers often help,
particularly in the TX audio path, but there are other tricks to reduce RFI.
VHF/UHF FM packet is a lot more immune, and I've always managed without audio
transformers.
The down-side of audio isolating transformers is, that they can easily pick up
stray magnetic fields from things like power supplies, from as far as half a
meter or so away. And you won't even hear that through tinny little
loadspeakers, but it will be bad enough to wreck your signal.
The RFI acid test is, to transmit idle flags or calibration tones continuously
whilst you vary TX power. You listen to the transmitted audio on a second
receiver nearby (on a dummy load). Use a pair of good quality HIFI
headphones. If the audio starts sounding raw when you increase power, you
have RFI.
If you kill the TX audio, you'll also hear stray 50 Hz pick-up.
FSK modes are much less susceptible than linear modes such as PSK.
Another thing to look out for: some people use handhelds for packet radio,
that leave the internal condenser mike connected. Shack noise then causes
packet corrupton.
Anyway, all of this has little to do with Linux...
HTH
Wilbert, ZL2BSJ
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