Dear Warren, LF Group,
The difference between your signal and the "locals" here is about 50 - 60dB
I guess; G3XDV is more of a challenge, being only about 11km from here. But
previous tests have shown that this kind of signal range should not be
impossible.
Part of the idea of this test was to find a way round the problem of people
wanting to use different sub-bands for different locations. With modern PCs
it is not difficult to display a number of spectrograms from a single
sound-card input. The whole 136kHz band can be accomodated in a normal SSB
filter bandwidth. So several sub-bands can be monitored simultaneously,
using different modes if desired, using a normal RX and PC. The main
difficulty of course is distortion effects caused by very strong signals in
the same passband as weak ones. This is probably best handled by directly
digitising receivers like SDR-IQ and Perseus, or "Softrock" type IQ
downconverters, where there are fewer non-linearities in AGC IF stages,
product detectors or audio stages to contend with. But it also seems to be
possible with ordinary rigs like the IC718 I used here. I selected a carrier
frequency of 135.6kHz USB, adjusted the passband shift to minimise the
levels of DCF39 and HGA22 as much as possible, and backed off the RF gain so
that hardly any sound comes out of the speaker apart from these utilities'
carrier tones at low level. I set the sound card "volume" so that the audio
was well below the clipping level on the Spec Lab input monitor.
Spectrograms can then be configured for audio frequencies centred on 577Hz
and 2180Hz. The noise level seems to be acceptably low, and the distortion
products are not a serious problem, it would seem.
I think using receivers more flexibly is part of the answer to the different
frequency ranges needed for various reasons. Also, I think receiving weak
amateur signals in the presence of strong locals is not as great a problem
as is sometimes made out.
Cheers, Jim Moritz
73 de M0BMU
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