First, a big thanks to Simon Lloyd-Hughes, G8AFN, G3XDV, G3RXH, G4FKK, G0MRF
and G3YXM for encouragement, comments and ideas. All very useful stuff which
is helping me to work on the noise problem, and proves the wonderful value
of
this reflector.
I have been quite busy over the weekend, so haven't had much time to persue
things, but here is an update of results so far. Unfortunately, the noise on
our own BC RX is just below the level to cause any noticable interference to
any of the main service broadcast signals (R4, R5 etc.) or the local BBC/IBA
MW stations (whose antennas I can see from here). Virgin 1214 does have some
noise on it, but is only intrusive during speech.
Although it is detectable from below 70kHz to above 20MHz on the TS-850, it
isn't as blanket 'wide-band' as I thought, and more careful tuning has
revealed some noticable peaks and troughs. It is, however, difficult to
detect a regular pattern, so I suspect these are due to resonances in wiring
connected to the source. A very interesting observation is that I was
previously tuning in far too narrow a bandwidth to properly analyse the
signal characteristics. Through a CW or even SSB filter, the noise sounds a
'roaring' hash and spectrally seems random. However, when I switch to AM
with
6KHz filters in, it becomes much more raspy, and feeding the audio into a
'scope reveals a sharp (but wobbly and ringing) pulse with a main repetition
rate of 10mS superimposed with a pulse at 20mS - surprise, surprise 100Hz
and
50Hz!!
Looking in that mode/B-W using a spectrogram shows the following lines:
50Hz weak
100Hz very strong
150Hz strong
200Hz strong
250Hz fairly weak
300Hz fairly weak
400Hz fairly strong
500Hz fairly strong
600Hz fairly weak
700Hz and above at 100Hz intervals, weak
This seems consistent with the 'wandering' nature of the PRF on the 'scope.
I went out again with the Walkman, listening to a peak around 1340kHz, but
still couldn't get a consistent fix on it. Walking around adjacent blocks,
the strength and nulls vary in a confusing way, but it is certainly audible
over a very wide area. What was especially interesting was that as I walked
past the numerous telephone poles around here, the noise level increased
significantly as I passed SOME of them. All the poles where the noise
increased had a wire running down the pole into the ground, but NOT all the
poles that had a drop-wire produced an increase in noise! Whether these are
acting as radiators or merely 'receive antennas' I wouldn't like to say. The
local digital exchange is actually on the corner of my block, but the noise
didn't noticibly peak as I walked past it and the bearing I get from home is
in the perpendicular direction!! I think this is going to be a tricky one to
DF......
If anyone has any further ideas from the above, I'd be really pleased to
hear
from you. The noise was on continuously all weekend, so I have been QRT.
John G4GVC near Leicester, IO92JP
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