From Dave G3YMC
Thanks for the detailed analysis of TV etc interference to 136.
I see drifty lines with 50Hz and 100Hz spacing, but a quick check at lower
frequencies with teh Rx or SA shows that these are mains modulated signals
with a funamental at 30 to 40kHz and emanate mainly from switch-mode power
supplies, PCs and TVs being the worst offenders.
In fact most TV power supplies operate at a switching frequency of around
70kHz, and
it is the second harmonic content which appears on 136. You usually also find a
strong carrier component at the ninth harmonic of the line frequency
(140.625kHz) and
as the power supply is often locked to the line timebase this will be modulated
with
power supply components and is often found to have sidebands spreading over the
band
(as is the case with my neighbour's wide screen TV). There is not likely to be
much
50Hz component with this radiation unless the power supply smoothing cap is
faulty.
Another gotcha is that loops dont always help, they will couple magetically
to TV and monitor scan coils at quite large distances (10m+)
I had thought my transmit loop was fairly good at not picking up these noise
sources,
and it is the case in many cases. However my neighbour's tv and its coax feed
is
quite close to the loop and clearly there is more pick up in that case - the
long
wire and vertical which are considerably further away pick up much less from
that
source.
Incidentally I still have a page on my website
(http://www.dsergeant.btinternet.co.uk/tvqrm.htm) to collate tv qrm problems and
indicate the ones to avoid. I have had very little input to this page - if you
want
a set listed (good or bad) let me know.
73s Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
| 