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SAT-Usenet 16/98

SAT-Usenet 
16/98 
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- SOMETHING ABOUT RECEPTION PROBLEMS AND CABLE LENGTH

On Thu, 23 Apr 1998 12:33:27 +0200, Lio FROST-AINLEY
<Lio.Frost-Ainley@cern.ch> wrote:

>Here's a problem I've had that I dont understand.
>
>I've just moved house and I was trying to locate the best site for my
>dish. I had a 100m run of cable and found the best spot to see the
>maximum of the Clarke belt. All the channels I expected to get came in
>fine, except that from Telecom 2B the signal for TF1 at 12.690V was very
>disturbed. Signals at higher frequency (e.g. FR3 @ 12.732V) were fine as
>were signals at lower frequency (e.g. Arte at 12.606V).
>
>Not being an electronics expert, I found this odd. The same equipment
>had worked fine at my previous house. The only thing that had changed
>was the length of cable. Since I had now found the ideal spot, I no
>longer needed the whole 100m run. When I cut it to c. 55m, the signal
>for TF1 was perfect again. So obviously that was the cause. 
>
>Can someone explain (in laymans terms) why I experienced this phenomenon
>? Also with my 55m cable, am I likely to have similar problems at other
>frequencies ?
>
>Thanks in advance
>
>Lio

There are several possibilities but the most likely would either be a
sharp bend in the cable or, if no bends, a kink during fitting that
was subsequently straightened out but left the inner part of the cable
deformed. Alternatively, a defect in the cable letting in moisture.
These could all give problems at certain frequencies - you just have
to hope you cut off the bit with the defect. Long lengths will
attenuate a signal and the degree of attenuation will vary with
frequency  but I'm not sure that this would just affect one 'spot'
frequency,  unless the lnb has a dip in its frequency response which
becomes evident when the signal level is borderline- perhaps some
installers can enlighten us.

I've just replaced my 50m run with CT125 - it appears to have made my
digital receiver's picture sharper but I'm not sure if I'm imagining
it - I can't see offhand how this would happen.

John Rainer

John Rainer <jrainer@netcomuk.co.uk> wrote in article
<353f78b4.1389394@nntp.netcomuk.co.uk>...
> On Thu, 23 Apr 1998 12:33:27 +0200, Lio FROST-AINLEY
> <Lio.Frost-Ainley@cern.ch> wrote:
> 
> >Here's a problem I've had that I dont understand.
Probably a standing wave in the cable from some damage or defect.
> 
> I've just replaced my 50m run with CT125 - it appears to have made my
> digital receiver's picture sharper but I'm not sure if I'm imagining
> it - I can't see offhand how this would happen.

Replacing the cable will reduce the losses, so more signal will arrive at
your receiver, also there will be less dispersion in CT125 than in CT100
and thinner cables.

Probably this results in a lower error rate in the digital stream (the gaps
would be interpolated by the decoder or just not applied (in MPEG)), so
your picture will look better, because more of the information to produce
it is there.

If you have Corel Photo House or similar you can see what I mean by looking
at the results of altering the settings on the JPEG save - the lower the
info content the mushier it looks, but no holes or stripes the whole
picture degrades in resolution.
> 
> John Rainer 
> 
-- 


R. Mark Clayton

MClayton@btinternet.com





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