Re: Shielded coax cable

In message <i9YrB9IJSRELFwhY@xxxxxxxxxxxx>, tony sayer <tony@xxxxxxxxxxxx> writes
In article <jVSDkYOEfCELFwzn@xxxxxxxxxxxxxxxxx>, Ian Jackson <ianREMOVET
HISjackson@xxxxxxxxxxxxxxxxx> scribeth thus
In message <50c0df2980charles@xxxxxxxxxxxxxxxxxxxxxxx>, charles
<charles@xxxxxxxxxxxxxxxxxxxxxxx> writes
In article <hep1d5$bl5$1@xxxxxxxx>, jamie powell <jamie_p84@xxxxxxxxxx>
wrote:

"John Rumm" <see.my.signature@xxxxxxxxxxxx> wrote in message
news:apWdncasfrHuyZLWnZ2dnUVZ8r5i4p2d@xxxxxxxxxxxxxxxxxxx
> jamie powell wrote:
>
>> However, for future reference, is there a well-shielded / high-grade
>> type of UHF coax cable which doesn't unintentionally pick up off-air
>> signals in strong signal areas?
>
> There are better ways...
>
> http://www.wrightsaerials.tv/resources/pre-echo.pdf


Well that's slightly more relevant. However, ignoring the issue of
tap-off units (which aren't present in my example), it only suggests: 1
- increase the amplifier gain to smother the pre-echo 2 - use CT100 cable
3 - use a decent outlet plate and flylead

The house I visited did have what appeared to be CT100 - there was gold
coloured foil wrapped around the dielectric, and plenty of outer braid.
It was fairly old though, apparently dating from 1993. There was no
outlet plate or flylead - the external coax ran directly into the back
of the TVs (TVlink units when I'd installed them).

The amplifier I fitted had (iirc) 8db gain, but obviously with the weaked
aerial reception, overall gain was compromised.

I figured someone might have designed an ultra-well-shielded type of
coax, specially to deal with the pre-echo issue, hence my original
question.

It's always possible that the direct (pre-echo) are getting straight into
the set without bing picked up by the downlead.

But yes, there is much better cable than CT100 - you do need to pay quite a
bit, though. Look at the pages of RS Components to see what is on the
market. URM67 is a good starting point.

No matter HOW good the cable screening is, it can't protect against
direct pick-up in the receiver itself.

Yeabut what about the effect of the receivers AGC which should turn down
the sensitivity in the presence of stronger signals?....

I've said here before that, in
the 1970, on the top floor of a 5-storey building in Hayes, Middlesex,
the lab 13" Sony received 19 locked UHF TV channels with no aerial
plugged in (and I don't think that plugging in a termination made much
difference). [And I believe that was in the days when we had only three
channels per transmitter.]

At the time, direct pick-up was considered to be the main cause of
left-hand ghosts, rather than poor cable. These days, TV sets and
especially STBs are probably screened a lot better, and the cable is
more likely to be the culprit.

I relate this tale simply to indicate how sensitive and prone some TV sets are/were to direct pick-up.

Yes, in the presence of a (relatively) strong signal arriving via the coax, the TV AGC will wind down its gain in order to present the required (and constant) signal to the detector. However, the left-hand ghost is still there in the background, XdB down.

If any ghost image is sufficiently separated from the wanted image, it will be just visible if X is about 40dB (regardless of what the separation is).

If the separation is reduced below a certain critical amount, the ghost starts to become less visible unless X is also lowered (ie the relative level of ghost is increased).

As the separation between ghost and the signal progressively approaches zero, X also reduces to zero, ie the ghost can be as strong as (or actually even stronger than) the wanted image.

There used to be a 'standard' graph showing the requirement for the (in)visibility of ghosting (separation vs signal/ghost ratio). Somewhere, I have a paper copy from the Dutch PTT, but I've never managed to find one on the internet.
--
Ian
.

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