Re: Is 50m too far....


"SatCure" <SatCure.3b853b8@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:SatCure.3b853b8@xxxxxxxxxxxxxxxxxxxxxxx
2. CT100 cable ceased manufacture when the Raydex company closed in
2005. The direct replacement is WC100 but WF100 has a solid "foam"
dielectric and is more resistant to kinking and rainwater. There are
other "equivalents" such as TX100 (by Triax, which is fine) and some
Chinese imports which may be questionable. The price is generally
governed by the price of copper so "cheap" usually equates to no copper
screen.
Yes, we have a curious situation in which the name 'CT100' has become
enshrined in thousands of specifications trotted out daily by construction
consultants and the like, yet the actual product doesn't exist. The thing is
though, that lots of people understand what 'CT100' means.


3. Cable attenuates the high frequency signals according as its length.
For a Sky minidish in the London area a sensible maximum length might be
40 metres before "rain drop-out" may occur to often. In other parts of
the country, 30m might be nearer the mark. The best way to compensate
for a long cable run is to use a slightly larger dish, such as the
"zone 2 Sky dish". From a distance, it looks like a minidish so you
shouldn't raise too much excitement with the neighbours!

I must take slight issue with you here. Signals from a minidish LNB are
likely to be only 11 to 14dB above the LNB's noise (plus a very small
contribution from sky noise). However the signals will be perhaps 30dB above
the level where receiver noise would become significant. To all practical
intents and purposes this means that the s/n ratio is set at the LNB, until
cable losses cause the signals to decrease by perhaps 15dB. This works out
at 50m of cable. To be really safe though a 50m run would benefit from a
line amp, if only because an amp with 'slope' will equalise the signal
levels across the band.

The relationship betwen rain drop-out and cable length is not so clear cut
as you suggest. The fundamental reason minidishes are prone to rainfade is
because they are not big enough. This means that the s/n ratio, at around 11
to 14dB, is only a gnat's knacker above the point where errors start to
overwhelm the decoder, this point being about 8 or 9dB. Sky dishes are
designed to work for most of the time, but not all the time. Anyone who's
familiar with installing good quality 60 or 80cm dishes will know that
gloomy feeling when they install a minidish and can only obtain BER and s/n
reasings that are by normal standards marginal, and thus unacceptable. So
rainfade occurs at the dish, where the s/n ratio is set. The clouds cause
the signal level to decrease marginally, but because the signal is only just
strong enough anyway to produce a useable s/n ratio this small reduction is
catastrophic. Subsequent cable losses are almost irrelevant, as long as the
signal remains strong enough at the bottom end of the cable to be above the
receiver's threshold. The problem with your suggestion that a larger dish
will compensate for a long cable and thus decrease rainfade is this: the
larger dish will produce signals that are perhaps 1 or 2dB stronger than the
smaller one, and thus will help a lot to improve the c/n ratio at the dish
and thus reduce rainfade. But this has nothing to do with cable length,
except that the signal, being 1 or 2dB stronger, will travel for a further 3
to 7 metres in the cable before its level is low enough to matter at the
receiver. In other words, in a situation where rain fade is on the threshold
of occuring, increasing the dish size will of course reduce it, but it will
only allow the cable to be 7 metres longer at best compared to a normal
minidish for the same results on the screen.

I have an installation to look after which provides evidence of this. To cut
a long story short the downleads from the switches to the flats vary from 5m
to a mind boggling 70m. (Yes the installers were pillocks; there were many
many other faults). When I first got involved the complaints were coming in
thick and fast. The 90cm dish was actually bent, as I was to discover.
Signals entering and leaving the switches had almost normal levels but the
BER and s/n were marginal. When it rained reception of about 75% of the
channels was unwatchable. The interesting thing was that that there was no
corelation between the complaint frequency and the downlead length. The ones
near the head end were complaining just as much. Incidentally, after we
fixed the dish and did a few other bits and bobs I issued a disclaimer: "I
haven't turned this into a good system; I've merely done a cheap fix. Those
with long cables will still have problems.If you want a complete solution
you had better start saving up." In fact, only two out of about fifteen of
the long downlead victims have complained, and satellite penetration is
getting on for 100% because the DTT is crap. Of those two, one didn't use
terrestrial so I went along and shoved a line amp on his switch output and
that solved it. Of course the whole thing is a horrible botch, but no-one
wats to pay for a proper solution. Oh, the other one decided to put up with
it when the agent told them they wouldn't pay. Bit sick in your new £250k
flat, now worth about £180k, and no Sky+ either!

In reality noise adds up, so in a marginal situation a rainfade-affected
LNB output, when compared to a non-rainfade affected LNB output (with both
sets of signal levels the same at the receiver) will pixellate first.
But this is a very marginal effect. To be practical, the only answer to
rainfade is to use a bigger dish. But the only answer to cable loss is to
use a line amp. Personally I'd want to use a 60cm dish anyway, and add a
line amp if necessary.

If it's impossible to use a larger dish, you could insert an
equalised-slope amplifier into each cable. However, this adds
significantly to the cost, adds noise to the signal, and isn't usually
a good answer.
Line amps work very well indeed when they are inserted at the right place on
the cable. A rule of thumb is to put them 20m from the LNB, although I have
used tham right next to the LNB. The amp's own noise is negligible as long
as the input levels are healthy. It's very hard to overload a decent line
amp. Here's a practical example:
http://www.wrightsaerials.tv/ourwork/021.html
In this case the signals available at the head-end were about 12dB below the
LNB output level. The head-end included four pre-amps to lift the signals 16
to 20dB, so that when they entered the multiswitches having passed through
six-way splitters they were at roughly the normal ex-LNB level. Incidentally
I wasn't pleased when I arrived at this site to discover that the
electricans had run in the very long downleads in CT100 (I would have
specified CT167) and had made it impossible for me to put line amps anywhere
exect at the very top. Not ideal at all.

Bill


.

Relevant Pages

  • Re: multiswitches (or not)
    ... I too am in a position where I am contemplating a multiswitch. ... have another dish with two LNBs aligned on Astra1/Hotbird 13E/19E. ... with about the same number of cables running into the house but would have more ... For the 28.2E signals I have a 1m dish & don't lose signal even in the heaviest ...
    (uk.tech.tv.sky)
  • Re: Dish for a Community
    ... satellite companies (such as DirecTV and DishNetwork) REQUIRE you to use an MDU installer for a community dish. ... Additionally, you may want to use existing cables, and the quality of the cable plays ... if you have 2 cables running to each unit, you can stack the signals ...
    (rec.video.satellite.dbs)
  • Re: DirecTV cabling
    ... I'd like to add DirecTV to this "home run" set-up. ... Does the dish have its own multiswitch for ... converging signals from the various satellites? ... cables to run from the dish to the multiswitch. ...
    (rec.video.satellite.dbs)
  • Re: B&B 11 bedrooms and Sky
    ... Your current tv cable almost certainly isn't satellite. ... >7) You can feed sat signals over ordinary tv cables for limited ... If using a large 90 cm dish - usual for communal ... In the case of very long downlead cables ...
    (uk.tech.tv.sky)
  • Re: Speaker Wire advise pls
    ... speaker can be moving at different speeds and directions. ... so the signals don't actually mix. ... But they start out the same at the amp. ... best suited to the driver is transposed into simply using dual cables. ...
    (uk.rec.audio)
Loading