Split coax signal from aerial to Set Top Box?

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dave

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Jul 29, 2007, 6:09:41 PM7/29/07
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I'd like to feed two STB's from the one aerial on the roof. Is it ok
to just splice the coax's that feed the STB's near the aerial? Or will
that result in a poor signal to the STBSs?
Thanks

Peter Crosland

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Jul 29, 2007, 6:24:16 PM7/29/07
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It depends on the signal strength at the aerial. Ask Bill Wright in
uk.tech.digital-tv for more details for a more detailed answer


Peter Crosland

g6...@yahoo.co.uk


meow...@care2.com

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Jul 29, 2007, 6:55:20 PM7/29/07
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no, use a splitter. That will deal with the impedance issue, thus
avoiding unnecessary signal loss and distortion due to reflections
etc. Theyre only 50p.


NT

The Natural Philosopher

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Jul 29, 2007, 8:22:25 PM7/29/07
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and are lossy..

better is a small two port distribution amp. Probably 20 quid or so.

>
> NT
>

Lurch

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Jul 29, 2007, 8:30:18 PM7/29/07
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On Sun, 29 Jul 2007 15:55:20 -0700, meow...@care2.com mused:

Mybe, depends what the signal is like in the first place.

As ever, a completely incomplete answer, basically just bollocks.
--
Regards,
Stuart.

Graham.

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Jul 29, 2007, 8:41:04 PM7/29/07
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<meow...@care2.com> wrote in message
news:1185749720.8...@b79g2000hse.googlegroups.com...

Hope you are taking notes Peter,
HI HI de G3ZVT

--
Graham.
%Profound_observation%


Peter Crosland

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Jul 29, 2007, 8:48:51 PM7/29/07
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I refer the honourable gentleman to the answer I gave earlier!

Peter Crosland

g6...@yahoo.co.uk


Mark Carver

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Jul 30, 2007, 4:07:39 AM7/30/07
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Peter Crosland wrote:
> Graham. wrote:

>> Hope you are taking notes Peter,
>> HI HI de G3ZVT
>
> I refer the honourable gentleman to the answer I gave earlier!

You should *never* split any RF feed by electrically paralleling it.
You'll create a mismatch, and risk generating standing waves in both of the
down leads.
This could give all sorts of problems, such as some channels noisier than
others, and garbled teletext reception for analogue. For DTT you could end up
with sections of the CODFM signal in one or more muxes notched out,
effectively rendering the whole signal unusable.

You must use either a splitter, or some form of distribution amplifier.

--
Mark
Please replace invalid and invalid with gmx and net to reply.

The Natural Philosopher

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Jul 30, 2007, 6:22:31 AM7/30/07
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Mark Carver wrote:
> Peter Crosland wrote:
>> Graham. wrote:
>
>>> Hope you are taking notes Peter,
>>> HI HI de G3ZVT
>>
>> I refer the honourable gentleman to the answer I gave earlier!
>
> You should *never* split any RF feed by electrically paralleling it.
> You'll create a mismatch, and risk generating standing waves in both of
> the down leads.

Not the risk, the actuality.

However in many cases the riposte 'so what?' elicits the actual answer,
'er, not much really'


> This could give all sorts of problems, such as some channels noisier
> than others, and garbled teletext reception for analogue. For DTT you
> could end up with sections of the CODFM signal in one or more muxes
> notched out, effectively rendering the whole signal unusable.
>

You MIGHT. Then again you might not. Mismatching a cable with a - say -
38ohm double load instead of 75ohm may actually give you a few -6db
nulls, bit not much worse than a passive attenuator/spillter does to ALL
signals.


> You must use either a splitter,

And degrade ALL signals 6dB IIRC..

or some form of distribution amplifier.
>

which is, as I said, the best solution. But costs more.

Mark Carver

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Jul 30, 2007, 7:07:18 AM7/30/07
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The Natural Philosopher wrote:

> You MIGHT. Then again you might not. Mismatching a cable with a - say -
> 38ohm double load instead of 75ohm may actually give you a few -6db
> nulls, bit not much worse than a passive attenuator/spillter does to ALL
> signals.

You can get sharper nulls than 6dB, and I've also seen teletext reception
totally obliterated on one or more channels, whilst the accompanying TV
pictures look perfect. But like you say it's a lottery.

>> You must use either a splitter,
>
> And degrade ALL signals 6dB IIRC..

Yes, but in a uniform manner. If you've got gallons of signal, it's no
problem, and even desirable to avoid overloading the tuner(s) if you're very
close to the transmitter.

> or some form of distribution amplifier.
>>
> which is, as I said, the best solution. But costs more.

Indeed.

The Natural Philosopher

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Jul 30, 2007, 7:13:00 AM7/30/07
to
Mark Carver wrote:
> The Natural Philosopher wrote:
>
>> You MIGHT. Then again you might not. Mismatching a cable with a - say
>> - 38ohm double load instead of 75ohm may actually give you a few -6db
>> nulls, bit not much worse than a passive attenuator/spillter does to
>> ALL signals.
>
> You can get sharper nulls than 6dB, and I've also seen teletext
> reception totally obliterated on one or more channels, whilst the
> accompanying TV pictures look perfect. But like you say it's a lottery.
>

With an open circuit sheath,yes..

>>> You must use either a splitter,
>>
>> And degrade ALL signals 6dB IIRC..
>
> Yes, but in a uniform manner. If you've got gallons of signal, it's no
> problem, and even desirable to avoid overloading the tuner(s) if you're
> very close to the transmitter.
>

Not if you are where I am yo.

Andy Wade

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Jul 30, 2007, 8:36:03 AM7/30/07
to
The Natural Philosopher wrote:

> Not the risk, the actuality.
>
> However in many cases the riposte 'so what?' elicits the actual answer,
> 'er, not much really'

True up to a point, but since the aerial and receivers/tuners also
present significant mismatches, there's all the more reason to use a
reasonably well-matched splitter. SWR ripple tends to build up
alarmingly when there are several mismatched devices on one run of coax.
(Read up on the flow-graph technique and work out some examples for
yourself.) Decent hybrid splitters are readily available[1], so there's
no excuse for not doing the job properly.

> You MIGHT. Then again you might not. Mismatching a cable with a - say -
> 38ohm double load instead of 75ohm may actually give you a few -6db
> nulls, bit not much worse than a passive attenuator/spillter does to ALL
> signals.

The dips can be a lot more than that. The worst case comes when one of
the split outputs is unused and is left open. A simple parallel
connection provides no RF isolation between the outputs, so at
frequencies where the open leg happens to an odd multiple of an
electrical quarter-wavelength long the junction will see a reflected
short circuit giving, in principle, an infinite notch. It's not quite
that bad in practice, because of the finite loss of the cable, but you
can get nulls of 10-20 dB.

A proper hybrid (aka "inductive" splitter) gives typically 20 dB
isolation between the output legs, and largely overcomes this objection.
It also helps reduce mutual interference between receivers resulting
from local oscillator leakage from the aerial inputs.

> And degrade ALL signals 6dB IIRC..

About 4 dB in practice for a 2-way splitter.

> or some form of distribution amplifier.
> which is, as I said, the best solution. But costs more.

If (and only if) there is enough signal, a simple passive splitter is
the better option.

[1] Such as
http://www.blake-uk.com/prod_products_proception_screenedmastheadsplitters.aspx
(in which I will declare having a vested interest).

--
Andy

tony sayer

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Jul 30, 2007, 9:36:57 AM7/30/07
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In article <46addbb2$0$15208$fa0f...@news.zen.co.uk>, Andy Wade
<spamb...@maxwell.myzen.co.uk> writes

>The Natural Philosopher wrote:
>
>> Not the risk, the actuality.
>>
>> However in many cases the riposte 'so what?' elicits the actual answer,
>> 'er, not much really'
>
>True up to a point, but since the aerial and receivers/tuners also
>present significant mismatches, there's all the more reason to use a
>reasonably well-matched splitter. SWR ripple tends to build up
>alarmingly when there are several mismatched devices on one run of coax.
> (Read up on the flow-graph technique and work out some examples for
>yourself.) Decent hybrid splitters are readily available[1], so there's
>no excuse for not doing the job properly.
>
>> You MIGHT. Then again you might not. Mismatching a cable with a - say -
>> 38ohm double load instead of 75ohm may actually give you a few -6db
>> nulls, bit not much worse than a passive attenuator/spillter does to ALL
>> signals.
>
>The dips can be a lot more than that. The worst case comes when one of
>the split outputs is unused and is left open. A simple parallel
>connection provides no RF isolation between the outputs, so at
>frequencies where the open leg happens to an odd multiple of an
>electrical quarter-wavelength long the junction will see a reflected
>short circuit giving, in principle, an infinite notch. It's not quite
>that bad in practice, because of the finite loss of the cable, but you
>can get nulls of 10-20 dB.


In fact this week we made up a filter to get rid of Band one frequencies
for a studio to transmitter link for a local radio station. The TX
system was overloading the aerial distribution amp and a couple of open
circuit notch stubs measured at 30 dB down WRT to band 2 88-108 !..

Complete with some harmonics up to UHF!...
--
Tony Sayer


meow...@care2.com

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Jul 30, 2007, 9:51:20 AM7/30/07
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On 30 Jul, 01:30, Lurch <myrealn...@sjwelectrical.co.uk> wrote:
> On Sun, 29 Jul 2007 15:55:20 -0700, meow2...@care2.com mused:

not sure what your issue is really.

A splitter is the usual good practice way to split an ae feed. If the
feed needs amplification, and if we're assuming a standard domestic
system, then the amplifier should go as near the ae as possible, which
is not generally at the split point.

Of course things get a fair bit more complex if you're looking at
large scale distribution, but that is going beyond standard domestic
ae systems.


NT

Phil B

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Jul 30, 2007, 12:08:02 PM7/30/07
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"The Natural Philosopher" <a@b.c> wrote in message
news:118575494...@demeter.uk.clara.net...
provided it doesn't overload the signal. I had to fit an attenuator on the
input of the amp here about 10km from my local transmitter.

Phil


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