Cross polarised UHF TV transmissions
 

(Further to the other thread)

It's certainly true that when two muxes are received on the same channel the
second one acts like pure noise. It can be confusing because of course the
unwanted mux is hidden behind the other. We had an example of this in Lincoln
last week, where our reception of the Emley Moor mux on ch49 was badly affected
by the Waltham mux on the same channel.

In the case of two co-channel signals of opposite polarity from the same
transmitter I would expect the following problems to arise:

If the receive aerial is partially screened from the transmitter the polarity
of the signals will be distorted. This is especially the case when the
screening is by trees, when polarity can be twisted through 90deg or the signal
can appear to have no polarity.

I doubt if transmitters always emit signals that are cleanly polarised enough
for this to work. It would only need a bounce off a supporting member for there
to be unwanted radiation of random polarity.

For a normal receive aerial to discriminate against an unwanted plane polarity
it has to be at exactly 90deg to it. Even then the discrimination is not really
very good, and the adjustment is so very critical. Other metalwork nearby or
reception on the feeder will have a severe effect. Low intesity multipath that
could otherwise go unnoticed would become significant, since these reflections
often have distorted polarity.

The 'noise' of the unwanted polarity would be in effect added to other noise,
so the c/n ratio would be that much worse. This would, I'm sure, greatly reduce
coverage.

Bill

In article , Bill
wrote:
(Further to the other thread)

It's certainly true that when two muxes are received on the same channel
the second one acts like pure noise.

Apologies, but I can't resist some 'nit picking' for the sake of clarity.
:-)

The problem is, I suspect, that a second MUX in the same channel may *not*
act like "pure noise". Noise will be random, and not structral relationship
with MUX modulation/coding. Whereas a second MUX will have a modulation
and structure which the RX is designed to be able to demodulate. Hence it
may be the case that a given power of a second MUX produces effects which
the same noise level would not.


In the case of two co-channel signals of opposite polarity from the same
transmitter I would expect the following problems to arise:

If the receive aerial is partially screened from the transmitter the
polarity of the signals will be distorted. This is especially the case
when the screening is by trees, when polarity can be twisted through
90deg or the signal can appear to have no polarity.

I'd prefer to avoid the term "polarity" here as in my experience it does
not mean the same as "polarisation". Hence its use may lead to confusion.

I doubt if transmitters always emit signals that are cleanly polarised
enough for this to work. It would only need a bounce off a supporting
member for there to be unwanted radiation of random polarity.

For a normal receive aerial to discriminate against an unwanted plane
polarity it has to be at exactly 90deg to it.

Not quite. For plane polarisations, 90deg would be required for
discrimination that approached 'perfect' (i.e. complete rejection of the
unwanted plane polarisation). However the amount of discrimination (in
simple power ratio terms) will vary smoothly with the angle. How much
discrimination would be needed for satisfactory dual-polarisation
operation, I've no idea, though...

The 'noise' of the unwanted polarity would be in effect added to other
noise, so the c/n ratio would be that much worse. This would, I'm sure,
greatly reduce coverage.

This is the area where I've seen no measurements or analysis, so it becomes
hard to say how much effect it would have.

Slainte,

Jim

--
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Many thanks for the paragraphs :-)
--
fred