wrote in message
...

If the picture never breaks up or disappears then the signal is good
enough. It doesn't take measuring equipment to show that.

FSVO "never" (such as "never, ever, ever") perhaps.

If you are a business installing an aerial then it's a whole
different ball game as you can't afford to prove the installation
is good enough by watching it for a week in varying weather
conditions. So measuring equipment is then useful, however not
nearly so useful/necessary for a DIYer.

A couple of points he firstly, many contributors to this thread have said
that they can get DTT reception, despite the postcode database (which does
err slightly on the cautious side) predicting otherwise. Be aware that
signal strength alone is not the only issue. Every UHF channel is re-used
hundreds of times over and coverage is mostly limited by co-channel and
adjacent channel interference from other TV transmitters, rather than by
thermal noise. Now, as many will be aware, UHF propagation is seriously
affected by certain weather conditions which give rise to 'tropospheric
ducting', increasing the signal levels from remote transmitters by large
amounts and causing severe interference[1]. The statistics of this effect
are well-studied and DTT coverage is planned to work for 99% of the time
(c.f. 95% of time for analogue TV). Some of those receiving in areas which
are not officially classed as 'covered' can expect to experience
tropospheric interference for more than 1% of the time.

Secondly, in the absence of any test equipment, a simple test of signal
margin is to insert a 3 dB attenuator pad at the receiver input. If this
leads to reception failure then your signal is somewhat marginal.

OK, but there are not so many areas where the digital MUXs are
transmitted on frequencies a long way away from the analogue ones,

Oh yes there a look at Belmont, Sandy Heath, Wenvoe and Waltham, to pick
four examples of main transmitters which have some of the mux's well outside
the original analogue group.

the frequency planning has tried to minimise that.

And has concentrated on population served: thus Crystal Palace, Sutton
Coldfield and Winter Hill get more-or-less in-group DTT channel allocations,
but for the rest of the country it's much more patchy.

What I was really trying to say though was that if you can get a
good channel 5 signal then you probably can get good digital
reception with a similar type of aerial, quite likely in most areas
the same aerial.

I think you're just introducing a red herring in suggsting that DTT
reception will be correlated with analogue C5 (which is not even radiated
from many TX sites). In any case very few people upgraded their aerials for
C5, so the existing one will most likely match the original 4-channel plan
group.

Is interference a serious problem with digital? Not only that but
does cheap cable *really* let in more noise than expensive cable?
Even at the frequencies involved the mesh of 'cheap' cable must be
pretty well impervious to signal getting onto the downlead, does the
extra aluminium foil of more expensive cable really help in reducing
interference pick up or is it much more to do with reducing
attenuation (or is it even more to do with selling soemthing
expensive unnecessarily)?

No, it's not a con. What Andy L was talking about there is 'impulsive
interference' - interference from sparking contacts - to which DTT is quite
sensitive. Impulses propagate on, and are radiated from, mains wiring, and
thus are much stronger in the house and loft than above the roof (which
provides some screening). A well-screened coax downlead from antenna to
receiver will largely prevent impulses entering the signal path. With poor
screening, a coupling mechanism exits to allow impulse energy into the
receiver, resulting in the familiar momentary 'freezes' and clicks.

Using well screened coax is necessary, but not sufficient. Any gaps in the
screening will allow the interference to get in, and poorly screened outlet
plates and flyleads are the other betes noir. At the other end of the coax,
the lack of a balun on the aerial will also provide a coupling mechnanism.
This is all sound theory, and has been verified experimentally in tests done
under controlled conditions. Oh, and an aerial in the loft is likely to be
close to mains wiring, so is asking for trouble.

With a really well-screened system and a decent receiver you can get solid
reception with signal levels down to 35 dBuV, even on the 64-QAM muxes
(ITV/C4 and SDN). With grotty brown open-weave coax and a typical outlet
and flylead, you may need 45 or even 50 dBuV before the signal is strong
enough to swamp the impulse interference.


[1] http://www.bbc.co.uk/reception/facts...on_weather.pdf

Also relevant:
http://www.dtg.org.uk/publications/books/r_book2.pdf
http://www.dtg.org.uk/publications/b...rk_aerials.pdf

--
Andy