wrote in message
...

If the screen 'goes blank' then (from my point of view anyway) that's
worse than acceptable. In reality 'interference' on a digital signal
tends to produce artefacts which are recognised by the viewer as
deterioration in the signal.

That's true of impulsive interference or other short-term effects, but
tropospheric co-channel interference (which was the context) will tend to
last for minutes or hours and if its strong enough to push you over the
'digital cliff' then you will get a blank screen (or frozen frame, or BSofD,
depending on the decoder).

You said "... end of the coax, the lack of a balun on the
aerial ...",
are you then saying that there should be a balun at the aerial end of
the coax? I thought that most aerials have a natural impedance of
around 75 ohms which is why 75 ohm co-ax is used.

A half-wave dipole has a 'natural impedance' of 75 ohms and that's what gave
rise to 75 ohm coax originally. 75 ohms is also the impedance at which an
air-spaced cable of a given o/d has minimum loss (c.f. 50 ohms for a solid
polyethylene dielectric cable).

A TV Yagi doesn't have a natural impedance of anything in particular since
adding reflector and directors tends to lower the feedpoint impedance; in
particular the spacing between the driven element and the first director can
have a dramatic effect. The design of TV antennas is still a largely
empirical black art (despite some impact from CEM techniques) and involves a
juggling act between impedance, gain and bandwidth. Just optimising for
maximum gain would give narrow bandwidth and an impractically low feedpoint
impedance.

Are you just saying that there should be a 75 ohm to 75 ohm (as
opposed to imedance matching) balun at the aerial because the aerial
is balanced and the coax isn't?

Almost -- I'm saying that there should be a balun at the aerial feedpoint
because the aerial is balanced and the coax isn't. The balun's impedance
ratio could be 1:1, or 4:1 or something else and that's a matter for the
designer that doesn't particulary concern the user.

Will this actually provide much improvement? If so it's
a fairly simple and cheap thing to do.

Yes, it provides a demonstrable improvement in immunity to impulsive
interference, especially if the downlead goes through the loft near to mains
wiring. To pass the CAI & DTG's 'benchmark' tests an antenna has to pass a
'feeder pickup' test, whcich amounts to a test of the effectiveness of the
balun. See
http://www.dtg.org.uk/publications/b...rk_aerials.pdf
for further details.

--
Andy