In article , Tony
scribeth thus
"Jim Lesurf" wrote in message
...

The problem here, I suspect (again as IIRC Hawker indicates), is that the
broadcasters and set makers were 'promoting' FM for many years and this
was uphill work [pun].
Eh? Sorry I am being thick here! Clue please.

This was fair enough as the competition in those days
was AM, and so FM was pretty likely to be better. But it may mean they
glossed over - and then forgot about - these problems and just how likely
they are. Again fair enough if the choice is FM with some multipath versus
the interference-ridden AM.

I don't remember Pat Hawker's articles, but I have done a bit of work in
the past with a multipath 'scope display. I have to say that I am a bit
surprised that the problem is as severe as you describe. I nearly always
found that multipath could be made acceptable with the right aerial in the
right direction. But I wonder if the change to mixed or circular
polarisation might have made multipath more difficult to get rid of.

As regards the engineering decisions, multipath is not nearly so much of a
problem in mono as in stereo, as it is usually worst for high audio
frequencies with a large S content. So I think the problem mainly arose
with the change to stereo rather than the change from AM to FM. There was
pressure on the BBC to do stereo, and I suppose they thought that anyone who
was interested enough to get a stereo receiver would also get (or already
have) a good outside aerial, which would nearly always be needed anyway for
adequate signal to noise ratio. But I'm sure the engineers never forgot
multipath, which is one reason why they thought DAB was such a good idea.

I expect that mixed polarisation might just make the problem worse.

One of the main reasons for its introduction was for the vertical
component to be used by cars the horiz to be used by fixed aerials.

But one thing it does do especially in urban areas is that when there is
multipath there is sometimes polarisation skew i.e. what was Vertical is
now Horiz and vice versa. Course take a Horiz TX and some reflections
coming as Vertical will now be discriminated against if you see what I
mean..

The good side is that it helps to reduce flutter for mobile systems in
that if the vertical component gets skewed then the horiz one will be -
bent- to Vertical and thus fill in the -missing- as it were....

You can see this on a spec analyser whilst driving around an area that
has mixed and vertical only stations. The mixed is quite stable in level
whereas the single Vert is UTP and down like the proverbial...
--
Tony Sayer