In article ,
Ian Jackson wrote:
My V expensive Blaupunkt doesn't have an aerial trimmer - nor have I seen
one for many a year. Thought most had some form of automatic matching
circuit these days?

In my 'shack', I've got a Philips RD525LEN LW/MW/FM car radio (bought
recently for one GBP, from a stall at a charity sale). The aerial is 5
foot of wire in the attic, with maybe 15 feet of 75 ohm TV coax (braid
connected to the attic water tank) down to the radio.

The coax is much longer than it would be in a car, and it's also the
'wrong sort of coax' (capacitance per unit length will be higher).
Nevertheless, it works very well, with no lack of 'liveliness' at the HF
end of the medium wave (where you normally expect to set the aerial
trimmer, peaking up a weak signal at (typically) 1500kHz.

I've got the instruction/installation manual, but there's absolutely no
reference to any aerial trimmer. There might be some form of 'automatic
matching circuit' but, if there is, what does it consist of? You would
need a varicap diode, driven from the AGC line, and some form of servo
loop which would automatically adjust and optimise the diode
capacitance. It all seems a bit complicated - bearing in mind that a
simple aerial trimmer capacitor has been satisfactory for some 60 or 70
years. I just can't see it myself.

I've not researched it so was only a guess. But I've not noticed quality
radios without a trimmer having inferior MW reception than before. Of
course I'm sure there are plenty of cheap units around that are poor - but
then they'll be poor in other ways too. And not surprising given just how
cheap they are these days. I read somewhere 10 dollars is an average for
factory fitted units...

However, don't most cars these days have an 'active' aerial? So any
matching would be done by that pre-amp? Can't think of any new UK supplied
car with the older telescopic type.

--
*Happiness is seeing your mother-in-law on a milk carton

Dave Plowman London SW
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