In message , "Dave Plowman (News)"
writes
In article ,
Michael A. Terrell wrote:
Most US car radios allowed the adjustment by removing
the tuning knob and using a small screwdriver to adjust the capacitor. A
lot of cheap imported radios had no adjustment, and had very poor AM BCB
performance.

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.
--
Ian