antenna trimming?

[mm]

Thanks for the helpful replies about the car radio that would only get
a couple AM stations. The guy ignored my advice about trimming the
antenna, and just bought another antenna, even though the one he had
had worked fine with the previous radio.

The new radio worked fine with the new antenna. Did he just get
lucky?

Thanks.

 

[mm]

___________________________
You forgot to mention that with age, the antenna coaxial cable would get
water in it, and
reception would suffer. Adjusting the trimmer would make little or no
difference in this situation. Replacement was the only fix.

But it worked fine and got lots of AM stations with the previous
radio. (He only changed the radio because he wanted something
fancier.)

 

[mm]

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?

Well that's related to my point. Maybe there is an automatic
matching circuit in the new radio that's not working, and he just got
lucky that it matched the new antenna. ??

So if he ever takes this radio to another car, again the AM won't work
if the matching circit doesn't work and he's not lucky with the new
car's antenna.


I asked him to read the manual to find out about antenna trimming, but
he didn't reply about that.

 

[Ian Jackson]

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.

 

[mm]

Lets remember that most people don't give a damn about listening
to AM radio anymore,

It's certainly heartwarming to think that conservatives aren't most
people, but even 10% of the market is usually enough to get some
attention by manufacturers.

In NY and Philly are AM stations I would listen to. Not sure about
the rest of the country.

 

[mm]

With the number of stations in the band these days, I find weak signal
performance is rarely the limiting factor. Usually the problem is
selectivity.

This discussion reminds me that my old Mercedes had a momentary contact
built into the power antenna switch that would let you adjust the
antenna length up and down from the dashboard. It was kind of cute but
never seemed to make much difference in reception. (Yet another
example of Germans inventing a problem so that they could solve it, I
think.) The radio itself was an old Blaupunkt and was a magnificent
piece of electronics, though. Vastly overengineered. Automatic
three-stage scan, direct frequency entry, and an alarm clock!

A good accessory. When I'm driving I like to set my alarm clock to
wake me up every 15 minutes.