From: mm on
On Sun, 8 Nov 2009 21:26:28 -0500, "James" <jnipperxxx(a)>

>MM wrote:
>>The car radio won't get AM unless one touches the antenna!
>>An internet acquaintance who still doesn't understand Usenet bought a
>>new radio for his Chrysler LeBaron, but AM reception was verrrrry low
>>volume, so as to be unusable. (FM is fine.)
>>He took it back to BestBuy and in the process of working on it, the
>>tech put his hand on the antenna and the volume went to normal and
>>reception was good.
>>He and the tech have concluded that he needs a new antenna, in place
>>of the one that has been there since the car was built, between 1990
>>and '95.
>>What are the odds this is true?
>>Or is the new radio likely defective?
>>Before 1965, I think it was, car radios had to have their antenna
>>"trimmed" to get good reception, by means of an antenna trimming
>>variable capacitor. In the Olds, there was an enlargement of the hole
>>under the on/off knob that gave access to a screw that adjusted the
>>trim. If touching the antenna increased radio volume, that meant the
>>antenna needed trimmming.
>>This is all I know about the electronics involved, but that may be
>>more than anyone else on the Chrysler yahoo list.
>>Is there a circuit that does this automatically today, that could be
>>broken in this new radio? Or has the need for trimming somehow been
>>eliminated? Or could the antenna be so bad that the radio can't
>>handle it.
>>Thanks for any help you can give him.

>Most likely, there is something wrong with the car antenna connections,
>either at the radio itself, or at the chassis point where it is mounted.
>It sounds like the antenna isn't property grounded.
>I would first have him check and double check the plug point where the
>antenna plugs into the radio. Make sure the plug is pushed all the way in.
>Also, be aware that on many car radio setups, there is a 2-3 foot antenna
>jumper wire which terminates in a female plug, and then then car antenna
>male plug fits into that female plug. That joinder should also be checked
>for tightness.
>Lastly, the antenna mount should be checked , to make 100% sure that it is
>grounded to the car chassis. If possible, remove the mount, scrap the paint
>or crud from the car body, and remount the antenna.
>Hope this helps. Please report back.

Sorry it took me so long. (that's why I restored my original question)
I was asking an open-ended question for someone else on a mailing
list. He wanted a new fancy car radio but the one he got had very low
volume, on AM, unless he touched the antenna. That sounded to me like
the antenna needed trimming (GregS made a reference to that.) and I
wanted him to read the manual and if it gave instructions for
trimming, to trim it, but he didn't get back to me on that.

Eventually he took it back to the store, which had installed it (at
first, he didn't tell us he'd had it installed) and they put in a new
antenna, and I'm sure charged him for that. Yet the antenna had
worked fine with the previous radio. I think one possibility was
that it was just chance that the new antenna worked with the new radio
and the old one didn't.

Or.... Apparently most radios don't have manual antenna trimming
anymore, but there is still automatic trimming needed to fit various
antennas. Maybe his radio was broken, or just didn't have that
circuit and only works with certain antennas. The latter is hard to

Anyone know more about this?

Thanks to you and to GregS for your replies.