AM / FM Antenna Problem No, I'm Not Crazy. :) :)

[hw999]

No, I'm not crazy and 4 or the 5 voices inside my head agree with me.

OK, I hope someone on this forum can help me with a problem I have. I have a new pick-up truck that has an AM/FM radio that works great.

Here is the problem. I also have a large slide-in type truck camper that sets in the bed of the truck, but also hangs out over the cab near the factory fender antenna. It looks something like this:

http://www.lassenrv.com/host camper.jpg

When I put the camper on the truck, my FM reception does not change, but my AM reception is almost completely lost. It’s is at best 10% - 15% of normal. The camper does not actually touch the camper, but the tip of the antenna is about 6” away. The camper exterior is aluminum siding.

I tried putting one of those little rubber ducky short antennas, but it didn’t help.

Now here is the weird part. If I grab the antenna with my hand, the AM reception goes back to 100% normal. Now if I had a really long arm I could ride down the road holding the antenna and everything would be fine, but my arm just ain’t long enough.

To make things even weirder, many years ago when I first noticed the reception improvement when I grabbed the antenna, I tried a few experiments.

I took a short piece of #18 insulated wire and put an alligator clip on the end. I then attached the alligator clip to the stainless steel antenna. I first tried attaching the other end of the wire to ground. That didn’t work. I then tried putting a couple of different resistors I had laying around in the circuit to ground. That didn’t work either.

But then I put a little capacitor I had laying around in the circuit and the AM reception improved to almost normal. In other words, I had an alligator clip on the end of the antenna, then a wire from the alligator clip to the capacitor and then a wire from the capacitor to the truck ground.

Does any of this make any sense? Why would that work? Why would me just grabbing the antenna make the AM reception almost normal?

Anyway, what I’m getting around to is that was many years ago and I don’t have that little capacitor any longer. I don’t remember the rating on it but it seems like it was 4,700 micro farad and 250 VAC? But I could be totally wrong on that. Maybe it was 4.7 micro farads. I just can’t remember.

I’m going down to my local Radio Shack sometime this week and pick up a couple of capacitors and see if I can reproduce my results from 8 or 9 years ago. Any thoughts on this or advice would be greatly appreciated. I am a gear head by trade and struggled to get through Electric Circuits 101 & 102.

HW

 

[prcguy]

What your describing is not all that uncommon. The typical whip used on cars is extremely short (in wavelengths) at AM broadcast frequencies and very high impedance. It will be somewhat sensitive to its surroundings and your new camper has created lots of capacitance to ground, which has detuned the antenna, coax and input circuit of the radio. At FM broadcast frequencies the antenna is much closer to a ¼ wavelength and not as critical to its surroundings. The clip lead and capacitor to ground is basically making the whip into a small loop of very low impedance and I suspect if you take the same clip lead and ground it to the vehicle body without the capacitor it would work about the same as with the capacitor. The capacitor you described would act like a short at these frequencies. Hopefully there is an easy cure. In my earlier days as a radio/stereo installer, AM/FM radios had a trimmer capacitor inside the radio to optimize the antenna and coax on the AM broadcast band. These were factory adjusted and usually not touched unless you replaced the antenna. On radios with a cassette player they were accessible through the cassette slot, otherwise there was a little hole somewhere on the side or back of the radio and I saw a few under the front panel bezel. The capacitor was usually quite obvious and will make lots of crackling noise on the AM band when touched with anything metal. If your radio has this adjustment, it should restore some if not most of the AM performance you had. I would suggest you ask the techs at your car/truck dealer before poking in holes randomly around the radio. Good luck.
prcguy