Car radio fade on AM - HELP

I have had a problem w/ my friend’s car radio that he’s been living with but it is getting to him. When he drive in certain places AM will fade in and out. If he stop in a ‘fade out’ zone it won’t get much until he move (it won’t fade back in). This seems to happen with the car on or off. The radio has been replaced (due to other reasons) but didn’t fix the problem.

Should the antennia be replaced? run and external ground? something else?

These places where the fade is is a little harder to get a signal then others but all cars I’ve used except for his has no problem.

Modern car radios (like most electronics of the last 10 years) are total crap. They weren’t designed to get weak signals since they knew that the average American Idiot Consumer wouldn’t notice.

Just double check the antenna connetion. Make sure that nothing is grounding the (inner) antenna wire. Salt and dirt on the antenna mast and base can cause problems. You can get a $20 antenna booster but it also boosts noise.

It’s not noise so much as loss of signal. I did notice if I touch the antenna in a ‘fade out’ area I will get much improved reception.

AM radio seems much more sensitive to antenna problems than FM. I’d repair or replace the antenna in the hope that it will solve, or at least improve, the problem.

What kind of car is it?

You just answered your original question. The antenna is junk. They can be easy to change or a pain in the butt. Unplug it from the back of the radio and pull it out as much as possible from under the dash. This should get you to the area where the antenna goes through the firewall or cowl. Tie a heavy cord or string to the end ot the antenna then tape it on with one wrap of electrical tape. Loosen the mounting nut from the antenna and remove it, then lower the antenna in the whole enough to turn the anchors. Pull out the old antenna and carefully pull out the coax lead. Cut off the cord when it comes through the mounting hole.

Follow the instructions for the new antenna. To get the new coax wire into the car, tie the cord on good and tight then a single layer of electrical tape, wrapping the tape tightly. A little light grease on the tape will help. Carefully pull in the new lead feeding the wire as you pull. Don’t just lower it all into the fender then expect to pull it through. Once the wire is into the passenge compartment, route the wire to the radio and complete the installation per the manufacturers recommendation.

Please keep in mind that touching an antenna with your hand can cause damage.

The most common failure of the receiver portion of a car radio is caused by scenarios like this:
Person slides out of car, getting a nice static buildup on themselves.
Brushes against the antenna, the spark is more than enough to fry the ultra-sensitive to low current first transistor off the antenna in the radio.

This is common enough that sometimes there’s a capacitor on the antenna or in the radio to block the DC surge of the spark. But since this adds .5 cents to the cost, not everybody does it.

So perhaps the transistor is partially weakened (not an easy fix) or the capacitor is bad, if it has one, which is only really fixable if it’s part of the antenna wire. In which case getting a new antenna will work.

Caution to Dopers regarding “Trying this trick at home.” If you for some reason need to touch the antenna, ground yourself first (to the car body).