Strange AM radio interference, improving reception?

[lferranteSWL]

Hello, new member with a quick question.

My AM radio antenna in the car (1997) has obnoxious sounding interference when passing under freeway overpasses or by bridges. There is a sample clip below. You can really hear it at the 0:08 second mark. It's much louder than how it sounds in the clip.

What I'm confused about is that I had another car (2005) that when passing through the same areas didn't exhibit this sound, it just sort of 'cut' it out or muted it. I guess this is due to the DSP in the newer radio? Or the antenna type? I don't know.

Here is the clip, is there any way to improve the signal? Like a ferrite core/bead, or something simple along those lines (I'm not extremely electronically proficient) - I'm not looking for a 6 foot antenna on the roof. The newer car with a window antenna worked, so any way to make the 1997 work?

Thanks

[W3GMS]

It sounds like based on your explanation that its always there when you drive through weak signal areas.  So it's probably not related to the road locations other than they may be weak signal locations.  It may be a locally generated noise within your car that the radio is hearing.  Its a strange sounding digital type encoded noise.  So do you hear it just on one frequency of the AM band or do you hear it all across the dial?  Also, as a test try and see if you hear the noise with the car engine not running and the ignition switch just in the ACC position.   From those observations maybe it can be zeroed in on a bit better. 

Joe W3GMS     

[flintstone mop]

Sounds like digital noises from the many on-board computers is today's cars. You were listening to AM radio? Not much you can do about that.
You might get lucky and find it and be able to use ferrite chokes to stop the interference.

[flintstone mop]

The window antenna car is a GM product? Your screen name and no call doesn't give any hint if you are in USA or elsewhere.
I do not know if other manufacturers use window antennas.
GM cars had a lot of engineering in the design of the radio, even the AM side. A front end and RF amp. And AM/FM pre-amp for the window antenna. I had a '97 Buick and the REAR window had built-in antenna and AM/FM pre-amp in the headliner. Perfect silence from electronic noises, and even played AM under bridges. Small fade-out, sometimes

REAR window antenna moves the antenna further from the engine and on-board computers. Only hope might be to drill  a hole in the 2005 and mount the antenna on the rear of the vehicle. But that's not a guarantee to get rid of the noises.

[lferranteSWL]

W3GMS - it happens when the ignition is in ON an ACC. Forgot to test other bands, just that's KFI 640AM.

I'm located in the US. Both cars were Lexus.

I guess the older 1997 just had a poorly designed AM antenna? It has the regular vertical motor assisted type.

The 2005 had a window antenna (in the glass)

[W3GMS]

W3GMS - it happens when the ignition is in ON an ACC. Forgot to test other bands, just that's KFI 640AM.

I'm located in the US. Both cars were Lexus.

I guess the older 1997 just had a poorly designed AM antenna? It has the regular vertical motor assisted type.

The 2005 had a window antenna (in the glass)

Ok, so if understand you correctly its only on one frequency and that is 640KHz.  With the ignition is on the ACC you still have the cars computer running.  Since it happens at multiple locations out on the road in weak signal area's, I think its very likely noise generated within the car.  The fact that your only hearing it on 1 frequency tells me its likely a harmonic of  some oscillator within your cars computer system.The only other remote thing that may be happening if the particular noise is a result from something being transmitted near the bridges.  Possible a security system or  another device that would have a transmitter.  You can drive near the interference location, turn the car off and get out of the car and use a small transistor radio and see if you hear the same noise that you hear in your car.  If not, then start the car and take the transistor radio near the car and see if you hear the same noise that you hear on your car radio.  If you can't hear the noise away from the car but do hear it near the car, then its originating from the car.

Joe, W3GMS     

[KA0HCP]

Everything you wanted to know about mobile installations at Alan's website:  www.k0bg.com

particular attention to 'bonding'.  I improved the performance of my car am/fm radio with a basic bonding job.

[flintstone mop]

Everything you wanted to know about mobile installations at Alan's website:  www.k0bg.com

particular attention to 'bonding'.  I improved the performance of my car am/fm radio with a basic bonding job.

It is really worth this effort. A little tedious. Use copper strap or the braided copper. Even the exhaust system needs bonding, cuz they hang that by rubber hooks to reduce noise inside the vehicle.
I was trying to run a mobile Ham radio (TS-50 and a Screwdriver antenna) in a Toyota pick-up and the electronic noises and hash was unbelievable. The 17M band was completely wiped out by the injector system. All I had to do was to move the ground wire that was the negative side of the four injectors to another location on the truck and the S-9 of hash was reduced to S-1. Many stations on the air, after that discovery.
And your 2005 window antenna is mounted at the rear? or is it in the windshield? Closer to the computers?
Fred