mobile antenna questions

[Gatekeeper007]

What I am wondering about involves car radio antennas. A couple of different times i have been with different friends listening to their car radios not so much for sound quality but for station seperation (no bleed over) and distance from stations on FM radio. Every once in a while you will find a car that can receive FM stations from a long way off much futher than what would be normal for most cars. At first I thought it was the radios that caused this but later found out it was the antenna system in question. I took a radio out of a car that could listen driving around to stations over 60 miles away when most other cars could barely hear them and tried it in other cars (use the word cars because that what I kept testing not trucks). What I found was that the radio didn't work any better than most any other good make of radio in the cars that worked normal but that three different radios all worked great in the one car above normal ( this one being a Transam with power up down antenna). The only thing I found in common between two of the cars I have checked is the fact that they had power up down antennas mounted on the rear connor panel of the cars. I would have thought this would be worse for reception because of the antenna design and the extra length of coax to reach the rear of the car but that wasn't the case not only did the radios get much greater listening distance they also had much better adjacent channel bleed over rejection. Anybody have any ideas as to why this might be?

 

[TomT]

Unless the antenna is smack dab in the center of the roof (nuisance if you frequent the drive through or low-ceilinged parking garages!) most car radios are directional (on FM) because of the effect of the roof pillars.
Hence on some cars it will seem like you can carry a station further away from the tower when you are driving away (or pick it up farther out as you return to town) because of this directional effect.

On AM, just moving the antenna farther away from the engine may also cut down on electronic noise from such things as the cruise control or engine computer. One Ford I had had little brass spring-like thingies on the underside of the hood, with teeth cut into them. This was supposed to ground the hood, but after a winter of road salt they would corrode and the AM radio would pick up all kinds of grunts and whines until I cleaned them off again.

 

[Doctor_Technical]

Gatekeeper, this may not apply in the situation you described, but I have played with one car -- think it was a Nissan -- that used a diversity antenna setup. IIRC one antenna was in the windshield (imbedded wire), the other was a standard whip. That radio was a signal-sucking monster that would pull clear stereo out of vanishing signal levels.

 

[littlejohn]

Nissan (and Infiniti) used diversity antennas on their FM radios - they worked marvelously. Whether they still do or not, I dunno.. haven't been in one for a while.
The Germans are much enamored of amplified antennas.. in Audis and VWs. These things have real problems in high signal areas, and tend to generate spurs in the radios.
I got something of a rep in the 70s for hotrodding GM auto radios. They were easy, just needed a drill and a half inch punch to make a hole in the front fender, and a Radio Shack 31 inch stainless whip. Replacing the windshield antenna helped them greatly.