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From digest.v7.n743 Wed Dec 10 16:19:15 1997
From: Jim Cash <j.cash_at_sympatico.ca>
Date: Wed, 10 Dec 1997 12:23:31 -0500
Subject: Re:1992 325i radio antenna?

John writes:
> This is just a question, but how many E36 owners have had problems with
> radio reception? I live in a large city close to several of the stations
> I listen to and sometimes get a lot of static. I remember hearing that
> '94 + 3 series have a diversity antenna. Does that help the problem any?
> Where is the antenna located anyway? I assumed it was the back window but
> it doesn't look obvious. I thought the reception was bad in my '96 Honda
> Accord EXL coupe, but the E36 is even worse. Is this antenna upgradable
> in any way? I know it's not the speakers cause everything sounds just
> fine when I'm playing cd's.
>

John
This is common to "all" cars. And it normally occurs only on the FM band.
Some car/radio/antenna combinations may be worse than others due to many factors.

The FM signal is a shorter wave frequency than AM, and it is very easily "reflected" off things like large buildings. When that happens you get the same signal bouncing around the city - and it sometimes happens that the same signal arrives at your car, from two different sources - the original, and the reflected.

Now the technical stuff.
These radio waves travel at the speed of light, and since the "reflected" waves travel farther than the direct ones, they arrive at slightly different times. When the 2 waves are at exact opposite phase (the top of one and the bottom of the other arrive at the same time) they tend to cancel each other. The radio sees "no signal", and the program fades.

Because the frequencies are short, the location where they perfectly cancel each other is usually very small. Sometimes if you stop at a light, and find the signal drops, you can restore it by just moving ahead about a foot.
As you car's antenna moves though these "spots" the signal will fluctuate, and the interference will sound like static.

There is one area in my city where this occurs regularly. I experience it in every car I drive through that 2 block section. There are only a few apartment buildings, but there is a large area of river, which can also reflect radio waves.

And sometime the waves will reflect off other objects like cars, trucks, trains and even planes. When these are moving the reflections are of very short duration.
It's an electronic jungle out there

Regarding the "Diversity' antenna. Yes they help. A diversity antenna is actually 2, or more, antenna elements, each connected to the antenna amplifier (may be in the radio - but some cars have separate amplifiers near the antenna. On my E39 there are 3 separate FM elements - all in the rear window. The amplifier is in the right rear C pillar. It continually monitors the signal strength on all of the 3 antenna's, and selects the strongest. It then passes this signal to the radio. This is done electronically, and very quickly. It may be switching between antennas several times each second.

Regarding whether you can upgrade the antenna in you E36. It will depend on your specific year, model, and other accessories, so you will have to discuss with your BMW retailer. There are other factors involved, such as the interference caused when the rear window defroster heater is on - it uses the same elements in the window as the antenna.

Some things to check are the antenna connections, and of course the radio itself may not be bang on its frequency selection. An automotive radio service shop can perform tests.

Just for interest sake there are 2 different rear windows for the E39.

Base model has:
1 AM antenna
1 FM antenna
1 FBZV antenna - for the remote locking

Diversity model has:
1 AM antenna (does not need diversity since AM waves are longer) 3 FM antennas
2 TV antennas (for TV in the OBM TV waves are similar to FM) 1 FBZV antenna - for the remote locking.

Cheers

Jim Cash
London, Ontario, Canada
BMW CCA 102929
E39 97 540iA

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