Radio tidbits to put diversity issue to

[tkomljan@mobility.com (Komljanec, Tony)]

Regarding the post for the radio sensitivity... same problem here... 
inherent with the in glass antenna.  There is only so much you can do with a 
compromise antenna.  That's why other manufacturer's went back to the 
standard quarter wave (75 cm.) fender mount antenna.   Simple physics.  By 
putting the antenna in the glass:

 -  There are directional qualities to the antenna (undesired)
 -  It's a compromise over a quarter wave vertical fender mount
 -  Better curb appeal (but I have 2 cellular antennae on my car)
 -  The rear defroster acts as both FM and AM antenna
 -  Audi and "BlueDot" bet on the diversity to tackle multipath.
 -  You can't seriously put 2 quarter wave fender mounts on a car.

Multipath lives in urban environments.  Two signals arriving out of phase, 
can cancel each other.  In a car, when you experience this, you move half a 
metre, and you are receiving again.  There are different "levels of 
implementation" with diversity.  A true space diversity system will have two 
parallel receivers.  Comparisons between signal quality is done at the 
baseband audio level, and the better signal chosen for listening.  I would 
not expect this to be the case with our units... it's rather expensive. 
 Other schemes simply combine the two antennae, or switch between them based 
on RF signal strength.  I don't know which our unit use, since I haven't 
seen a schematic.

In Europe, listening to distant (DX) stations is not the norm.  Most FM 
stations have multiple transmitters to blanket a large area.  Their 
transmitters also provide supplementary data on a sub-carrier.... 
identification, advertizing, road and weather info (RDS), ***and the other 
frequencies***  they are transmitting on in neighbouring areas.  Europeans 
also use FM subcarriers for paging, so signals must be strong everywhere. 
  When the signal you are listening to gets weak, your radio automatically 
scans to find another transmitter providing the same program.  This is 
really helpful in mountainous areas, or when you drive through a country... 
you can listen to the same program without having to manually re-tune.  With 
this in mind, it's easy to see why the Audi gods put more emphasis on 
diversity and hidden antennae.  In Europe, signal strength is not a problem 
for most broadcasters using multiple transmitters.

It's too bad we don't use the RDS system in North America.  By the way, 
digital radio (Eureka 147 @ 1.5 GHz) is coming to Canada and Europe.  It 
will provide all these features and more, including CD like sound.

This is my 2 cents rationalization as to why Audi chose this radio and 
diversity combo.

Regards,

Tony K

Toronto, ON
'93 100 CSQ