"David" wrote in message
...
On 31 Mar 2007 11:02:47 -0700, "ve3..." wrote:


In WWII Nazi submarines used TRF receivers as the oscillator in a
superhet put out enough signal that air and naval sub chasers could
home in on the signal and deep-six them.

In fact, there is a ratings service, called MobilTrak which installs
antennas on utility poles where cars go by, and can tell what FM station is
being listened to based on the IF offset, but it does not work on AM.

http://www.mobiltrak.com/ has an explanation of the technology.

Mostly, this is used by retail business to see what station is listened to
at point of purchase (POP) for ad effectiveness studies. It is of limited
value in quantifying audience, as 2/3 of radio listening is not in the car.