I work on fire alarm systems in outlying areas that transmit a signal to the
fire department via 72 Mhz AM. The signal period is very brief and the
carrier is modulated by a series of tones which first pertain to the box ID
and then successive tones corresponding to which zone or zones are in alarm.
The transmit power is one watt and cannot be increased.

This particular site is about 9 miles from the receiver. The transmit site
is low and the terrain is hilly.

** Sounds impossible.


Five years ago we installed a three element beam to replace the existing
omni directional antenna they had. Communication with that omni was spotty
at best. With the beam, although the received signal was low, test
transmissions were consistent and reliable.

** I bet it was low and badly affected by rain too.


I'm concerned about the modulation factor because this box seems to have
suffered some type of event to lock it up the way it was when I first found
it. I also have no idea what caused the transmitter fuse to blow.

Other than my service monitor/spectrum analyzer which tunes .450Mhz to 1.0
Ghz FM, (only) I have no way to hear this AM signal. Does anyone have any
ideas?

** Err - ever heard of a radio scanner ?



.... Phil