Perhaps a polarization change would help.
Sometimes as radio signals get reflected/refracted they can swing 90degree
pol.

Perhaps try mounting the Tx or one of the Rx antennas horizontally as a
test?


P
wrote in message
...
We have a system in place in the local area whereby the fire
departments monitor fire alarm signals at the fire stations in most of
the area towns. It is mandatory that all businesses as well as
commercial properties are equipped with fire alarm systems. Two of the
most common methods of communicating with these fire departments is
either by master box telegraph which sends a coded signal by operating
a wind up eccentric interrupter via municipal overhead wire, or by
radio transmitters which are connected to these fire alarm systems.

With the radio system, in the event of a fire, three short bursts of
data are sent over approximately a one second period. The location,
event, and zone information are all sent during this period. The
maximum allowable power output is one watt The signal is AM at a
frequency of 72MHZ. The terrain is hilly. Most transmitting antennas
are omni directional affairs which seem to work adequately however
there are some cases where it is necessary to use a directional
antenna.

It should also be noted that most fire departments have more than one
receiver with antennas located in different locations. Sometimes these
antennas are miles apart. Depending on topography, it is sometimes not
possible for a transmitter to hit one receiver, however it may hit the
other with a good signal. Redundancy is important but most fire
departments understand propagation somewhat and are satisfied with a
good signal at one receiving location.

We have one transmitter location which is about 5 miles from the fire
station. The terrain is somewhat hilly however not pronounced. The
fire station has two receivers with both antennas that were installed
on the roof about ten feet apart. This makes no sense to me. It seems
like they should have been put in two different locations as other
towns do it to be able to receive signals coming from other locations
which may be blocked to the other antenna.

In this instance, these two receiving antennas and the transmitter
location form a straight line. This particular fire department is
insisting on reception on both its receivers. The complaint is that
the fire department is receiving a good signal on one of its receivers
but not the other one.

We have tried moving the antenna to another part of the building,
raising it 25 feet, and replacing the RG 58U cable with RG 213.
Nothing seems to work. I've also considered using a Yagi however It
has been suggested to me that in this case perhaps the second
receiving antenna may be blocked by the first, which has created a
dead spot to this particular transmitter location.

In my opinion, this two antennas in the same location arrangement also
eliminates any advantage the fire department would realize of
receiving any signals from a blocked transmitting location. I'm also
wondering if two receiving antennas operating on the same frequency
should have been installed that close together in the first place.

Does anyone have any suggestions The fire inspector knows little
about radio and is a real pain in the ass. Thanks, Lenny.



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