On Sun, 24 Jan 2010 10:54:09 -0800 (PST),
" wrote:

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.

Diversity receiver. It's much the same as with the common wireless
router that has two antennas. One antenna is considered the main
antenna, the other the auxillary antenna. The receiver sits on the
main antenna until it hears something. If the error rate is high on
what it's hearing, it switches to the aux antenna.

The idea is to eliminate frequency selective fading. That's where the
direct path and a reflected path both converge on an antenna, but due
to the differences in path length, may cancel or distort the received
signal. It's only necessary to move 1/2 wavelength in order to
eliminate this effect, which is what the aux antenna does.

There are pleny of other methods of accomplishing the same thing. For
example, redundant receivers, both listening to the same channel, and
comparing results are possible. Same with two receivers on different
RF frequencies, in order to accomidate congestion.

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.

See receiver voting systems. These are common in municipal
communications systems, where it is difficult to hear a handheld radio
at all points of the city. Mutliple receivers are used as you
describe. The hard part is deciding which receiver should be heard by
the dispatcher, which is what the voting system does.

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.

Are you sure that there are two receivers and not one receiver and an
antenna switch? Some makers and model numbers would be useful. At
72Mhz, two closely spaced antennas should hear roughly the same thing.
If one doesn't, it's broken somewhere. What's not clear is whether
the signal is weak or just not there.

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

A 72Mhz Yagi is rather large and expensive.

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.

It's possible, but not likely. How far apart are the fire departments
two antennas spaced? Are they perhaps on opposite sides of a tower or
pole?

With careful construction, it's possible to create a cardioid pattern
with two omni antennas and a phaseing harness, but the beast is so
critical in construction, that it's unlikely to have been constructed
by accident. Similarly, if one antenna were acting as a reflector for
the other, the maximum gain or loss would be about 3dB, which should
not be noticable.

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 doubt it but it's possible. If the antennas are side mounted on a
tower, the tower will do some blocking. Spacing of the antennas away
from the tower also have a big effect. There's a section in the ARRL
antenna handbook and other publications showing the effects to an
omnidirectional pattern caused by various antenna to tower spacing.

A photograph of the fire dept antenna structure would be helpful.

I'm also
wondering if two receiving antennas operating on the same frequency
should have been installed that close together in the first place.

It's not a problem.

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

Replace the sick receiver with a cheap scanner receiver and see if it
hears your transmissions. If it does, fix the receiver.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558