Jim Lux wrote:
"Is there a low cost (possibly passive) way to port 900 MHz radio
transmissions into these buildings?"

I`m assuming full-duplex (simultaneous transmissions and receptions are
required. Half-duplex 2-way extensions indoors, through tunnels, etc.
are routinely obtained using a land-mobile station antenna port
connected to TV twinlead run through the building, tunnel, etc. It works
like gangbusters. Special Andrew Corp. rafiating Heliax is not required.

For full-duplex, it is only slightly more complicated to do something
silimiar. A metal building which can keep radio signals out should also
keep them in.

First I would try the simple expedient of antennas back-to-back, one
outside the building and the other inside. Problem is in the huge signal
loss in the first few feet from the re-transmission antenna. As much as
99% of the signal is likely lost. That may or may not be acceptable.

For the full-duplex solution, you will need two amplifiers, one for each
direction of transmission. These go inside the buildiing. The input of
one amplifier is connected to one of the outside antennas. The amplifier
output is connected to 300-ohm TV twinlead which is run around the
interior of the builrding so that some part of the twinlead is never
very far from anyplace a vehicle may be parked in the buillding.

The input of the second amplifier is connected to a similarly placed run
of twinlead. The two twinlead runs are separated so that the input of
the second amplifier isn`t overloaded by the output of the first
amplifier and that a feedback loop isn`t created. output of the second
amplifier is fed to its antenna outside the building.

Separate frequencies are used in full-duplex systems so the probability
of feedback is small.

I suppose the AVL system is using a satellite and highly directive
antennas can be used. 900-MHz is a low microwave frequency so cheap
consumer-electronics amplifiers may be applicable. A 900-MHz dish
antenna with significant gain is much larger than those used for the
Dish TV network. Ordinary microwave systems use circulators to connect
their receivers and transmitters to a common antenna. The vehicles in
your AVL system surely don`t use big dishes so there is probably no need
for high-gain antennas. Your system may be a one-way system for all I
know. You can use applicable parts of what I described above.

Best regards, Richard Harrison, KB5WZI