Thanks, this helps. Yeah, looking for "better than nothing" here for sure. I also gotta watch out for taking on liabilities myself. Disclaimers of no responsibility will need to be clear.

I'm not understanding the connecting pairs together in your response.

I was thinking of a single directional antenna per floor connected into a 2.4Ghz Wilkinson power divider with 22 dB isolation for the splits going to adjacent floors. This way each floor would reach one floor up and one floor down and no further. (I haven't yet measured the power outputs of the devices and of a heavily trafficked 2.4Ghz noise floor to make sure 22dB is sufficient.) I'd connect the bottom floor to the top floor the same way so basically each floor has one peer up and one peer down.
                                                                            |--> (3dB less per power split) to floor above
Fire alarm (free space)  Ant--->PD (0.5dB loss) |                                                                   -- 22dB isolation between
                                                                            |--> (3dB less per power split) to floor below

where PD is a Wilkinson power divider, 5 floors for 5 PDs.

Then the LMR connects the Wilkinson's splits to each other. The LMR is external to the building (along the rear ladder) and the antennas internal. 

The floor logical topology becomes 5<-1<->2<->3<->4<->5->1 such that any single hop failure only impacts that floor. Passive materials will be military grade, hermetically sealed and inside conduit. The building has two sets of egress stairs so alerting per one set being compromised can direct to the other. The internal stairwell is connected floor to floor - that was required by the fire department (and was costly to the owners.)

I was hoping to be able monitor the devices' topology and detect any changes using remote servers in the cloud. So far no luck in getting that information via an API that I can find. I think this is a critical piece, i.e. having always on servers bugging someone to pay attention to an unexpected topology change or a need for battery replacements.

I don't think there'd be fire marshal approval for this but, even without that, I think this meets the "better than nothing" goal and isn't prohibitively expensive.

Bob

On Tue, May 18, 2021 at 5:09 PM David Lang <david@lang.hm> wrote:
passive repeaters (two antennas connected together with no electronics) work
much better than most people realize.

I would do floor-by-floor antenna pairs, and if each device going off should
trigger the one on the next floor, I'd consider spacing them out so that you
don't have one antenna pair feeding a strong signal to the next pair.

If the only purpose of this is to relay the fire alarm signals, use high-gain,
narrow beamwidth antennas pointed at the alarms, this will do wonders at
overcoming the general noise on 2.4GHz

I have too much radio experience to be happy trusting anything wireless for
critical things, but where the alternative is no connectivity, it's better than
nothing.

David Lang

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