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I have two buildings. Building A has wifi (2.4GHz) infrastructure, Building B doesn't. One side of building B can pick up the wifi signal from building A. I want to passively extend the signal through to the rest of building B, using two antennas and a short piece of coax.

Supposing I put a decent directional antenna (24dBi) in building B, pointing at a WAP in building A, then joined that to an omnidirectional antenna with a short (50cm) piece of coax. In this scenaria, for the omni antenna, would it be better to have a large (e.g. 12-15 dBi) antenna, or a small one (~7 dBi) (assuming the smaller one can still cover the range of the building)?

I am aware an active repeater would be better, but I'd like to get some thoughts on this scenario.

EDIT: background reading: http://www.netscum.com/~clapp/wireless.html#passive

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I didn't want to answer until I validated my thoughts with a RF expert I know.

While what you describe is possible based on the physics, you are most likely going to face issues in the real world application of such a repeater. You are going to lose a fair amount of signal, probably in the 2-6dB range (or worse with low quality components).

Further, there is nothing to filter out any other interference or RF signal with something like this, so you will also be repeating that as well. This will potentially introduce additional interference into Building A and impact the wireless experience there. Depending on the actual environment, the higher gain omni may help ("flatter" coverage may avoid some interference above/below) or hurt (repeat more interference on the same plane with the antenna).

Another issue you may face is that depending on the components themselves, some amount of the signal may get "reflected" back out toward the original source. Any directional filter you used to prevent this would also prevent bidirectional communication.

Without testing, I think the real problem may be that you will find this works better in one direction than the other. Ultimately, if you want to go down this road, you should test heavily to make sure this works as intended. However, as wireless environments are always in flux, I wouldn't depend on this link too heavily.

I didn't even touch on some other problems this may introduce, such as the hidden node problem.

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The antenna still needs to be connected to something that will function as the bridge in Building B. This will allow a bridge connection between A and B allowing clients to talk between the buildings. As the clients will need to associate with something locally as well as they won't just bounce off the passive antenna.

I'd put an AP in Building B that can bridge to an AP in Building A to provide a wireless uplink (bridge connection), ideally in the 5GHz spectrum. This connection can be omnidirectional depending on distance between buildings, if it is a fair distance away (>150m then I'd probably start looking at directional antenna instead). Then have that AP also function as a local AP in the 2.4GHz spectrum for building B clients to associate with for connectivity.

Omindirectional 7dBi should be plenty... but again there is no mention of area/distance/size so it is hard to provide a recommendation without these measurement factors.

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  • I'm specifically looking at a passive repeater. – askvictor Sep 14 '14 at 23:37

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