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I looked at the BCM4312 file - available as as doc or a pdf - and it says:

BCM4312 has 2 radio chains, one is a dedicated Rx chain, the other is shared by TX and > RX chain using an RF switch Support Antenna Diversity

RF Output Power 802.11b: 16.5 dBm ± 1.5 dBm 802.11g: 15.5 dBm ±1.5 dBm

Can I hook up two Yagi antennas to each u.fl connector (it has 2 u.fl male connectors) and will "diversity" work? How exactly does all this work?

Wiki says:

Spatial diversity employs multiple antennas, usually with the same characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, sometimes a space on the order of a wavelength is sufficient. Other times much larger distances are needed

Transmit/Receive diversity uses two separate, collocated antennas for transmit and receive functions. Such a configuration eliminates the need for a duplexer and can protect sensitive receiver components from the high power used in transmit.

All of the above techniques require some sort of post processing to recover the desired message - Switching – In a switching receiver, the signal from only one antenna is fed to the receiver for as long as the quality of that signal remains above some prescribed threshold. If and when the signal degrades, another antenna is switched in. Switching is the easiest and least power consuming of the antenna diversity processing techniques but periods of fading and desynchronization may occur while the quality of one antenna degrades and another antenna link is established.

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  • \$\begingroup\$ additionally could someone clarify why iwconfig shows output power as 30dBm when the max juice that this can kick out is 15dBm \$\endgroup\$ – user41067 Jul 27 '14 at 13:33
  • \$\begingroup\$ How about a hyperlink for the clever but lazy people? \$\endgroup\$ – Andy aka Jul 27 '14 at 14:13
  • \$\begingroup\$ @Andy added the link \$\endgroup\$ – user41067 Jul 27 '14 at 14:26
  • \$\begingroup\$ That's a link to a .doc word file and I'm not going to open it in case of viruses. I suggest you provide a better link. \$\endgroup\$ – Andy aka Jul 27 '14 at 15:14
  • \$\begingroup\$ .pdf's can contain viruses same as doc - I suggest you tell us what file format you prefer, that is virus free, instead of asking open ended questions. I'd also like to point out that i have no way of knowing if you are merely trolling. The question clearly stated that I had viewed a DOC file - you asked for a hyperlink (to the DOC file) - that's what I posted. \$\endgroup\$ – user41067 Jul 27 '14 at 17:12
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From the datasheet it always transmits over one of the antenna connectors and a software algorithm is used to determine the best one at the time to use for receiving:

BCM4312 has 2 radio chains, one is a dedicated Rx chain, the other is shared by TX and RX chain using an RF switch. For RX, It supports antenna diversity and provides four options. A software diversity algorithm is used to determine which option is used.

It doesn't mention what the four options are but in general the idea of using diversity mode is when you think the reception from one antenna is likely to be better than the other, it won't increase range otherwise because only one is actually being used at a time. For further reading here's an article Multipath and Diversity from Cisco that goes into quite a bit of detail on the use cases.

So it's certainly possible to connect a Yagi to each antenna connector and it shouldn't cause any problems as long as you don't use it to try and expand coverage to a wider area. But if it's in a location where multipathing isn't likely to be issue like say outdoors and line-of-sight it won't gain you anything either. In that case you'd be better to spend the extra money on a better antenna and low-loss coax and connectors.

From a practical point of view determining the best setup is mainly a matter of testing various configurations and doing a site survey to make sure all the areas you're interested in have good coverage. The last time I did something similar it was an embedded system that had logging and GPS anyway, but there are a number of phone apps around that may prove useful if the clients will be moving around. For point-to-point you could just measure throughput and reliability over a period of time to make sure it meets your needs.

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