AC66u antenna connector?

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I want to use a retired AC66u (the old model that is not compatible with newer Merlin builds) for a low power RF experiment. It will not be connected to the internet. Of course, the router has three RP-SMA connectors. This experiment requires one antenna exclusively.

So my question: Is there a setting in any firmware that will best accommodate this? I remember seeing Antenna Diversity in one firmware or another, but I am not sure how that works.

I will buy a small cheap Alfa to do the same thing, but it is more fun to dust off and re-purpose something from the bottom of the closet.

 

[distilled]

I completely forgot that a HackRF can do what I need, so this thread can be ignored.