I have yet to find anything that tells how much power a BNC connector can
take though. I found one that gave it a voltage rating of 500V and a
current rating of 3.5A DC. If you use Ohms law 500V at 50 ohms would be
10 amps which gives 5,000 watts, I seriously doubt it can really handle
that. That calculation probably didn't help any. If you take the 3.5A
with 50 ohms you get 175 V and 612.5 watts. However that 3.5A was DC and
generally it is easier to carry the same number of amps with AC, but not
sure how that changes as the frequency goes way up.

I have certainly used TNC's at 1300MHz at over 1kW pulsed, in military
applications, so breakdown is not the problem!!

73
Jeff