| There's some discussion here:  http://www.broadband-hamnet.org/hsmm-mesh-forums/view-postlist/forum-344/topic-344-wrt54g-power-plug-dimensions.html It seems to be a "Type M 5.5x2.1mm barrel connector". The antenna connector is called "RP-TNC" or Reverse Polarity TNC. 
 Technically, the antenna has the female connector, since the center connector is female, but it seems most people mislabel it as "male."  Always get a picture or diagram of what you're buying.   I've seen it listed both ways.  
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