I often read about "RP-SMA" connectors. What is that?

RP steht für „Reverse Polarity“ und bedeutet nichts anderes als das der Innenleiter eine umgekehrte Bauform (Kelch oder Stift) hat als man üblicherweise erwartet. RP stands for „Reverse Polarity“ and means nothing more than that the inner conductor has an inverted design (calyx or pin) than is usually expected. This variation became common in the early 2000s with the growing popularity of WLAN antennas in order to somewhat limit the proliferation of very powerful antennas - and thus much too high levels for the shared frequency band. In the meantime, RP connectors are available for very many designs (RP-BNC, RP-TNC, RP-SMA, RP-N, etc.), they are mostly used in the Wifi area.