Hello Walt,

We have just a voltage source of 212Vrms with 100 Ohms in series. This
is all in a black box and called my PA. That black box has a UHF
socket and that mentions "100 W into 50 Ohms".

When you connect 50 Ohms to the SO 239 socket, you get a voltage
divider. 2/3 of the EMF is across the internal 100 Ohms and 1/3 is
across the 50 Ohms load. This results in 70.7V across the load (so
100W).

Does this make sense?

Wim
PA3DJS