Wes and Richard. I've realized my conceptual mistake here. I have an
antenna analyzer, not just an SWR meter. I can MEASURE the resistive
part of the feedpoint impedance. I can calculate the radiation
resistance using EZNEC or more simply for simpler antennas.

The resitive part of the feedpoint impedance minus the radiation
resistance is the loss resistance, and then I can calculate the antenna
efficiency.

I've complicated things by getting the Q involved, though maybe if all
you have is an SWR meter...

Thanks for the replies.

73,
Dan