J.B. Wood wrote:

The problem here is an impedance analyzer can't distinguish between
resistive losses (antenna, ground, tuner (if considered part of the
antenna)) and the radiation resistance of the antenna. To determine
efficiency you'd have to make some field strength measurements (usually
performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space. The only "approximation" would be to measure the
resistive part of the antenna feedpoint impedance and then subtract from
this the radiation resistance obtained from calculation. Sincerely, and
73s from N4GGO,

A good deal of, if not the majority of, the loss in a short antenna
system is in the matching system components. So even if this method
allowed you to get a reasonable estimate of the efficiency of the
antenna itself(*), you still wouldn't know what fraction of the
transmitter power is getting radiated, since you can't tell how much is
lost in the tuner.

(*)My limited experience in doing careful antenna measurements leads me
to be very skeptical of the ability to determine antenna efficiency even
very roughly by a single impedance measurement. I think comparison of
measured bandwidth to lossy model results is probably the best indicator.

The bottom line is that the impedance meter won't tell you much about
the efficiency of the antenna or system. About the only practical way
available to most amateurs is comparison of received signal strengths
between the antenna and a known reference antenna, using a step
attenuator to measure the difference.

Roy Lewallen, W7EL