*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 radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks

So if the base current entering the whip itself can be measured, and
whether or not a loading coil was used at the base to resonate* the
antenna system, and regardless of the loss in the r-f ground used by
the whip -- then for practical purposes the power radiated by the
antenna system will be the product of the square of that base current,
and the radiation resistance of the whip.

* However "resonated" antenna systems using a short whip do not have
the same radiation performance as those using a naturally resonant 1/4-
wave monopole. Resonance just means that the capacitive reactance at
the feedpoint of the short whip has been exactly offset by the
inductive reactance of a loading coil, which can allow for the most
efficient power transfer into the radiator.

However the radiation resistance of the whip is not changed by this
process -- it is still very low compared to a naturally resonant 1/4-
wave monopole. Therefore the radiation efficiency of such a short,
loaded, whip antenna system cannot approach that of a naturally
resonant 1/4-wave monopole unless the losses in the loading coil and r-
f ground are nearly zero.

Loading coils do not replace the "missing degrees" of an electrically
short radiator, as far as its radiation resistance is concerned. The
radiation resistance of a whip depends only on the electrical length/
OD of the whip, itself.

RF