Hi Hugh,Gary,
The efficiency of all antennas depends on three resistances
Rrad.the radiation resistance determined mainly by the height of the
vertical section,formulas for calculating this in various text books.However
it is unlikely to exceed 0.1 ohm probably 0.01 ohm especially on 73kHz.The
earth loss resistance which depends on many factors and which is the RF loss
resistance not the DC resistance although they are related.It must be
measured at the frequency in use since there is more or less ground
penetration ,more at low frequencies. Finally the environment loss
resistance due to trees structures etc close to the antenna.
As Alan says a simple bridge can be used to measure this TOTAL
resistance,which is unlikely to be less than 25 ohms in am amateur situation
and may well exceed 100 ohms. Since radiated power depends on the current in
the Antenna (vert sect) it is important but difficult to reduce this
resistance.
Hope this helps, mail me direct if I can explain  more at length.
73 Laurie.