Cecil, W5DXP wrote:
"We can deduce the ratio of voltage to current from the feedpoint
impedance."

Kraus says:
"The current I1 at a distance from the nearest current maximum, as shown
in Fig. 6-11 is given by

I1 = Io cos beta x

where I1 = terminal current,

Io = maximum current

When x=0, R1 = Ro; but when x=lambda/4, R1 = infinity if Ro is not zero.
However, the radiation resistance measured at a current minimum
(x=lambda/4) is not infinite as would be the calculated from:

R1 = Ro/ cos squared (beta x), since an actual antenna is not
infinitesimally thin and the current at a minimum point is not zero.
Nevertheless, the radiation resistance at a current minimum may in
practice be very large, i.e., thousands of ohms.

I think the voltage may be measured at the end of a dipole using a diode
probe spaced from the dipole by an insulator. The ground return can be
provided by the exterior of a 1/2-wavelength coax which is well
grounded.

For calibration, the easily determined voltage at the dipole feedpoint
can be indicated by the diode probe.

The coupling can be kept small if there is sufficient antenna power. The
antenna does not need to be reconfigured into a halo to measure its
voltage at its open-circuit end.

Best regards, Richard Harrison, KB5WZI