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

What`s the difference between a 1/4-wave of transmission line and a
1/2-wave dipole?

The transmission line is designed for low radiation and the antenna is
designed for high radiation.

Current will be turned around by the open-circuit at the end of the
transmission line. At the end of the antenna, displacement current will
flow between oppositely polarized parts of the antenna, the earth, and
other surrounding objects, and some conduction current will be
redirected in the opposite direction on the antenna.

Impedance of the transmission linee is uniform in its presentation to
either one of the two traveling waves traversing the line, forward or
reflected..

SWR changes along an antenna due to radiation, inefficiencies,
irregularities, etc. An average characteristic (surge) impedance is used
for antenna calculations.

SWR for the usual open-circuit 1/2-wave dipole:
SWR = Avg. Zo of the antenna / radiation resistance.
Avg. Zo = 276 log 1/periphery of antenna conductor
log is the base 10 unit
periphery is measured in the same units as length, usually in
wavelengths.

Loaded antennas tend to be high Q resonant circuits and this limits
their bandwidths. Low resistance allows a large current as reactance is
cancelled at resonance. The large current causes large voltage drops
across the antenna inductance and capacitance. I`ve seen mobile antennas
which were ready to fire up coronas at their tips at the slightest
excuse. This demonstrates high reactance (energy storage) and low
resistance (radiation and loss).

There are several variables which determine the voltage and current at
the end of the dipole. The current does not drop to zero simply because
the conduction in the forward direction stops. Displacement current
continues the flow to some extent. The dipole has a Q, and the wave
traveling on the dipole generates a voltage to current ratio related to
the size and configuration of the antenna.

You can predict that forward voltage on a transmission line will double
at its open-circuit end, and you can predict the forward voltage. I
think prediction of voltage at the open-circuit end of an antenna is
harder.

Best regards, Richard Harrison, KB5WZI