Calltrex wrote:
We see here, as in all antennabooks, at the leftside the
voltage and current are in phase,

Those plots are for a *standing-wave antenna*. What you
are missing is that there is no phase shown in those plots.
Pure standing wave voltage and and pure standing wave current
have a constant phase. What you have plotted is a snapshot in
time of voltage and current *amplitude envelopes* in which the
phase is irrelevant. Those plots are not time domain plots.
They are plots of the voltage *envelope* and current *envelope*.

But in the right side the voltage and current are in
antiphase, hence my conclusion that

180 degree "antiphase" is still purely resistive with zero
reactance. All that has happened to the voltage is that
the sign of the voltage has changed. It is a snapshot in
time. 1/2 cycle later that same plot would be upside down.

the antenna must be reactive ! From your explanation at 180
degrees, why doesn't
current at the right side flip over then ? Should be.

When all the voltages and currents are either in phase or
180 degrees out of phase, the resulting impedance is purely
resistive. There is *no reactance* in an ideal resonant dipole!
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com