Amos Keag wrote:
But, what does feedpoint impedance have to do with
resonance?

On a standing wave antenna, like a center-fed dipole, the
feedpoint impedance is (Vfor+Vref)/(Ifor+Iref) where Vfor
is forward voltage, Vref is reflected voltage, etc. and
the plus sign denotes superposition, i.e. phasor addition.

On a wire dipole, the resonant feedpoint impedance will
occur only when all the phases line up, i.e. If Vfor is
at zero degrees, Vref must be at 180 degrees, Ifor must
be at zero degrees, and Iref must be at zero degrees. That
way, we get minimum voltage divided by maximum current with
a resultant phase angle of zero degrees. Eureka! The dipole
is resonant because the feedpoint impedance is purely
resistive.
--
73, Cecil http://www.qsl.net/w5dxp