Zombie Wolf wrote:
Any antenna will be either "inductive" or "capacitive", depending on its
length as relative to the frequency its designed for. If it is short for the
frequency, it is usually capacitive, and if it is long for the frequency, it
will exhibit inductive qualities. The combination of these inductive and
capacitive factors, added vectorially, results in the "Z", or impedance , of
the antenna.

Simply put, the feedpoint impedance of an antenna is the ratio of
the total voltage to the total current at the feedpoint. In a
standing-wave antenna, like a center-fed dipole, the total voltage
is the vector sum of the forward voltage and reflected voltage.
The total current is the vector sum of the forward current and
the reflected current. The reflected current undergoes a 180 deg
phase shift at the tips of the dipole but the reflected voltage
does not. For a resonant 1/2WL center-fed dipole, the feedpoint
impedance is:

Zfp = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) = low

i.e. Vfor is 180 degrees out of phase with Vref and Ifor is
in phase with Iref.

For a (resonant) one-wavelength center-fed dipole, the feedpoint
impedance is:

Zfp = (|Vfor|+|Vref|)/(|Ifor|-|Iref|) = high

i.e. Vfor is in phase with Vref and Ifor is 180 degrees out
of phase with Iref.
--
73, Cecil http://www.qsl.net/w5dxp


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