Ian Jackson wrote:
What is a 'terminated dipole'?

In an anechoic chamber, the dipole has a resistor to chassis at
both ends. When the value of the resistor equals the characteristic
impedance of the dipole, the dipole becomes a traveling wave
antenna instead of a standing wave antenna. Your infinite dipole
in free space will be a traveling wave antenna, i.e. no reflections.

And why 600-800 ohms?

That's a ballpark Z0 for traveling wave antennas not located
near ground. It's the Z0 of a 1/2WL dipole during the transient
state before the arrival of the first reflections from the ends.

The resonant feedpoint impedance of a center-fed 1/2WL dipole is
(the difference between the values of the forward voltage and the
reflected voltage) divided by (the sum of the values of the forward
current and reflected current). Note: the two voltages are out of
phase and the two currents are in phase.

This is the (B) definition of "impedance" in the "IEEE Dictionary",
i.e. impedance as a *result*, not a cause. Destructive wave interference
at the feedpoint of a center-fed 1/2WL dipole is what causes a Z0-match
to a low resistive value, e.g. 50 ohms or 70 ohms.
--
73, Cecil http://www.qsl.net/w5dxp


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