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## The "effective height" parameter of an antenna.

The effective height of an antenna multiplied by the incident electric field strength
gives the terminal voltage at the antenna feed.

This simple idea is very useful, and applies historically to vertical tower
antennas which stretch upwards to a certain height above the ground.

For other antennas the better nomenclature might be "effective length".
A discussion of "effective height" may be found in the second edition of the
classic text on Antennas by J D Kraus (ISBN 0-07-100482-3 McGraw Hill 1988)
on pages 40-42, section 2-19.

Krauss identifies the "effective height" as the actual height, times
the antenna current averaged along its length divided by the
peak current which is assumed equal to the feed current.

He further identifies this parameter as 2*sqrt((effective area for capture)*(radiation resistance)/Zo)
where Zo = 377 ohms is the impedance of free space.

#### COMMENT

Suppose the feed is open circuit and no current is drawn from the antenna
wire or rods. We might expect that the terminal voltage would be different
in this case from that pertaining to the case where the antenna is matched
to its own radiation resistance. Immediately we can see that there are
problems with the definitions above.

Another insight may be gained from considering that the perfectly
conducting antenna wire shorts out the parallel electric field, and so
in the case of an open circuit antenna feed, the integral of electric field
from one end of the antenna to the other must be just equal to the
voltage across the feed gap, and therefore that the physical and effective lengths of
the antenna are the same.

Copyright D.Jefferies 2005

10th April 2005
D.Jefferies