4.
Changes in Radiation Resistance.
a. The radiation resistance at the center of a half-wave
horizontal antenna erected in free space is 73 ohms.
The actual
resistance of the same antenna erected over varying ground
conductivity and heights is zero to approximately 100 ohms.
b. See Figure 42. The change in resistance occurs because of the
ground reflected wave. It occurs in the following manner: Let's say
that a given power is applied to an antenna in free space.
The
radiation resistance is 73 ohms because there was no ground
reflection. But, suppose that the same antenna is erected at a given
distance above the ground.
The ground reflects part of the energy
back to the antenna, adding to the existing current and lowering the
resistance.
It is assumed that the ground reflected wave was in
phase with the direct wave; therefore, adding to the original
current. If the two waves are not in phase, the overall current is
less, resulting in a higher radiation resistance.
c. The change in radiation resistance of a vertical half-wave
antenna is much less than that of a horizontal antenna. The maximum
resistance is 100 ohms at the center of the antenna at a height of a
quarter-wave above ground. It decreases to about 70 ohms at a height
of a half-wave length.
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