Figure 36. Beam with measured on relative field
strength and relative power patterns.
PRACTICAL HALF-WAVE ANTENNAS
1.
Introduction.
a. We have discussed how to calculate a half-wave.
Now, let's
discuss the patterns half-wave antennas make.
We have shown in
figures 34 and 35, the radiation pattern of an antenna in free space.
Since our antennas must be erected over earth, the patterns created
are different.
b. The ground has the greatest effect on the medium and high
frequency antennas which are mounted fairly close to it in terms of
2.
a. If a horizontal antenna is erected some distance above ground,
its radiation pattern is as shown in figure 37. Notice that some of
the energy travels directly to the distant station. Notice also that
part of the energy strikes the ground directly in front of the
antenna. As we have learned earlier, phase reversal takes place and
may cancel out the direct wave if the ground-reflected wave and the
direct wave arrive at the same time and are out of phase.
If they
arrive in phase, the ground reflected wave adds to the direct wave,
making it stronger.
As the height of the antenna is increased, the
ground reflected signal either adds to the direct wave or creates a
null.
This action results in a series of radiation lobes.
As we
have also
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