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The purpose of the ground plane is to
form a virtual antenna on the opposite side of the monopole; this is referred to
as an image antenna. It does this by effectively inverting the polarity and
reflecting the wave up into the monopole to add to its incident energy. Metal
surfaces do not reflect energy. This apparent reflection is due to current being
generated in the ground plane surface by the incident rf wave, which
then acts as an antenna and re-radiates the signal. If the ground
plane material is a poor conductor, or is not making continuous contact
throughout, it is much less effective as a radiator. These electrically small
ground planes do not work well at low grazing angles, which is where most
of the received signal is coming from in level flight. Patterns of monopoles
over an electrically short or poorly conducting ground plane show decreased gain
normal to the monopole axis. Curved, rather than flat, ground planes throw
something else into the mix, with hard-to-predict behaviour resulting. Look!
ground-plane antennas work, but are several dB below the gold-standard of the
half-wave dipole for horizontal radiation. If you are not worried about Tx-Rx at
long distances, live with it! If you want the best Tx-Rx, experiment with the
half-wave dipole through the horizontal stab I referred to in a previous
posting. Also, use the best low-loss, low leakage coax available,
especially to make up for the deficiencies of the monopole's ground
plane.
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