also aids in obtaining a slight degree of security, makes enemy direction finding more difficult, and
reduces noise, interference, and fading.
g. Antennas used in the VHF and UHF ranges.
(1) The VHF frequency range begins at 30 MHz and the UHF frequency range ends at 3000
MHz. The antennas used in multi-channel systems are designed to match those frequencies. The wire
rhombic and log periodic antennas used in the high frequency (HF) (3-30 MHz) range are not effective
when used in the VHF and UHF ranges. Therefore, rhombics and log periodic antennas are seldom used
at frequencies above 30 MHz. Reflective antennas, such as corner reflectors and parabolic reflectors,
are preferred.
(2) At frequencies above 100 MHz, corner reflectors may be used. Above 400 MHz, the
wave length is such that even a parabolic reflector, excited by a dipole at the focal point, is practical.
h. Parabolic reflector antenna. A parabolic reflector antenna (figure 17) consists of a saucer-
like reflecting surface (parabola) with a small waveguide opening or a dipole (feed device) placed at its
focal point. The wave from the primary source is reflected forward by the parabolic reflector as a plane
wave. The larger the surface, the greater the antenna gain and the narrower the beam width. When a
dipole source is used, a parasitic element or a small auxiliary reflector is often placed in front of the
dipole so that more of the energy is directed toward the large parabolic surface. The presence of the
dipole and auxiliary element has some blocking effect on the forward pattern, but the loss is small in
large reflector antennas. The polarization of the field in the horizontal and vertical planes is the same as
that of the primary source. In other directions, a cross polarized wave is developed that has both
horizontal and vertical components. The reflector surface may be solid or perforated. To reduce wind
resistance, some of the larger reflectors often have perforated surfaces or grid wires in which the
spacings are a precalculated fraction of a wave length. Perforated and grid reflectors serve as well as
solid reflectors. Parabolic reflector antennas are not restricted to the VHF portion of the spectrum.
They are equally effective in line-of-sight microwave (frequencies at 1000 MHz or above) paths and
tropospheric scatter paths. In microwave multichannel systems, parabolic reflector antennas are used
with microwave radios using frequencies from 1500 MHz upwards, and with microwave and
tropospheric scatter radios using frequencies up to and above 5000 MHz. Tropospheric scatter antennas
must be sited to obtain the lowest possible line-of-fire angels while microwave antennas must be sited to
obtain the best possible line-of-sight paths between stations.
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