TC 9-64 _________________________________________________________________________
4-45. Radiation is somewhat less affected by reflections from aircraft flying
over the transmission path when vertical polarization is used instead of
horizontal polarization. With horizontal polarization, such reflections cause
variations in received signal strength. This factor is important in locations
where aircraft traffic is heavy.
4-46. When vertical polarization is used, less interference is produced or
picked up because of strong VHF and UHF broadcast transmissions
(television and FM). This is because VHF and UHF transmissions use
horizontal polarization. This factor is important when an antenna must be
located in an urban area having several television and FM broadcast stations.
Advantages of Horizontal Polarization
4-47. A simple horizontal antenna is bidirectional. This characteristic is
useful when you wish to minimize interference from certain directions.
Horizontal antennas are less likely to pick up man-made interference, which
ordinarily is vertically polarized.
4-48. When antennas are located near dense forests or among buildings,
horizontally polarized waves suffer lower losses than vertically polarized
waves, especially above 100 megahertz. Small changes in antenna locations
do not cause large variations in the field intensity of horizontally polarized
waves. When vertical polarization is used, a change of only a few meters in
the antenna location may have a considerable effect on the received signal
strength. This is the result of interference patterns that produce standing
waves in space when spurious reflections from trees or buildings occur.
4-49. When simple antennas are used, a horizontally-mounted antenna less
affects the transmission line, which is usually vertical. When the antenna is
mounted at right angles to the transmission line and horizontal polarization
is used, the line is kept out of the direct field of the antenna. As a result, the
radiation pattern and electrical characteristics of the antenna are practically
unaffected by the presence of the vertical transmission line.
4-50. Radiated energy is the useful part of the transmitter's signal. However,
it represents as much of a loss to the antenna as the energy lost in heating
the antenna wire. In either case, the dissipated power is equal to I2R. In the
case of heat losses, the R is real resistance. In the case of radiation, R is an
assumed resistance; if this resistance were actually present, it would
dissipate the same amount of power that the antenna takes to radiate the
energy. This assumed resistance is referred to as the radiation resistance.
4-51. Radiation resistance varies at different points on the antenna. This
resistance is always measured at a current loop. For the antenna in free
space, that is, entirely removed from any objects that might affect its
operation, the radiation resistance is 73 ohms. A practical antenna located
over a ground plane may have any value of radiation resistance from 0 to
approximately 100 ohms. The exact value of radiation resistance depends on
the height of the antenna above the ground. For most half-wave wire
antennas, the radiation resistance is about 65 ohms. It will usually vary
between 55 and 600 ohms for antennas constructed of rod or tubing. The
actual value of radiation resistance, so long as it is 50 ohms or more, has
little effect on the radiation efficiency of the antenna. This is because the
4-14
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