TC 9-64 _________________________________________________________________________
waves, and a horizontal antenna is used for the reception of horizontally
polarized waves. In some cases the orientation of the electric field does not
remain constant. Instead, the field rotates as the wave travels through space.
Under these conditions both horizontal and vertical components of the field
exist and the wave is said to have an elliptical polarization.
Polarization Requirements for Various Frequencies
4-37. Ground-wave transmission is widely used at medium and low
frequencies. Horizontal polarization cannot be used at these frequencies
because the electric lines of force are parallel to and touch the earth. Because
the earth acts as a fairly good conductor at low frequencies, it would short out
the horizontal electric lines of force and prevent the radio wave from
traveling very far. Vertical electric lines of force, on the other hand are
bothered very little by the earth. Therefore vertical polarization is used for
ground-wave transmission, allowing the radio wave to travel a considerable
distance along the ground surface with minimum attenuation.
4-38. Sky-wave transmission is used at high frequencies. Either horizontal or
vertical polarization can be used with sky-wave transmission because the sky
wave arrives at the receiving antenna elliptically polarized. This is the result
of the wave traveling obliquely through the earth's magnetic field and
striking the ionosphere. The radio wave is given a twisting motion as it
strikes the ionosphere. Its orientation continues to change because of the
unstable nature of the ionosphere. The relative amplitudes and phase
differences between the horizontal and vertical components of the received
wave also change. Therefore, the transmitting and receiving antennas can be
mounted either horizontally or vertically.
4-39. Although either horizontally or vertically polarized antennas can be
used for high frequencies, horizontally polarized antennas have certain
advantages and are therefore preferred. One advantage is that vertically
polarized interference signals, such as those produced by automobile ignition
systems and electrical appliances, are minimized by horizontal polarization.
Also, less absorption of radiated energy by buildings or wiring occurs when
these antennas are used. Another advantage is that support structures for
these antennas are of more convenient size than those for vertically polarized
antennas.
4-40. For frequencies in the VHF or UHF range, either horizontal or vertical
polarization is satisfactory. These radio waves travel directly from the
transmitting antenna to the receiving antenna without entering the
ionosphere. The original polarization produced at the transmitting antenna is
maintained throughout the entire travel of the wave to the receiver.
Therefore, if a horizontally polarized antenna is used for transmitting, a
horizontally polarized antenna must be used for receiving. The requirements
would be the same for a vertical transmitting and receiving antenna system.
4-41. For satellite communications, parallel frequencies can be used without
interference by using polarized radiation. The system setup is shown in
figure 4-9. One pair of satellite antennas is vertically polarized and another
pair is horizontally polarized. Either vertically or horizontally polarized
transmissions are received by the respective antenna and retransmitted in
the same polarization. For example, transmissions may be made in the 3.7 to
3.74 GHz range on the vertical polarization path and in the 3.72 to 3.76 GHz
4-12
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