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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.
Q7. The various properties of a transmitting antenna can apply equally to the same antenna when it is
used as a receiving antenna. What term is used for this property?
Q8. The direction of what field is used to designate the polarization of a wave?
Q9. If a wave's electric lines of force rotate through 360 degrees with every cycle of rf energy, what is
the polarization of this wave?
Polarization Requirements for Various Frequencies
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.
Since 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.
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.
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.
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.
For satellite communications, parallel frequencies can be used without interference by using
polarized radiation. The system setup is shown in figure 4-8. 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 range on the horizontal polarization path without adjacent frequency (co-channel)
interference.
