_______________________________________________________________________ Antennas
Figure 4-8. Reciprocity of Antenna
ANTENNA GAIN
4-34. Another characteristic of a given antenna that remains the same
whether the antenna is used for transmitting or receiving is gain. Some
antennas are highly directional; that is, more energy is propagated in certain
directions than in others. The ratio between the amount of energy propagated
in these directions compared to the energy that would be propagated if the
antenna were not directional is known as its gain. When a transmitting
antenna with a certain gain is used as a receiving antenna, it will also have
the same gain for receiving.
POLARIZATION
4-35. Let us review polarization briefly. In chapter 2 you learned that the
radiation field is composed of electric and magnetic lines of force. These lines
of force are always at right angles to each other. Their intensities rise and fall
together, reaching their maximums 90 degrees apart. The electric field
determines the direction of polarization of the wave. In a vertically polarized
wave, the electric lines of force lie in a vertical direction. In a horizontally
polarized wave, the electric lines of force lie in a horizontal direction. Circular
polarization has the electric lines of force rotating through 360 degrees with
every cycle of RF energy.
4-36. The electric field was chosen as the reference field because the intensity
of the wave is usually measured in terms of the electric field intensity (volts,
millivolts, or microvolts per meter). When a single-wire antenna is used to
extract energy from a passing radio wave, maximum pickup will result when
the antenna is oriented in the same direction as the electric field. Thus, a
vertical antenna is used for the efficient reception of vertically polarized
4-11
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