TC 9-64 _________________________________________________________________________
Figure 4-31. Unidirectional End-Fire Arrays
Parasitic Arrays
4-139. If a small light bulb were placed in the center of a large room, the
illumination would be very poor. However, if a reflector were placed behind
the bulb, the space in front of the reflector would be brighter and the space
behind the reflector would be dimmer. The light rays would be concentrated.
Also, if a lens were placed in front of the bulb, the light would be even more
concentrated and a very bright spot would appear on the wall in front of the
lens. A flashlight is a practical combination of the small bulb, the reflector,
and the lens. The energy from an antenna can be reflected and concentrated
in a similar manner.
4-140. Although we do not usually discuss the gain of a flashlight, we can
continue the comparison of an antenna and a flashlight to explain the
meaning of antenna gain. Suppose the spot on the wall in front of the
flashlight becomes 10 times brighter than it was when only the open bulb
was used. The lens and reflector have then produced a 10-fold gain in light.
4-42
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