b. The AN/FPN-33 height-finding antenna is similar to the AN/SPN-5
antenna.
The big difference between the two antennas is the way their
reflectors are mounted.
Notice in Figure 149 that the AN/FPN-33 height-
finding reflector is mounted vertically to provide a narrow beamwidth.
Remember that the antenna must have a narrow vertical beamwidth to give
accurate elevation information.
c. The AN/FPN-33 uses waveguide to carry the RF pulses from the
magnetron to the reflector. The waveguide terminates at the focal point in
a horn.
This horn has a plastic cover to protect the waveguide from bad
weather and moisture. The gain of this antenna is about 8,000 over a simple
dipole.
d. A third type of antenna that you will work with has a reflector
called a parabolic cylinder.
28. Parabolic reflectors.
a. A spherical wavefront spreads out as it travels, producing a pattern
that is neither too sharp nor too directive.
A plain wavefront does not
spread out because all of the wavefront moves forward in the same direction.
For a sharply defined radar beam, the need exists to change the spherical
wavefront from the antenna into a plain wavefront.
accomplishes this.
b. Radio waves behave similarly to light waves.
Both microwaves and
light rays travel in straight lines and are focused and/or reflected.
Figure 150 shows a point-radiation source placed at the focal point.
The
field leaves this antenna with a spherical wavefront. As each part of the
wavefront reaches the reflecting surface, it shifts 180 degrees in phase and
moves outward at angles causing all parts of the field to travel in parallel
paths.
Because of the shape of a parabolic surface, all paths from the
focal point to the reflector and back to line XY are the same length.
Therefore, all parts of the field arrive at line XY the same time after
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