Figure 150.
Parabolic Cylinder Reflector.
c. There is radiation from the antenna into space as well as toward the
reflector when using a dipole as the source of radiation.
Energy not
directed toward the paraboloid has a wide-beam characteristic that destroys
the narrow pattern from the parabolic reflector.
Using the hemispherical
shield that directs most radiation toward the parabolic surface, prevents
this occurrence. This eliminates direct radiation, makes the beam sharper,
and concentrates power in the beam.
Without the shield, some of the
radiated field leaves the radiator directly. Since it is not reflected, it
does not become a part of the main beam and serves no useful purpose. The
same is accomplished using a parasitic array that directs the radiated field
back to the reflector, or using a feed horn pointed at the paraboloid.
d. The radiation pattern of a parabola contains a major lobe directed
along the axis of revolution and several minor lobes as shown in Figure 151.
Very narrow beams are possible with this type of reflector.
29. Radar set AN/MPQ-4A antenna uses a parabolic cylinder.
Figure 151 shows a radar antenna and its radiation pattern. Radar
set AN/MPQ-4A locates mortar and artillery shells, so, the antenna must send
out a narrow beam.
Actually, this set has two beams.
Each beam has a
vertical beamwidth of 0.8 degrees and a horizontal beamwidth of 1 degree.
Notice that the reflector is a parabolic cylinder fed along a line instead
of at one point.
Waveguide carries RF pulses from the magnetron to the
reflector. The pulses are then fed to the reflector from a horn that runs
the length of the reflector.
207
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