Practical_Antenna_Handbook_0071639586

484 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Figure 20.28A Parabolic antenna
conventional feed.
Reflector
Primary feed horn
Waveguide or
transmission
line
• Shorter transmission line length
• Lower sidelobes
• The open horn sees sky instead of
earth (hence, a lower temperature)
On the negative side, galactic and
solar noise might be slightly higher on a
Cassegrain dish, depending on where
it’s aimed.
Figure 20.29A shows the monopulse
feed geometry. In this system, two radiator
elements placed at the focal point are
fed to a power splitter network that produces
both sum and difference signals.
When these are combined, the resultant
beam shape (Fig. 20.29B) has an improved
–3-dB beamwidth as a result of
the algebraic summation of the two.
Main
reflector
Primary
feed horn
0111057 FIG 18-29A
Waveguide
Subreflector
Incoming
wavefront
Figure 20.28B Parabolic antenna Cassegrain feed.