Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 485
F 1
Sum
3 dB BW 3 dB BW
F 2
Difference
A
Figure 20.29A Monopulse feed.
Sum
Figure 20.29B Monopulse patterns.
B
Array Antennas
Microwave arrays can be formed from dipole elements, as in the broadside array of Fig.
20.30 (which is used in the UHF region), or a series of slots, horns, or other radiators.
The overall gain of an array antenna is proportional to the number of elements and the
details of their spacing. These and other antennas require a method of phase shifting for
proper beam formation by the array. In Fig. 20.30, the necessary phase shifts are created
by the crossed feeding of the elements,
but in more modern arrays
3
2
Wire mesh
reflecting
screen
Half-wave
dipole
elements
other forms of phase shifter are used.
Two other methods of feeding an
array are shown in Fig. 20.31. The corporate
feed method connects all elements
and their phase shifters in
parallel with the source. The branch
feed method breaks the waveguide
network into two (or more) separate
paths.
Figure 20.30 Reflector array antenna.