precision types, the resistive element is a metalized-glass plate which may be inserted either from one of the
narrow sidewalls or through a slot milled in the center of the upper wall. Two variable attenuators using
dissipative elements are shown in figure 89.
2-18.
CIRCULATORS
a. The circulator allows up to three transmitters and three receivers to use a common antenna. This is
done in such a way that no transmitter interferes with either of the others. In a dual-frequency-diversity system,
however, only three of the four ports are used, and the remaining port is blocked by a plate, as shown in figure 90.
Figure 90. Dual-frequency-diversity microwave system.
b. The transmitter path through the circulator is as follows: microwave energy entering any port passes
through the circulator in the direction of the arrow (clockwise) to the next port, where it emerges.
(1) Assuming connections as shown in figure 91, microwave energy from transmitter A enters the
circulator at port 4 and emerges at port 1. The transmitter and receiver bandpass filters associated
with transmitter and receiver B reflect the microwave energy from transmitter A back to the
circulator. The transmitter A energy reenters the circulator at port 1. This energy is circulated in
a clockwise direction, emerges at port 2, and is delivered to the antenna.
(2) Microwave energy from transmitter B enters the circulator at port 1, is circulated clockwise,
emerges at port 2, and is delivered to the antenna.
344 L2
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