2-29Figure 2-22.—Hybrid-ring duplexer.During transmission the E field from the transmitter enters arm 3 and divides into two fields 180degrees out of phase. One field moves clockwise around the ring and the other moves counterclockwise.The two fields must be 180 degrees out of phase at the entrance of an arm to propagate any energy downthe arm. The field moving clockwise from arm 3 ionizes the tr tube in arm 4, and the energy is blockedfrom the receiver. The tr tube reflects a high impedance equivalent to an open circuit. This highimpedance prevents any energy from entering the receiver - even though the two fields are out of phase atthe entrance to arm 4. The field moving counterclockwise from arm 3 ionizes the tr tube in arm 2, whichreflects a short circuit back to the ring junction. No energy is sent to the receiver, however, because thefields arriving at arm 2 are in phase. The clockwise and counterclockwise fields arrive at arm 1 out ofphase by 180 degrees. They are then propagated through the arm to the antenna.During reception, the relatively weak field from the antenna enters arm 1 and divides at the junctioninto two out-of-phase components. Neither field is sufficient to fire the tr tubes in arms 2 and 4; since thefields arrive at these arms out of phase, energy is propagated to the receiver. The energy arriving at arm 3is in phase and will not be coupled to the transmitter. Since the operation of the arms of a hybrid ring isthe same as the operation of E-type waveguide T-junctions, you may find it helpful to review NEETS,Module 11, Microwave Principles.
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