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