WAVEGUIDE DUPLEXERS usually consist of tr tubes and atr tubes housed in a resonant cavity
and attached to a waveguide system in some manner. Resonant-cavity tr tubes may be applied to
waveguides, either directly or indirectly, to obtain switching action. The indirect method uses a coaxial
line system, and then couples the coaxial line into the waveguide that feeds the antenna. If large losses are
incurred by the use of a coaxial line, the resonant cavity can be coupled directly to the waveguide. Figure
2-20 shows a direct method of cavity tr switching in a waveguide system. The waveguide terminates in
the antenna at one end and in a shorting plate at the other. The magnetron uses a voltage probe to excite
the waveguide. The transmitted pulse travels up the guide and moves into the tr box through a slot. The
cavity builds up a strong electric field across the gap, breaks it down, and detunes the cavity. This action
effectively seals the opening and passes the pulse energy to the antenna.
Figure 2-20.Waveguide duplexer with cavity tr tube.
The signals received during the resting time travel down the guide to the magnetron and the shorting
end plate where they are reflected. The slot coupling the waveguide to the cavity is located at a point
where the standing-wave magnetic field produced by reflections in the waveguide is maximum. The
maximum magnetic field, therefore, energizes the cavity. The echo signals are not strong enough to cause
an arc, and the cavity field is undisturbed by the gap. Therefore, the cavity field couples rf energy into the
receiver coaxial line and provides maximum energy transfer.
The cavity tr switch can also be applied to branch lines of the waveguide, as shown in figure 2-21.
The magnetron is coupled to the guide by a voltage probe to produce proper excitation.