Radar System Engineering

SEC. 115] DUPLEXING AND TR SWITCHES 409
The practical method of accomplishing this impedance transformation
is by means of a resonant cavity. Figure 11.15 shows a section through
a 1B27 TR tube and associated cavity, with input and output couplings.
The gap across which the discharge takes place is formed by two reentrant
cones on the axis of symmetry of the approximately cylindrical cavity.
The cones add capacity to the resonant circuit and the cavity is smaller
for the same wavelength than it would be without them. Tuning is
accomplished by pushing one cone in and out on a flexible diaphragm.
The unloaded Q of the cavity is lower than that of a cylinder because of
From T-iunction Flexible
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1~ dtam. flange
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term,nals
FIG. 11.15 .—1B27 TR tube and cavity assemblywith loop coupling,
the presence of the cones and the glass of the gas enclosure; it is about
2000. With normal input and output loading, the loaded Q is about 35o.
Both input and output coaxial lines end in coupling loops which play the
role of the step-up and step-down transformers. They can be thought
of as single-turn windings which, in proportion to their area, loop more
or less of the magnetic field existing in the cavity. The smaller the loop,
the higher the step-up ratio and the higher the loaded Q.
As a result of the impedance transformation, the arc coupling (i.e.,
the power going to the receiver as a result of the voltage across the arc
discharge) is well below the danger point and is independent of input
power. There is a second mechanism of coupling, called direct coupling,
which gives leakage power proportional to input power. At the higher
transmitter powers, direct coupling becomes more important as a source
of leakage power than arc coupling. Direct-coupling power is the power