Radar System Engineering

SEC. 11.5] DUPLEXING AND TR SWITCHES 411
Figure 11.17 shows a cut-away view of another type of TR tube, the
1B24, which is widely used in the 3-cm band. It is of the integral-cavity
type—that is, all of the cavity contains gas, not just the central portion.
A gas reservoir on the side increases the total volume of gas over that
contained in the relatively small 3-cm cavity. The tube is clamped
between standard 3-cm rectangular choke joints, and coupling into and
out of the cavity is by means of round windows or irises. Glass is
soldered across them to seal the gas enclosure. Such windows play the
same role in coupling cavities to waveguide as do coupling loops for
coaxial line. On 10-cm systems using
waveguide, the input coupling may be
by means of an iris, and the output by
a coupling loop.
Switches exactly like TR switches
with the output coupling omitted may
be used as ATR switches. However,
since the double adjustment of the two
switches is not always made correctly,
fixed-tuned, low-Q ATR switches are
preferable. Figure 11.18 shows the
1B35 tube, designed for the 3-cm band,
and its mounting. One such tube will
cover a frequency band of 3 per cent,
and pairs of these tubes will cover a
band of 6 per cent. The loaded Q of
the cavity is made very low (approximately
5) by a large windowin the end. FIG. 11.lS.—1B35 broadband ATR tube
and mount; 3-cm band.
The cavity is placed in series with the
magnetron line by simply substituting the face of the tube containing the
window for a portion of the broad side of the waveguide. The breakdown
takes place across the inner face of the elongated glass window.
A duplexer that represents in waveguide the basic scheme of Fig.
11.14 appears in Fig. 11.29 as part of an r-f transmitting and receiving
system. The TR-tube input window is in contact with the narrow side
of the waveguide. From the resemblance of a waveguide to a stubsupported
two-wire transmission line, illustrated in Fig. 11.8, it is
apparent that a T-junction on the narrow side of the guide is a parallel
connection, and that the plane of the narrow side is a quarter wavelerigth
from the effective center of the waveguide circuit.
MICROWAVE COMPONENTS OF THE RECEIVER
After emerging from the TR switch, the received signal is mixed with
the local-oscillator signal and the combination applied to the crystal, in