Radar System Engineering

SEC. 113) WA VEGUIDE 403
types of metal hose or conduit are produced. A molded rubber sheath
pressurizes and protects the piece, as well as holding the adjacent turns
in tight contact. For short lengths, convolutions small compared with
the wave-length can be formed hydraulically, and thereby give flexibility
to a continuous metal tube. Figure 11.1 la! shows an example.
Transitions between waveguide and cooxial lines usually take the
form of a quarter-wave stub antenna on the coaxiai line projecting into
the waveguide a quarter guide-wavelength from an endplatej as shown in
FIG. 11.12.—Coaxial line to waveguide transitions. (a) Probe transition, coaxial line to
waveguide. (b) Doorknob transition.
Fig. 11 12a. The endplate reflects the energy going in that direction
back in phase with that going down the guide. Expressed in terms of
impedances, the short-circuited quarter-wave section of guide presents
an open circuit at the probe. Then th~ load seen at the probe is only a
single unit of guide impedance, rather than two units in series. Such a
probe lowers the breakdown potential of the guide. The “doorknob”
transition of Fig. 11. 12b is designed to minimize breakdown. It can be
thought of as a quarter-wave probe with a special form of stub support
for the tip of the probe in which capacitive and inductive effect have been
balanced against each other so that as a support it presents no loading
of the waveguide. Rounded contours reduce the electrical gradient as
much as possible.
Rotary joints between pieces of waveguide may consist of a coaxial
rotary joint of the type described in Sec. 11.2 with transitions to waveguide
at each end. Thk is common in the 10-cm region. The large
1~-in. OD line and the doorknob transitions just mentioned are used