Radar System Engineering

i?EC. 11.3] WA VEGUIDE 401
too gradual an attenuation of the higher modes inevitably excited at
discontinuities such as T-junctions and diaphragms. For these reasons,
waveguides are ordinarily used only for frequencies where the broad
dimension lies between 0.60 and 0.95 of the free-space wavelength.
Since the maximum electric field comes across the narrow dimension
of the guide, it is undesirable to choose this dimension too small. In
fact, the power that can be transmitted for a given breakdown field is
directly proportional to the guide dimension
in the direction of the field.
The height is limited by the requirement
that the narrow dimension be
less than half a free-space
wa~.elength
in order to avoid the possibility of
propagating the simplest mode ha\--
ing polarization at right angles to
that shown in Fig. 11.9,
FIG. 1l.10.—TY~.-eguirlerllo!ie cou~li,,g,
~-ClllLxind.
There is no unique or generally accepted definition of the impedance
of a waveguide. This might be expected from the lack of definite
localized terminals at which the voltage and current could, in principle
at least, be measured. Howe\-er, the usual procedures of impedancematching
are carried over from transmission-line theory, and calculations
are made on the basis of normalized impedances. The impedance of a
standard waveguide is defined as unity, and resistive or reactive elements
inserted in the guide are computed relative to the standard, rather than
in ohms.
A typical choke joint between pieces of ~~aveguide is shown in Fig.
11.10. The principle is identical with that discussed in Sec. 112 for the
outer conductor of a coaxial rotary joint. The diameter of the radial
section spreading out at right angles to the rectangular tube is chosen so
that the average or effective distance from the inner surfaces of the
waveguide is a quarter wavelength. .$ circular groo~-e, likF\\-isea qufirtcr
wavelength deep, forms the short-circuited terminating section. .i rLlbber
gasket in the outer groove serves to keep the wavegyide airtight. By
careful choice of dimensions, such a joint can be made to be a good match
over a frequency band 12 to 15 per cent wide. Since no rurrent floii-s
across the gap between the choke and its mating flange, physical contact
is not necessary. The power flows across a small gap with negligible loss,
However, in such a case the leakage of radiation, although small compared
to the transmitted po\~cr, may still o~-erwhelm sensiti~-e energ~- detectors
nearby. In cases ~vhere electrical leakage must be minimized and the
outer gasket groove is not needed for pressurization, an electrical gasket
is substituted. Such a gasket is made by pre,s~ing a ring of }~oven metal
gauze into the proper form.