Radar System Engineering

398 R-F COh[PONENTS [SEC. 113
dielectric of nominal outer diameter 0.280 in. A section of the cable with
the standard type N connector is shown in Fig. 11.7. The connectors
match the 50-ohm impedance of the cable at 10-cm and longer wavelengths.
The mismatch at 3 cm is not great. Breakdown in the connectors
limits the peak power to a few kilowatts; the most common use
of such cable is in test equipment. Attenuation data are given in
Table 11.1.
11.3. Waveguide.—Although a metallic pipe of almost any shape
will transmit or guide electromagnetic waves if their wavelength in air is
short enough, rectangular tubing whose internal dimensions have a ratio
between 2.0 and 2.5 has been almost universally adopted where the problem
is simply the transfer of microwave energy. (Use of round guide in
the special case where axial symmetry is required is discussed in a later
paragraph.) A detailed understanding of the propagation of waves in a
region bounded by conducting walls can only be obtained from the solution
of Maxwell’s equations. Practically, however, the results of the
mathematical analysisl have come to be used in a procedure which retains
most of the concepts of transmission-line theory, with equivalent lumped
reactance connected at suitable points to account for the effects of
discontinuities.
The resemblance between a rectangular waveguide and a two-wire
transmission line is shown in Fig. 11.8a to 11.Sd. In Fig. 11.8a is shown
a single quarter-wave stub support, analogous to the coaxial stub support
described in Sec. 11.2. At the proper frequency the input impedance of
the short-circuited stub is extremely high and there is no effect on the
propagation of the wave on the line. In Fig. 11.8b a great many stubs,
extending both ways from the two-wire line, have been added, still without
affecting the propagation of the frequency in question. In Fig. 11.8c
the stubs have coalesced into a rectangular tube which looks like a wave-
Wide. For a single stub, a slight correction to the length is necessary to
allow for the inductance of the crosspiece, but when the stubs become a
solid tube, no lines of force can link the narrow side, and the quarterwave
distance becomes exact. This also implies that the length of the
narrow side of the tube is not critical.
The two-wire transmission-line model explains how a waveguide can
transmit all frequencies higher (wavelengths shorter) than that for which
the quarter-wave stubs were designed. In such a case, as shown in Fig.
11.8d, the two wires”become broad busbars with only as much of the wide
side of the guide given over to stubs as is required by the now shorter
wavelength. However, wavelengths greater than twice the broad
dimension cannot be propagated because then the stubs become less
Waveguide Handbook, Vol. 10; Microwave Trammi.ssimi Circuits, Vol. 9.