Radar System Engineering

404 R-F COMPONENTS [SEC.113
where powers of the order of a megawatt are handled. In the 3-cm
region transition is made to round guide, where the next-lowest mode has
axial symmetry. Currents flow across the junction between the rotating
and nonrotating parts of the round tube by means of the same folded
choke arrangement used for the outer conductor of the coaxial rotary
joint described in Sec. 11.2. Figure 11.13 shows a typical joint. The
lowest mode in round guide has diametral rather than axial symmetry,
FIG. 11.13.—Waveguide rotary joint usingroundwaveguideand axially symmetrical
mode.
and if present to an appreciable extent will cause serious variation in
voltage standing-wave ratio as a function of angle of rotation. The
transitions from rectangular to round guide are designed to avoid exciting
the undesired mode as far as possible. The guide wavelengths of the
desired and undesired modes are sufficiently different so that a suitable
choice of length of round guide will minimize coupling of the undesired
mode from one rectangular guide to the other. Various absorbers for the
undesired mode are sometimes used.
The power-handing ability of a waveguide, calculated from the electric
fields involved, is approximately twice that of the largest coaxial line that