Radar System Engineering

396 RF COMPONENTS (SEC.11.2
The standard connector for joining coaxial lines is shown in Fig. 11.5.
Since longitudinal currents cross the junction plane on both inner and
outer conductor, good contact must be assured. For the outer conductor
this is done by pulling together two mating cones of differing taper by
means of the strong outer clamping rings. The fittings that solder to
FIQ.11 .5.—Coupling for coaxial line.
the outer tube also contain a gasket groove for keeping the line airtight.
The inner conductor is itself a tube. The two pieces are joined by a
beryllium copper “bullet” which is soldered into one piece and makes
tight contact with the inner surface of the other by means of expanding
prongs on the rounded tip.
In order to transfer r-f power to a rotating scanner, a rotary joint for
a coaxial line is necessary. Early designs involving wiping contacts on
both the inner and outer conductors
gave difficulties arising from poor contact,
sparking, and wear. The superiority
of the noncontact type employing
choke joints has led to its universal
(a)
adoption. The principle is shown in its
simplest form in Fig. 11.6a. The gap
between the stationary and rotating
parts of both conductors is situated at
the end of an open-ended quarter-wave
coaxial line. As was mentioned in
Sec. 11”1, the impedance at the input
(b)
FIG. 11.6.-C!hoke-type rotary joints.
end of an open quarter-wave section (in
this case across the gap in the line) is
zero. Power flows across without loss
or sparking. Closer analysis shows that the open end of the quarterwave
section on the outer conductor is not an infinite impedance, because
there is some radiation, producing a finite radiation resistance. This
effect can be reduced, and the match improved, by adding a short-circuited
quarter-wave line in series with the outer gap, as in Fig. 11.6b.
The outermost gap can now be very small, or even a rubbing contact,