Radar System Engineering

SEC.11.2] COAXIAL LINES 395
lines have been almost completely supplanted by stub-supported lines
in the 10-cm region.
The principle of the stub support is shown in Fig. 11.2. As was mentioned
in Sec. 11.1, the input impedance of a quarter-wave line shorted
at the far end is the same as an open circuit. When placed in parallel
with the main line such a connection has no effect at all on the impedance
FIG. 112.-Simple quarter-wave stub FIG. 11.3.—Broadband stub support.
support.
and causes no reflection. The mechanical and electrical superiority of a
solid piece of metal as a support and insulator is obvious, and at 10 cm
the length of the stub (about 1 in. ) is such as to make the projection short
and unobtrusive.
Obviously a quarter-wave stub can have the desired property at only
one frequency. Deviations of only 1 to 2 per cent in frequency cause the
stub to have a reactance that presents
an appreciable mismatch.
Figure 11.3 shows a broadband stub
support where the frequency sensitivity
is compensated over a band of
i 15 per cent.
At the center of the
band the stub has an effective
length
of exactly a quarter wave, and the
two quarter-wave sections in the FIG. 11.4, —Stu&supported elbow.
main line transform to an impedance
lower than normal and then back to normal. The conditions for no reflection
are satisfied. At a frequency lower, for example, than the center frequency,
the stub is less than a quarter wave, but the inductive reactance
thereby presented at the T-junction is made just enough to compensate
for the fact that the quarter-wave transformers in the main line are also
less than a quarter wave long, and would present a mismatch in the
absence of the stub. Similar, but converse, conditions obtain for frequencies
higher than band center. Figure 11.4 shows how a broadband
stub can be used to make an elbow. There is an added complication
because the sharp elbow introduces a reactance which must be compensated
for in the construction of the stub.
Such supports are standard in coaxial lines used in the 10-cm region
and regularly have a VSWR less than 1.03 over a baad of ~ 15 per cent
from the center frequency.