Radar System Engineering

394 R-F. COMPONENTS [SEC.II 2
conduct or. These are used at microwave frequencies also, especially for
short, low-power interconnecting cables in the lo-cm region. However,
the attenuation even in the best dielectric, as seen in Table 11~1, begins
to be serious at 10 cm and gets worse at shorter wavelengths. Substitution
of air for the solid dielectric eliminates dielectric losses, and only the
much smaller conductor losses remain. But the center conductor must
somehow be supported mechanically. Thin dielectric beads have been
used as supports, but the disintegrating effect of dielectric breakdown
over the surface of the bead is hard to avoid. Reflections from the beads
can be largely canceled by proper spacing. 1 However, bead-supported
TABLE11.1.—S1,ANDA~DMICROWAVETRAMMISSIONLINE
Dimensior,s 01), in.
Wall, i:,.
Wavemgth,
crr
,faximur
power, *
MW
attenuation,
t
db/ra
Wavelength
rrrnge,cm
.4. Waveguide
1*X3
ixli
*X1
ix%
.0.080
0.06!!
0.050
0,040
10.0
3,2
3.2
1.25
10.5
1.77
0.99
0,22
0.039
0.15
0.24
o.5~
7.6-11,8
3,+4.7
2.4-3,7
1.1–1.7
B. Rigid coaxial lines, stubsupported
Outer l;
Inner ~
0.049
0.035
10.0
4.2
0.08
9.3-11,7
Outer ~
Inner ~
0.032
0.03”
10.0
1.3
0.15
9.1–11.7
Outer +
Inner &
0.032
0.032
3.2
0.36
0.49
3.1-3.5
C
Flexible coaxial cable, RG-
9/U, polyethylene dielectric
Dielectric OD 0.280 in.
Inner conductor 7 strands
#21 .4VVG,
Outer conductor double
braid
‘IO. O
, 3.2
0.31$
0.31$
0.56
1.12 )
3.o-@
● Comnuted for maximum gradient of 30 kv/cm. No allowance in coaxial lines for increaaed field
around stub supports. Practical operating point + to j of value. given.
t For bnam walls. For copper or ,ilver walk, attmuatmn is about half that liitsd, and for dver
plating has an intermediate value, depending on finish.
t Experimental value for coin-sih,er tubing, generally used at this wavelength. Surface finish
aKecta value.
f Specification limit for cable alone. Connectors hrnit safe power to a few kilowatta.
~ Microwave Transmission Circuits, Vul. 9.