Practical_Antenna_Handbook_0071639586

C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 111
Figure 4.1C Twin-lead transmission line.
Outer
shield
Parallel
conductors
Dielectric
Figure 4.1D Shielded twin-lead transmission line.
Figure 4.1e Horseshoe parallel line spreader.
S
ing between the two wires is maintained at a
fixed distance by rigid insulators. Early radio
experimenters employed wood dowels
soaked in paraffin for protection against the
elements, but, as the market grew, ceramic
spacers became available. More recently,
molded plastic formulations have been employed
to reduce both the cost and the weight
of the total assembly.
Parallel lines have been used at VLF, MF,
and HF for a century. Even antennas for the
lower VHF bands are often fed with parallel
lines. For years, the VHF, UHF, and microwave
application of parallel lines was limited
to educational laboratories, where they are
well suited to performing experiments (to
about 2 GHz) with simple, low-cost instrumentation.
Today, however, printed circuit
and hybrid semiconductor packaging have
given parallel lines a new lease on life and a
burgeoning market presence.
Figure 4.1C shows a type of parallel line
called twin-lead. This is the venerable television
antenna transmission line. It consists of two
parallel conductors encased in, and separated
by, a flexible plastic dielectric. The dimensions
of TV-type twin-lead were established years
ago in conjunction with the chosen dielectric to
give the final product a characteristic impedance
of 300 Ω to simplify impedance matching to the
folded dipole driven element of the typical TV
antenna. Other kinds of twin-lead are manufactured
with other target impedances; some
forms of transmitting twin-lead have an impedance
of 450 Ω, for instance.
Figure 4.1D shows a form of parallel line
called shielded twin-lead. This type of line uses
the same structure as TV-type twin-lead, but
adds a shield layer surrounding it. The shield
may be braided, a thin layer of aluminum foil,
or both. This feature can make it less susceptible
to electrical noise and other problems.
Some users of open line prefer the spacer shown in Fig. 4.1E. Generally formed from
plastic or ceramic, its U shape reduces losses, especially in wet weather, by providing
increased leakage path length between the two conductors relative to their spacing, S.
Occasionally one will run across an installation using multiwire parallel line. Fourwire
and six-wire transmission lines most often find use when the distance between
transmitter and antenna is extremely long (i.e., a substantial portion of a mile).