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 139
feedpoint impedances substantially higher or lower and/or more reactive than the
ideal l/2 dipole. So we can be reasonably certain that a matching network at the junction
of the feedline and the antenna is likely to be needed.
For most of us, the transmitters, transceivers, and amplifiers we are apt to buy or
build are designed for maximum power transfer to a 50-Ω resistive load. In many cases
we can simplify our task by choosing to use 52-Ω or 75-Ω transmission line from the
transmitter to a matching network closer to the antenna. Most of the following techniques
for obtaining a match assume just such a configuration.
In general there are two broad categories of matching techniques:
• Distributed matching systems (stubs, transmission line sections, etc.)
• Lumped element tuners (antenna tuning units, or ATUs)
The second category is covered in Chap. 24.
R 2
Distributed Matching Networks
Networks that depend on the propagation of the signal along a carefully chosen length
of conductor for their matching capabilities are called distributed networks. The popular
examples described in the following sections are typically employed either as a series
element placed between the antenna and feedline or as a shunt element directly across
the antenna feedpoint or a short distance along the feedline from it.
Keep in mind that since the dimensions of most
distributed matching networks are a function of the operating
frequency (or wavelength), a specific implementation
will work as advertised only over a limited
frequency span centered on the design frequency. Most
of the networks described here are most useful, therefore,
as matching devices for single-band antennas and
arrays.
R 2 4R 1
Coaxial Cable Baluns
Figure 4.11 describes a balun that transforms impedances
at a 4:1 ratio, with R 2 = 4 × R 1 . The length of the
balun (or U-shaped) section of coaxial cable is
L = 492 F
(4.41)
Fv
where L = measured physical length of U-shaped
section of cable, in feet
v F = velocity factor of coaxial cable (typically
0.66–0.85)
F = operating frequency, in megahertz
R 1
Figure 4.11 Coaxial balun transformer.