Practical_Antenna_Handbook_0071639586

CHAPTER 8
Multiband and Tunable
Wire Antennas
Most communications operators require more than one band, and that makes
the antenna problem exactly that—a problem to be solved. Amateur, commercial,
and military operators are especially likely to need either multiple
antennas for different bands or a multiband antenna that operates on any number of
different bands. This situation is especially likely on the high-frequency (HF) bands
from 3.5 to 29.7 MHz.
Another problem regards the tunability of an antenna. Some amateur bands are
very wide (several hundred kilohertz), and that causes the feedpoint impedance of
most antennas to be highly variable from one end of the band to the other. It is typical
for amateurs to design an antenna for the portion of the band that they use most often
and then tolerate a high voltage standing wave ratio (VSWR) at the other frequencies.
Unfortunately, when you see an antenna that seems to offer a low VSWR over such a
wide range, it is almost certain that this broad response is the result of resistive losses
that are reducing the antenna efficiency. However, it is possible to tune an antenna over
a wide bandwidth using an adjustable antenna tuning unit (ATU) similar to those described
in Chap. 24.
Multiband Antennas
For users whose interest (or license) is limited to narrow frequency ranges at a number
of different places in the frequency spectrum, multiband antennas provide a reasonable
approach to putting a good signal out on
each of the desired bands. The sections that
B
A
A
B
follow provide examples of some timetested
ways to do this.
Trap Dipoles
Perhaps the most common form of multiband
wire antenna is the trap dipole shown
in Fig. 8.1A. In this antenna category, paral-
A10-meter segment
AB15-meter segment
Figure 8.1A Trap dipole for multiband operation.
(Courtesy of Hands-On Electronics and Popular
Electronics)
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