Practical_Antenna_Handbook_0071639586

CHAPTER 6
Dipoles and Doublets
A
common misconception about radio communication is that costly antennas
and supports are required in order to hear or “get out” well. To the contrary,
many of the world’s strongest signals and most competitive amateur radio enthusiasts
employ simple but effective antennas that can be erected by inexperienced
people without any special tools or equipment. The key to success in these cases is not
money—it is knowledge of a few key fundamental concepts, coupled with intelligent
planning and siting of the antenna, all based on the user’s objectives.
Leading all other antenna types in performance versus simplicity is the half-wave
dipole. The half-wave dipole—consisting of nothing more than a length of wire—is an
amazing antenna in its own right. For that reason alone, a solid understanding of how
a λ/2 dipole functions is perhaps the single most important piece of knowledge to take
away from this book. But it’s so good, in fact, that it forms the basic building block for
countless other types of antennas that we know by other names: the Yagi, the quad, the
collinear, the loop . . . and so on. In short, whether used individually or in concert with
one or more additional dipole-like elements, the half-wave dipole excels at efficiency of
radiation and ease of matching to common types of feedlines.
The half-wave dipole is one member of a class of antennas named Hertz, or hertzian,
for radio pioneer Heinrich Hertz. Hertz used a short dipole in his 1887 experiments that
first demonstrated the existence of radio waves predicted by James Clerk Maxwell
nearly a quarter-century earlier. In general, a hertzian antenna is one that does not rely
on the presence of a connection to ground and is usually constructed and erected such
that its two sides maintain a symmetrical relationship to ground. The most common
example of an antenna with hertzian origins is the ubiquitous TV antenna. In contrast,
Marconi antennas as a class typically are unbalanced with respect to ground and depend
upon fields induced in either the natural ground or an artificial ground system
beneath them for their proper operation. Commonly seen examples of a Marconi antenna
are AM broadcast towers and citizens band ground-plane antennas.
The original hertzian dipole was not a half-wave dipole; Heinrich Hertz’s experimental
apparatus had dipole arms that were much shorter than the fundamental wavelength
produced by his spark generator. However, over the years, usage of the term hertzian
has been narrowed to refer primarily to antennas whose total length is approximately a
half-wavelength. Today, the terms dipole and half-wave dipole are frequently used interchangeably.
Occasionally you may hear also the half-wave dipole called a half-wave doublet. Historically,
the terms dipole and doublet referred to two elements of charge, one positive and
one negative, driven to opposite ends of a fictitious infinitesimally short hertzian dipole
by an equally fictitious ideal signal source. As noted in Chap. 3, many, if not most, grad-
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