Practical_Antenna_Handbook_0071639586

CHAPTER 12
The Yagi-Uda Beam Antenna
As we have discussed in the past few chapters, antenna arrays deliver an increased
signal to or from one or more favored directions at the expense of signal
in other directions. We have seen examples of both all-driven (or driven)
arrays and parasitic arrays. In the latter, one or more elements of the array are not directly
connected to the feedline; instead, they cause RF energy to be redirected through
careful choice of certain of their characteristics—in particular, their length relative to a
half-wavelength at the operating frequency and the spacing between them and adjacent
elements of the array.
Arguably the most popular parasitic array in common use today is the Yagi-Uda
beam antenna or, more commonly, the Yagi (thus showing the advantage of having your
name appear first on research papers and book covers). Amazingly, the Yagi has been
with us for 85 years, having been first described publicly by its inventors in 1926! The
Yagi is inherently a balanced antenna, making it a natural for horizontal polarization,
but many tens of thousands of vertically polarized Yagis for the 11-m citizens band have
dotted the landscape here in the United States since the 1960s. Nor is it difficult to recognize
certain parasitic arrays of verticals used by broadcast stations and amateurs as
ground-mounted equivalents of Yagi “half-elements”.
Despite its age, the Yagi was not popularly employed on the high frequencies until
after World War II, when the availability and affordability of aluminum tubing in appropriate
lengths, diameters, and thicknesses made the backyard assembly and erection
of these arrays feasible for the average radio amateur. Before then, only dedicated
DXers seeking “that extra edge” made Yagis from whatever was available to them. In
fact, the first full-size rotary three-element 20-m Yagi seen by one of the authors was
made of cast-iron pipe; its owner reported it weighed 300 lb! It was mounted on a
heavy-duty tripod on the roof of his house, directly over his radio room, and rotated by
hand from inside the room. Other builders of early Yagis stapled ordinary copper wire
to structural frameworks made from cheap, easily obtainable materials, such as wood
two-by-fours.
Certainly there were many other horizontal HF arrays in use prior to 1940. But
most were usually fixed in position and had only one or two favored directions. The
W8JK two-element driven array was very popular—so much so that a few enterprising
amateurs had designed and built rotatable versions. But by 1950 it was clear that
the Yagi-Uda design with the need to directly feed only one element, coupled with the
use of aluminum for the entire structure (originally described as the “plumber’s delight”
method by the late Bill Orr, then-W6SAI, in a February 1949 QST article), provided
the basis for a lightweight and economical beam antenna on 14 MHz and above
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