Practical_Antenna_Handbook_0071639586

282 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
ance. This new design uses two driven elements to provide maximum gain and
directivity over a wider bandwidth. In general, it can be considered a very wideband
three-element Yagi.
Over the years, there have been commercial two-element Yagi antennas designed
and sold that employed either a reflector or a director in combination with the driven
element—especially for the 40- and 30-m bands. If a reflector is chosen, peak performance
(i.e., forward gain and front-to-back ratio) will increase very rapidly from the
lower-frequency side of the operating range of the beam, then fall off slowly as the frequency
is increased through the operating range. If a director is chosen, the opposite is
true. All major manufacturers of two-element HF Yagis at the time of this writing offer
only the reflector + driven element version.
Three-Element Yagi Antennas
For a three-element HF Yagi with full-size elements, there are six important dimensions:
• Length of the driven element (DE)
• Length of the reflector (R)
• Length of the director (D)
• Diameter of the elements (d)
• Centerline spacing between the driven element and the reflector (R-DE)
• Centerline spacing between the driven element and the director (DE-D)
Of course, what appears in the preceding list as a single dimension (d) may be many
dimensions, depending on whether the elements are constructed of nested or telescoping
tubing of more than one outside diameter (OD). Also, at VHF and UHF (and even for
some HF designs) the specific dimensions of the boom-to-element attachment method
need to be incorporated in the design calculations. Finally, note that the sum of all the
interelement spacings (measured to the centerline of each element) plus the width of
one boom-to-element bracket determines the minimum required boom length for a specific
design.
All six of these dimensions interact with each other. That is, if we have a Yagi design
that meets our specifications for forward gain, front-to-back ratio, and SWR at 14.1
MHz, for instance, a change in the spacing between the reflector and the driven element
may well worsen the performance of any or all of those parameters at 14.1 MHz. If we
are lucky, we may be able to find another combination of element lengths that allows
our Yagi to meet or exceed the performance at that frequency, but it’s also quite possible
that we won’t.
As you might guess from the inclusion of element diameter in the preceding list, the
dimensions for a multielement Yagi constructed of #12 copper wire are not likely to be
identical to those for the same beam with elements of 1-in aluminum tubing. In fact,
because of the close coupling between elements, multielement Yagis are far more sensitive
to changes in all dimensions than simple dipoles, as we can see with the next two
examples.
Example 12.1 A l/2-dipole for 20 m constructed from #12 copper wire is trimmed for
a purely resistive feedpoint impedance at 14.1 MHz. The resulting length of the dipole