Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 301
At this writing, the only adjustable-element Yagi on the market is the SteppIR family,
manufactured by SteppIR Antennas, Inc. Small motors at the center of each element
lengthen or shorten the elements of the company’s two-, three-, or four-element Yagis
in response to microprocessor commands from the radio room below. As a result, a
single set of fixed-mounted elements can be optimized at all frequencies between 7 and
54 MHz, depending on the specific model purchased. Two- and three-element Yagis can
almost instantly switch the direction of maximum forward gain by 180 degrees simply
by commanding the parasitic elements to swap lengths, thus swapping their roles as
reflector and director. Since the antenna dimensions do not have to be optimized for
more than a single frequency at any one time, element spacing along the boom is virtually
immaterial, although the SteppIR models that cover 6 m do include additional
fixed-length parasitic elements for that band, interlaced with the motorized elements.
The SteppIR is clearly more expensive to manufacture and sell than a conventional
monoband HF Yagi and, as you might expect, has a higher purchase price. But if you
require a gain antenna capable of operation on any frequency between 7 and 54 MHz,
it could well be the most economical approach when compared with the cost of multiband
Yagis or a bevy of monobanders requiring multiple towers and/or stronger and
taller masts. A bigger concern for some is the potential reliability and maintenance costs
associated with having electronic circuitry located high in the air. Nonetheless, there are
today many satisfied SteppIR users around the world.
Yagis with Loaded Elements
One of the most popular Yagi implementations on the market today is the monoband
beam with shortened elements—especially on 30, 40, and 80 m, where beams with fullsize
elements pose mechanical challenges to the rotor and support system. Typically,
elements are shortened in either of two ways:
• Lumped loading coils
• Linear loading wires
Keep in mind that while lumped loading coils may appear very similar to traps
(especially when viewed from 50 to 100 ft below!), they differ from traps in that they are
not meant to form a resonant circuit or act like a trap anywhere near the operating frequencies
of the beam. A loading coil is simply replacing part of the radiating element
with a lumped component inductor.
Like traps, loading coils have loss. The coils found on beams with shortened elements—notably
the Cushcraft 40-2CD and its successor, the XM240—are not high-Q
coils. By comparison, the Force 12 Delta series of “shorty 40” loaded beams uses extremely
high Q coils that are, not surprisingly, extremely bulky and heavy. Similar high-
Q coils designed and sold by individuals are currently available; they are primarily of
interest to a handful of amateurs who are building their own 40- and 75/80-m low-loss
shortened Yagis.
But it is easy to overstate the practical effects of loss in the stock coils. The Cushcraft
two-element 40-m beams have enjoyed an outstanding reputation for performance. One
of the authors has owned four different brands of “shorty 40” two-element Yagis at one
time or another during the past four decades—two with loading coils and two with linear
loading wires—and has nothing but praise for the Cushcraft’s electrical performance.