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 293
Of course, aluminum is used for antenna booms and elements for the same reason
it is used in airplanes: weight versus strength versus cost. Aluminum has other advantages
that we often take for granted: It oxidizes in a fairly benign and easily reversible
way, and it’s easily cut by hand with a hacksaw or drilled with a hand drill, to name but
two. Recognize, however, that aluminum is a broad umbrella for a variety of alloys
prepared in many different ways. Aluminum tubing you find at the local hardware
store may not have the strength you need, at least for larger Yagis. In his excellent book
Physical Design of Yagi Antennas, David B. Leeson, W6NL (ex-W6QHS) warns readers
that 6061-T6 and 6063-T832 are better choices for antenna builders than the more commonly
stocked 6063-T5. If you must send away for aluminum tubing stock, see App. B
for a list of suppliers.
Total boom length and the spacing of elements along the boom are important parameters
in the design of any multielement Yagi. Boom lengths for three-element Yagis
typically lie between 0.15 l and 0.5 l. As a “glittering generality”, for a fixed number of
elements, longer boom lengths are capable of wider SWR bandwidths and make it easier
for the beam designer to achieve maximum gain and high F/B ratios across the same
frequency range. But if narrower bandwidths and a slight reduction in maximum forward
gain are acceptable to the owner, excellent performance can be obtained from
beams with shorter booms.
Rotatable Three-Element 20-Meter Yagi
At frequencies below about 24 MHz, the elements of rotatable Yagis are best formed
from multiple sections or segments of tubing having diameters that get smaller with
distance from the boom or center of the element. Such a design approach can provide
maximum wind and ice survivability with minimum weight elements. For 15 m, each
half-element will likely consist of a (largest diameter) center section and perhaps three
progressively smaller diameter sections. On 20 m, a center section and four to six progressively
smaller diameters are common. Below 20 m, survivability issues usually lead
to design approaches involving substantially more complicated methods (trusses, for
example) of strengthening booms and elements alike, so 20 m represents just about the
lowest frequency for which a simple design like the one presented here is mechanically
adequate for the majority of wind environments.
Stepped-diameter correction of each element’s length can be done manually, using
a spreadsheet technique developed by Leeson and described in his book. Alternatively,
antenna modeling software (such as W7EL’s EZNEC 5+) that incorporates the Leeson
correction can simplify the task. Once an element taper schedule such as that of Fig.
12.4A has been created based on mechanical (strength) considerations, the lengths and
diameters of the various element sections are entered into the EZNEC wire table, and
EZNEC automatically incorporates the Leeson corrections in its calculations to arrive at
the correct electrical length for each element half. (Just be sure to first turn on the
stepped-segment option!)
A convenient boom length for a rotatable three-element 20-m Yagi is 20 ft, since the
boom can be a single length of 3-in irrigation tubing or a pair of 2-in OD × 10-ft aluminum
tubes butted together with a short internal or external reinforcing tube and a
boom-to-mast bracket of suitable dimensions, as depicted in Fig. 12.4B. Also shown is
an alternative approach to constructing the boom from multiple sections of aluminum
tubing if the longest that can be obtained conveniently are 6-ft sections. (See App. B for
suppliers that routinely ship boom and element tubes by UPS.) Regardless of the exact