Practical_Antenna_Handbook_0071639586

296 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
diameter step at the junction of certain pairs of tubes. Some commercial antenna manufacturers
accomplish this by swaging one end of each tube, but most of us do not have
the tools or the skills to do this at home. The other approach is to insert a few inches of
tubing having the right “in-between” diameter at the junction of the two tubing segments.
But if we’re going to have to obtain tubing for the “unused” diameters to avoid
having to swage joints, we might as well use it for element segments, instead.
Caution The half-element taper schedule of Fig. 12.4A has good wind and ice survival
characteristics only because critical regions of the half-element are double thickness by
virtue of inserting the next smaller diameter inside the larger tube for at least the distances
shown in the figure. If you attempt to scrimp on tubing material and do not overlap tubes at
least as much as shown in the drawing, your Yagi will fail at very low wind speeds! All
telescoping joints and doubled thickness regions must be pinned so as to prevent relative
slippage between any two tubes. Popular techniques for pinning include dimpling “hidden”
reinforcing tubes with a punch, stainless steel sheet metal screws, and pop rivets. Unless
you’re extremely proficient at slitting the ends of tubing and properly applying hose clamps,
using hose clamps on telescoping sections is not recommended by the author.
An important design decision is whether to shunt-feed the driven element. If so, the
element can be continuous metal tubing across its center point as it passes over or under
the boom, but the added complexity of a gamma match or other shunt-feeding system
may be a potential deterrent to the backyard builder.
Instead, this beam employs a split driven element, thus allowing the use of a simple
feed technique, the 4:1 balun. This is not exactly a trivial decision because an 18-ft length
of tubing, whether tapered or not, supported only at one end represents quite a cantilevered
beam. The penalty for choosing this approach is that the boom-to-element bracket for
the driven element must be long enough to allow proper spacing of four U-bolts (two
properly spaced on each side of the driven element) along its length and the U-bolts and
bracket must be strong enough to withstand the moment arm created by the force of the
wind over the entire length of the element half. Also, insulating “tubes” must surround
the element halves where they are grasped by the U-bolts, and the insulation must extend
to just beyond the outer edges of the bracket. Purchase of appropriate boom-to-element
insulating bracket assemblies from the antenna manufacturers or accessory suppliers
listed in App. B may be the best option for the first-time builder of an HF Yagi.
For the director and reflector brackets, flat aluminum plates that are at least ¼-in
thick, 3.5 in wide (the edge dimension that is parallel to the boom length) and 6 to 8 in
long (the edge dimension parallel to the element length) should be fine for normal wind
speeds. Alternatively, obtain aluminum rectangular (bathtub) channel of roughly the same
dimensions for vastly increased strength.
Table 12.2 is used in conjunction with the element taper drawing of Fig. 12.4A to
assemble each element of the 20-m Yagi. The table includes three separate sets of dimensions,
each having a distinct focus:
• The “CW” column emphasizes performance across the 14.000 to 14.100 end of
the 20-m band. In particular, when fed with 50-Ω transmission line, the SWR is
less than 1.5:1 across that 100-kHz range and the F/B ratio exceeds 30 dB over
the bottom 80 kHz or so.
• The “Mid” column is a design that maximizes SWR bandwidth while maintaining
reasonably flat gain across the entire 20-m band. As a result, maximum gain at the