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 291
The best procedure for completing a Yagi is to initially assemble the dipole or Yagi
to recommended nominal dimensions with hose clamps and slotted tubes, attach the
feedline (discussed later), raise the antenna up in the air, and check the VSWR at the
transmitter end of the feedline before bringing the antenna back down to add the machine
screws or pop rivets. Whether this is possible or practical in any given installation
will depend heavily on the specifics of the site, the support, and the ground crew available.
Sometimes a stepladder or garage roof can act as a temporary support. Another
trick for minimizing the effect of nearby ground is to aim the Yagi straight up in the air
with its reflector nearest the ground (although this is of no benefit for the rotary dipole).
Remember, it’s no crime to secure the initial dimensions with the screws, then remove
the screws and redrill for a different element length, if necessary.
Plastic caps supplied for the element tips by many Yagi manufacturers are both a
blessing and a curse. While keeping spiders and other insects out, they can trap moisture
inside the element. Natural element sag is perfectly capable of draining moisture
on its own, so a small piece of wire mesh, held in place with a small hose clamp, is a
good way to prevent insects from setting up shop inside Yagi elements. Alternatively,
drill small holes in the stock plastic caps and install them with the holes toward ground.
Even the plastic caps should be held in place with good electrical tape or hose clamps
because they are easily dislodged during the antenna raising process. Also, many of
these caps tend to crack and lose their tight grip on the element when exposed to the
sun over long periods.
If the rotatable dipole is constructed of full-length elements so that no center loading
coil is employed, feeding it is simply a matter of connecting a 75-Ω coaxial cable
(RG-6, RG-59, RG-11, or equivalent) to its center. The center conductor of the cable goes
to one side of the element and the shield or braid goes to the other, using attachment
techniques detailed in Chap. 28. As described before, the two element halves should be
separated about 2 in by the supporting hardware. For someone with access to a wood
lathe, a 2-ft length of hardwood dowel turned down to the inside diameter (ID) of the
element tubing and treated with preservative (such as an exterior polyurethane paint)
or other water repellant is an excellent center insulator. Strain relief for the cable must
also be provided.
Note As discussed in Chap. 3 and elsewhere, using a loading coil as a substitute for part of an
antenna element reduces the element’s effectiveness as a radiator. Remember, the difference
between an efficient radiator and a discrete (lumped) circuit component is primarily in the
physical configuration of the conducting material. When you coil up a portion of an antenna
element into a small volume, you convert that portion of the radiator into a nonradiating
lumped component. The deleterious effect on the antenna is greatest when this is done within
the high-current section of the element. Therefore, whenever possible, loading of elements
close to their centers should be avoided. Often, other forms of loading can attain the same
reduction in length with far less decrease in radiated signal strength.
Multielement Yagi Construction
Each element of a rotatable multielement Yagi antenna can be constructed in a manner
similar to that used for the rotatable dipole described earlier. The only significant difference
is that the parasitic elements (reflector and directors) do not have a feedline attached
to them and the metal is continuous across the centerline of the element.