Practical_Antenna_Handbook_0071639586

286 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
• Perched at the top of a support, Yagi antennas are a prime target for lightning
discharges and wind-driven precipitation static. In addition, Yagis on a tower
may (inadvertently or deliberately) be part of a capacitive top-loading system
for 160 or 80 m when the tower is used as a transmitting vertical on either of
those bands. The Yagi designs that best address these concerns have both sides
of all elements grounded to the boom—either directly or through the matching
network.
Yagi Antennas with Four or More Elements
As additional elements are added to a Yagi, they typically are directors. For decades,
until the advent of the dual-driven element variant previously described, a four-Â
element Yagi almost automatically meant a reflector at one end of the boom, followed
by the driven element and the first and second directors, in that order. Similarly, a fiveelement
Yagi usually employed three directors. “Why not multiple reflectors?” you
may ask. The nonmathematical answer is this: If the first reflector is properly designed
and positioned on the boom relative to the driven element, virtually all of the energy
radiated off the “back of the beam” (i.e., beyond the reflector) will be canceled by the
phased combining of radiation from all the elements. Thus, there will be very little net
RF passing beyond the reflector and, hence, very little useful work for a second or third
reflector to do! Nonetheless, some very long, high-performance VHF and UHF Yagis
use three reflectors mounted in a plane orthogonal to the plane of the other elements.
Yagi Construction
In this section we will briefly discuss some simple construction methods and inexpensive
materials for Yagis that can be built relatively easily—i.e., practical ones. It is assumed
that most readers who want a triband multielement Yagi will prefer to buy a
commercial product, rather than build a homebrew model, because the traps or multibanding
circuits employed likely involve the use of construction techniques not available
to most hobbyists. The projects in this chapter are meant to be within the reach and
capabilities of most readers and assume the availability of little metalworking equipment
in the home workshop beyond drill, screwdriver, nutdriver, and hacksaw or tubing
cutter.
As the design frequency of a rotatable Yagi drops below 30 MHz or so, mechanical
issues come to the fore. In particular, reliable element self-support dictates the use of
multiple tubing diameters for the elements, sometimes in conjunction with truss systems.
It’s not uncommon, for instance, to find as many as five or six different tubing
diameters in each element of a 20-m beam. And, as element lengths increase, the
strength of the associated boom-to-element clamps (brackets) becomes a critical design
matter, as well.
For designs requiring more extensive workshops, the most practical—and the least
expensive—approach for many of us is to start by buying one or more used Yagis, if for
no other reason than to have a stock of suitably premachined parts and element sections
of many different diameters. Don’t overlook the parts value of a “junked” beam from a
fellow hobbyist or your local CATV system’s head-end! Lacking such sources, a few
specialty components (certain types of boom-to-element brackets, for instance) are
probably best obtained from the parts department of one of the antenna manufacturers