Boom Influence on Yagi Antenna

 

The boom of the Yagi antenna is an inevitable part of its construction.

Theoretically, Yagi antennas can work fine without a boom. Practically, it is

possible only if the boom is made of non-conducting material (wood, fiberglass,

etc.) along with some mechanical maintenance and certain other limitations. In fact, a

conducting boom is not the intended radiating part of antenna but only an inevitable part

needed to support its weight.

The Yagi antenna can be built so that elements are insulated and separated by some safe
distance from any conducting boom, or passing through it. The latter method can be done
in two different ways: first, the elements being electrically bonded to boom, and second,
the elements are electrically insulated from boom. All of these element mounting
methods have their advantages and disadvantages and result in different boom influence
How does this unnecessary but inevitable “intruder” influence the Yagi antenna
performance? This is the question which provoked me to start an extensive work on
antenna software simulations which could further enlighten us about this problem.
How to do the task?
Antenna simulation programs based on Numerical Electromagnetics Code (NEC) and its
derivatives are the most common and straightforward way to simulate the Yagi antenna
performance. These programs use Method of Moments (MoM) antenna analysis codes
and have many advantages but a few limitations. Some of limitations are completely
unacceptable for simulations of boom influence on antenna performances. The most
problematic of them all is inaccurate simulations when wire conductors are spaced very
close relative to its diameter, and inability to simulate transversal currents (currents
flowing orthogonal to wire conductor axis) in thick conductors as in NEC-2 Manual [1]
explained:
“…only currents in the axial direction on a segment are considered, and there is no
allowance for variation of the current around the wire circumference”.
L.B. Cebik in one of his articles [2] in his popular Antenna Modeling series explains this
problem with doing Yagi antenna boom simulations:
“One potential arena in which the validity of these approximations may be tested is the
modeling of a boom connected directly to the parasitic elements of a Yagi antenna. In
practice, the connection or the very close proximity of a boom to the parasitic elements
alters the required length of the elements to preserve array performance. However, in
NEC-2 and NEC-4—when modeled within the other limitations of the software—the
boom has no effect upon the parasitic elements.”