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 281
These induced currents make each parasitic element into a radiator in its own right.
That is, each director or each reflector now radiates an RF field just as the driven element
does. However, the RF radiation from a reflector, for instance, has a different phase from
that of the driven element, and it originates at a different point in space than the driven
element occupies. The radiated fields created by the induced fields and currents in the
parasitic elements travel in all directions into space. And at every point in space, they
combine linearly and vectorially (i.e., in both amplitude and phase) with the original radiated
fields from the driven element. Because the radiation from each element has its own
unique phase and amplitude, the resultant is a radiation pattern that exhibits peaks and
nulls depending on where in the space beyond the Yagi the receiving antenna is located.
Up to this point in the discussion, there is nothing particularly special about what
we have described. For instance, if you hang a 20-m dipole in your attic along your
ridgeline, you have probably created a multielement parasitic array comprised of your
dipole and any house wiring circuits running around the attic. Of course, you don’t
have much control over the distance to the house wiring or the length of these branch
circuits, but they are electrical conductors and they can have a major effect on the radiation
pattern and input impedance of your dipole.
The “magic” worked by Yagi and Uda was that they derived equations (without
benefit of electronic calculators or high-speed computers, it should be noted) capable of
accurately predicting the radiation patterns that would result from various physical
arrangements of elements, ultimately arriving at the configuration that now bears Yagi’s
name. From these equations and generally accepted engineering objectives for a
directional antenna (high forward gain, high front-to-back ratio, wide bandwidth, reasonable
input impedance, etc.), they then provided formulas for calculating practical
and manually optimized dimensions for element diameter, length, and spacing.
As a matter of definition, if a reflector is used, it is placed “behind” the driven element—where
“behind” is taken to mean on the side of the driven element away from,
or opposite, the direction of desired maximum signal radiation—and it is typically a few
percent longer than a half wavelength in the conductor at the operating frequency.
If a director is used, it is placed in “front” of the driven element (i.e., on the side of
the driven element that is in the direction of desired maximum radiation). In a traditional
Yagi design, the director nearest the driven element is typically about 3 percent
shorter than a half wavelength in the conductor.
Spacings between adjacent elements of a Yagi typically are between 0.1 l and 0.25 l,
where l is the wavelength in air. The exact spacings depend on what the designer intends
to optimize. For example, if weight, turning radius, and wind load are more important
than bandwidth, a three-element Yagi can fit on a boom that is only 0.2 l in
overall length, but the designer and user will find that other attributes, such as standing
wave ratio (SWR) bandwidth, front-to-back ratio, and that last ounce of forward gain,
have been compromised.
The basic Yagi is a planar antenna, with all elements lying in the same plane and
sharing the same orientation. Although there is no fixed rule regarding the number of
either reflectors or directors, it is common practice to use a combination of a single reflector
and one or more directors, in addition to the driven element. Thus, from the
definitions and “rules” of the preceding two paragraphs, the most common threeelement
beam consists of a driven element flanked by a reflector at one end and a director
at the other end of a boom supporting all three. Most four-element Yagis have two
directors, but in recent years a different kind of four-element Yagi has made its appear-