Practical_Antenna_Handbook_0071639586

280 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
Direction of
propagation
Director
D1
S
Driven
element
Feedpoint
DR
S
Reflector
R
Figure 12.2 Basic Yagi-Uda antenna.
In addition to having a driven element—which is similar to a standard half-Â
wavelength dipole fed at or near the center by any of a number of different means that
we will discuss later—Yagi antennas employ either or both of two additional types of
elements: reflectors and directors. These are called parasitic elements because they are
not directly connected to the feedline but instead receive energy radiated from the
driven element and then reradiate it. The combination of this reradiated energy from
the parasitic elements with the original radiation from the driven element causes
peaks and nulls in the radiation field at a distance from the antenna as a function of
both the azimuthal (or compass) heading and the elevation angle of the receiving point
from the antenna.
The mathematical equations to support and “prove” the operation of Yagi antennas
are beyond the scope of this book, but here’s a short qualitative description of what
goes on:
The driven element of a Yagi is, for all intents and purposes, nothing more than a
half-wave dipole. RF energy fed to it via the feedline connected at its center terminals
creates RF currents and magnetic fields that radiate from the element, as we learned in
Chap. 3. When these radiation fields reach a parasitic element (either a reflector or a
director), they induce fields and currents in that element because it is, after all, a conductor.
But because each parasitic element is deliberately made shorter (when used as a
director) or longer (when used as a reflector) than an exact half-wavelength at the frequency
of operation, the induced fields do not establish the same amplitude and phase
relationships on the parasitic element as exist in the driven element.