Practical_Antenna_Handbook_0071639586

CHAPTER 9
Vertically Polarized
Antennas
In previous chapters, we defined the polarization of an antenna as being the same as
the orientation of the electrical (E) field for the antenna. The direction of the electric
field for a specific antenna design is a function of the geometry of the radiating
element(s). For a complex structure with different portions creating E-fields having different
orientations and phasing, the “effective” E-field is the vector sum of all the individual
E-fields and may not even be stationary. However, for the simple case of an
antenna (or radiating element) consisting of a single wire or thin rod oriented vertically,
the E-field points in the same direction as the long dimension of the antenna, so the
antenna polarization is also vertical, by definition.
But what do we mean by “vertical”? In free space, just as an astronaut freely floating
outside the space shuttle has no particular up or down or sideways reference, a wire
in free space is neither vertical nor horizontal nor anything in between. In fact, the
words “vertical” and “horizontal” have meaning for antennas only in respect to some
reference plane possessing some amount of electrical conductivity. For most of us, there
are only a few such reference planes that we will ever be concerned with:
• The earth’s surface (for most land-based or fixed-station antennas)
• The roof or trunk of an automobile (for most mobile antennas)
• The skin of a satellite (for most satellite-mounted antennas)
• The roof of a tall building (for a city dweller’s antennas)
We define the polarization of the antenna primarily because many antenna characteristics
(radiation pattern and input impedance, to name just two) are greatly affected
by any metallic or conducting bodies in close proximity to it. All of the surfaces listed
here are conductors to one degree or another, so the orientation and proximity of the
antenna relative to any of these surfaces are potentially critical to the proper functioning
of the antenna.
Throughout this book, we will assume—unless otherwise stated—that we are always
talking about antenna orientations relative to the earth below, with the assumption
that our globe is perfectly spherical. In other words, “up”, “down”, and “vertical”
all coincide with the direction of gravity’s pull directly beneath the antenna. “Horizontal”
will be, by definition, 90 degrees away, parallel to the surface of the earth.
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