magnetic properties, they are called electromagnetic
waves.
Most of the electromagnetic energy on the earth
originates from the sun. The sun’s electromagnetic
waves propagate through space and into the earth’s
atmosphere. The sun actually radiates electromagnetic
energy at several different wavelengths and
frequencies, ranging from gamma rays to radio waves.
Collectively, these wavelengths and frequencies make
up the electromagnetic spectrum, as shown in figure
2-2. Here on earth, radar systems transform electrical
energy into electromagnetic energy in the form of
radio waves.
Each region of the electromagnetic spectrum can
be subdivided into narrower frequency bands as shown
in figure 2-2. As you can see, electromagnetic waves
from radar energy normally fall between 200 MHz and
300 GHz. A radar transmitter emits this energy into the
atmosphere through an antenna. While only a fragment
of the energy returns, it provides a great deal of
information. The entire process of energy propagating
through space, striking objects, and returning occurs at
the speed of light. Targets struck by electromagnetic
energy are said to have been radiated, and the return
signals they produce are called radar echoes.
PROPERTIES OF ELECTROMAGNETIC
WAVES
An electromagnetic wave consists of two fields, an
electrical field and a magnetic field, which are
perpendicular to each other and to the direction of
propagation of the wave front (fig. 2-3). Polarization
refers to the orientation of the electrical field
component of an electromagnetic wave. Polarization
can be either linear or circular. With linear
polarization, the electromagnetic waves are either
horizontally or vertically polarized relative to the
earth’s surface (fig. 2-3).
Most weather radars, including the WSR-88D, are
horizontally polarized. There are two major benefits to
this. The first is that energy returns from man-made
ground targets that have a greater vertical extent than
horizontal extent (like buildings) are greatly reduced.
The second benefit relates to the returned energy from
raindrops. Since raindrops tend to flatten as they fall,
the surface area that the radar is able to detect
increases, thus increasing energy return. Other
important terms relating to electromagnetic waves you
need to know are wavelength, amplitude, frequency,
and power.
Figure 2-2.—The electromagnetic spectrum.
2-2
