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


 


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