Antenna Characteristics

ANTENNA CHARACTERISTICS As you will learn in this section, all antennas exhibit common  characteristics.  The  study  of  antennas involves the following terms with which you must become   familiar: Antenna Reciprocity The ability of an antenna to both transmit and receive  electromagnetic  energy  is  known  as  its reciprocity.  Antenna  reciprocity  is  possible  because antenna characteristics are essentially the same for sending  and  receiving  electromagnetic  energy. Even though an antenna can be used to transmit or receive, it cannot be used for both functions at the same time.  The  antenna  must  be  connected  to  either  a transmitter or a receiver. Antenna Feed Point Feed point is the point on an antenna where the RF cable is attached. If the RF transmission line is attached to the base of an antenna, the antenna is end-fed.  If the RF transmission line is connected at the center of an antenna, the antenna is mid-fed or center-fed. Directivity The directivity  of an antenna refers to the width of the radiation beam pattern. A directional antenna concentrates its radiation in a relatively narrow beam. If the beam is narrow in either the horizontal or vertical plane, the antenna will have a high degree of directivity in that plane. An antenna can be highly directive in one plane only or in both planes, depending upon its use. In general, we use three terms to describe the type of directional  qualities  associated  with  an  antenna: omnidirectional,  bidirectional,  and unidirectional. Omnidirectional  antennas  radiate  and  receive  equally well  in  all  directions,  except  off  the  ends.  Bidirectional antennas radiate or receive efficiently in only two directions.  Unidirectional  antennas  radiate  or  receive efficiently in only one direction. Most antennas used in naval communications are either omnidirectional or unidirectional. Bidirectional antennas are rarely used. Omnidirectional antennas are used to transmit fleet broadcasts and are used aboard ship  for  medium-to-high  frequencies.  A  parabolic,  or dish,  antenna  (figure  2-14)  is  an  example  of  a unidirectional antenna. As you can see in the figure, an Figure  2-14.—Principle  of  parabolic  reflection. antenna (normally a half wave) is placed at the “focal” point and radiates the signal back into a large reflecting surface (the dish). The effect is to transmit a very narrow beam of energy that is essentially unidirectional. Figure  2-15  shows  a  large,  unidirectional  parabolic antenna. Directional antennas are commonly used at shore  installations. Wave  Polarization Polarization   of   a   radio   wave   is   a   major consideration   in   the   efficient   transmission   and reception of radio signals. If a single-wire antenna is Figure  2-15.—Unidirectional  parabolic  antenna. 2-15


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