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Quarter-Wave Antenna

 
  
 
are ~/4  (one-fourth  wavelength)  at  the  operating frequency. Together, of course, the sections make the effective   length   of   the   antenna  L/2   (one-half wavelength) at the operating frequency. One feature of the dipole antenna is that it does not need to be connected to the ground like other antennas. Antennas shorter than a half wavelength must use the ground to achieve half-wave characteristics. The half- wave  antenna  is  already  long  enough  to  radiate  the signal  properly. Because of sophisticated antenna systems and tuning   processes,   half-wave   antennas   can   be electrically   achieved   aboard   ship.   Therefore wavelength is becoming less and less the criteria for determining the types of antennas to be used on ships. Dipole  antennas  can  be  mounted  horizontally  or vertically,  depending  upon  the  desired  polarization,  and can be fed at the center or at the ends. Because it is ungrounded, the dipole antenna can be installed above energy-absorbing structures. QUARTER-WAVE   ANTENNA A  quarter-wave  antenna  is  a  grounded  antenna  that is  one-fourth  wavelength  of  the  transmitted  or  received frequency.  You  will  hear  the  quarter-wave  antenna referred to as a “Marconi antenna.” The quarter-wave antenna is also omnidirectional. As we mentioned earlier, a half-wave antenna is the shortest practical length that can be effectively used to radiate  radio  signals  into  free  space.  The  natural question, then is, “How do we use a quarter-wavelength antenna if a half-wavelength is the shortest length that can be used?” The answer is simple. Figure  2-20.—Direct  and  image  signal  of  a  quarter-wave antenna. Figure  2-21.—Current  distribution  in  a  real  antenna  and  its image. Two components make up the total radiation from an antenna. One component is that part of the radiated signal  which  leaves  the  antenna  directly.  The  other  is  a ground  reflection  that appears to come from an underground image of the real antenna (figure 2-20). This image is sometimes called the mirror image and is considered to be as far below the ground as the real antenna is above it. Figure 2-21 shows basic current distribution in a real and image antenna. There are certain directions in which the direct wave from the real antenna and the reflected wave from the image are exactly equal in amplitude but opposite in phase. Conversely, there are other directions in which the direct and reflected waves are  equal  in  amplitude  and  in  phase.  Therefore, depending on the direction and location of the point at which the field strength is measured, the actual field strength may be (1) twice the field strength from the real antenna alone, (2) zero field strength, or (3) some intermediate value between maximum and minimum. It is this “real” and “image” radiated field that forms the basis for using quarter-wavelength antennas. This  reflected-energy  principle  is  very  useful  in  the lower frequency ranges, although ground reflections occur in the high-frequency range as well. The antenna does not always  need to be placed at the Earth’s surface to produce an image. Another method of achieving reflected images is through the use of ground planes. This means that a large reflecting metallic surface is used as a substitute for the ground or Earth.  This  method  is  frequently  used  in  the  VHF/UHF 2-19

   


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