Figure  2-19.—Inverted  cone  antenna. greater  than  2:1.  They  are  considered  medium-  to high-power radiators, with power handling capabilities of  40  kW  average  power. CONICAL   MONOPOLE   ANTENNA Conical  monopoles  are  used  extensively  in  hf communications.  A  conical  monopole  is  an  efficient broadband,  vertically  polarized,  omnidirectional  antenna in a compact size. Conical monopoles are shaped like two truncated cones connected base-to-base. The basic conical monopole configuration, shown in figure 2-20, is composed of equally-spaced wire radiating elements arranged in a circle around an aluminum center tower. Usually,  the  radiating  elements  are  connected  to  the top  and  bottom  discs,  but  on  some  versions,  there  is a center waist disc where the top and bottom radiators are  connected.  The  conical  monopole  can  handle  up to  40  kW  of  average  power.  Typical  gain  is  -2  to  +2 dB,  with  a  vswr  of  up  to  2.5:1. RHOMBIC   ANTENNA Rhombic   antennas   can   be   characterized   as high-power,   low-angle,   high-gain,   horizontally- polarized,   highly-directive,   broadband   antennas   of simple, inexpensive construction. The rhombic antenna (fig.  2-21)  is  a  system  of  long-wire  radiators  that depends on radiated wave interaction for its gain and directivity.  A  properly  designed  rhombic  antenna presents to the transmission line an input impedance insensitive   to   frequency   variations   up   to   5:1.   It maintains  a  power  gain  above  9  dB  anywhere  within a  2:1  frequency  variation. At  the  design-center frequency,  a  gain  of  17  dB  is  typical.  The  radiation pattern  produced  by  the  four  radiating  legs  of  a rhombic  antenna  is  modified  by  reflections  from  the earth under, and immediately in front of, the antenna. Because   of   the   importance   of   these   ground Figure  2-20.—Conical  monopole  antenna. reflections  in  the  proper  formation  of  the  main  lobe, the  rhombic  should  be  installed  over  reasonably  smooth and   level   ground. The  main  disadvantage  of  the rhombic  antenna  is  the  requirement  for  a  large  land area,  usually  5  to  15  acres. QUADRANT   ANTENNA The   hf   quadrant   antenna   (fig.   2-22)   is   a special-purpose receiving antenna used in ground-to-air-to-ground  communications.  It  is  unique among  horizontally-polarized  antennas  because  its 2-11


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