spherical   waves   emerge   at   the   exit   side   of   the conducting lens (lens aperture) as flat-fronted parallel waves.   This type of lens is frequency sensitive. The dielectric type of lens, shown in figure 1-10, view B, slows down the phase propagation as the wave passes through it.   This lens is convex and consists of dielectric  material.    Focusing  action  results  from  the difference between the velocity of propagation inside the dielectric and the velocity of propagation in the air. The result is an apparent bending, or refracting, of the waves. The  amount  of  delay  is  determined  by  the dielectric   constant   of   the   material. In   most   cases, artificial dielectrics, consisting of conducting rods or spheres that are small compared to the wavelength, are used.  In this case, the inner portions of the transmitted waves are decelerated for a longer interval of time than the outer portions. In a lens antenna, the exit side of the lens can be regarded  as  an  aperture  across  which  there  is  a  field distribution.    This  field  acts  as  a  source  of  radiation, just as do fields across the mouth of a reflector or horn. For a returning echo, the same process takes place in the lens. ARRAY   ANTENNAS .—An   array   type   of antenna  is  just  what  the  name  implies—an  array  or regular   grouping   of   individual   radiating   elements. These  elements  may  be  dipoles,  waveguide  slots,  or horns.   The most common form of array is the planar array,  which  consists  of  elements  linearly  aligned  in two  dimensions—horizontal  and  vertical—to  form  a plane (fig. 1-11). Unlike  the  lens  or  parabolic  reflector,  the  array applies   the   proper   phase   relationship   to   make   the wavefront flat before it is radiated by the source feed. The  relative  phase  between  elements  determines  the position   of   the   beam;   hence   the   often   used   term, phased array.   This phase relationship is what allows the beam to be rotated or steered without moving the antenna.  This characteristic of array antennas makes it ideal  for  electronic  scanning  or  tracking.     (We  will discuss scanning shortly.) Radomes The  term   radome   is  a  combination  of  the  words radar    and    dome. Radomes   are   used   to   cover   and protect radar antennas from environmental effects such as   wind,   rain,   hail,   snow,   ice,   sand,   salt   spray, lightening,  heat,  and  erosion. The  ideal  radome  is transparent to the RF radiation from the antenna and its return   pulses   and   protects   the   antenna   from   the environment. A   radome’s   design   is   based   on   the expected  environmental  factors  and  the  mechanical and electronic requirements of the RF antenna. Although, in theory, a radome may be invisible to RF  energy,  in  real  life  the  radome  effects  antenna’s performance in four ways.  These are; beam deflection, transmission   loss,    reflected   power,   and    secondary effects.    Beam deflection  is the shift of the RF beam’s axis.   This is a major consideration with tracking (i.e. FC)  radar. Transmission  loss   is  the  loss  of  energy associated  with  reflection  and  absorption  within  the radome.   Reflected power can cause antenna mismatch in   small   radomes   and   sidelobes   in   large   radomes. Depolarization and increased antenna noise are a result of  secondary effects. As   an   FC,   you   will   be   primarily   responsible maintaining   the   radome   associated   with   your equipment. This   normally   will   include   routine cleaning and inspection according to your prescribed preventive maintenance schedule.  Some minor repairs may be authorized by your technical manuals, but most repairs will normally be done by an authorized factory representative.     You  may  be  required  to  repaint  the radome  because  of  normal  environmental  wear  and tear.    If  so,  be  especially  careful  to  use  only  paint(s) authorized   by   the   manufacturer   and   to   follow   the authorized step-by-step procedures. Figure  1-12  is  an  example  of  a  radome  in  use  in today’s Navy.  Other systems that use radomes include, the   Combined   Antenna   System   of   the   Mk   92   Fire Control System, the AN/SPQ-9 series antenna for the Mk 86 Gun Fire Control System, and the Mk 23 Target 1-10 HORIZONTAL   LINEAR SUBARRAY TRANSMITTER   AND   RECEIVER SLOT ANTENNA FCRf0111 Figure 1-11.—Planar array antenna.