LENS ANTENNAS

•     The most common type of radar antenna is the parabolic reflection in one of its various forma the microwave paraboloid reflection is analogous to an automobile headlight or to searchlight mirror. The analogy of an optical lens is also found in radar.Three types of microwave lenses applicable to radar are  
• dielectric lens
•     metal-plate lenses

• lenses with nonnuifrom index of refractin                                                                                       

     Dielectric lenses. The homogeneous, solid, dielectric-lens antenna of  is similar to the conventional optical lens. A point at the focus of the lens produces a plane wave on the opposite side of the lens. Focusing action is a result of the difference in the velocity of propagation inside the dielectric as compared with the velocity of propagation in air. The index of refraction n of a dielectric is defined as the speed of light in free space to the speed of light in the dielectric medium. It is equal to the square root of the dielectric constant. Materials such as polyethylene, polystyrene, plexiglas, and Teflon are suitable for small microwave lenses. They have low loss and may be easily shaped to the desired contour. Since the velocity of propagation is greater in air than in the dielectric medium, a converging lens is thicker in the middle than at the outer edges, just as in the optical case.

One of the limitations of the solid homogeneous dielectric lens is its thick size and large weight.Both the thickness and the weight may be reduced considerably by stepping or zoning the lens. Zoning is based on the fact that a 360(deg ) change of phase at the aperture has on effect on the aperture phase distribution. Starting with zero thickness at the edge of the lens the thickness of the dielectric is progressively increased toward the lens axis as in the design of a normal lens. However, when the path length introduced by the dielectric is equal to a wavelength, the path in the dielectric can be reduced to zero without altering the phase across the aperture.The thickness of the lens is again increased in the direction of the axis according to the lens design until the path length in the dielectric is once more 360(deg), at which time another step may be made. The optical path length through each of the zones is one wavelength less than the next outer zone.

 

      Artificial dielectrics    Instand of using ordinary dielectric materials dielectric materials for lens,it is possible to construct them of artificial. The ordinary dielectric consists of molecular particles of microscopic size, but the artificial dielectric consists of discrete metallic or dielectric particles of macroscopic size. The particles may be spheres, disks,or rods imbedded in a material of low dielectric constant such as polystyrene foam. The particles are arranged in some particular configuration in a three-dimensional lattice. The dimension of the particles in the direction parallel to to the electric field as well as the spacing between particles should be   small compared with a wave length.If those  conditions are met, the lens will be insensitive to frequency.When the particles are metallic spheres of radius a  and spacing s between centers, the dielectric of the artificial dielectric is approximately assuming no interaction between the spheres. An artificial dielectric may also be constructed by using a solid dielectric material with a controlled pattern of voids. This is a from of Babinet inverse of the more usual artificial dielectric  composed of particles imbedded in a low-dielectric-constant material. The voids may be either spheres or cylinders,but the latter are easier to machine.

    Metal-plate lens     An artificial dielectric may be constructed with parallel-plate wave guides.The phase velocity in parallel-plate wave guide is grater than that in free space:  hence the index of refraction is less than unity.This is opposite to the usual optical refraction medium. A converging metal-plate less is therefore thinner at the center than at the edges, as opposed to a converging dielectric lens which is thinner at the edges .The metal-plate lens is an E-plane lens since the electric- field vector is parallel to the plates.Snell"s law is obeyed in an  E-plane lens,and the direction of the rays through the lens is  governed by the usual optical laws involving the idex of refraction.The surface contour of a metal-plate lens is in general,not parabolic as in the case of the reflector.For example, the surface closest to the feed is an ellipsoid of revolution if the surface  at the opposite face of the lens is plane                                                                                                                                        

Even with an index of refraction in the vicinity of 0.5 to 0.6 the thickness  of the metal-plate lens becomes large unless inconveniently long focal lengths are used.The thickness may be reduced by zoning just as with a dielectric lens.The bandwidth of a zoned metal-plate lens is larger than that of an un zoned the gain, and increase the sidelobe level.An example of an X-band metal-plate zoned lens                                                                                                                                                             

Another class of metal-plate lens is the constrained lens or path-length lens, in which the rays are guided or constrained by the metal plate. In the H-plane metal -plate constrained lens, the electric field is propagates through the plates is relatively unaffected provided the plate spacing is greater than     The direction of the rays if the not affected by the refractive index. and Snell"s law dose not apply .Focusing action is obtained by constraining the waves ti pass between the plates in such manner that the path length can be increased above that in free space. In one type of cylindrical constrained lens with the E field parallel to the plate ,A 1 beam could be scanned over a 100(deg)sector by positioning the line feed.The lens was 72 wavelength in size had. and operated at a wavelength of 1.25 cm

Evaluation of lenses as antennas.   One of the advantage of a lens over a reflector antenna is the absence of aperture blocking. Considerable equipment can be placed at the focus of the lens with out interfering with the resultant antenna pattern.The first monopulse radars used lenses for this purpose, but with time the monopulse RF circuitry  was reduced in size and the reflector antenna came to be preferred over the lens or the homogeneous sphere can beam  over a wide angle. The lens is usually less efficient than comparable reflector antennas because of loss when propagating through the lens medium and the reflecting from the two lens surface.In zoned lens there will be additional, undesired scattering from the steps The lack of suitable  solid or artificial dielectric materials has limited the development of lenses.The problem of dissipating heat from large dielectric lenses can sometimes restrict their use to moderate-power or to receiver applications.Conventional lenses are usually large and heavy, unless zoned.To reduce the loss caused by scattering from the steps in a zoned lens, the ratio of focal length Fto the antenna diameter D must be made large f/d ratio. The mechanical support of a lens is usually more of a problem than with a reflector                                                                          

The wide- angle scanning capability of a lens would be of interest in radar as a competitor for a phased array if there were available a practical means for electronically switching the transmitter abd receiver among fixed feeds so as to achieve a rapidly scanning beam