VHF, UHF and Microwave Antennas – II

LENS ANTENNA :-

TRANSMITTING MODE RECEVING MODE

FUNCTIONS OF LENS ANTENNA It controls the illumination of aperture. It collimates the electromagnetic rays. It provides directional characteristics. In receiving mode ,it converges the incoming wave front at its focal point. It produces plane wave front form a spherical wave front.

ANTENNA CONFIGURATIONS Lens antenna with refractive index n>1 Lens antenna with refractive index n<1

LENS ANTENNAS CLASSIFIED AS DEALY LENS FAST LENS

Dielectric Lens There are two types of lens antenna Delay lances (electrical path length increased by lens medium) Ex. Dielectric lances, H plane metal-plate lenses Fast lances (electrical path length decreased by lens medium) Ex. E plane metal-plate lenses Lens Antenna

Dielectric Lens There are two types of dielectric lenses Non Metallic Metallic/artificial Non Metallic Dielectric lens This types is similar to optical lens It may designed by ray analysis methods of geometrical optics. Metallic or Artificial Dielectric lens This types is less heavy compare to nonmetallic lens It consist of discrete metal particles of macroscopic size. Lens Antenna

Non Metallic Dielectric lens (Fermat’s Principle) Lens Antenna

Non Metallic Dielectric lens (Fermat’s Principle) Lens Antenna

Non Metallic Dielectric lens Reflection Coefficient Lens Antenna

Metallic/Artificial Dielectric lens Lens Antenna

Reflector Lens Antenna

Polyrod A dielectric rod or wire can act as a guide for EM waves. This tendency to radiate is turned to advantage in the polyrod antenna. (made of Polystyrene) Lens Antenna

PRINCIPLE OF LENS ANTENNA

ZONING OF LENS(Z) Curved surface zoning Plane surface zoning

Advantages of zoing:- Dis advantages of zoing:-

Advantages of lens antenna Lens are bulkier Design of lens antenna is complicated Lens are expensive

Applications of lens antennas Lens antenna used as microwave antenna For large bandwidth requirements unstepped dielectric lens antennas are used For narrow bandwidth applications ,dielectric lens antennas are used

LUNEBURG LENS ANTENNA

EINSTEN’S GRAVITY LENS The incident electromagnetic waves passing around stars are deflected through an angle which is proportional to mass of the star and then they brought into focus on the far side this is know as Einsten’s gravity lens