Reflector antenna

FOR THIRD YEAR ECE STUDENTS

1. Unit-V VHF,UHF and MICROWAVE ANTENNA PREPARED BY B.SIVA NAGESWARARAO DEPT OF ECE ASST PROF ESWAR COLLEGE OF ENGINEERING NARASARAOPET ANDHRA PRADESH

2. Reflector antennas most important in microwave radiation applications At microwave frequency size of anteena small Frequency and size are inversely proportional Any shape of reflector produce desire directivity

3. In Reflector antenna, another antenna need to exite it. Dipole Horn Slot used for exite so called primary antenna Reflector called as secondary antenna

4. Reflector represent any shape Most common geometrics are Plane Corner Curved or parabolic reflectors

5. f lat sheet reflector or plane reflector Simplest form of reflector antenna Is kept infront of the feed Energy is radiated in the desired direction To increase the directivity, large flat sheet placed infront of the half dipole Main advantage is Reduce backward radiation and increase gain in forward direction

6. Corner reflector Dis-advantage of flat reflector is back and side radiation Modified, join two flat reflector with some angle concentrate radiation in forward direction Called corner reflector Angle called include angle Angle is 90 degrees, some times angle other than 90 degrees used

7. Two types of corner reflectors 1)active (with source) 2)passive( without source)

8. System Effiency depend on the spacing between the vertex of the reflecto r and feed element S THAT IS ‘d’ Distance is adjusted by include angle Include angle decreases ,spacing between feed and r eflector increased

9. Most of the time feed element is dipole or array of collinear dipoles placed parallel to the vertex placed at distance ‘d’ Dimensions are Aperture of coner reflector is Da Length of reflector is L Height h

10. Da is lamda < Da< 2 lamda Lamda /3 <d< 2lamda/3 L=2d

11. Analysis of radiation field of source with corner reflector Alpha=pi/N, N IS Integer Apply image principle Field produced between two corner reflectors is same as an array of 2N CURRENT ELEMENTS SPACED EQUIDISTANCE ON CIRCULAR PATH WITH RADIUS ‘d’ with adjacent elements opposite polarities

12. Number of images ,polarities and position depend on include angle and polarization feed element Multiple images on circular path for different angles

13. Same method can be extended to analyze corner reflector with corner angles

14. For 60 degree corner reflector one driven and five images For 45 degree corner reflector one driven and 7 images Field strength of corner reflector along axis due to a driven element and three image elements is 4 times the field strength of the half wave dipole In the free space All image elements add to driven element

15. Power radiated by the driven antenna of corner reflector is 1.7 times than the half wave dipole in free space Gain of the corner reflector antenna along the axis is almost 9db or more

16. END