Helical antenna

FOR THIRD YEAR ECE STUDENTS

1. HELICAL ANTENNA PREPARED BY B.S.NAGESWARARAO ASST PROF ESWAR COLLEGE OF ENGINEERING NARASARAOPET GUNTUR ANDHRA PRADESH It is broad band VHF and UHF Antenna It provide circular polarization

2. It contains HELIX of thick copper wire or Wounding in the shape of screw thread Used as antenna in conjuction with metal plate Metal plate acts as ground plane

3. Helix is fed by coaxial cable One end of helix connects to inner conductor of coaxial cable Outer conductor connects to the ground plane

4. Mode of radiation depend on Diameter of the helix D and turn spacing S Feed wire lies in the plane through helix axis( between A and B) Length of feed wire equal to S/2

5. Various dimensitions of helix

6. MODES OF OPERATION 1) NORMAL MODE (or) PERPENDICULAR MODE 2)AXIAL (or) BEAM MODE

7. Normal mode In this mode radiation pattern is same as broad side array Perpendicular to the axis of helix Circular polarization This mode obtain when dimensions of helix are less than LEMBDA

8. SUCH SMALL HELIX BAND WIDTH NARROW AND RADIATION EFIIENCY IS LOW

9. RADIATION PATTERN IS COMBINATION OF SMALL DIPOLE AND LOOP ANTENNA

10. HELIX IS SERIES COMBINATION OF LOOP AND DIPOLE ANTENNAS LOOP DIAMETER IS EQUAL TO HELIX DIAMETER HELIX SPACE IS EQUAL TO LENGTH OF DIPOLE

11. RADIATION OF HELIX IN NORMAL MODE IS THE COMBINATION OF DIPOLE AND LOOP RESPECTIVELY

12. AXIAL RATIO (AR)

13. WHEN

14. THIS IS THE CONDITION FOR PITCH ANGLE TO GET CIRCULAR POLARIZATION THIS MODE IS LIMITED USE BECAUSE OF LOW BANDWIDTH AND RADIATION EFFIENCY

15. AXIAL MODE (CONDITIONS)

16. DESIGN PROCEDURE IMPORTANT PARAMETERS BEAM WIDTH GAIN IMPEDENCE AXIAL RATIO

17. END