Example: Design a helical antenna operating at f = 475 MHz given pitch angle of 12.5° and turn radius of 10 cm such that the HPBW=39º. Solution: 1st find C and compare with λ to select either normal or axial mode. 2nd find # of turns, axial ratio, directivity. Cupped ground plane AXIAL-MODE HELIX D = 15 LA Circularly polarized L λ 100 Beam S C=лD ✓ Lo 50-2 coaxial feed T 2-to-1 (71%) bandwidth An Axial Mode Helix Invented by Kraus 10