Effect of ground plane on the input impedance of Quadrifilar Helix Antenna

In this research work, the variation in the input impedance of Quadrifilar Helix Antenna (QHA) versus height of perfect conducting ground plane (PEC) is investigated. The results show that when the height of ground plane varies from 0.05λ to 1.0λ, maximum variations are observed in real part of input impedance below 0.3λ (44Ω to 50Ω in simulations and 41Ω to 52Ω in measurements). From 0.3λ to 1.0λ the change in the real part of input impedance of QHA is insignificant (varies between 48Ω to 50.6Ωin simulations and 48 Ω to 52Ω in measurements). On the other hand the imaginary part of input impedance changes between -10Ω to 0Ω in simulations and 7Ω to 0Ω in measurements and is insignificant above the height of 0.3λ (changes between -2.0 Ω to 0.2Ω in simulations and 3Ω to 0Ω in measurements). The comparatively stable input impedance of QHA with respect to height of ground plane was further compared with horizontal dipole and rectangular loop with different orientations. As height varies from 0.05λ to 1.0λ, variations in the input impedance (real and imaginary part) is maximum for horizontal dipole (HD) and horizontal and perpendicular loop (H&PL). For other two orientations, vertical loop (VL) and horizontal and parallel loop (H&∥L) the variations are small. However still the smallest deviations in the input impedance about its mean value are that of QHA which makes it very suitable in applications where the antenna had to be placed above a ground plane such as in satellite platform or aerial platforms.