Quadrifilar helix antenna (QHA) has its applications in satellite communications. This paper presents the performance optimisation of input and radiation characteristics of QHA in the presence of infinite and finite metallic ground planes. For the infinite ground plane, it has been observed that input parameters such as impedance and voltage standing wave ratio (VSWR) are stable, and the antenna has broader half power beamwidth (HPBW). Smaller metallic platforms that act as finite ground planes produce better 3-dB axial ratio beamwidth and boresight axial ratio. Deployment of QHA on smaller metallic platforms such as nanosatellites and CubeSats enhances the circularly polarised beamwidth of the antenna with improved boresight axial ratio. However, on large low earth orbit (LEO) satellites, stable input characteristics and broader HPBW have been achieved at the cost of narrow circularly polarised beamwidth and degraded boresight axial ratio.KEYWORDS helix antenna, satellite antennas, satellite communication, RF measurements, wire antennas, wideband antennas 1 | INTRODUCTION Quadrifilar helix antenna (QHA) has a wide range of applications and is deployed on various space and terrestrial communication platforms. 1-5 It is used for global positioning system (GPS), 5 radio frequency identification (RFID), 6 and low earth orbit (LEO) satellites. 7 In authors' opinion, further study is required on the QHA deployment over terrestrial and satellite platforms.In literature, the effect of ground plane on the QHA radiation characteristics is explained based on two theories: image theory and uniform theory of diffractions. Image theory explains the performance of QHA above large infinite ground planes that is helpful in examining QHA for large platforms. 8 If the ground plane is a perfect electric conductor, it will reflect radiations caused by any antenna installed over it. The image theory assumes that this reflected signal is originated by another antenna placed at the mirror image of the real antenna. This assumption helps to analyse ground reflections as the superposition of two radiation sources. Uniform theory of diffraction (UTD) is applicable to the finite ground plane models to describe the diverging behaviour of QHA from an expected response. 9,10 UTD is derived from the geometrical theory of diffraction (GTD) that has a significant impact on the high frequency computational This work was supported by COMSATS University Islamabad, Lahore Campus, Lahore Pakistan.