Miniaturisation of quadrifilar helical antenna: impact on efficiency and phase centre position

This study addresses the intrinsic limitations of the miniaturisation of very-high frequency-band quadrifilar helical antennas. Several designs with a relative size (i.e. the ratio between antenna's heights after and before the miniaturisation) down to 27% are obtained using an optimisation process based on intensive full-wave electromagnetic simulations. Reported simulation and experimental results underline the impact of height reduction on the antennas main descriptors: radiation pattern, directivity, polarisation purity, input matching, efficiency and maximum achievable gain. It was observed that the efficiency and the gain do not decrease linearly with the reduction of the antenna height and the input matching is slightly improved for the compact antennas. The shape of radiation pattern and the polarisation purity are not significantly degraded for relative size down to 27%.