Optimisation of a spherical helical antenna

Cylindrical helix and planar spiral antennas produce circularly polarised (CP) radiation over a wide bandwidth. Unfortunately this circular polarisation is not maintained in off- axis directions. This research investigates the Spherical Helical Antenna (SHA) which is essentially a helix which has been tapered to give it a spherical envelope. The SHA is shown to produce good quality CP over both a wide bandwidth and a wide beamwidth. Custom FORTRAN code has been written which implements the Method of Moments (MoM) technique in order to model the antenna. Curved segments are used and these are shown to substantially reduce the number of segments required to accurately represent the current distribution and shape of the antenna. The code is carefully validated against both published results and control software. Modifications are proposed to the SHA including a novel scheme to define the spacing between turns, the consideration of non-integer number of turns, truncation, and a novel balanced feed configuration. Each of these modifications is shown to improve the performance of the antenna, particularly its polarisation properties. A multi-objective non-dominated genetic algorithm (GA) is applied to optimise the perfonnance of the antenna in terms of its axial ratio (AR) in the boresight direction and its half-power and AR beamwidths. With the aid of a purpose-milled mould which allowed wires to be shaped to exact curvatures, a prototype twin-arm SHA has been constructed. Radiation patterns and axial ratio measurements for this antenna are presented.