Axial ratio of a curl antenna

The axial ratio of a curl antenna composed of vertical linear, horizontal linear, and curled sections is investigated using the method of moments. Axial ratio contours are presented for various values of antenna height h at a test (centre) frequency of 11.85 GHz. As the horizontal linear section length L_f increases, the number of curl turns n required for radiation of a circularly polarised (CP) wave decreases. It is also found that the ranges of choices for L_f and n for radiation of a CP wave becomes wider as the antenna height increases. Further calculations show that, as the antenna height increases: (i) the frequency bandwidth over which the axial ratio is less than 3 dB becomes wider, and (ii) the cross-polarisation component increases in a direction off the z-axis normal to the antenna plane, reducing the gain. A curl antenna of (height h, spiral constant a_sp , horizontal linear section length L_f, number of curl turns n, =4.6 mm, 0.26 mm/rad, 3.01 mm, 1.23 turns respectively) has a frequency bandwidth of 10.3% for a 3 dB axial ratio criterion, with a maximum gain of approximately 8 dB