Miniaturised high-frequency and very-high-frequency antennas based on optimised non-uniform helical structures

Helical windings are often used to shorten the length of a resonant low-frequency antenna, but this results in an antenna with narrow bandwidth for the normal mode. The windings are usually uniform, but in contrast the current study investigates the possible advantage of optimised non-uniform windings for improved bandwidth. Optimisation can involve a prohibitive number of variables when continuous variations along the antenna are described in terms of wire segment geometry. This study, however, reduces such requirements by describing the winding variations (pitch and radius) in terms of a limited number of parameters. These parameters are the coefficients of expansion of the variations in terms of radial basis functions. By this artifice, the optimisation problem is made tractable with limited computer resource. An evolutionary optimiser is used to achieve the optimal winding shape with respect to radius and pitch. Measured results from prototypes demonstrate the optimisation process to be effective. Further, simulations using non-ideal ground planes show the designs to be robust.