Extending the bandwidth of a superdirective first-order probe for spherical near-field antenna measurements

A superdirective array of electrically small dipole radiators can effectively be used as a compact and lightweight first-order probe (a directive antenna radiating predominantly spherical modes with the azimuthal index |μ| = 1) in spherical near-field antenna measurements at low frequencies. This contribution shows that a very narrow frequency bandwidth peculiar to superdirective antennas can be extended to practical values by the proper design of the array elements as well as by relaxing the maximum directivity condition, while keeping |μ| = 1 modes dominating in the radiation spectrum of the antenna. The resulting probe has the relative bandwidth of 3% and directivity above 9 dBi; its height is 0.5λ over a 1λ circular ground plane.