Techniques to compensate mutual coupling effects in a Multiple Switched Beam Smart Antenna with Beam Shaping Capability

A smart antenna for mobile applications incorporating an array of ± 45 polarised stacked patch elements 4 columns wide excited by a multi-beamforming and beam shaping network is described. Four narrow overlapping beams, one wide “broadcast channel” beam and right and left shaped beams can be provided. The later shaped beams are to provide high capacity coverage in a specific narrow angular sector while low capacity coverage is maintained over the remainder of a 120 sector. Results are presented for the simulation of this smart antenna using CST EM simulation software. In addition, a demonstrator array has been constructed and tested which has yielded a positive conformation of the simulation results. It will be shown that the effects of mutual coupling degrade the beam shapes, particularly for the broadcast beam producing a significant reduction in gain over the centre of the pattern. Results are included to show the effects of applying modified complex excitation weights for compensation of mutual coupling. Whilst this technique can be shown to restore beam patterns it is also shown to introduce unacceptably high return loss at the antenna input ports. An alternative technique for producing Wide Angle Impedance Matching (WAIM) is described which is shown to improve return loss but degrade beam shapes. This work describes how both techniques can be combined to maintain good beam shapes and preserve good impedance matching, with consequent good return loss, over wide scan angles and also for shaped beams.