Anomalous mutual coupling between microstrip antennas

An asymptotic expansion of the microstrip Green´s function is used to facilitate a study of mutual coupling between two printed antennas, and coupling versus element separation is presented for substrate parameters of practical interest. For certain substrate parameters, it is found that the magnitude of mutual coupling does not decay monotonically with increasing element separation. Instead, the magnitude exhibits a quasi-periodic oscillation which can be attributed to the interference of surface and space waves. The presentation and discussion of this anomalous behavior is the purpose of the paper. The asymptotic representation of the Green´s function, which differentiates space wave terms from surface wave terms when source and observation points are largely separated, is used to characterize the behavior of mutual coupling in the principal planes