Out-of-Band Mutual Coupling Suppression for Microstrip Antennas Using Characteristic Mode Analysis and Shorting Pins

A method combining characteristic mode analysis and shorting pins is presented to reduce out-of-band mutual coupling of linearly polarized rectangular microstrip antennas. Virtual feeds are applied to the antennas for depicting the contributions of each mode to the normal operation and mutual coupling of the antennas. Expressions of the modal self and mutual impedances are deduced for coaxial probes that represent the electric current excitation. Shorting pins are used to suppress specific modes with large modal mutual impedance and the suitable positions are suggested by the eigenfield distributions. Normal operation of the antennas is hardly affected by the shorting pins. The numerical and experimental results are presented for verification. Several prototypes are presented for explaining the presented method and a reduction of 5~14 dB is achieved for mutual coupling. Furthermore, it is found that the presented method is hardly affected by the distance of the antennas, which makes it useful for the design of shared aperture antennas.