Self-Decoupled Linear and Planar MIMO Microstrip Patch Antenna Arrays Operating in the Fundamental TM01 Mode

The H-plane coupled multi-input multi-output (MIMO) rectangular microstrip patch antenna (MPA) array is investigated. To ensure compact size as well as good radiation performance, the excited MPA is designed to operate in the single fundamental TM01 mode. On the other hand, due to the near-field coupling effect, the TM01 mode and the adjacent TM11 mode are simultaneously excited in the coupled MPA. The superposed field of the two coupled modes forms a null-field region at the position of feeding probe, generating an intrinsic coupling dip with low <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}_{21}\vert $ </tex-math></inline-formula> above the resonance frequency of TM01 mode. To make use of this intrinsic coupling dip, the decoupling band is moved downward to overlap with the impedance passband by simply truncating two rectangular corners from the rectangular MPA. A self-decoupled convex MPA array is thus achieved without requiring any additional decoupling structure. This self-decoupling method is also applicable to the E-plane coupled MPA array, and more importantly, it is extendable to a large-scale planar MIMO array, showing great universality. For validation, a <inline-formula> <tex-math notation="LaTeX">$2\times2$ </tex-math></inline-formula> MPA array is simulated, fabricated, and measured. A 4% usable bandwidth with <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}_{ii}\vert &lt; -10$ </tex-math></inline-formula> dB and <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}_{ij}\vert &lt; -20$ </tex-math></inline-formula> dB is achieved, and the maximum isolation is over 50 dB.