A Miniaturized Coupler Decoupling Network for Two-Element Tightly-Coupled MIMO Antenna Array

In this paper, a miniaturized coupler decoupling network is designed for reducing the mutual coupling of two-element tightly-coupled multiple-input and multiple-output (MIMO) antenna array. To make the antenna structure more compact, the center-to-center distance of the elements is reduced to
$0.3\lambda_{0}($
where
$\lambda_{0}$
is the free-space wavelength at the center frequency). A decoupling network is inserted between the two elements, which is made of two couplers connected by two transmission lines with different lengths and widths. A new coupling path is built through the proposed decoupling network which makes surface wave coupling and spatially wave coupling cancel out. The dimension of the coupler is
$0.112\lambda_{0}\times 0.216\lambda_{0}$
for miniaturization. As a result, an improvement of isolation from 4.75 dB to 43.76 dB at the center frequency is achieved.