A Flexible Miniaturized Wideband Branch-Line Coupler Using Shunt Open-Stubs and Meandering Technique

This article presents a flexible miniaturized wideband branch-line coupler printed on a biodegradable paper substrate. The proposed design consists of a combination of series transmission lines and shunt open-stubs along with the meandering technique as a substitute to the conventional quarter-wavelength transmission line (QWTL). The horizontal QWTLs of the traditional three-section branch-line coupler are replaced by a proposed equivalent structure, and the vertical QWTLs have been meandered. The developed prototype occupies an area of
$0.04~\lambda _{\mathrm {g}}^{2}$
, and has been tested where the measurements agree well with the simulations, and the obtained fractional bandwidth is greater than 60% at 0.9 GHz operating frequency. In addition, for flexibility analysis, the prototype is bent in different directions, and the tested results are obtained and compared to the flat design. The proposed device can be used in antenna feeding networks, microwave and conformal applications that require both compact size and frequency wideband response simultaneously.