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In this study, the authors introduce a methodology for low-cost simulation-driven design optimisation of highly miniaturised branch-line couplers (BLCs). The first stage of their design approach exploits fast concurrent optimisation of geometrically dependent, but electromagnetically isolated cells that constitute a BLC. The cross-coupling effects between the cells are taken into account in the second stage, where a surrogate-assisted fine-tuning procedure is executed. The tuning process is based on space-mapping-enhanced low-fidelity model of the entire BLC. The latter is constructed by cascading local response surface approximations (RSAs) of the BLC building blocks. The efficiency of their technique is demonstrated through the design of two compact BLCs. A considerable scale of miniaturisation has been achieved in both cases (83.7 and 87.4%, respectively), at the computational cost corresponding to a few EM simulations of the respective BLC. Comparison of numerical results with several surrogate-based design approaches as well as an experimental verification of the final designs are also provided.

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4. N. Talwalker, C.P. Yue, and S.S. Wong, Integrated CMOS Transmitreceive switch using LC-tuned substrate bias for 2.4-GHz and 5.

In this work, a step‐by‐step development of a compact microstrip rat‐race coupler (RRC) has been presented and discussed. A high degree of miniaturization has been obtained by substituting six quarter‐wavelength uniform atomic building blocks of a RRC by their nonuniform counterparts. The miniaturization procedure has been realized in three progressive steps: (i) the first layout solution of a miniaturized RRC has been acquired using an accurate mathematical model of a nonuniform transmission line (NUTL); (ii) the second layout solution has been obtained by a manual refinement and electro‐magnetic (EM) fine‐tuning of the NUTL geometry aimed at the utilization of the interior of the RRC in a highly efficient manner; (iii) the final layout, occupying only 8.5% estate area in comparison to a conventional RRC, is the result of the EM optimization. The experimental results confirm the theoretical predictions, proving the vital usefulness of the approach. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:970–974, 2014