The synthesis of wide-and multi-bandgap electromagnetic surfaces with finite size and nonuniform capacitive loading

A method is presented that allows for the efficient design of capacitively loaded finite-size electromagnetic bandgap (EBG) structures, which can target a wide range of design objectives. The design flexibility is achieved by adding arbitrary nonuniform capacitive loading to an underlying periodic EBG structure. This system can be interpreted as having an effective aperiodic structure, which allows more design flexibility in terms of bandgap engineering. To choose the proper capacitances, a powerful global optimization technique known as the covariance matrix adaptation evolutionary strategy is employed that is aided by a fast port-reduction strategy. This approach avoids the need to carry out multiple computationally expensive full-wave simulations during the course of the optimization process by requiring only a single full-wave simulation be performed prior to initiating the optimization. To demonstrate the utility of this method, the capacitive loading of a mushroom-type EBG structure in a parallel-plate waveguide is optimized to reduce transmission from 2.4 to 7 GHz. This design was fabricated and the measured response was found to be in good agreement with the simulations. Using the same initial full-wave simulation, another structure was designed to improve isolation at the 2.4-, 3.6-, and 5-GHz WLAN bands to below-22 dB. An additional set of structures are also designed using capacitively loaded mushroom-type EBG surfaces without placing them inside of a parallel-plate waveguide.