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Liquid metal
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Keeping Circuit Designs Liquid

Microwave circuit designers have long depended upon the tunability of different components (such as resonators) to optimize the final performance of a design (such as a radio transceiver). Traditionally, circuits have been based on solid materials like printed circuit boards (PCBs), with conductive metal traces and dielectric materials for isolation. But an alternative approach to designing tunable microwave circuits involves the use of liquid-based materials, such as liquid metal conductors and liquid dielectric materials. Such materials are mechanical in nature and highly linear—not to mention, well-suited for high-power applications.

To explore the suitability of fluidics as an alternative to electronic or mechanical tuning—e.g., with PIN diodes or microelectromechanical-systems (MEMS) components, respectively—researchers at Texas A&M University (College Station, Tex.) and Ossia (Bellevue, Wash.) examined the benefits and drawbacks of fluidics in tunable microwave circuits and how to overcome any of the problems presented by liquid circuit technologies.

Liquid dielectric materials, which have included water, have been employed when there was a need to alter the dielectric constant. Liquid metals, for conductors, have been used in different ways, such as additional loading or switching components on a traditional microwave circuit or as the main conductor in a microwave circuit, where flexibility or even circuit stretchability was essential. Designers who have sought the right materials for flexible circuits know all too well that conventional conductive materials, such as copper, can experience fatigue and crack over time when subjected to bending. Liquid metals, when exploited in prototype designs such as coplanar-waveguide (CPW) resonators, experienced no fatigue or change in performance with bending. This conclusion was based on S-parameter measurements and computer simulations performed with the High Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansys.

The researchers explain how fluidic channels can be integrated on top of planar microwave circuits to serve as reactive loading structures. Liquid metal structures can be placed on polydimethysiloxane (PDMS) substrate material (with dielectric constant of 2.68) to create fluidic microwave circuit structures. Different insulator materials can be used in place of PDMS, but a challenge remains in attaching these substrates to conventional microwave circuit materials when these fluidic circuit structures are used as loading or tuning circuits. The research examined performance characteristics, including power-handling capabilities, for several high-frequency components (including antennas and filters). The results showed impressive broadband tuning capabilities for some wide frequency bands, such as 1.7 to 4.9 GHz. See “Fluidics in Microwave Components,” IEEE Microwave Magazine, Vol. 17, No. 6, June 2016, p. 50.

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