This article reviews the application and technology advancement of gallium (Ga)-based liquid metal (LM) in high-frequency circuits and antennas. It discusses the material properties of common LMs, the fluidic channels used to contain LM and their manufacturing techniques, and the actuation techniques, which are all critical for the design and implementation of LM-based devices. LM’s fluidic and pliable nature, together with its excellent electrical, thermal, and rheological (i.e., fluid flow) properties, provides some unique and innovative solutions to flexible/wearable electronics, reconfigurable circuits, and antennas. This article provides a comprehensive review of a wide range of LM-enabled high-frequency circuits and antennas, including interconnects and transitions, reconfigurable passive circuits (such as resonators, filters, and couplers), switches, phase shifters, reconfigurable antennas, flexible and wearable antennas, and metamaterials (i.e., periodic materials with properties not found in nature). This article presents various design concepts and implementation techniques, highlights key capabilities, and discusses the challenges and opportunities with the use of Ga-based LM materials.

A Ka‐band substrate integrated waveguide bandpass filter has been designed and fabricated using low temperature co‐fired ceramic (LTCC) technology. The in‐house developed SICCAS‐K5F3 material with a permittivity of 6.2 and a loss tangent of 0.002 was used. The size and surface area of the proposed bandpass filter are reduced by exploiting vertical coupling in vertically laminated three‐dimensional structures. The coupling between adjacent cavities is realized by a narrow slot. A vertical transition structure between the coplanar‐waveguide feed line and the substrate integrated waveguide is adopted to facilitate the internal signal connection. The demonstrated third‐order filter has a compact size of 6.79 mm×4.13 mm×1.34 mm (0.63λ0 × 0.38λ0 × 0.12λ0) and exhibits good performance with a low insertion loss of 1.74 dB at 27.73 GHz and a 3 dB fractional bandwidth of 10 %.