Application of the theory of characteristic modes to the design of compact metallic strip antenna with multilayer technology (LTCC)

For compact wireless sensors, compact antennas embedded in the sensor are necessary. But it is known that the reduction of the size of an antenna reduces the radiation efficiency and the frequency bandwidth. Then, depending on the application, it is difficult to predict the radiation performance of a compact antenna and to be sure that it is optimum. In this paper, a methodology is proposed and illustrated with the design of a compact antenna for ISM (Industry, Scientific and Medical) applications at 868MHz. For the analysis, the principle is to split the antenna in two parts: the radiating body and the feeder. The radiating part without feed is analyzed with the Theory of Characteristic Mode. The analysis of the modes or current distributions on a structure allows identifying resonant modes. Depending on the size of the sensor, the metallic strip that acts as the antenna could not have “natural” resonance at the desired frequency. However, it is possible to force this resonant frequency by adding short circuit or lumped components to the strip. Through this approach, the optimal shape of the antenna, the position of the lumped components and the feed can be defined in order to find the best comprise between antenna size and electrical performance (bandwidth, efficiency, complex input impedance).