Pattern Synthesis for Lossy Antennas Based on N-Port Characteristic Mode Analysis

Abstract

In this paper, a pattern synthesis methodology for lossy antennas is proposed based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -port characteristic mode analysis (CMA), with the radiation efficiency of the antenna guaranteed at the same time. Reactive loads are chosen as physical implementations. To perform <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -port CMA, N ports are firstly added to the lossy and lossless structures to obtain the impedance matrices. Modal efficiencies are then calculated, based on which the locations of the feed and the loads are selected. With the characteristic solutions and one set of modal weighting coefficients (MWC), where inefficient modes have been excluded, corresponding loads and radiation pattern can be determined. Afterwards, an objective function considering physical implementation and the target pattern is establish and optimized. With optimal MWCs, the lumped components loaded at each port are calculated and implemented. The method is applied to an arm-worn smartwatch antenna operating at GPS L1 band, which aims at radiating towards the sky direction for reliable link with satellites. Following the proposed method, the power radiated towards the target area has been increased by more than 40% compared with the original design. The total efficiency is above -6.1 dB when it is worn on a CTIA arm. The prototype of the proposed antenna has been fabricated and measured, which showed similar performances to the simulated one both in free space and in the arm-worn scenario.