Alleviating User Interaction for Handset Antennas with Reactive Loads

Alleviating user interaction in handset antennas is challenging. In this communication, a systematic approach is proposed to improve the radiation efficiency of antennas in a lossy environment, exemplified by hand-loaded mobile terminal antennas. The strategy is based on N-port characteristic mode (CM) analysis for composite lossy structures. To construct the network, numerous ports are strategically incorporated into the lossy antenna. Modal efficiencies are then derived for any possible subnetworks, identifying an efficient network with a smaller number of ports. By optimizing the exciting coefficients of the CMs of the subnetwork, the reactive loads at each port can be determined, leading to optimized currents and higher antenna efficiency. The approach proves effective for ultrathin handset antennas operating at multiple bands (GSM 900, DCS 1800, and PCS 1900). Without altering the structure, the proposed antennas can provide an efficiency improvement of more than 3 dB compared with the reference antenna. Antenna prototypes were fabricated, with the measured results agreeing well with the simulated ones.