Novel tapped feed structures for extremely small normal mode helical antennas

Abstract

Extremely small normal mode helical antennas (NMHA) for RFID tags were developed. By introducing folded structures, antenna efficiencies of -5 dB and -1.5 dB were achieved in antenna sizes of 0.025 wavelength and 0.05 wavelength, respectively. However, antenna input resistances were very low such as 5 ~ 8 Omega. Impedance matching structures were requested. In this paper, novel impedance matching structures are proposed. A tap feed is composed around a helical wire. A very compact matching structure is accomplished. Excellent impedance matching capabilities to 50 Omega coaxial cables are ensured through numerical and experimental studies