Novel tapped feed structures for extremely small normal mode helical antennas

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