Simple design equations of tap feeds for a very small normal-mode helical antenna

A very small normal-mode helical antenna (NMHA) has been developed for sensor antennas of the tire pressure monitoring system (TPMS) and tag antennas of the radio frequency identification (RFID), because of the usefulness in metal proximity. In order to achieve an actual antenna applicable to radio units, an impedance matching structure was necessary. A simple tap structure was developed in the works of Dinh et al. (2008). However, design method of a tap feed was not clarified. In this paper, important design data such as mutual inductances between a tap and the NMHA, step-up ratios of input resistances and simple design equations are shown.