Impedance matching design of small normal mode helical antennas for RFID tags

The normal mode helical antenna (NMHA) is widely used on RFID applications because it fits well with the requirements of smaller physical size and high inductive impedance. In this paper, a power transmission coefficient method of mapping a modified impedance function onto the conventional Smith chart is applied to conjugate matching design between NMHA and chips. Moreover, the effect of various geometrical parameters of NMHA on resonant frequency and input impedance is investigated. Some significant characteristics of NMHA in the Smith chart are obtained by varying each parameter. It could provide an effective guidance to tune the antenna to desirable complex impedance in the Smith chart. Finally, impedance matching design of a specific NMHA is conducted according to the previously mentioned method.