Design method of a tap feed for a very small normal-mode helical antenna

Very small normal-mode helical antennas (NMHA) can achieve a high antenna gain in the metal proximity use. So, the NMHAs were developing to use for the RFID (Radio Frequency Identification) tags and the TPMS (Tire Pressure Monitoring System) sensor antenna placed in a tire. In order to achieve impedance matching to the radio unit, a tap feed structure is inevitable. In this paper, the usefulness of the tap feed is shown by an actual example of a RFID tag antenna. Next, the performance of the tap feed is analyzed with the equivalent circuit expression. The input impedance of the tap feed is clearly expressed in a closed form equation. By applying the self resonance condition to the preceding equation, the simple expression of the impedance step up ratio is derived. Moreover, comparisons of input impedances by equations and electromagnetic simulations are shown. Simulated and equation results agree well.