Modified Wheeler Cap Approach for Radiation Efficiency Measurements of On-body RFID Antennas

In this paper, the radiation efficiency of a differentially fed loop antenna is measured using the modified Wheeler Cap (WC) technique. The antenna is matched to the Alien Higgs 9 RFID chip, when placed on a human body-mimicking gel. Characterization of the used gel is first performed, followed by measurements of the wake-up power and impedance of the RFID chip. The WC method using the modified Johnston’s equation published in an earlier work, is applied for radiation efficiency measurements of the proposed loop. Read range measurements are performed for the tag as well, and radiation efficiency is extracted using Friis formula. It is showcased that, for balanced antennas fed by a differential port, and placed over lossy matter giving low overall antenna efficiency, the WC modified method can be applied to measure the radiation efficiency by normalizing the $S 11$ plots to the chip resistance.