This paper proposes an improved mathematical formulation of Johnston's approach to measure the radiation efficiency of an antenna, based on the Wheeler Cap (WC) technique. The proposed modifications allow the measurement of the radiation efficiency of small antennas matched to complex loads implemented on Radio Frequency IDentification (RFID) tags. The studied structure is a low-cost, silver-printed, differentially-fed RFID dipole antenna. The antenna is printed on a flexible PET (polyethylene terephthalate) paper that is conformable on various objects. Link budget measurements validate the accuracy of the formulation, which can be applied to any dipole antenna matched to an RFID chip with a complex input impedance.
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