This paper proposes an improved mathematical formulation of Johnston’s approach tomeasure the radiation efficiency of an antenna, based on the Wheeler Cap (WC) technique. Theproposed modifications allow the measurement of the radiation efficiency of small antennas matched tocomplex loads implemented on Radio Frequency IDentification (RFID) tags. The studied structure is alow-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 measurementsvalidate the accuracy of the formulation, which can be applied to any dipole antenna matched to anRFID chip with a complex input impedance.