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Many near field communication (NFC)-based reader / smart card applications are operated at a maximum magnetic field strength to increase the smart card's operational stability. However, a maximum magnetic field strength is worthwhile only in situations of high smart card power requirements (e.g., performing cryptographic operations) or long distance communications. As a result, electrical power is wasted, which limits the run-time of mobile battery-operated reader devices. Here we present an adaptive field strength scaling (AFSS) methodology. The strength of the reader's emitted magnetic field is modified depending on the instantaneous power consumption requirements of the smart card. When the smart card consumes less power, the magnetic field strength is reduced. Whereas when it consumes more power, the magnetic field strength is increased. Thus, the power consumption of the reader / smart card system as a whole is optimized while preserving the smart card's operational stability. In this work, we present the design and implementation of two different AFSS approaches. A reader / smart card hardware emulation platform is used to prove the AFSS technique's feasibility and proper functionality. Experimental tests demonstrate that the energy consumption of the AFSS enhanced reader / smart card system can be reduced by up to 54% compared to current commonly used approaches. Furthermore, we show that the smart card's stability is preserved if the AFSS technique is applied.