How to Convert from Magnetic Field Strength Measurements to Radiated Power for Short Range Device Transmitters

Abstract

With the rapid development of short range devices (SRD), it becomes critical to limit the electromagnetic radiation from the transmitters inside them. For both inductive loop coil transmitters and electric field transmitters, the conversion from magnetic field strength to radiated power in both near field and far field regions are discussed. Three power concepts, total radiated power (TRP), effective isotropic radiated power (EIRP) and effective radiated power (ERP) are clarified. Two concerns in current European Telecommunications Standards Institute (ETSI) and Australian / New Zealand StandardTM (AS/NZS) standards regarding SRD transmitter magnetic field strength or ERP / EIRP limits in the frequency range of 9 kHz to 30 MHz are addressed, formulae to convert from magnetic field strength to EIRP are derived.