RFID is a technology for wireless identification of objects. More recently, much attention is paid to application areas within biomedical engineering, in which wearable tags for on-body use could provide real-time remote bio-monitoring of humans. New types of antenna materials and antenna structures are emerging to fulfil the requirements encountered within the new RFID application areas. Tag designs where the tag antenna structure is formed from conductive ink or conductive threads have been proposed as competitive materials to conventional etched copper. The new materials used to form the complex antenna materials are challenging to model accurately. In this book, a novel radiation efficiency measurement method is developed and verified for measurement of passive UHF RFID dipole tag antennas. The measurement method provides a powerful tool for characterisation of complex antenna material structures losses in practise. The acquired information can be used to optimise tag antenna material structures and to improve tag antenna performance and reliability, which is crucial for widespread use of RFID to become reality.