Electrically Small Loop Antennas for RFID Applications

The electromagnetic properties of electrically small loop antennas for two different applications are discussed: near-field and far-field RFID systems. In the first case miniature loop antennas are designed for the near field wireless biological and medical sensing application. The magnetic field in the near-field zone is used for coupling between the loops. Using regularly placed capacitors between segments of the loop provides a homogeneous current distribution along the electrically small loop resulted in the improved magnetic coupling. In another case the loop antenna with the improved radiation characteristics is designed for far distance RFID reader antenna. The antenna consists of a combination of the noninteracting loop and dipole. Such a combination shows a cardioid-type radiation pattern and doubled antenna gain. Results of simulations and measurements of test structures of the loop antennas are presented.