Rotating field antenna with a magnetically coupled quadrature loop

摘要

A rotating field antenna is provided which includes a figure eight shape loop, a center loop magnetically coupled to the figure eight shape loop, and a drive element for driving the figure eight loop. The figure eight shape loop has an upper loop, a lower loop and a crossover region therebetween. The center loop overlaps at least a portion of the crossover region and at least a portion of one or both of the upper and lower loops. The center loop has no direct or physical electrical connection to the offset figure eight shape loop. Magnetic induction produces a 90-degree phase difference between the phase of the figure eight loop and the phase of the center loop. The antenna thereby produces a rotating composite field when driven by the drive element. The figure eight loop and the center loop are coplanar. The drive element may be an amplified voltage source which has a fundamental frequency of about 13.56 MHz, thereby providing a multiple loop antenna which is useful for electronic article surveillance systems that use RFID tags which resonate at 13.56 MHz.