Security tag system with improved range consistency

摘要

A radio transmitter used to transmit an activation signal for radio tags held by individuals or the like provides multi-dimensional polarization to an activation signal so as to allow more consistent range detection of an individual with a tag having a single axis of sensitivity. Circular or spherical polarization can be obtained using the system which may include an antenna design permitting decoupling of antennas necessary for the polarization, a quadrature locking system simplifying proper phase control of the necessary signals and a system for tuning the same.

主权项

1. A control unit for a wirelessly activated security tag comprising:
a first and second antenna having mutually perpendicular first and second transmission axes respectively; a first radiofrequency amplifier communicating with the first antenna to provide a first activation signal at a first center frequency; a second radiofrequency amplifier communicating with the second antenna to provide a second activation signal at the first center frequency and having a 90 degrees phase shift with respect to the first activation signal; and including a third antenna orthogonal to the first and second antennas and a third radiofrequency amplifier communicating with the third antenna to provide a third activation signal deviating from the first center frequency to provide a varying phase shift with respect to the first and second activation signals; a radiofrequency receiver for receiving an identification signal from a security tag activated by one of the first, second and third activation signals to provide an output control signal; further including a microprocessor for controlling the first, second, and third radiofrequency amplifiers to vary a power output provided by each of the first second, and third amplifiers to modulate an activation region and to monitor the radiofrequency receiver for receiving an identification signal from the security tag from at least two different power outputs provided to each of the first, second, and third amplifiers to deduce a range of an activated security tag.