Abstract: A method and apparatus for locating and tracking a portable transmitter that may be deposited with currency or other items desired to be tracked, comprising a doppler antenna array, analog antenna switching, radio frequency and intermediate frequency circuitry and a digital signal processor, exhibiting increasing sensitivity, accuracy, and range.

Abstract: A radio frequency transmitter is hidden within a packet of currency which may be mingled with other currency packets in the cash drawer of a bank, convenience store or other such commercial institution. The transmitter is automatically activated by unauthorized removal from the cash drawer or other place of safekeeping. The signals from the transmitter activate alarms in one or more receivers located in police vehicles in the area. The receivers employ phase comparison techniques to indicate relative direction and distance to the transmitter thereby facilitating apprehension of the possessor of the packet.

Abstract: A radio frequency receiver is responsive to signals radiated by a remote transmitter. The receiver includes at least one pair of antennas separated by approximately one-half wavelength at the transmitter frequency. The phases of the signals detected by the antennas are compared for the purpose of determining the relative direction to the transmitter.