RFID system with barriers and key antennas

A radio frequency identification (RFID) system, comprising:

• a) an RFID reader having an antenna located at a reader-antenna location, the RFID reader adapted to transmit a security signal at a selected RF security frequency and a downlink signal at a selected RF downlink frequency and to receive an uplink signal at a selected RF uplink frequency;
• b) a first RF-attenuating barrier spaced apart from the antenna of the RFID reader, wherein:
• i) the first RF-attenuating barrier includes a port having first and second apertures, each aperture having a respective centroid and a respective selected shortest dimension, and the centroids being spaced apart by a respective centroid spacing, so that a security interference pattern is formed on a side of the first RF-attenuating barrier opposite the antenna of the RFID reader when the security signal passes through the port, a downlink interference pattern is formed on a side of the first RF-attenuating bather opposite the antenna of the RFID reader when the downlink signal passes through the port and an uplink interference pattern is formed on a side of the first RF-attenuating barrier towards the RFID reader when the uplink signal passes through the port;
• and
• ii) the RF-attenuating first barrier is positioned with respect to the reader-antenna location to define a link-antenna location at which the downlink interference pattern provides a selected downlink power at the link-antenna location, and at which the uplink interference pattern provides a selected uplink power at the reader-antenna location;
• c) a second RF-attenuating barrier spaced apart from the first RF-attenuating barrier and on the opposite side thereof from the antenna of the RFID reader, wherein:
• i) the link-antenna location is between the first and second RF-attenuating bathers;
• ii) the second RF-attenuating barrier attenuates the security interference pattern less than the downlink interference pattern;
• and
• iii) the second RF-attenuating barrier is positioned with respect to the first RF-attenuating barrier to define one or more peak-antenna locations at peaks of the security interference pattern;
• d) an RFID tag including:
• i) a link antenna disposed at the link-antenna location;
• ii) first and second key antennas disposed beyond a side of the second RF-attenuating barrier farthest from the antenna of the RFID reader, wherein the first key antenna is disposed at one of the one or more peak-antenna locations and the second key antenna is not disposed at one of the one or more peak-antenna locations;
• and
• iii) a controller adapted to transmit the uplink signal using the link antenna after first receiving the security signal at the first key antenna but not at the second key antenna, and thereafter receiving the downlink signal using the link antenna.