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Tunable High Frequency RFID Coupler via Saturating Transformer Core

IP.com Disclosure Number: IPCOM000201711D
Publication Date: 2010-Nov-18
Document File: 8 page(s) / 858K

Publishing Venue

The IP.com Prior Art Database



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To realize the maximum power transfer, RFID couplers, especially near-field couplers, must be tuned to match the impedance of the source. Ideally, the coupler should always be matched in impedance to the source across all operating conditions.

            Creating an impedance matched coupler under static conditions is a fairly straight forward process to those skilled in the art. Unfortunately, small changes in RFID tag placement (relative to the coupler), design, or even production variations in the coupler itself can cause impedance mismatches resulting in the inability to read or write a RFID tag. Such an impedance mismatch may be caused by inductance variation in the coupler coil.

            An adjustable inductor added in series with the coupler coil may allow the coupler to be tuned to the proper impedance. If the coupler coil loses inductance, the adjustable inductor may be adjusted to increase inductance to compensate. If the coupler coil increases inductance, the adjustable inductor may be adjusted to decrease inductance. This adjustability allows the overall inductance to be maintained at the properly tuned level to retain the impedance match. A transformer may be used to create the adjustable inductor where the core may be shifted up or down its hysteresis curve, thereby changing the inductance. Direct current is relatively easy to adjust and controlled such that it can be used to create an RFID coupler capable of tuning itself.


            High frequency RFID reader systems operating at 13.56MHz use near-field magnetic energy to communicate to the RFID tags. The magnetic energy “couples” the RFID reader loop antenna to the RFID tag loop antenna. Due to the “coupling” action, the name “coupler” is used to describe the reader’s loop antenna.

            As with any RF circuit, maximum power transfer occurs when the source impedance equals the complex conjugate of the load impedance. For example, if the source has an impedance of 50 +j0 ohms, then the load must also be at 50 +j0 ohms. If there is a complex component then the signs of the imaginary components must be opposite (which then cancel).

            Impedance matching between RFID reader (source) and coupler (load) is crucial to the operation of the RFID reader system. Impedance match is the point where maximum energy is radiated per given input power. The implication is the amount of radiated energy drops as the mismatch between source and load impedance grows, which eventually results in cessation of communications between RFID reader system and RFID tag.


            Impedance matching the coupler and source is a well known process to those skilled in the art. Typically the impedance matching is done in a static configuration, with the coupler installed and RFID tag fixed in the desired position near...