Computation of a directional coupler for RFID systems

The question of establishing a connection point between the antenna circuit and the receiving part of the reader is of major importance for the reliable working of an RFID system, particularly for one of long-range type. Many systems used so far a direct or a voltage divider connection. A novel approach employing a directional coupler with transformers was pioneered by NXP and, according to [2], represents a state-of-the-art as concerns RFID. The computations of the coupler performed in [1] and [2] assumed ideal transformers, i.e., infinite inductances of the transformer coils. We present here a computational model based on admittance matrices that allows for finite inductances.