As the left side drawing, The response of two TAG with different Q value, The lower Q one can operate the wider frequency range but the reading distance is shorter, The higher Q one that has the longer reading range but the frequency range for the TAG receive is small.

For design the RFid Tag, Reader or NFC in the mobile device these different application may need consideration about the reading range and the frequency accuracy of the reader and TAG, The left side drawing that TAG activation level at Y1 for yellow curve the operation frequency range wider from F1 to F4 but less sensitivity to blue curve.

The Air Transformer Consisted by the Reader and TAG of RFID System as the drawing, The TAG resonant frequency (fr) is depended on the reader output impedance (R1 and L1)of the Air Transformer and the Mutual Coupling M (depended on the Distance of TAG)

For avoiding the testing result depended on the coupler & reader, TRC develop the wideband Coupler that great reduce the primary impedance and the repeatable testing each TAG, it is the same reason for the RFID Reader.