The MicroRWD MF (Mifare) module has been designed to interface to a simple high Q antenna coil of around 1uH inductance together with some capacitor and resistor components. The antenna coil and passive components form the tuned RLC (ResistorInductor-Capacitor) circuit, which is designed to resonate at the 13.56 MHz carrier frequency and have a Q factor of 35-40. For the 125 kHz family of MicroRWD modules, the low Q antenna (700uH) has a relatively wide tolerance of inductance so the capacitor components are fixed and mounted on the RWD module itself pins AN1 and AN2 connect to an external antenna coil directly. For the 13.56 MHz MicroRWD modules, the antenna Q is higher and the inductance tolerance is narrow. For this reason
the capacitor components are external to the module to allow fine-tuning and adjustment of the LC circuit as shown below. The external resistors are used to limit the antenna current and correct the antenna Q to 35-40 range.