A pet monitoring device (101) for monitoring a sub-dermal RFID microchip (103), the pet monitoring device comprising: a wearable item (1) bearing 1 to 5 turns of electrical conductor (7) wound circumferentially to form a wearable item resonator; and an RFID reader (9) attachable and detachable to said wearable item, wherein said RFID reader comprises: a driving circuit (1100) comprising a primary inductance (Lp) inductively coupled to said wearable item when said RFID reader is attached to said wearable item; a secondary inductance (Ls) and resonance capacitor (Cs) conductively coupled to said wearable item when said RFID reader is attached to said wearable item, wherein the secondary inductance and resonance capacitor form the wearable item resonator with said electrical conductor, wherein the wearable item resonator comprises a circuit (1004) to automatically adjust said resonance capacitor to compensate for a size of said wearable item when fitted to said pet; wherein the driving circuit is operable to dri
