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This letter presents a novel dual-band (HF and UHF) radio frequency identification (RFID) tag antenna. Two key challenges are faced: the compact dimension and the impaired bandwidth due to the couplings between two band structures. To enable the implementation of whole design on a credit-card-size single-layer support, a spiral coil along the edges of the card is designed to handle the HF (13.56 MHz) near-field coupling, and the UHF antenna is placed inside the coil instead of outside. Meanwhile, a diagonal symmetric design consisting of two meander lines inside the spiral coil is adopted for UHF band. This structure supports multiple resonance modes around 915 MHz to broaden the working bandwidth under the large inductive circumstance from the HF coil. The proposed antenna is easy for adjustment with ways of handling both coarse tuning and fine-tuning. Its overall dimension is of a credit card size on a single-layer thin substrate. The design methodology and antenna measurement results are both presented and discussed in the letter.