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A wireless power transmission system for 1.8MHz, 20W LED display board using coupled magnetic resonances is presented. The proposed wireless power transmission system consists of a 50W class-F power transmitter, two magnetic resonance couplers and 20W full bridge RF rectifier receiver. The transmitter is composed of DDS (Direct Digital Synthesizer) block, class-F power amplifier and VSWR measurement block to implement the frequency searching and power tracking function. Two magnetic resonance couplers are designed with a small spiral resonator. The diameter size of the spiral coil is 30 cm (1.8λ/1000). The receiver is composed of RF rectifier, overvoltage protection (OVP) with active dummy load and DC to DC converter. The power consumption of the LED display board is about 20W and the entire efficiency of this WPT system is 43.5%.