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Photonic bandgap (PBG) structures can stop propagation of electromagnetic waves of certain frequencies and hence can be applied to construction of microstrip filters. In this paper, a novel PBG microstrip filter based on YBa2Cu3O7 high-temperature superconductor films is presented. The PBG patterns forming the filters were etched on a 0.3- and a 0.7-μm-thick film. Experimental results showed that the transmission coefficient S21 of the filter is dependent on the thickness of the YBa2Cu3O7-x film. The thinner PBG filter exhibits an effective stopband of a center frequency of 20 GHz, a bandwidth of 4.7 GHz (at -40 dB), an attenuation of -55 dB, and a rolloff rate of 38 dB/GHz, while the thicker structure exhibits narrow notches only in the S21 characteristics.