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[en] We report the experimental measurement of wakefields in a 17.14 GHz metallic photonic bandgap accelerator structure and its comparison with theory. Damping of wakefields is a critical issue in the next generation of high gradient electron accelerators, and photonic bandgap (PBG) structures have unique properties for suppressing and damping wakefields. In the experiments the wakefields were generated by passing an 18 MeV electron beam through the six cell 17.14 GHz structure. The wakefield radiation was measured at two locations: the structure output port, and at a window viewing port located at the side of the structure.The electron beam consisted of a train of bunches spaced at 17.14 GHz, so that only radiation at 17.14 GHz and its harmonics was observed. Wakefields at up to the fifth harmonic (85.7 GHz) were detected by a heterodyne receiver system. The wakefield power levels were measured at 17 and 34 GHz for average currents between 20 and 300 mA. The results were compared with full wakefield simulations using the code ANALYST. The measured power level at 17 GHz was in excellent agreement with the theoretical estimate, but the measured power at 34 GHz was significantly lower than the theoretical estimate. At both 17 and 34 GHz the measured power level increased as the beam current squared, as expected from theory. The experimental results demonstrate the weak excitation of high order mode wakefields in a photonic bandgap structure.