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Utilizing the cutoff properties of certain lossy periodic waveguide structures, low reflection absorptive filters for harmonic power at microwave frequencies have been obtained. Various approaches to specific absorptive filters are discussed, and the experimental results presented demonstrate the validity of the concepts developed. Insertion loss of up to 50 db for second harmonic power (TE10 or TE20 mode) and less than 0.1 db for the fundamental at S-band could be accomplished using a waveguide filter of eight inches length. The VSWR is of the order of 1.5 or better throughout the entire useful frequency range. The wide band performance of the filters is characterized by the fact that satisfactory operation results if the frequency of the fundamental falls within the range of dominant TE10 mode propagation in rectangular guide, excluding the extreme edges of the band.