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This paper presents two highly compact filter configurations using concentric open-loop resonators. A description of each filter configuration is presented, where a linkage between structural features and coupling coefficients is outlined. A third order trisection and a fourth order source-load coupled filter were designed and fabricated at 1.0 GHz, and each filter was shown to occupy an area of just 18 by 18 mm. The measured insertion loss of the third and fourth order filters was 0.83 and 1.20 dB, respectively. Moreover, these structures may also be cascaded to produce higher order compact filters.