A system comprising: a rectangular-to-circular waveguide transition comprising a front section and a back section opposite the front section, the rectangular-to-circular waveguide transition defining a circular hole extending from the front section through the back section, the rectangular-to-circular waveguide transition further having a first cavity extending from the circular hole to a first arcuate region and extending from the face of the front section partially towards the back section, the first cavity being integral with the circular hole, an arc of the first arcuate region of the first cavity beginning and ending with the circular hole, the rectangular-to-circular waveguide transition also having a second cavity extending from the circular hole to a second arcuate region and extending from the face of the front section partially towards the back section, the second cavity being opposite the first cavity with respect to the center of the circular hole, the second cavity being integral with the circular hole, an arc of the second arcuate region of the second cavity beginning and ending with the circular hole, the rectangular-to-circular waveguide transition further comprising a first and second recess (214a and 214b) integral with the first and second cavity, respectively, and extending from the face of the front section, partially towards the back section, the first recess being opposite of the second recess with respect to the center of the circular hole; and a filter card configured to be placed in the first and second recesses (214a and 214b) and across the circular hole of the rectangular-to-circular waveguide transition. a rectangular-to-circular waveguide transition comprising a front section and a back section opposite the front section, the rectangular-to-circular waveguide transition defining a circular hole extending from the front section through the back section, the rectangular-to-circular waveguide transition further having a first cavity extending from the circular hole to a first arcuate region and extending from the face of the front section partially towards the back section, the first cavity being integral with the circular hole, an arc of the first arcuate region of the first cavity beginning and ending with the circular hole, the rectangular-to-circular waveguide transition also having a second cavity extending from the circular hole to a second arcuate region and extending from the face of the front section partially towards the back section, the second cavity being opposite the first cavity with respect to the center of the circular hole, the second cavity being integral with the circular hole, an arc of the second arcuate region of the second cavity beginning and ending with the circular hole, the rectangular-to-circular waveguide transition further comprising a first and second recess (214a and 214b) integral with the first and second cavity, respectively, and extending from the face of the front section, partially towards the back section, the first recess being opposite of the second recess with respect to the center of the circular hole; and a filter card configured to be placed in the first and second recesses (214a and 214b) and across the circular hole of the rectangular-to-circular waveguide transition.