A combiner/divider comprising: a first plurality of input/output waveguides distributed in a plane and diverging in at least a partially common direction away from a central point, wherein each input/output waveguide extends from a respective single outer node disposed distal of the central point to a respective single inner node proximate to and spaced from the central point; an output/input waveguide having an inner port proximate to the central point and facing the inner nodes of the first plurality of input/output waveguides; and a transition waveguide defining an open cavity extending from the inner port of the output/input waveguide to the inner nodes of the first plurality of input/output waveguides, wherein the transition waveguide flares outwardly in the plane from the inner port toward the inner nodes and communicatively couples the output/input waveguide with the first plurality of input/output waveguides; wherein the first plurality of input/output waveguides includes all of the input/output waveguides of the combiner/divider, each input/output waveguide of at least a second plurality of the input/output waveguides included in the first plurality of the input/output waveguides has a respective inner dimension, H, in the plane and transverse to the length of the respective input/output waveguide that increases over the entire length from the respective inner node to the respective outer node; wherein the first plurality of input/output waveguides comprise transverse walls defining the input/output waveguides including transverse wall faces that are transverse to the plane, a transverse wall face of the transverse walls is disposed between each pair of adjacent input/output waveguides of the first plurality of input/output waveguides, and each transverse wall has a respective end at the inner nodes of the respective adjacent input/output waveguides; wherein the first plurality of input/output waveguides are communicatively coupled at the respective inner nodes to the transition waveguide; and the inner nodes of the first plurality of input/output waveguides are all disposed at the same radial distance from the central point. a first plurality of input/output waveguides distributed in a plane and diverging in at least a partially common direction away from a central point, wherein each input/output waveguide extends from a respective single outer node disposed distal of the central point to a respective single inner node proximate to and spaced from the central point; an output/input waveguide having an inner port proximate to the central point and facing the inner nodes of the first plurality of input/output waveguides; and a transition waveguide defining an open cavity extending from the inner port of the output/input waveguide to the inner nodes of the first plurality of input/output waveguides, wherein the transition waveguide flares outwardly in the plane from the inner port toward the inner nodes and communicatively couples the output/input waveguide with the first plurality of input/output waveguides; wherein the first plurality of input/output waveguides includes all of the input/output waveguides of the combiner/divider, each input/output waveguide of at least a second plurality of the input/output waveguides included in the first plurality of the input/output waveguides has a respective inner dimension, H, in the plane and transverse to the length of the respective input/output waveguide that increases over the entire length from the respective inner node to the respective outer node; wherein the first plurality of input/output waveguides comprise transverse walls defining the input/output waveguides including transverse wall faces that are transverse to the plane, a transverse wall face of the transverse walls is disposed between each pair of adjacent input/output waveguides of the first plurality of input/output waveguides, and each transverse wall has a respective end at the inner nodes of the respective adjacent input/output waveguides; wherein the first plurality of input/output waveguides are communicatively coupled at the respective inner nodes to the transition waveguide; and the inner nodes of the first plurality of input/output waveguides are all disposed at the same radial distance from the central point.