A multimode radiation source is disclosed. One embodiment includes a waveguide radiator and an orthomode transducer coupled to the waveguide radiator to provide a first signal to the waveguide radiator. The waveguide radiator is configured to receive the first signal and to radiate the first signal at a first location as a first spherical wave signal with a first phase center. The multimode source also includes transmission medium coupled to the waveguide radiator and configured to radiate a second signal and a third signal from the first location as a second spherical wave and a third spherical wave with substantially the first phase center.