A radially-fed RF power combiner combines a plurality of input signals to generate a single fundamental-mode transverse electromagnetic (TEM) output. The combiner comprises a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals. The feedthroughs are arranged radially around the vacuum coaxial transmission line. The inner conductive surface of the vacuum coaxial transmission line may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line. Each of the conductive ridges may be coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs. The conductive ridges may have a taper to provide an increasing gap between the top of the conductive ridges and an outer conductive surface of the vacuum coaxial transmission line. The increasing gap may gradually transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope allowing the quasi-TEM mode signals from each conductive ridge to spread azimuthally within the vacuum envelope and combine to generate a substantially pure TEM mode signal.

An axially-fed RF power combiner combines a plurality of input signals to generate a single fundamental-mode transverse electromagnetic (TEM) output. The combiner comprises a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals. The feedthroughs are arranged axially around the vacuum coaxial transmission line. An increasing gap is provided between the inner conductive surface and the outer conductive surface of the vacuum coaxial transmission line to gradually transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line. In some conductive-ridge embodiments, the inner conductive surface of the vacuum coaxial transmission line may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line. In some ridge-less embodiments, the inner conductive surface includes a tapered region within the vacuum envelope to provide an increasing gap between the inner conductive surface and the outer conductive surface.

An axially-fed RF power combiner combines a plurality of input signals to generate a single fundamental-mode transverse electromagnetic (TEM) output. The combiner comprises a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals. The feedthroughs are arranged axially around the vacuum coaxial transmission line. An increasing gap is provided between the inner conductive surface and the outer conductive surface of the vacuum coaxial transmission line to gradually transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line. In some conductive-ridge embodiments, the inner conductive surface of the vacuum coaxial transmission line may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line. In some ridge-less embodiments, the inner conductive surface includes a tapered region within the vacuum envelope to provide an increasing gap between the inner conductive surface and the outer conductive surface.