Abstract: A coaxial support structure (10) is provided to providing an interface and support for microwave transmission components. The preferred coaxial support structure (10) includes a hollow cylindrical outer conductor (12), a hollow cylindrical inner conductor (14) disposed coaxially therewithin, and a dielectric spacer (16) providing separation and mutual captivation. The dielectric spacer (16) has odd symmetry, preferably trilateral, and has an end gap (70) separating the spacer end surface (68) from the end planes (38) in which the respective end surfaces (36,50) of the conductors (12,14) are disposed. The inner conductor (14) is provided with spiral grooving (52) on its peripheral surface (46) to provide excellent captivation by the central tube portion (64) of the dielectric spacer (16) with minimal effect on the transmission. Optional features include threading (58) on the interior surface (48)of the inner conductor (14) and a dust washer (72) for disposal within the end gap (70).