In some embodiments, a feed structure to couple RF energy to a target may include a body having a first end to receive RF energy and a second end opposite the first end to couple the RF energy to a target, the body further having a central opening disposed through the body from the first end to the second end; a first member coupled to the body at the first end, wherein the first member comprises a first element circumscribing the body and extending radially outward from the body, and one or more terminals disposed in the first member to receive RF energy from an RF power source; and a source distribution plate coupled to the second end of the body to distribute the RF energy to the target, wherein the source distribution plate includes a hole disposed through the plate and aligned with the central opening of the body.