b extending from the upper portion 160 a the diameter or cross section of the upper portion 160 a is greater than the diameter or cross section of the lower portion 160 b the resonator 160 further includes a transverse through hole 161 disposed in one wall of the upper portion 160 a the capacitive coupling element 156 which may have a rod configuration or other shape is inserted into the hole 161 so that the capacitive coupling element 156 projects horizontally from the side of the upper portion 160 a of the resonator 160 if desired known fastening techniques such as glue tape screws etc can be used to secure the position of the capacitive coupling element 156 within the hole 161 of the resonator 160 in another embodiment the hole 161 does not completely pass through the wall of the upper portion 160 a in still another embodiment the hole 161 and one end of the capacitive coupling element 156 are matingly threaded so that the capacitive coupling element 156 can be rotatably inserted into the hole 161 this feature can enhance the interchangeability of capacitive coupling elements in and out of the hole 161 without requiring re machining or other costly processes for instance all the capacitive coupling elements of varying thickness and or length can have the same sized end that is threaded to change the em coupling coefficient between the resonators all that is required is to remove the existing capacitive coupling element from the hole 161 e g by unscrewing it from the hole 161 and insert a new capacitive coupling element of appropriate thickness and or length into the hole 161 e g by screwing it into the hole.