Experimental evaluation of lossy coatings of different materials on helix support rods for a high-efficient TWT

All reflections present in the device may cause oscillations and hence instability in a high efficiency traveling wave tube (TWT) amplifier. In a helix TWT, a coating of lossy material is deposited on the helix support rods to absorb these reflections. Carbon is a very commonly used material for this type of coating, and may be deposited by a number of techniques. But other materials may also be employed for this purpose. In the present work, coatings of carbon, nickel and iron have been made and evaluated for their attenuation performance at microwave frequencies. Three APBN rods have been coated with carbon in different thicknesses by a pyrolytic deposition method, while two rods of alumina have been coated with an electro-deposition method. An experimental set-up involving a narrow-height wave-guide with a hole was used to measure the attenuation of the coated rods at 6.0 GHz. Very significant results have been obtained in the experimental study and it is concluded that carbon coated rods have maximum attenuation but nickel and iron can be used also for attenuator purposes.