Coating of a lossy material is done at the helix support rods to improve the stability in a traveling wave tube amplifier.Helix is supported by three dielectric rods normally of APBN (Anisotropic Pyrolytically deposited Boron Nitride) or Alumina, which have high thermal conductivity. A coating of lossy material applied on the helix support rods serves as attenuator to absorb the reflections. Carbon is a widely used material for this type of coating, and is commonly deposited by a number of methods (including pyrolytic deposition, thin film vapor deposition or sputtering method). But other materials like nickel and iron may also be employed for this purpose. Conducting polymers have recently evinced much interest in potential applications as electromagnetic interference (EMI) shielding screens, as coatings or jackets for flexible conductors, and as broadband microwave absorbers. From the military and aerospace point of view especially, the absorption characteristics in particular have many applications including stealth and even a few materials are capable of dynamic microwave absorption. Conducting polymers offer the properties of high conductivity combined with very light weight, flexibility and reasonably facile processibility. Due to these properties and their high conductivity / weight ratio, one potential applications may be seen as attenuator coatings in the helix TWT. In order to characterize the coating for loss performance in TWT applications four rectangular substrate of alumina (1 inch x 0.5 inch) have been coated with the following polymer combinations.
1. Polyaniline with p-toulene sulphonic acid (PTSA)
2. Polyaniline-polypyrrole with FeCl3 and PTSA
3. Polypyrrole with FeCl3
4. Polyaniline with anthraquinone sulphonic acid
An experimental set-up is made for measuring the attenuation of the coated films in the C-band frequency range using the partially filled wave-guide method. The microwave power is measured at the input and output of the wave-guide to find the return loss and the insertion loss. The work is very significant in the designing of good quality attenuators in the microwave and millimeter wave frequency tubes. Other properties of these materials are being explored for evaluating their suitability as good quality and long life attenuators.
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