The filters were fabricated from Invar primarily because of its low coefficient of thermal expansion (CTE) of this material as will be discussed in the next section. However, since it was plated with silver, no loss penalty was incurred by its use, as was verified by fabricating test cavities with it and comparing them with aluminum, brass, copper and nickel cavities. It is instructive to compare the unloaded Q s of cavities fabricated from several of the considered materials. In fact, once silver plated and polished, the highest unloaded Q values were obtained with Invar.