a temperature stable bandpass folded filter for microwave radio communication , made from standard waveguide sections . two waveguide sections are divided into cavities using metal plates made from a temperature stable material , to construct the filter . the sections are connected juxtaposed and the cavities of each section are coupled through holes in the coupled broadwalls of the section . low insertion loss and very good temperature stability is obtained with the use of copper clad invar waveguide material . the method of constructing the temperature stable folded waveguide filter of reduced length is also disclosed .