Adjustable transmission line filter and method of constructing same

Transmission line filter adapted to respond to a predetermined frequency, having a resonator conducting layer with dielectric material on opposite sides thereof and a ground plane conducting layer on the outside surfaces of the dielectric material. The filter may be of the stripline or microstrip type having a resonator constructed so that it tends to operate at a frequency different from the desired frequency, and which is not accessible for trimming after the filter is constructed. The ground plane layer is accessible so that a portion thereof can be removed after the filter is constructed to change the electrical parameters of the line formed by the resonator and the ground plane layer to change the response frequency of the filter. Alternatively, conducting material can be added to an opening in the ground plane conducting layer to change the response frequency of the filter. The ground plane layer can be removed by a sand blast, laser beam or other means, and can be added by the use of conductive paint, to trim the frequency of the filter after it is constructed.