A planar filter of a one-side open type such as a microstrip-line type has the advantage that the planar filter can be easily manufactured and adjusted. Having one side open, however, such a planar filter is likely to be affected by the outside environment, and emits electromagnetic waves to disturb the outside environment. This tendency becomes more conspicuous at higher frequencies. One simple technique for eliminating this drawback is to seal a planar filter in a metal case. Then, unnecessary propagation of electromagnetic waves inside and outside the metal case is completely shut off.
However, a metal case housing a planar filter leads to another problem. This problem is caused by cavity mode resonance of the metal case. For example, input power into a planar filter induces cavity mode resonance, and the cavity mode resonance is picked up by the output line of the planar filter. As a result, a passband is formed in a frequency band that is not included in the design of the planar filter, or the filter frequency characteristics are greatly degraded.
As one of the methods for solving the above problems, there has been a method by which unnecessary higher-order mode shield plates made of a conductor or a high-permittivity dielectric material are used.