A bandpass filter makes use of at least one waveguide cavity that is
thermally compensated to minimize drift of a resonant frequency of the
cavity with thermal expansion of cavity components. The compensation
relies on deformation of the shape of at least one cavity surface in
response to thermally-induced dimensional changes of the cavity. A
control rod is used to limit the movement of a point on the deformed
surface, while the rest of the surface moves with the thermal expansion.
The control rod is made of a material having a coefficient of thermal
expansion that is significantly different than that of other filter
components. The rod may also be arranged to span more thermally
expandable material than defines the filter such that, as the filter
expands, the point of deflection is moved toward the interior of the
filter beyond its original position. A similar effect may be accomplished
by connecting the control rod to an end deflecting rod that does the
actual limiting of the movement of the deflection point. If the end
deflecting rod has a coefficient of thermal expansion that is higher than
that of the control rod, the end deflecting rod will expand with
temperature relative to the end of the control rod, forcing the
deflection point inward.