A method of tuning the frequency of a waveguide filter including the step of removing dielectric material from one or both of the first and second opposed exterior side surfaces of the waveguide filter to cause a change in the center frequency of the waveguide filter. The cooling device comprises: a cooling panel of a hollow structure adapted to contacted by at least one electronic unit for passing the cooling air therethrough; and a housing box housing the cooling panel and the electronic unit and having an opening for passing the cooling air to the cooling panel. Inside the housing, the light bulb attachment means is electrically connected to the two electrical contact posts.