A simple method of producing far-infrared high-pass filters

A method of making high-pass waveguide filters for the far infrared is described. The filters are formed from arrays of tightly packed narrow-bore metal tubes. Such filters feature sharp cut-off transitions in the region between 20 cm -1 and 40 cm -1 . Transmission above cut-off is comparable to that in similar filters produced by drilling holes in thin metal sheets. Typically the transmissions just above cut-off are about 5% while that just below was measured as less than 0.1%.