A section of waveguide:

A waveguide is a hollow metal tube used to carry radio frequency energy, primarily at microwave frequencies. Its physical dimensions dictate a specific 'cutoff frequency' below which electromagnetic waves simply cannot propagate. Because it blocks all frequencies below this cutoff point and allows higher frequencies to pass through, a section of waveguide acts mathematically and practically like a high-pass filter. Distractors such as 'low-pass' or 'band-stop' are incorrect because there is no inherent upper frequency limit caused by the waveguide's basic geometry. Furthermore, traditional waveguides are usually made of rigid copper, brass, or aluminum, making them relatively heavy and difficult to install compared to flexible coaxial cables.