An enclosed physical structure that contains and guides a signal. Conducted transmission media fit this definition, as insulation and shields variously serve more or less effectively to confine a signal to the electrical or optical conductor and guide it along a physical path. Unshielded twisted pair (UTP), shielded twisted pair (STP), coaxial cable, and optical fiber are all waveguides. Some radio systems make use of waveguides comprising hollow metal pipes made of a good conductor such as copper, aluminum, brass, or silver, surrounding a dielectric region, usually of air.The pipes generally are rectangular in form, although they sometimes are circular and can take a variety of other shapes, as well. Such radio waveguides are restricted to use in very high power or very high frequency applications over short distances due to their size, weight, inflexibility, and cost. See also transmission medium.