waveguide, device that controls the propagation of an electromagnetic wave so that the wave is forced to follow a path defined by the physical structure of the guide. Waveguides, which are useful chiefly at microwave frequencies in such applications as connecting the output amplifier of a radar set to its antenna, typically take the form of rectangular hollow metal tubes but have also been built into integrated circuits. A waveguide of a given dimension will not propagate electromagnetic waves lower than a certain frequency (the cutoff frequency). Generally speaking, the electric and magnetic fields of an electromagnetic wave have a number of possible arrangements when the wave is traveling through a waveguide. Each of these arrangements is known as a mode of propagation. Waveguides also have some use at optical frequencies.