In order to transmit an electromagnetic wave along a certain path, it has to be configured in the vicinities of the boundaries, where it propagates by successive reflection. A hollow conducting metallic tube of uniform cross-section for transmitting an electromagnetic wave by successive reflection from the inner wall of the tube is called a wave guide.

The propagation in the wave guide can be considered as a phenomenon in which waves are reflected from wall to wall and hence pass down the wave guide in zig-zag fashion. The resultant wave travels longitudinally down along the wave guide and has no component of either electric or magnetic fields in the direction of propagation. A large number of distinct field configurations or modes are theoretically possible in the wave guide, which can be determined from the solution of Maxwell s equations and which satisfy the boundary condition.

An essential feature of the waveguide is that it exhibits a frequency characteristic similar to that of the high-pass filter, in that there is a lower limiting frequency below which there is no transmission. At frequencies below the cutoff value, the wave is simply reflected backward and makes no forward progression.