Why is it possible to transmit an electromagnetic wave through a waveguide?

Answer 1

First of all we have to consider the relationship between current flowing in a twin wire conductor, its resultant magnetic field, the EMF and the electric field between the conductors. The current flow produces a magnetic field that exists at right angles to it. Furthermore, a moving magnetic field will produce the reverse i.e. current. Additionally, the electric field produced by the EMF, exists at right angles to the magnetic field and the conducting surface. This relationship exists for DC, AC and Radio Frequencies (RF). It is essentially the 'golden rule', as electrical transmission cannot exist without these in place. Additionally, Nicola Tesla discovered that at frequencies at and above the radio spectrum, current does not exist throughout the cross-sectional area of the conductor - progressively only on the surface. This is known as skin effect. Thus at radio frequencies, if we shave the outside of a conductor and turn it inside out, the electro-magnetic wave moves along a metallic waveguide on the skin of the inner surface without the outer surface of the waveguide being affected i.e. acting as an insulator. The wave on the inside conforms to the requirements set out above: Current flows on the inside 'skin' at right angles to the magnetic field. This is known as wall current. An electric field exists at right angles to the magnetic field and the conducting surface. The waveguide is a rectangle and its dimensions are designed to support particular wavelengths. The wave formation inside is composed of alternating 'H' loops (magnetic filed) and 'E' diamonds (electric field). Extraction of the energy is only possible by providing an interruption to the wall currents. A slot cut at 90 degrees across the wall currents produces the greatest coupling and therefore the strongest output. It is possible to have a rectangular slot in a waveguide transmitting at great powers without any output i.e. its longer length aligns or is parallel with the wall currents. So the output can be controlled from provided a slot with no output at zero degrees to maximum output when a slot cuts the wall currents at 90 degrees. In summary, as long as the basic rules are met, as described in the first paragraph, a waveguide can transmit energy along its length.