Basics of waveguides

There is something almost comical about a waveguide – the notion that electromagnetic signals or usable power could pass through a hollow pipe and that techniques appropriate to the plumber’s trade would be needed to bend the pipe and make it leak proof. Flanged joints and gaskets are actually used!

To understand what is going on, we must consider the various principle transmission modes for electrical energy. As frequencies rise, inductive (series) and capacitive (parallel) losses conspire to create attenuation in ordinary wires. At radio frequencies, Heaviside’s coaxial cable is commonly used, and it is good up to about one gigahertz for short distances. Above that level, a tubular waveguide is needed.

There are variations in waveguide construction, but the general idea is that a conductive metal pipe of rectangular cross section with a polished inside surface forms a channel that conveys high-frequency electrical energy. The size of the waveguide is directly proportional to the wavelength, and inversely related to the frequency, of the signal that is to be conveyed.

Optical fiber is a waveguide for light frequencies. The signal transmission medium is solid rather than the air of an RF waveguide. The material properties of optical fiber differ from those of its cladding (basically high permittivity vs. low permittivity), a necessary condition for reflection of light so it propagates within the fiber. Waveguides also exist in nature. An example is the acoustic path formed by thermal layers in the ocean, accounting for the fact that the cries of whales may be heard thousands of miles away.

Increasingly large waveguides are needed as the frequency declines. This reality imposes a lower limit on their usage. Waveguides for low-frequency applications (for example, radio telescope arrays) must be set with a crane, while those designed to convey the highest frequencies are held in place with tweezers for soldering on a printed circuit board.

At least one waveguide is found in nearly every U.S. home. It is behind the small mica window near the top of a microwave oven where it transfers power from the magnetron microwave source to the cooking chamber. Another familiar waveguide is a part of TV and Internet-access satellite dish antennas.

A waveguide is needed to convey the microwave frequencies used in transponder-to-dish transmission. It conveys the high-frequency signal as reflected from the parabolic dish to the feed horn, down to the low noise block (LNB) where the signal is converted to a more manageable lower frequency so it can travel 50 ft or so via coax into the home and to the receiver or modem. The LNB, because of its size, cannot reside at the focus of the dish, where it would block transmission.

Electrical energy propagates in a waveguide, a single-conductor path, in a radically different fashion from the familiar two-conductor electrical connection. A signal’s electrical and associated magnetic fields are always perpendicular to one another. When a signal propagates through an electrical transmission line, its associated EM fields are also perpendicular to the line of travel. This common type of transmission is the principle mode, also known as the transverse electric and magnetic (TEM) mode. Its transmission requires two conductors. The diameter of the conductors is small compared to the wavelength of the electrical energy conveyed. This transmission mode works for dc as well as ac, although as mentioned above, performance falls off at higher frequencies due to capacitive and inductive losses. At higher frequencies the waveguide becomes applicable.

In waveguide transmission, there is no second (return) conductor. In this type of transmission, the electric and magnetic fields are still perpendicular to one another, but only one of these fields is perpendicular to the direction of travel. The names for the two subcategories of this type of transmission are transverse electric (TE) mode and transverse magnetic (TM) mode, depending on which field is transverse to the direction of travel.

High frequency signals are inserted at the upstream end of the waveguide and extracted at the downstream end by small dipole or half-dipole antennas, simple segments of wire.

A future article will discuss cavity resonators in which waveguides are also used.