Waveguide Microwave RF Feeder Tutorial

- RF waveguides are used as feeders or transmission lines in microwave applications where they are able to carry high power levels while offering low signal loss figures.

RF waveguides are a form of RF feeder or transmission line used for microwave applications.

The basics of waveguide technology are easy to comprehend, although the mathematics involved can become complicated when wave theory and the like are used.

Waveguides, as the name indicates are a guide for electromagnetic waves and this way they enable them to feed power from one location to another.

Waveguides effectively confirm and direct a wave within a given boundary. Within many electronic circles, waveguides are most commonly used for microwave RF signals, the same principles can be used for many forms of wave from sound to light.

RF & microwave waveguide basics

Waveguides are used in a variety of applications to carry radio frequency energy from one pint to another. In their broadest terms they can be described as a system of material that is designed to confine electromagnetic waves in a direction defined by its physical boundaries. This definition gives a very broad view of their properties, but it indicates that waveguide theory can be applied in a number of areas and in a variety of different ways.

Electromagnetic waves propagating in open space travel out in all directions and can be thought of as spherical waves travelling out from a central source. As a result the power intensity decreases as the distance increases - it is proportional to the power of the source divided by the square of the distance. The waveguide operates by confining the electromagnetic wave so that it does not spread out and losses resulting from this effect are eliminated.

Typically a waveguide is thought if as a transmission line comprising a hollow conducting tube, which may be rectangular or circular within which electromagnetic waves are propagated. Unlike coaxial cable, there is no centre conductor within the waveguide. Signals propagate within the confines of the metallic walls that act as boundaries. The signal is confined by total internal reflection from the walls of the waveguide.

Diagram of a rectangular waveguide showing its construction and rectangular cross sectional shape.
Rectangular waveguide

Waveguides will only carry or propagate signals above a certain frequency, known as the cut-off frequency. Below this the waveguide is not able to carry the signals. This is obviously an important parameter, and one of the most basic specifications for its operation.

Types of RF waveguide

There is a number of different types of RF waveguide that can be used, bought and designed.

Typically they are thought of as being rectangular in cross section as this is the most common form of waveguide. However other types and approaches may be used.

  • Rectangular waveguide:   This is the most commonly used form of waveguide and has a rectangular cross section.
  • Circular waveguide:   This is less common than rectangular waveguide. They have many similarities in their basic approach, although signals often use a different mode of propagation.
  • Circuit board stripline:   This form of waveguide is used on printed circuit boards as a transmission line for microwave signals. It typically consists of a line of a given thickness above an earth plane. Its thickness defines the impedance.

In addition to these basic forms, there are also flexible waveguides. These are most widely seen in the rectangular format. Flexible waveguide is often used to connect to antennas, etc that may not be fixed or may be moveable.

Waveguides are more expensive than other forms of RF feeder. However they offer a number of advantages that mean they are the only feasible solution in many applications. Although waveguide is not nearly as widely used as other forms of feeder such as coax, it still forms and essential method of transferring RF power, especially are microwave frequencies.

By Ian Poole

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