Waveguide components are among those unique structures developed to pass microwave energy from one point in a system to another with minimal losses. Although restricted to specific bandwidths by the relationship of their dimensions to wavelength, waveguide transmission lines are still unparalleled in their low signal loss characteristics. Depending upon the construction materials, waveguide transmission lines, flanges, and components are also capable of handling higher RF power levels than coaxial connectors and components.

Waveguide transmission lines, often called "microwave plumbing," can be rectangular, circular, or elliptical in form. Rigid waveguide provides the highest electrical performance but, because of its lack of mechanical flexibility, can be difficult to install, requiring precisely drilled mounting holes and careful planning at the system level. By trading off some of the performance of rigid waveguide, flexible waveguide provides considerably greater ease of installation.

Rigid waveguide sections are capable of almost negligible insertion loss and VSWR performance that is usually a maximum at 1.01:1. Depending upon surface finish and type of conductive plating, waveguide can be designed to handle power levels in excess of 200 kW.

The width of a waveguide is approximately one-half the wavelength of the frequency of the signals to be propagated through the waveguide, while the height of the waveguide essentially determines its power-handling capability. Since signal wavelengths only become small at microwave frequencies, waveguide dimensions for lower-frequency applications become prohibitively large. For example, assuming the half-wavelength requirement, a waveguide designed for 1-MHz operation would be about 500-feet wide. On another negative note, to minimize insertion losses, the inside surfaces of waveguide are often plated with conductive metals, such as gold or silver, which can increase the cost.

The half-wavelength width supports transverse-mode propagation through the waveguide, such as transverse-electric (TE, with no electric field in the direction of propagation), transverse magnetic (TM, with no magnetic field in the direction of propagation), transverse electromagnetic (TEM, with no electric or magnetic fields in the direction of propagation), and hybrid modes (with both electric and magnetic fields in the direction of propagation). Signals with a half-wavelength larger than the width of the waveguide will not propagate through the waveguide, and are said to be beyond (below) the cutoff frequency of that waveguide.

For that reason, waveguide are generally designed for use above 1 GHz, and reaching frequencies as high as 325 GHz. Waveguide sizes have long been standardized, by the Electronic Industries Association (EIA) in the United States and the Royal Civil Service Commission (RCSC) in the United Kingdom, into a set of designations by frequency range (see table). For the EIA designations, for example, the trend is for larger-number designations to represent lower operating frequencies. For example, WR430 supports frequencies from 1.7 to 2.6 GHz while WR5 handles signals from 140 to 220 GHz. The reverse trend is true for the RCSC designations.

Rigid and flexible waveguide are typically used in military, avionic, and satellite-communications (satcom) systems. In addition to the use of conductive plating on inner waveguide surfaces, waveguide can be pressurized with or without internal gases to increase the power-handling capabilities.

Suppliers of rigid and flexible waveguide transmission lines and components are plentiful and include some of the oldest names in the industry, such as ARRA, Inc. (www.arra.com) and Waveline, Inc. (www.wavelineinc.com). ARRA, one of the longest-running suppliers of waveguide sections and waveguide components, offers a comprehensive selection of bends, couplers, phase shifters, switches, tees, terminations, and twists at frequencies to 40 GHz and beyond. The firm also specializes in supplying custom waveguide assemblies for demanding airborne, shipboard, and space applications. In the component area, for example, the models 42-400 and 42-420 are WR42 E-plane and H-plane waveguide bends, respectively, for use from 18.0 to 26.5 GHz (Fig. 1).

One of the more complex standard waveguide components in the ARRA catalog is the model 90-603 cross-guide coupler (Fig. 2). The company's series of cross-guide couplers feature two or more output ports with customer-specified coupling values. Values of 20, 30, and 40 dB are standard, with other values from 20 to 60 dB available upon request. The model 90-603 is designed with WR90 waveguide for applications from 8.2 to 12.4 GHz. As with other of the firm's cross-guide couplers, minimum directivity is 15 dB with frequency sensitivity of ±1.3 dB or better. The mainline VSWR is 1.15:1 or better while the coupled-port waveguide output VSWR is also 1.15:1 or better. Units are available with a coaxial output, with VSWR of 1.25:1 or better.

The company also manufactures waveguide standard gain horns from 2.6 to 40.0 GHz for communications systems transmission and reception and test applications (Fig. 3). The standard waveguide horns feature a straight-line pyramidal design for low VSWR of 1.25:1 or less. Constructed of aluminum with waveguide sizes from WR284 (2.60 to 3.95 GHz) to WR28 (26.5 to 40.0 GHz), the horns offer 15 dB gain at the lower frequencies and 18 dB gain at the higher frequencies.

Since many systems require a mixture of coaxial and waveguide connections, among the more useful products in the ARRA catalog are waveguide-to-coaxial adapters, such as the model 75-460.

It includes a WR75 waveguide flange at one end 10 to 15 GHz) and Type N connector at the other end. It is also available with an SMA connector as model 75-462.