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.

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Waveline, Inc. (www.wavelineinc.com) offers extensive lines of waveguide products as part of its long history as a quality supplier of waveguide transmission lines and components. For example, Waveline's calibrated variable precision rotary wave attenuators deliver precisely adjustable values of attenuation with almost negligible variations due to temperature and humidity. The Series 22R rotary vane attenuators feature a specially designed cylindrical waveguide section with matched metallized attenuating elements that are rotated to achieve the precise attenuation values. The firm has reduced the size of the attenuators by the use of newly developed waveguide cylindrical sections and improved transition techniques.

The Series 22-R rotary vane attenuators are available in panel-mount and rack-mount versions with an attenuation range of 0 to 60 dB for waveguide bands from WR284 (2.60 to 3.95 GHz) to WR22 (33 to 50 GHz). The insertion loss for these precision attenuators is 1 dB or less and typically only 0.5 dB The attenuation accuracy is either ±0.1 dB or within ±0.2 dB of a reading, whichever is greater. The maximum VSWR is 1.15:1. The attenuators handle as much as 15 W power at lower frequencies and 0.5 W at 50 GHz.

Antennas for Communications (www.afcsat.com) is perhaps best known for their satellite-communications antennas and antenna feeds, although the company also offers a range of waveguide transmission lines. For example, the company's TALLGUIDE® low-loss waveguide is designed as a replacement for standard rigid waveguide. The technology is used for a variety of components, including transitions, adapters, and antennas. The company claims transmission-loss improvements as much as a factor of 10, with improvements in signal linearity of 3 to 6 times compared to standard waveguide.

Instruments For Industry (www.ifi.com) produces a variety of waveguide components through 50 GHz including terminations, standard gain horns, power dividers/combiners, and broadwall directional couplers. The couplers, which can be specified with coupling values of 3, 6, 10, 20, and 30 dB, feature mainline VSWR of 1.10:1 and better and coupled-port VSWR of 1.15:1 or better. The nominal coupling value is accurate within ±0.5 dB while the directivity is typically 40 dB.

Flann Microwave (www.flann.com) offers lines of tunable waveguide filters in manual (Series 288) and programmable (Series 287) versions. The filters, which can be specified with bandpass or bandreject (notch) responses, operate over bandwidths as narrow as 0.5 percent. The passbands can range from 1 to 45 GHz with 15-percent frequency tuning ranges. For example, model 15287-AA is a waveguide bandpass filter that tunes from 4.3 to 5.1 GHz.

Microwave Communications Laboratories, Inc. (www.mcli.com) has developed a variety of coaxial and waveguide microwave components, including the model WS2-X crossguide directional coupler with WR430 waveguide (1.7 to 2.6 GHz). The three-port terminated design achieves directivity of 40 dB with frequency sensitivity of ±0.6 dB. Available in standard and custom coupling values, the nominal coupling value is accurate within ±0.5 dB while the main-port VSWR is 1.05:1 or better and the coupled-port VSWR is 1.10:1 or better.

Microwave Development Laboratories (www.mdllab.com) recently announced a new WR28 in-line waveguide rotary joint for applications from 27.5 to 31.0 GHz. The rotary joint features insertion loss of 0.3 dB or less and VSWR of 1.30:1 or less. The firm also offers waveguide rotary switches, waveguide-to-coaxial adapters, attenuators, power dividers, bends, directional couplers, magic tees, and hybrids.

Waveguide components are also used in industrial heating systems, with companies such as Gerling Applied Engineering, Inc. (www.2450mhz.com) offering extensive lines of waveguide components for industrial heating applications. The firm's products operate at specific bands set aside for the purpose, such as 2.45 and 5.8 GHz. The firm's website contains an excellent article on the optimal selection of waveguide components for industrial heating systems.

Additional suppliers of waveguide components include Advanced Technical Materials, Inc. (www.atmmicrowave.com), with extensive lines of attenuators, shorts, phase shifters, isolators and circulators, waveguide sections, horn antennas, power combiners, terminations, rectangular waveguide assemblies, and flexible waveguide; Aerowave, Inc. (www.aerowave.net); A. T. Wall Co. (www.atwall.com), with one of the industry's broadest lines of rigid rectangular and circular waveguide transmission lines; Coleman Microwave (www.colemanmw.com), with the model 603025 directional coupler with 6 dB coupling (±0.5 dB) from 10.7 to 11.7 GHz and 25-dB minimum directivity, as well as extensive lines of filters and diplexers; HD Communications Corp. (www.hdcom.com); MEM Research (www.memresearch.com); Microwave Devices, Inc. (www.mwdevices.com), which offers the WF series of waveguide bandpass and harmonic filters for applications through 60 GHz as well as adapters, terminations, isolators/circulators, and directional couplers; Micro Metalsmiths Ltd. (www.micrometalsmiths.co.uk); Microtech, Inc. (www.microtech-inc.com), with extensive lines of flexible rectangular waveguide from WR650 (1.12 to 1.70 GHz) through WR22 (33 to 50 GHz) and with peak power ratings as high as 10 kW for the WR650 waveguide; Millitech, Inc. (www.millitech.com), with lowpass, highpass, and bandpass filters from 18 to 140 GHz; Renaissance Electronics Corp. (www.rec-usa.com), with waveguide couplers, filters, and adapters; The Waveguide Solution (www.waveguidesolution.co.uk); and Vector Telecom (www.vectortele.com). Of course, for a complete listing of waveguide suppliers, consult the online version of the Microwaves & RF Product Data Directory at www.mwrfpdd.com