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Waveguide Has Many Miles to Go

March 21, 2017
Waveguide technology has been written off as obsolete for some time, but nothing has replaced it yet.

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Waveguide suppliers are truly the “old-timers” of the RF/microwave industry, selling a transmission-line technology that most users would rather replace with something newer. For all its bulk compared to an alternative, such as a coaxial cable with connectors, waveguide is still the lowest-loss way to get a high-frequency electromagnetic (EM) signal from point A to point B. And it does get smaller as the frequency pushes higher.

Manufacturers of metal waveguide have been likened to blacksmiths working in a forge, hammering out these circular and rectangular tubes with their precise dimensions related to the wavelengths of the frequencies of interest. For years now, this is a technology that many have predicted would soon vanish, but it is still here, and still in demand. Can’t anyone find a replacement?

Millimeter-wave frequencies may be a best friend to metallic waveguides, mainly because coaxial cables—those lighter-in-weight, lower-cost transmission-line alternatives to waveguide—tend to suffer higher losses at higher frequencies, notably at millimeter-wave frequencies above 30 GHz.

Some impressive advances have been made in coaxial-cable and connector technologies over the past decade. They include relatively phase-stable, low-loss cables with connectors commercially available for applications to 110 GHz, such as for connection to a millimeter-wave vector network analyzer (VNA). However, they still can’t come close to the superior electrical characteristics at millimeter-wave frequencies (for more on how waveguide are used in high-frequency testing, see “Testing Automotive Radar Brings mm-Wave Challenges”).

Effective But Bulky

In terms of performance, waveguide has extremely low signal losses and reflections, with very little distortion of signal phase—a critical characteristic for phase-modulated communications signals. Waveguide is completely shielded compared to other transmission-line formats, but it is unwieldy, even in its various formats of rectangular, circular, or double-ridge waveguide, in rigid and flexible forms.

Waveguide are also traditionally more expensive than other transmission-line formats, including microstrip and stripline on printed-circuit boards (PCBs). Consider just the costs of materials when comparing waveguide to other transmission-line formats.

1. This compact waveguide component is a transmitter for 60-GHz communications links. (Courtesy of Pasternack)

Waveguide are fabricated entirely of conductors, such as copper. Compare this to the amount of copper used as the center conductor in a coaxial cable or as the copper laminate used on a PCB material. In both cases, it is the most expensive material cost for both of those components, so it is easy to understand why waveguide can be more expensive. Waveguide also tend to be manufactured in smaller quantities, so there is no economy of scale such as when manufacturing long runs of coaxial cables.

Handling those “Last Mile” Links

Nonetheless, waveguide appear to be growing in popularity, most likely due to the growth of applications at millimeter-wave frequencies. The 60-GHz band has provided reliable, short-range unlicensed communications links at multi-gigabit data rates using traditionally expensive waveguide components. Such links are often referred to as “the last mile” in communications systems.

Growing demand for those links (expected to be widely used in the much-heralded 5G wireless communications networks) is enabling larger, more cost-effective production runs of waveguide components, such as integrated waveguide transmitters (Fig. 1). In what once was realized as separate components, this single housing with waveguide connection includes the frequency synthesizer and amplifier needed to generate transmit signals for unlicensed communications at 60 GHz.

Developed and supplied by Pasternack, the waveguide transmitter is tunable from 57.0 to 64.8 GHz. Tim Galla, Active Component Product Manager at Pasternack, explains, “The PEM010 is a high-performance millimeter-wave transmitter module in a compact package that supports a WR-15 waveguide interface and is ideal for use in developing multi-gigabit, high-speed, point-to-point wireless communication links. Best of all, this 60-GHz transmit module is available in stock and ready to ship to our customers today.”

At one time, waveguide components were considered specialty items and often manufactured as custom products. Galla’s last remark, that the transmitter is in stock, is evidence of the growing production quantities of waveguide components, especially at frequencies like 60 and 77 GHz.

Low Frequencies Work, Too

However, even at lower frequencies, there is sometimes a demand for waveguide components. The 24-GHz band is also an unlicensed portion of the frequency spectrum that is used for radar and communications applications. Transmitters and signal sources are also needed for those applications and waveguide transmission lines to help conserve signal power better than coaxial transmission lines.

2. This waveguide Gunn oscillator provides a tunable output at a center frequency of 24.125 GHz. (Courtesy of Fairview Microwave)

As an example of the growing number of waveguide components supporting that frequency band, a Gunn diode oscillator designed and developed by Fairview Microwave can be used as a local oscillator (LO) in transmitters and receivers at 24 GHz and beyond for a wide range of millimeter-wave applications, including commercial and military radar systems, radio communications systems, and security screening systems.

The waveguide Gunn oscillator (model FMWGN1001) generates a signal at a center frequency of 24.125 GHz, which can be tuned by 1 GHz above and below that center frequency. Randy Leenerts, senior RF engineer at Fairview Microwave, notes, “Our FMWGN1001 Gunn oscillator module offers designers a cost-effective and high-performing K-Band waveguide frequency source solution for use with sensitive communications, radar, and test-and-measurement applications.”

Waveguide technology may be a means of achieving that “last mile” of wireless communications at 60 GHz, and a part of 77-GHz automotive radar systems, but it appears to have more than a few miles left in it. In fact, for a technology that has been considered “obsolete” for some time now, it would seem that waveguide has lots of life left.

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