When Waveguide Buried the Competition

As waveguide has the lowest insertion loss of any type of RF and microwave transmission medium it would seem to be a perfect fit for long-haul communications. Or so it seemed in the 1960s and 1970s when AT&T and Bell Laboratories developed the details for building, installing, and operating a 4,000-mi.-long network of buried circular waveguide for voice, data, and video communications. The massive initiative that evolved over more than a decade was ultimately abandoned after optical fiber made light wave communications possible, but even by today’s standards the waveguide system’s performance would have been remarkable.

The WT4 Long-Distance Buried Waveguide System would have accommodated 238,000 two-way voice circuits or various combinations of voice, data, and video using two-level, differentially coded, phase-shift-keyed modulation to achieve a continuous data rate of 274 Mb/s. The system could be upgraded to four-level modulation to provide capacity of 476,000 voice channels by modifying the electronics, without adding repeaters.

There would be 124 broadband channels of which 59 would be used for protection in each direction and was designed to have higher reliability than the current transmission media, such as buried coaxial cable and microwave repeaters, with predicted outage of 0.025 per two-way, 4000-mi. connection. A fully loaded system was expected to have a cost per circuit mile far lower than any existing long-haul system and was to be installed on rights-of-way using standard construction techniques.

One of the factors driving development of WT4 was the world’s first videoconference system called Picturephone (Figure 1), on which AT&T would ultimately spend more than $500 million (nearly $2.3 billion today), that was introduced at the 1964 New York World’s Fair. Although a Picturephone system was commercially deployed in Pittsburgh in 1970 and Chicago the following year, it was taken off the market a few years later.

Figure 1 image of AT and T advertisement in time magazine promotion WT4

Figure 1: FAT&T had great expectations for WT4, which is was none too shy in promoting in this ad in Time magazine

Its failure was the result of several factors, not the least of which was the cost to the user, which initially was $160 per month for the equipment and 30 min. of calls. Additional calls cost 25 cents per minute. This was later reduced to $75 per month for 45 min. of calls to increase the number of customers, but at its peak the Picturephone service generated only 453 calls. As it was usable only if parties at both ends had the equipment and service, there was obviously little demand. And it turned out that most people really didn’t like being seen while they were talking.

A Massive Challenge

Bell Laboratories’ scientists were old hands at making the seemingly impossible a reality, and as waveguide had the bandwidth and exceptionally low loss required in such a network when excited in the TE01 mode, the idea of creating such a huge expanse of waveguide seemed achievable. Like all emerging applications, there were enormous technical hurdles to be overcome as well as those encountered in building a 4,000-mile-long, waveguide-filled pipeline in widely varying terrain and environmental conditions.

The first exploratory development work was conducted in 1959 using 2-in. waveguide and traveling-wave-tube (TWT) repeaters but was abandoned in 1962 because of TWT cost and reliability problems. There also wasn’t enough Bell System traffic at the time to warrant continued work. But in 1968, after six years of continuous long-haul growth and the emergence of solid-state devices such as IMPATT diodes, an all-solid-state system was undertaken as a joint project of Bell Laboratories, Western Electric Company, and the Long Lines Department of AT&T. The new system, the WT4A, would use all-solid-state regenerative repeaters, replacing the problematic TWTs.

The dielectric-lined circular waveguide consisted of a steel tube copper-plated on the inside and lined with a thin layer of polyethylene that could reduce mode coupling (and thus loss) in bends. This type of waveguide would be used in 98% of the transmission path with sections of helix waveguide inserted periodically along the length to limit transmission deviations. Both types had an inside diameter of 60 mm, which was determined to provide the least loss across the frequency band from 38 GHz to 104.5 GHz. Loss was calculated to be an incredibly low 1 to 2 dB per mile, which would allow repeater spacing to be every 37-mi. in gentle terrain and 31 mi. in rugged terrain.

Figure 2 image of former bell telephone president john de butts talking to lady bird johnson

Figure 2: Illinois Bell Telephone President John de Butts in Chicago talks to First Lady Lady Bird Johnson in Washington in 1964.
(Source: AT&T Archives and History Center)

The final shot across the bow of WT4 was the first commercial fiber-optic communication system developed in 1975 that along with the GaAs semiconductor laser made long-distance communication possible and effectively eliminated the need for other transmission media. When compared with the enormous capacity and speed of optical fiber waveguide seems archaic, but more than 40 years ago there was simply nothing that could compete with its extraordinary low loss and high throughput.

This article was originally published in Microwave Product Digest

When Waveguide Buried the Competition

A Massive Challenge

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