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Using Calibration to Optimize Performance in Crucial Measurements
Oh, the Way We Were!
As Microwave Journal celebrates its 40th anniversary this year, we take a look at how things were (or how we remember them as having been) when it all started, way back then.
Alexander E. Braun
West Coast Correspondent
It was a time of Pickett & Eckel slide rules, the Boeing Superconstellation and 93¢-per-pound sirloin (including S&H green trading stamps). Things like lasers, virtual instruments and cell phones kept Buck Rogers and Flash Gordon company in an improbable 21st-century comic book future. Normandy, Iwo Jima and Pork Chop Hill had not made it into the history books yet and were still places of personal significance to young men and their families. The biggest threat we faced came from a country that has since vanished, and it would be another three years before a whippersnapper senator with presidential ambitions and a graceful turn of phrase would excite the imagination and start us on our way to the stars. It was 1958.
"The Cold War was full on, and we'd been kicked in the pants by Sputnik the year before. The government was going all out supporting microwave research and advancing the technology, all technologies. A whole new crop of young, enthusiastic engineers - World War II and Korean conflict GI Bill beneficiaries - were entering the profession. We'd come through the war and were now eager to get on with our lives."
The Birth of Microwave Journal
The demands of the Second World War and the cooling of relations with the Soviet Union had catapulted microwave technology from abstract science to engineering and from theory to application. The microwave world was quickly growing away from the small, close-knit community it had started off as, although most of the members still knew each other - or at least about one another's work - and delighted in referring to themselves as plumbers. At that time, most of the exciting work taking place was in waveguides, with developments rapidly taking shape in coaxial lines and octave bandwidths. Radar still was the technology's principal application, but microwave towers were beginning to make their appearance for point-to-point, long-distance applications by the then monolithic Bell System. Microwaves was coming of age.
It was into this world that the idea for Microwave Journal came to be as a result of discussions between William Bazzy and Theodore S. Saad, president and founder of Sage Laboratories. (Currently, Bazzy is CEO of Horizon House Publications Inc. and Saad is retired.) Microwave Journal would be a magazine for the profession, for the plumbers who worked at Sylvania, Raytheon, Varian and HP, and places like Stanford, Lincoln Labs, MIT and Livermore.
As associate editor Seymour Cohn put it in the publication's first editorial, "Taking a broad view, microwave engineering covers activities on signal sources and amplifiers, on waveguides, transmission lines and components, on antennas and propagation, on instrumentation and on numerous system applications. Much of this work has been exploited to a high point of development and use in the past two decades. We can clearly see that the future for microwave engineers depends upon the birth of new ideas and upon our alertness to use these ideas to advance our field." The Journal (as it was referred to by the early enthusiasts) would be the vehicle to disseminate new ideas and developments.
"It was the start of an entirely new way of doing things. I remember at the time I was working at Hewlett-Packard and I had bought a transistor from Fairchild - a lousy little thing. I just wanted to play with it, to learn what these things were and did. Then my boss saw it and read me the riot act. 'If you're thinking of using that in any of our instruments,' he bellowed, 'you're going to have to do it over my dead body!' A year later he had to swallow his pride."
Changes and Developments
By 1958, work that had been started by Robert Barrett at the Air Force Cambridge Research Center in 1949 was coming to fruition, and microwave printed circuits began to be mainlined into system designs. The original concept had originated as a realization by Barrett that he could use a flat strip coaxial line to construct components such as filters, directional couplers, hybrids and more. Ferrites also were coming on line and being used in one-way transmission lines, circulators, switches, variable attenuators and modulators. Reciprocity was dead and microwave engineers were celebrating by breaking new ground with designs unthought of as little as a few years before.
The gas maser was four years old but, although outstanding as a frequency standard, it was limited as a tunable broadband amplifier. In 1958, H.E.D. Scovil of Bell Labs published a paper entitled "The Three-level Solid-state Maser," which opened up an entirely new horizon of applications.
"When the [backward-wave oscillator] came in, it was a good news/bad news situation. Before, we'd make point-to-point measurements using slotted lines and [standing-wave ratio meters]. There were some signal sources with single-knob adjustments - some even mechanically swept. We also now had electronically swept sources with good power output, and octave frequencies became available. We could do faster measurements but the downside was that many of our products previously tested and spec'd on a point-to-point basis had to be cleaned up. The customer could now look at performance between the discrete points previously specified."
Size of the Industry
In 1958, the Journal boasted that "microwaves are currently being used in nearly every branch of the military service and for radar, communication and navigation as well as for the launching, guidance and fusing of missiles. While this military use dominates all others, other applications include microwave spectroscopy, atomic clocks, radio astronomy, medical and industrial heating, x-ray generation by microwave-assisted particle acceleration, and commercial telecommunication and radar."
The Journal attempted to quantify this picture by showing the size and growth of the microwave industry. However, at the time there still was no defined microwave industry so figures had to be derived, indirectly, from various sources. According to the guest editorial in Microwave Journal's first issue, an April 1957 Fortune magazine article indicated that the government had bought $3 B worth of electronic equipment in 1956. From this figure the editorial's author estimated that the microwave portion probably ranged between $175 M and $300 M. "It is difficult to define the exact point where our industry starts and ends," he admitted. "However, it would include the manufacture of microwave antennae [sic], waveguides, duplexers, mixers, microwave tubes and solid-state devices, but would definitely exclude indicators, power supplies, pedestals, associated computers, etc."
Another startling fact brought to light by the editorial was that "approximately seven percent of the membership of the Institute of Radio Engineers are also members of their Professional Group on Microwave Theory and Techniques, whereas 21 percent of the product advertising in the May 1958 issue of the Proceedings was devoted to microwave devices, a tribute to the creativity of the microwave engineer." The editorial ended by regretting that the commercial uses of microwaves represented only a small portion of the industry's effort, amounting to a total annual commercial volume of $15 M to $20 M.
"The introduction of the tunnel diode came as a shock to many of us. There had been speculation about the tunneling phenomenon, but nothing had come out of it. Then, Dr. Leo Esaki noticed a very small resistance at voltages less than 0.1 V in a heavily doped germanium alloy p-n junction diode. He got a Nobel Prize, and we got another tool to work with."
The Coming of MICs
The concept of MICs originated with planar transmission lines when, back in 1936, Harold Wheeler put two flat coplanar strips side by side to form a low loss transmission line that could be rolled up to save space. The concept was used and expanded during the Second World War and, by 1950, the transmission line was being discussed in the literature of the period. Four years later, a symposium on microwave strip circuits (stripline, microstrip, tri-plate and microwave printed circuits) was held under the cosponsorship of the Air Force Cambridge Research Center and the Research Laboratory of Physical Electronics of the department of physics at Tufts College. In 1955, Seymour Cohn wrote a landmark paper: "Problems in Strip Transmission Lines."
AIL was working with a different type of transmission line using a thin substrate of dielectric material suspended between two ground planes that had a double-registered pattern on them, making air the major dielectric. AIL trademarked the construction, calling it Stripline. In the years that followed, AIL did not defend its trademark as aggressively as Xerox defends its own today, and Stripline became stripline, the generic name that it is today for any flat transmission line between two shielded ground plates.
AIL built complex ICs using the suspended substrate stripline technique. The first commercial product introduced was a seven-channel, C-band balanced mixer assembly with an LO distribution network that was first used in an Army radar surveillance system.
Sanders Associates was working on a different strip transmission line concept with solid dielectric two-layer construction it trademarked Tri-Plate. The Tri-Plate modules were sold as a kit as components for breadboarding. In 1956, Sanders published A Handbook of Tri-Plate Microwave Components, which compiled much of the information published previously along with other work Sanders had done. It would remain the only text on stripline design until Harlan Howe (the current publisher/editor of Microwave Journal) published Stripline Circuit Design 18 years later. All of these activities culminated in the growth and acceptance of MICs as the building blocks of today's systems.
"Looking at 1958 from 1998, there's no question that there have been a great many changes. The introduction of computer design is one of the major ones. But the foundations were laid back then. I believe any one of us at the time, had a time machine transported us from then to now, would not be overwhelmed by present technology. There haven't been any advances of a magnitude that would require re-education. It's not like the case of a Middle-Ages alchemist suddenly finding himself working for present-day DuPont."
Ferrites Make their Mark
The first 20 years of microwave radar development took place without ferrites. It was not until 1949 that the first experimental microwave ferrite device was demonstrated. Continued advances in the field gave birth to a new generation of components. It was now possible to increase switching speeds, and components exceeding the pre-War dreams of design engineers became not only practical, but simple to execute. Soon, ferrites could be found in missile-guidance radar, tracking antennas and microwave relays.
By 1958, the initial problems ferrites presented had been mostly solved. Hysteresis effects in the magnetic materials and variations with temperature and frequency range were controlled using closed-loop servo systems. Die casting, precision investment and plaster casting, new constructional techniques for flexible waveguides, jacketing materials, and epoxies and plastics, plus then-new low loss high temperature dielectric materials opened the door for miniaturization and compact structures.
That time also saw the problem of interference due to an increasingly crowded spectrum begin to truly become a factor. This change spurred the development of high accuracy tracking antennas, which did not solve all the problems being tackled but made things easier than they were before. Techniques for electronic scanning of antenna arrays using microwave ferrite phase shifters began to be developed, but although everyone agreed this goal would be attained, it was not yet possible to employ any of those techniques satisfactorily enough in system applications.
"To be effective in what we were doing, we often had to do fast catch-up work in areas that had not been considered pertinent or even important while we were going through college. What microwaves needed then was a chimera: a kind of monstrous creature who not only could engineer, but was also part mathematician and part theoretical physicist."
Coaxial Line Development
At the end of World War II, coaxial transmission line circuits and components were second only to waveguides in breadth of application and sales volume. However, the various connectors that existed at the time were unable to meet the demands of the newer, more complex and powerful systems. Coaxial lines and components were ignored during the war years mostly because there was limited need for broadband systems, and reduction in system size and weight had not been a priority. Coaxial line and connector technology did not advance much during this time. As systems continued evolving to greater bandwidth and functional complexity, size, weight and power consumption became issues. Coax entered the spotlight, particularly as it was being selected over waveguide. However, to allow the modularizing, testing, assembling and packaging of components, a rugged connect/disconnect capability was needed that was reflectionless and transparent with zero physical length.
In 1956, the TNC connector made its appearance, replacing the N connector, the old standby, which had been a de facto standard whose highest frequency of use was 12 GHz. Precision connectors finally brought coaxial transmission line and coax components into their own. Eventually, the IEEE Group on Instrumentation and Measurement standardized the 14 and 7 mm precision connectors, which were usable to 8.5 and 18 GHz, respectively.
"My regret is that although the quality of the engineer and the engineering he - or she - does today hasn't changed, the loyalties have. It used to be that if you were any good your company wanted to keep you, groom you, help your career along. As a result, you got peace of mind and they got your loyalty. Now, we're disposable. We're single entries on the long line of numbers that add up to the corporate bottom line and if, regardless of experience, we can be replaced by someone cheaper we're history. Small wonder that today's engineers feel little or no loyalty toward the company. In this respect, much of what was human has left our industry and our profession."
The future, as predicted in that first Journal issue so long ago, was not that much off. The editors believed that, "Large-scale use of mm-wave transmission in circular waveguides will carry thousands of messages simultaneously, communication systems that will make communication possible with space stations, spaceships and other planets. Microwave spectroscopy will revolutionize industrial control, and microwaves will be used in thermonuclear power generation."
Some things have not changed. As Cohn stated at the end of his editorial 40 years ago, "We cannot see past the horizon, but we know that the admirable inventive genius of man will never be satisfied and that there will be new vistas to explore that we cannot predict or imagine now. If we accept change eagerly and constantly work to improve our background of knowledge, we may confidently expect just as much excitement of discovery and application as has stimulated our minds and heightened our interest in the past."
"I learned about the slide rule by reading Robert Heinlein's science fiction - all his snotty young heroes used one and I wanted to be just like them. It was the only thing I ever taught myself to do, and by the time I reached college, I could calculate results to the third decimal place. Now I'd be lost without my calculator but, somehow, it's not the same thing. In the younger borders of my mind, where Harry S. is still president and Tranquillity Base perhaps a 2001 possibility, there still stands the lone hero, surveying the desolate Martian landscape through a shattered faceplate, his patrol ship disabled, worriedly fingering his trusty slide rule to figure out a way in which to bend natural law and turn back the dark tides of the galactic warlord..."
The author is grateful to all the microwave "old elephants" (they know who they are) who were kind enough to share their wisdom and reminiscences with him and whose quotes provide interesting perspectives throughout this piece. Without their candid recollections of the era, this article would not have been possible. Thanks also go to Theodore Saad for his willingness to provide access to presentations he has made dealing with the history of our industry. Another important source for this article was the Special Centennial Issue, Historical Perspectives of Microwave Technology, published in the 1984 IEEE Transactions on Microwave Theory and Techniques, which proved to be an invaluable window to the past. Photographs appear courtesy of Raytheon Co., Lexington, MA.
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