His Impact on Mid-20th Century Microwave Technology
To say that the world and our industry has lost a titan with the passing of Dr Seymour B. Cohn would be an understatement. Seymour Cohn was a major contributor to microwave engineering from his first job at the Harvard Radio Research Lab (HRRL) during World War II and throughout his long and fruitful career. His exposure to microwave technology there in those early years of the blossoming microwave industry presented him with many issues that needed exploring. He responded with major contributions to stripline, isolators, waveguide-to-coax transitions, power dividers, ridged waveguide, directional couplers,\ and, most importantly, filter theory. His chapter in the book published by HRRL was an important contribution to filter design.
Even though he was not a member of academia per se, his research and writings provided part of many engineers on the job education and greatly enhanced their ability to design difficult components. At least two of his many papers can be considered to be classics for their impact on technology so significant at that time.
- Problems in Strip Transmission Line, which was published in the March 1955 MTT Transactions, derived formulas for the characteristic impedance and attenuation loss in shielded strip line. This early work helped engineers to design strip line circuits and was a big factor in making printed circuits a viable option at microwave frequencies.
- Direct Coupled Resonator Filters which was published in the February 1957 IRE Proceedings. This paper put forth the concept of a low pass prototype filter and showed how it can be used to design lumped element and transmission line band pass filters using frequency transformations. It paved the way for subsequent advances by others that took advantage of the Richards transformation and Kuroda’s identity that generated other microwave filter structures.
His major contribution in the 1950’s and 1960’s were done well before the computer became a ubiquitous design tool. His outstanding ability to use his strong grounding in field and network theory to analyze, gain an understanding of a microwave component, and create practical results and inventions, makes him one of the few giants of microwave engineering. Furthermore, his papers were always written to be easy for the design engineer to use. Quite often an involved mathematical formula was reduced to a handy design graph. Before the computer era, the microwave engineering field was blessed to have the contributions of Seymour Cohn.
Dr Cohn’s developments were so significant that by 1959, just eleven years after receiving his PhD, he was elected a Fellow of the IRE (IEEE), "For contributions to the theory and design of microwave components."
Seymour Cohn was born at Stamford, CT, in 1920. His parents were surprised and concerned, when at age14, he told them he wanted to become an electrical engineer. No one in the family had a scientific bent, and besides, these were The Depression Years and engineers couldn’t find jobs. But he persisted and became one of the most prolific, inventive and respected engineers in the mid-20th Century microwave industry.
He earned is BEE degree at Yale in 1942 then joined HRRL developing military electronics for the war effort. There he participated in, and later managed, development of a radar receiver. He spent seven months as an HRRL technical observer at the Army Air Force headquarters, in Italy, where he supported a radar group. There he devised circuit changes and electronic attachments to thwart new German countermeasures. Upon returning to the US, he was appointed leader of a microwave filter and tuner project where he did considerable original research and development. His group mastered techniques of designing all types of low-, high-, and bandpass filters using coaxial line and waveguide elements. He wrote that “since most of my work at (H)RRL has been in new fields, I have been able to invent a great number of useful devices of fundamental importance” . Seymour wrote chapters 26, 27, and most of 28 of the McGraw-Hill publication: “Very High-Frequency Techniques” authored by the HRRL technical staff.
In 1946 Seymour was awarded a pre-doctorate fellowship by the National Research Council for full time graduate study at Harvard. He was awarded his PhD in January, 1948. His thesis subject was on periodic structures contained within waveguide structures “for use as filters, and for slowing electromagnetic wave velocity in electron accelerators and traveling wave tubes” 1. (He soon after married his fiancé Florence. They had decided to wait until he finished his doctoral studies). After graduation, he joined Sperry Gyroscope Co. until 1953 and then Stanford Research Institute until 1960. Wanting a better balance of research and management, he left SRI and joined the Rantec as Vice President and Technical Director. At this point he had published 53 papers and had been awarded 25 patents.
At Rantec Dr Cohn oversaw the engineering group, supported engineers with his knowledge and expertise, and responded to business opportunities, especially those that used his areas of expertise. One of these was a filter that, over a band, would have a flat (constant) phase response instead of flat amplitude response (to the author’s (GO) knowledge it was the first of it’s kind.) This development indirectly led to a new, major product line for Rantec, a Waveguide Network Analyzer with Smith chart and related displays. After the author (HGO) constructed the filter, we asked ourselves how are we going to test and prove to the customer that the filter created an output with a flat phase response. With Dr Cohn’s ideas, we created a network that would measure phase over a band and plot the result. Later, Dr Cohn and other Rantec engineers conceived the first Network Analyzer product in the US, an instrument that measured both signal amplitude and phase and present it as a Smith chart of other graph.
In about 1964 Rantec was purchase by Emerson Electric Co. Dr Cohn stayed with Rantec for 2 or 3 more years but grew in disagreement with changes being introduced under the new management; in 1967 he decided to become a consultant. He incorporated as S. B. Cohn Associates and presented himself as a Microwave Component “Specialist” instead of “Engineer” to steer clear of potential legal issues. He wanted only four core clients, two of which were Wavecom and Narda Microwave.
With Narda, he designed multi-octave directional couplers and got his taste of designing matching circuits for active components. Two very difficult tasks were to match a step recovery diode to a YIG filter over 4 octaves, and the output of an RCA power transistor over an octave. These accomplishments were key to the development of the first ever, broadband, all solid state, microwave sweep generator, done under Jerry Hausner, the project engineer for this instrument. His achievements are truly remarkable and our community is fortunate that he documented and published that work so that all of us can benefit. He continued to consult for Narda Microwave until his retirement.
During this period Dr Cohn, Frank Coale and Terry Cisco formed a consulting company (CAED). Their biggest contract was with NASA for developing a collection of computer programs for microwave design. Cisco created the programs which included some based on Dr Cohn’s algorithms and his microstrip, stripline and slot line programs to determine the properties of physical structures including couplers. All these were published in a NASA Monograph. When Coale died the company closed.
Dr. Cohn consulted with Wavecom for about 20 years supporting their filter and passive component line. Clark Bell, Wavecom’s primary interface with Seymour, noted that Seymour was well known for solving very complex problems by paying close attention to every detail. This observation is supported by the contents of his consulting notebooks, one or more for each client. These notebooks have been donated to the Historical Exhibit of the MTT Society and are displayed at it’s site in Maryland, and (in part) during each IMS.
Seymour Cohn sought very high accuracy in those areas where it was necessary, but he also appreciated the use of simple approximations whenever possible, as he noted in two articles, “Bones from the Technical Graveyard” and “Beating a Problem to Death” .
In 1955 Seymour joined the Administrative Committee (AdCom) of IEEE PGMTT (now Microwaves Theory and Techniques Society – MTT-S). He was MTTS Chairman/President in 1962-63 and was elected Honorary Life Member of the MTT-S in 1978. He received many awards including the MTT-S Microwave Prize in 1964, IEEE Lamme Medal in 1974 and MTT-S Microwave Career Award 1979.
Dr Seymour B Cohn died exactly 6 weeks short of his 95th birthday on September 9, 2015. He was married to his beloved wife, Florence (Hoffman), for 65 years until her death in December, 2013. He is survived by sons Bill (Lisa), Ric (Carol), and Peter; grandsons Rob and Miles, and his brother Leonard.