Characteristic impedance, or not

Article By : John Dunn

When asked to certify that a twisted-pair cable would exhibit a particular characteristic impedance, this engineer could not oblige.

From Wikipedia, we read the following excerpt:

“The Science Advisor to the President is an individual charged with providing advisory opinions and analysis on science and technology matters to the President of the United States. The first Science Advisor, Vannevar Bush, chairman of the Office of Scientific Research and Development, served Presidents Franklin Delano Roosevelt and Harry S. Truman from 1941 to 1951. President Truman created the President’s Science Advisory Committee in 1951, establishing the chairman of this committee as the President’s Science Advisor. This committee continued under Presidents Dwight D. Eisenhower, John F. Kennedy, Lyndon B. Johnson, and Richard M. Nixon until 1973. Nixon terminated the committee rather than appointing a replacement for his advisor who had resigned. The US Congress established the Office of Science and Technology Policy in 1976, re-establishing Presidential Science Advisors to the present day.”

Nixon didn’t like something he was being told so he got rid of the messengers. Something like that happened to me at a would-be client.

I was asked to certify that a particular twisted-pair cable would exhibit a particular characteristic impedance. There were two wires inside of a braided shield and in examining the cable, I could feel the two wires through the braid. The wires were not dimensionally consistent along the cable’s length. Instead they were kinky, gnarled, and whatever other adjectives you might care to apply.

I pointed this out to my would-be client:

Figure 1
Several cable structures

I explained that to be able to assign a characteristic impedance to any particular cable, for whatever cable structure we might be discussing of which the above are just a few examples, the physical dimensions of the conductors and their inter-conductor spacings had to be known and controlled, which those of his twisted-pair cable sample were not.

We ended our meeting without coming to a consulting agreement and the next thing I learned was that someone else had taken the job, declared that the twisted-pair cable’s characteristic impedance was 90 ohms and collected a fee.

I didn’t get the consulting assignment, but in hindsight, that was a good thing because in my view, an engineering fraud had been committed and I was not a part of it. I wouldn’t want to have to defend that engineering report in a court of law.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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