Even in these days, when news travels instantly and there are few secrets, it’s possible to miss an important or interesting story. That’s what happened to me recently: I was doing some online research and came across an item from October 2015, published at IEEE Microwave Magazine : “The Smith Chart Comes Home [President’s Column].” Editor’s note: Readers will need to register for a paid subscription—well worth it—Bill and I both love the excellent tech white papers
The news was simple: the column explained that IEEE MTT-S [Microwave Theory and Techniques Society] was buying “the rights from the Smith family of the Smith trademark belonging to Analog Instruments, along with the copyright. In return, the MTT-S would make the Smith chart available to students, practitioners, and indeed people all over the world involved in microwave technology.”
Whoa…how did I miss that? I’ve been a basic-level user of the Smith chart for many years, and was always fascinated by its design and history. If you have done any RF work at all, you’ve seen, used, been saved by, or been mystified by this graphical tool. It’s an essential guide to performing complex impedance transformations for transmission lines and matching circuits.
The Smith chart would be classified as a “nomograph,” (also called a nomogram) which were widely used graphs for solving numerical problems in the days before calculators, computers, and software packages. There were nomographs for everything from deciding how much paint you needed on a wall of a given size, to the resistance value needed in parallel with another known resistor to yield the final desired value.
Although nomographs have been made largely obsolete or quaint antique artifacts due to our cheap and easy numerical-computing power, they do have this useful aspect: they make it relatively easy to see the effect of variations around a given parameter value. For example, if you change that parallel-matching resistor by a few percent (perhaps due to tolerance or to make it into a standard value), you can quickly see the effect on the parallel combination’s resistance.
The Smith chart was invented by Phillip H. Smith in 1936 while at Bell Telephone Laboratories, and was publicized in the January 1939 issue of Electronics magazine. It uses a grid with orthogonal circles to represent values ranging from zero to infinity through the technique of conformal mapping. A simplified representation of the Smith chart, Figure 1 , shows the basic layout. The full chart, Figure 2 , is easily intimidating at first. (I won’t try to explain how to use it, since that is a complicated story and many tutorials are available online and in books.)
The Smith chart structure has curves of constant resistance and inductance (capacitance, inductance) ranging from 0 to infinity.
The formal Smith chart details the structure with gradations and other annotations.
Why did MTT obtain the rights to the chart? It’s not for the royalty or licensing money, that’s for sure. Although the chart was registered as a trademark soon after its development, the registration was apparently never enforced. Many “publishers” of the chart did so without getting approval, and these appropriations were not pursued legally. Therefore, the chart has fallen into a quasi “public domain” zone and it would be difficult, if not impossible, to start collecting any fees now.
Of course, use of the chart on paper is not what it was back in the day, as they say. But that doesn’t mean the Smith chart itself is dying out. It is used in many RF CAD programs and on the display screen of RF instrumentation such as the vector network analyzer (VNA).
So, why bother to get the rights? It’s a legacy issue and a recognition of how much Smith and his chart have contributed to RF and microwave design insight. In fact, MTT-S is incorporating the chart into their website, overlying it with a map of the world and the Society’s logo, Figure 3 . To me, that looks a lot more meaningful than the logo which IEEE Spectrum (the IEEE “flagship” publication) adopted a few years ago, Figure 4 . That one is supposed to somewhat conjure up the electromagnetic spectrum or something like that, but I defy anyone to make any sense of it.
The MTT-S website logo pays homage to the Smith chart and its importance to RF/microwave engineering
The logo for IEEE Spectrum is supposed to allude to the spectrum, but that’s a leap of visual imagination.
What’s been your experience with the Smith chart? Did you learn about it in school, or was it via self-discovery and learning?
- Maxim Integrated, Tutorial 742, “Impedance Matching and the Smith Chart: The Fundamentals”
- National Instruments, “Impedance and Impedance Matching”
- University of Kansas, EECs Dept., “Chapter 5 – Impedance Matching and Tuning”
- San Jose State University, “Impedance Matching and Matching Networks”
- Keysight Technologies, Smith Chart format