Phillip Smith: From Amateur Radio Operator to Creator of the Smith Chart

Phillip Hagar Smith was an electrical engineer who became famous for inventing the Smith chart, a graphical calculator for solving problems with transmission lines and matching circuits. He also contributed to fields including radar, FM, and antennas—quite the transformation for an amateur radio operator. 

 

Phillip Hagar Smith. Image courtesy of the Smith Chart Amateur Radio Society

 

Phillip Smith's Launching-off Point: Bell Labs

Phillip Smith was born in Lexington, Massachusetts, in 1905, not far from the Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory. Not only was he on the doorstep of a world-famous research university, but he also descended from a famous inventor: Eli Whitney, the creator of the revolutionary cotton gin machine. From an early age, Smith had a keen interest in radio, creating his own amateur radio station using homemade components and contributing articles to the radio section of the Boston Traveler. One thing was clear: Smith was destined to pursue a career in electrical engineering. 

In 1924, Smith went to Tufts College to study electrical communications. When he wasn't studying, Smith was an electronics and mechanical hobbyist, owning and operating an amateur radio station and reconstructing motorcycles. However, it was upon graduating in 1928 with a B.S. degree in electrical engineering that he was offered a job at Bell Telephone Laboratories in the Radio Research Department. Combined with his growing interest in graphical representations of mathematical relationships, this opportunity gave him a chance to truly make his mark. 
 

Bell Telephone Laboratories (now Nokia), the industrial research and scientific development company where Phillip Smith embarked on his career. Image (modified) courtesy of Ckaack [CC BY-SA 4.0]

 

From the Smith Chart to the Cloverleaf Antenna

Smith’s first assignments at Bell Laboratories required him to take countless measurements of standing waves, inspiring him to develop a quick method to compute the input impedance of transmission lines. In 1931, he created his first graphical solution in the form of a rectangular chart. However, Smith recognized the limits of this chart, which could only accommodate so much data. He continued to search for ways to improve his chart, constantly refining it. 

Smith’s technical work was first published in 1932 as a footnote to another article, and in the same year, he filed his first patent on a single-stub matching network. He later obtained a patent on the optimum conductor diameter ratio for a coaxial transmission line.

After enhancing his chart with a grid made up of orthogonal circles, he could finally accommodate all data from zero to infinity. And thus, the Smith chart was born. After a slow acceptance process, his article describing the chart was published in 1939. Of course, as was Smith’s way, he went on to incorporate further improvements.

 

A diagram of the Smith chart. Image (modified) courtesy of Sbyrnes321 via Wikimedia Commons [CC BY-SA 3.0]

 

In addition to constructing the Smith chart, Smith also developed charts for L-type impedance transformation circuits. He worked with Signal Corps Laboratories before and during the war, focusing on radar by designing antennas and related components. After the war, he worked on the design of FM broadcasting antennas and invented the "Cloverleaf" antenna.

Smith retired from Bell Labs after a 42-year career. During his lifetime, Smith was awarded many accolades, including his election as a Fellow of the IRE and receiving the Special Recognition Microwave Application Award from the IEEE-MIT. When he died in 1987, more than nine million copies of his chart had been sold. More than that, Smith held 21 U.S. patents and published over 35 technical papers on antennas and transmission lines during his lifetime—a huge feat in the electrical engineering community.

 

Smith Charts Persist Today

The Smith chart is a tool for visualizing the impedance of a transmission line and antenna system as a function of frequency. The charts can be used to improve understanding of transmission lines and how they behave from an impedance viewpoint while also helping with impedance matching.

While the Smith chart was originally developed as a useful tool for making the equations involved in transmission lines easier to manipulate, modern computers and software have made the necessity for paper charts obsolete. However, Smith charts are still widely used today and are incredibly useful for showing how RF parameters behave at one or more frequencies. In fact, most RF circuit analysis software includes a Smith chart option for displaying results. Instead of the digital era diminishing the importance of the Smith chart, it has become the ultimate background for both computer and measurement instrument displays. 

 

---

 

Want to learn more about Smith Charts? Check out these Technical Articles by our own Dr. Steve Arar—listing here in reading order.

The Smith Chart and Its Relation to the Reflection Coefficient and Impedance

Learn by Example—Using an Impedance Smith Chart

Analyze RLC Components Using the Admittance Smith Chart and Examples

Design a Two-element Matching Network Using the ZY Smith Chart

Using the Smith Chart to Design a T and Pi Matching Network

Learn Stub Tuning With a Smith Chart

Exploring Single Stub Impedance Matching Through Smith Chart Examples