The Smith Chart is one of the most useful tools in
radio communications, but it is often misunderstood. The purpose
of this article is to introduce you to the basics of the Smith Chart.
After reading this, you will have a better understanding of impedance
matching and VSWR — common parameters in a radio station.
The Smith Chart was invented by Phillip Smith, who was born in Lexington, MA
on April 29, 1905. Smith attended Tufts College and was an active amateur radio
operator with the callsign 1ANB. In 1928, he joined Bell Labs, where he became
involved in the design of antennas for commercial AM broadcasting. Although
Smith did a great deal of work with antennas, his expertise and passion focused on
transmission lines. He relished the problem of matching the transmission line to the
antenna; a component he considered matched the line to space.1
Smith developed the first graphical solution in the form of a rectangular plot
from his measurements of the maxima and minima voltages along the transmission
line. He used a thermocouple bridge and voltmeter to make the measurements.
The first graphical chart was limited by the range of data
so he came up with a polar plot that was a scaled
version of the first plot. According to his
biography, his impedance coordinates
were not orthogonal — which means
perpendicular — and there were
no true circles, but the standing
wave ratio was linear.
This chart closely
Phillip Smith — Inventor of
the Smith Chart.