A diagram designed by Phillip Hagar Smith in 1937 and published in 1939 in the January issue of Electronics magazine. This circular chart often used by electrical engineers shows graphically the complex impedance anywhere along a transmission line. The chart is simply the unit circle of the complex plane representing the generalized reflection coefficient. Constant real and imaginary line impedances are represented by overlayed contours. The chart can be used to directly determine the normalized line impedance at any position of
the transmission line as well as the reflection coefficient. The chart can also be used as a units converter. No math is needed to use the chart because all quantities are simply determined graphically.
A collection of Smith chart resources can be found at http://sss-mag.com/smith.html