Wednesday, April 15, 2009

Smith Chart

Why do we need it? This is answered very well by David J. Jefferies in his article where he says:

"The SMITH chart is a graphical calculator that allows the relatively complicated mathematical calculations, which use complex algebra and numbers, to be replaced with geometrical constructs, and it allows us to see at a glance what the effects of altering the transmission line (feed) geometry will be. If used regularly, it gives the practitioner a really good feel for the behaviour of transmission lines and the wide range of impedance that a transmitter may see for situations of moderately high mismatch (VSWR)."

Many articles about the Smith chart seem to focus first on proving its mathematical validity. It is not necessary to have a rigorous understanding of its mathematical basis to get useful results from it as a tool.

A nice approach to understanding the format and applications of a Smith chart, without getting bogged down in math, is provided in the December 2007 QST, page 48. This is the first of a 3-part series that totals only 6 pages.

Another good introduction is the Smith Chart Tutorial suggested by John W4BXI. This one makes nice use of colors to illustrate the construction of the chart and includes several good simple applications. Don't be put off by the first two pages.

An impressive collection of Smith chart resources was suggested by Phil KB4XX. It includes a list of 20 tutorial articles, including the one referenced in the previous paragraph.

Several web sites provide downloadable copies of the Smith chart that you can print locally for use in working your own transmission line and matching problems. Several are included at the collection link above. My favorite sources are:

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