From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 17 Jan 2005 13:45:33 -0000
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Subject: LF: RE: Re: Re: SWR-off topic
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Dear Peter, LF Group,

I just had a flick through "Electronic Applications of the Smith Chart"
(1969) by Phillip H Smith in the university library - I suppose he ought to
know about it...

The first version of the Smith chart seems to have been published in 1936.
In the introduction, Smith says it was "...intended only to assist in the
solution of transmission line problems inherent in the design of directional
shortwave antennas for Bell System applications of that period; its broader
application was hardly envisioned at that time". This first chart was
slightly different from the usual one in that the scales were modified to
have a linear graduation in SWR, which was expressed as Vmin/Vmax, and so
was between 0 and 1 instead of 1 and infinity. This distorts the shape of
all the graduation lines, although it is still circular overall. But it
shows that it was intended to use SWR measurements from the outset.

The conventionally-shaped scales appear in the 1939 version, and the radial
scales for reflection and transmission loss, etc appear in the 1944 version,
which is more or less the current form. Smith puts the popularization of his
chart down to its adoption by Radar workers at the MIT Radiation Lab during
WW2.

There are a bewildering array of modifications to the Smith chart, including
expanded regions of low or high SWR, an inverted form with unity SWR at the
outer edge of the chart (very odd scales on that one!) and versions for
negative resistance (apparently useful for tunnel diode designs).

Cheers, Jim Moritz
73 de M0BMU