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{[(Amateur -ET- Land)]} 'The Smith Chart'

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Posted by Brian Kirk Parquette on January 13, 2001 21:17:00 UTC

'The mathematics of transmission lines and certain other devices becomes cumbersome at times, especially when dealing with complex impedances and "non-standard" situations. In 1939 Phillip H. Smith published a graphical device for solving these problems, followed in 1945 by an improved version of the chart. The graphic aid, somewhat modified over time, is still in constant use in microwave electronics and other fields where complex impedances are found. The Smith chart is indeed a powerful tool for the rf designer.'-Practical Antenna handbook,Joseph J. Carr.
Phillip Hagar Smith was born in Lexington, Massachusetts on April 29, 1905, to George and Rose Whitney Smith of Scotch and English
ancestry. Rose Whitney was a descendant of Eli Whitney, the inventor of the cotton gin. While attending Tufts College, Phil was an
active amateur radio operator with the call sign 1ANB. He also played the cornet in the Tufts College band. To commute between
Lexington and Tufts, he drove a reconstructed model T Ford and later a 4 cylinder Harley Davidson motorcycle. He received the BSEE
degree from Tufts College (now Tufts University) in 1928, majoring in electrical communications.

In 1928 he joined the technical staff of Bell Telephone Laboratories with the Radio Research Department, Deal, NJ where he worked
under J.C. Schelleng and E.J. Sterba. In these early days, Phil became involved in the design and installation of directional antenna
equipment for commercial AM radio broadcasting. In 1929 he was working in Lawrenceville, NJ, on an antenna system which was
designed to communicate by shortwave with Europe and South America. The antenna was connected to the transmitter by a two wire
transmission line. Perhaps the major reference at the time was J.A. Fleming's 1911 telephone equation, which expressed the impedance
characteristics of high frequency transmission lines in terms of measurable effects of electro-magnetic waves propagating theron, i.e, the
standing wave amplitude and the wave position.

In the reprint of an article entitled "Transmission Lines for Short-wave Radio Systems," presented at the IRE 20th anniversary
convention in April 1932, there was a footnote which read "Disclosed to the writers by P. H. Smith, Bell Telephone Laboratories". The
footnote referred to a paragraph in the article which began, "There is another effective way for transforming line impedance by means of
short line devices...." It was the first published report of work done by Phil, work that ultimately led to the creation of the Smith Chart.

In spite of his long identification and association with antenna activities, Phil was basically a transmission-line type. He relished the
problem of matching the transmission line to the antenna, a component which he considered matched the line to space. Considering the
frequency and the consequent large size and resultant cumbersomeness of the antenna, the measurements were not simple. In those early
days, the sensing element was a thermocouple bridge with about 6 or 8 thermocouples coupled to two coils, whose dimensions were
determined by the frequency of transmission. The indicator was a microvoltmeter which measured the magnitude of the signal. The entire
assembly was then moved along the transmission line to determine the relative magnitude and location of the maximum and minimum
signals. For transmission lines high in the air, this required one individual to move the sensing device along at the end of a long pole, while
a second individual would read the signal through a telescope. It was primitive, but it worked. This was the early environment that Phil
faced as an electrical engineer with the Bell Telephone Laboratories. For those who knew him best, it was no surprise that he would
doggedly pursue his goal of creating a chart to simplify the work. From Fleming's equation, and in an effort to simplify the solution of the
transmission line problem, he developed his first graphical solution in the form of a rectangular plot.

Phil persisted in his work, the diagram gradually evolved through a series of steps. The first rectangular chart was limited by the range of
data it could accommodate. He was aware of the limitations and kept working on the problem until some time in 1936, when he
developed a new diagram that eliminated most of the difficulties. The new chart was a special polar coordinate form in which all values of
impedance components could be accommodated. The data for this diagram was scaled from the earlier rectangular diagram. The
impedance coordinates in this case were not orthogonal and were not true circles, but, in the form chosen, the standing wave ratio was
linear. The chart closely resembled what ultimately became the final result.

Phil, however, suspected that a grid made up of a system of orthogonal circles might be more practical. He felt it would have distinct
advantages, particularly as regards reproducibility. With this in mind, he spoke to two of his co-workers, E.B. Ferrell and J.W. McRae.
Because they were familiar with the principles of conformal mapping, they were able to develop the transformation whereby all data from
zero to infinity could be accommodated. Fortunately, curves of constant standing wave ratio, constant attenuation and constant reflection
coefficient were all circles coaxial with the center of the diagram. The scales for these values, while not linear, were entirely satisfactory.
A diagram designed along these lines was constructed in early 1937. It was essentially the form still being used today.

Smith approached a number of technical magazines with regard to publication of the Chart, but acceptance was slow. There were not
many technical magazines at the time, and none in the microwave area. However, in January of 1939, after a delay of two years, the
article was printed in Electronics magazine.

A fact one cannot ignore is that many highly competent people proposed charts for use in solving transmission line problems. Some of
their charts had brief periods of popularity, but it is a comment on Phil's persistence in searching out the ultimate solution, that his Chart
stands out above all others in its use and usefulness.

It took a while for Phil to convince other people of the utility of his chart. One of the first individuals to see its value was A.G. Fox at Bell
Labs, who in 1939 found it useful in some early work he was doing on the new subject of waveguides. When the M.I.T. Radiation
Laboratory was formed in 1940, the value of the Smith Chart was recognized immediately and it was put into general use. According to
Phil, the M.I.T. workers were his first customers. It would be hard to visualize many of the achievements of the M.I.T. Rad Lab without
some help from the Smith Chart. For microwave people at that period, the Smith Chart had the equivalent impact of turning on a bright
light in a previously dark room.

Phil published a second article in 1944 which incorporated further improvements including the use of the chart with either impedance or
admittance coordinates. In 1958, in the first issue of the Microwave Journal, a biography of Phil was published to acknowledge the
importance of his contribution. In a series of 6 subsequent issues of the magazine, Dr. George Southworth described the importance and
some of the applications of the Smith Chart.

According to Dr. Southworth, the Smith Chart, even in its earliest form, was no sudden flash of genius. Phil's first ideas were imperfect
and they required time for full maturity. However, as Dr. Southworth wrote, "it was to his everlasting credit that he did not allow his idea
to die on the vine, but nourished it until he had brought it to a high degree of perfection".

Today's emergence of the digital computer as a dominant design tool has in no way diminished the importance of the Smith Chart. The
Smith Chart has become the ultimate background for both computer and measurement instrument displays.

Phillip Smith Beyond the Smith Chart

Had he not invented the Smith Chart, Phil would still deserve to be honored for his many contributions to technology. Just before
America's entry in World War II, he was sent with a small group of engineers to Fort Hancock to work with the Signal Corps
Laboratories on a most important secret weapon - radar. He spent a year on Sandy Hook designing antennas and related components for
production of the SCR-268 radar. Later, he worked on early microwave radar antenna developments for submarine use under W.H.
Doherty at Whippany, NJ. In his early professional career, while developing 500 kw coaxial line components for radio station WHAS in
Louisville, Kentucky, he obtained a basic patent on the optimum conductor diameter radio for a coaxial transmission line. This is the
outer to inner diameter ratio of a coaxial line which results in maximum power handling capability for a given outer conductor diameter.
Smith said this was one of the simplest patents ever granted - the only claim was the single number 1.65. Another basic patent he
obtained was for the adjustable matching stub tuner.

After World War II he worked on the design of FM broadcasting antennas for Western Electric broadcasting equipment. During that
period he invented the famous "Cloverleaf" antenna. Later he became involved in military weapon radar systems studies and designed
and supervised groups responsible for the electrical design of the DEW LINE, NIKE ZEUS and the ABM System, which became

One of the programs he worked on that can help to illustrate his creativity in microwave technology was an acquisition radar system on
the Island of Kwajalein, in the South Pacific. This was an experimental system in the early days of the SAFEGUARD program. The
design of the antenna involved using a Luneburg lens technique. The classical Luneburg lens is a spherical lens that has the property that
when the lens intercepts a plane wave, the focal point of the wave will always appear at a point perpendicular to the wave itself on a line
through the center of the sphere at a point on the opposite surface of the sphere, regardless of the direction from which the plane wave
approaches the lens.

This made it possible that when a signal was received, by virtue of the location of the receivers and the action of the Luneburg lens, one
could determine the azimuth and elevation of the target.

The technique that was used at Kwajalein was to build one half of the sphere - that is a hemispherical Luneburg lens - with a ground
plane significantly larger than the diameter of the sphere itself. The lens was made up of a series of polyfoam cubes about 2' x 2' x 2'
loaded with aluminum slivers, so that the polyfoam block had a uniform dielectric constant throughout. By varying the amount of
aluminum slivers, one could vary the dielectric constant of the block. The required values of dielectric constant were then determined to
achieve the Luneburg lens performance. It turned out for their system they needed about 10 to 12 different values of dielectric constant
and perhaps dozens of each value. The system worked as predicted by theory.

The operation of the antenna relied on the ability to build the homogeneous aluminum-loaded polyfoam blocks of different, but precise
dielectric properties. The idea for the blocks came from Phil. This episode helps to highlight one of Phil personality traits. As a friend of
his commented, "he could be oh so stubborn." And "on occasion that stubbornness had a profound effect." Against the wisdom of some
of the most distinguished consultants at Bell Labs, Phil maintained that by the random distribution of the aluminum slivers the dielectric
constant could be controlled both as to homogeneity and value so as to serve the needs of the project. The test proved he was right.

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