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Planar formats began to be used with the invention of [[stripline]] by [[Robert M. Barrett]]. Although stripline was another wartime invention, its details were not published<ref>Barrett & Barnes (1951)</ref> until 1951. [[Microstrip]], invented in 1952,<ref>Grieg and Englemann (1952)</ref> became a commercial rival of stripline; however, planar formats did not start to become widely used in microwave applications until better dielectric materials became available for the substrates in the 1960s.<ref>Bhat & Koul, p. 3</ref> Another structure which had to wait for better materials was the dielectric resonator. Its advantages (compact size and high quality) were first pointed out<ref>Richtmeyer (1939)</ref> by R. D. Richtmeyer in 1939, but materials with good temperature stability were not developed until the 1970s. Dielectric resonator filters are now common in waveguide and transmission line filters.<ref>Makimoto & Yamashita, pp. 1–2</ref>
Important theoretical developments included [[Paul I. Richards]]' [[commensurate line theory]], which was published<ref>Richards (1948)</ref> in 1948, and [[Kuroda's identities]], a set of [[Transformation (function)|transforms]] which overcame some practical limitations of Richards theory, published<ref>{{multiref|First English publication:|Ozaki & Ishii (1958)}}</ref> by Kuroda in 1955.<ref>Levy & Cohn, pp. 1056–1057</ref> According to Nathan Cohen, the [[log-periodic antenna]], invented by Raymond DuHamel and [[Dwight Isbell]] in 1957, should be considered the first fractal antenna. However, its self-similar nature, and hence its relation to fractals was missed at the time. It is still not usually classed as a fractal antenna. Cohen was the first to explicitly identify the class of fractal antennae after being inspired by a lecture of [[Benoit Mandelbrot]] in 1987, but he could not get a paper published until 1995.<ref>Cohen, pp. 210–211</ref>
== References ==
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