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The step- by- step advances in the ability to design and fabricate circuits at higher frequencies have produced a technology for the use of microwaves. Although there is no set definition for the microwave range, it is taken to be starting at approximately 1 GHz and extending to several hundred GHz. A planar circuit technology has evolved and become dominant for these circuits in this frequency range.Early microwave equipments used rectangular waveguides and coaxial cables to interconnect bulky microwave devices, resulting in designs which were large and costly to construct. This began to change in the late 1940s and early 1950s, when the idea of using a flat strip of metal between two conducting planes as a waveguide for microwaves was conceived and promoted by R.M.Barett. Independently in 1942, a similar form of line was used by Wheeler. These evolved into strip lines. This configuration consists of a metallic strip on a dielectric slab which, in turn, is backed by a conducting ground plane. Because of its planar geometry, microstrip readily lends itself to fabrication. Further development of this waveguide has resulted in designs that are capable of integrated circuit construction with reduced size and cost. These are called microwave integrated circuits.Microstrip is probably the most commonly used method of achieving a waveguide structure. This is due to its simplicity of manufacture, light weight compatibility with integrated circuit process, reasonable range of characteristic impedances, low loss, relatively broad bandwidth, etc.

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