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.