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An effective method for improving the characteristics of the microstrip parallel coupled line coupler is proposed. Some grooves are oriented parallel to the strips and just next to them. It can be shown that an appropriate depth of the grooves can be used for equalizing the even- and odd-mode phase velocity in a coupled microstrip lines with proper geometrical dimensions. Sets of design graphs are derived for various depths of grooves as a parameter and with curves that implies phase velocity equalization curves. The simulated scattering parameters for couplers in conventional and proposed topology show the efficacy of the new grooved substrates.
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Moradian et al. (2008) studied this question.
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