Defected-ground coupled microstrip lines and its application in wideband baluns

Defected-ground coupled lines (DCL) is introduced. It consists of edge coupled microstrip lines with a defected ground. The equivalent circuit model, which is based on separating the two modes of the coupled lines into the common and differential modes, is also developed. The model is validated by comparing the results of ten conventional coupled-line circuits using DCL with EM simulations and excellent agreement was achieved (error is less than or equal to 1.5%). In DCL, the propagation characteristics of the common and differential modes are controlled separately. This permits the design of novel wideband baluns that have a reduced area and a superior performance compared with the multisection quarter wavelength coupled-line baluns. Design guidelines of the proposed baluns are detailed, and single-section and three-section DCL-baluns are implemented. The single-section DCL total length is 0.16lambda, it provides 10% increase in fractional bandwidth, and it occupies 60% of the area when compared with the five-section quarter wavelength balun bandwidth and area, respectively. The single-section and three-section baluns have a fractional bandwidth of 40 and 70%, a maximum amplitude balance of 0.5 dB (for both) and a maximum phase balance of 10deg and 3deg, respectively. Measurements, circuit and EM simulations of one conventional coupled-line circuit with DCL and the two baluns are in excellent agreement.