Мiкросмужковий двосмуговий балун шлейфного типу з довiльними в смугах комплексними частотно-залежними iмпедансами входу та навантаження

This article proposes a planar microstrip circuit configuration for designing a compact dual-band balun, whose two output signals have equal amplitudes and opposite phases in two operating frequency bands. At the same time, such circuit provides the transformation of complex load impedances different in these bands into the complex source input impedances, which also differ in these bands. The proposed balun circuit is based on a symmetric four-port branch line-based planar structure with one open port, formed by single-line segments with three reactive elements and an isolation scheme. To implement these reactive elements with different values across the frequency bands, transmission-line stubs are used. By applying even-odd mode analysis to the symmetrical four-port network, expressions were obtained for calculating the electrical parameters of the dual-band balun circuit elements. To remove restrictions on the transformable complex impedance values (both load and input), additional line segments are also introduced into this circuit, the parameters of which are set arbitrarily. The choice of the values of these parameters allows to provide as a result of calculations the electrical parameters of the line segments suitable for technical implementation. To verify the proposed circuit and calculation method, a prototype dual-band balun operating at 2.4 and 5.2 GHz was implemented and measured. To manufacture a balun based on microstrip transmission lines, a dielectric substrate with a dielectric constant of 2.6 and a thickness of 1.45 mm was used. Measurements showed that for amplitude and 180°-phase difference mismatches below 0.6 dB and 5°, respectively, the bandwidth of the dual-band balun is 290 MHz in both bands. Simulated and measured results agree well, proving the design concept.