Baluns are class of passive microwave components used to transform transmission line mode from balanced to unbalanced and vice versa
by providing the required impedance transformation as the balanced and unbalanced port impedances are usually unmatched. Given the important role of baluns in many classical and modern applications [1
], such as common mode rejection in balanced architectures, push-pull amplifiers and double balanced mixers, broadening a balun operational bandwidth is still an ongoing research topic. In this context, scholars worldwide reported several wideband balun designs in support of the emerging broadband technologies and multiband integrated systems.
To this end, Marchand, multi-section coupled-lines, composite right/left handed transmission lines, and CPW baluns have been recently introduced. In [2
], wideband compensated and uncompensated Marchand baluns were proposed using step transformers. However, the vias accompanying the design increases its structural complexity, sensitivity, and fabrication cost. Multi section coupled-line baluns were realized in [3
] using two-line edge coupled microstrips. Hence, simplifying the complexity order by eliminating the vias was obtained at the expense of the operating bandwidth. Lim et al.
proposed a wideband CPW balun with the aid of a multistage out-of-phase Wilkinson power divider (WPD) [4
], which is although interesting is limited to the number of consecutive stages in the WPD that may degrade the overall electrical performance due to the resulting crossovers. Fractal shape composite right/left handed transmission line broadband balun [5
] was designed. Such a technique resulted in a compact design, but the added complexity may introduce ports overlapping if proper care is not taken during simulations. Another planar balun was designed on a single layer substrate in [6
]; however, the relatively narrow bandwidth was a major drawback. Furthermore, a multi-layered wideband balun [7
] in the C- and X-band with a number of holes was designed making the design highly complex. A wideband balun using parallel strips was designed in [8
], which is synthesized using a third order Chebyshev response. The design has a symmetrical four-port structure incorporating vias with relatively moderate operating bandwidth. Microstrip to slotline transitions was discussed in [9
], where the balun implemented was compact in size covering ultrawide bandwidth. The disadvantage of the design though was the use of high dielectric constant (ɛr
> 10) material in order to match the slotline transition to 50 Ω microstrip line. Additionally, high radiation loss is associated with such transitions.
In this work, we propose a broadband three-port coupled-line balun derived from a four-port symmetrical network. The proposed balun covers the frequency spectrum of 0.8–8 GHz by the virtue of utilizing tightly coupled tapered lines that have the benefit of being straightforwardly handled, compact, and exhibits broad frequency spectrum with applications in WiMAX, WLAN, Bluetooth and ISM bands.