A Broadband Balun Using Meander Line

In this study, a broadband branch-line balun with meander lines is presented. It is implemented by using meander line structure to improve the size and performance of the conventional branch-line balun. The parametric analysis of the proposed balun is carried out using the available electromagnetic solver IE3D. Using the meander line structure, experimental measurements show that the fabricated balun has an amplitude imbalance of 0.09dB and a phase imbalance of 0.24o operation at 1.98GHz. Moreover, the proposed balun is found to occupy only 58% of the conventional branch-line balun's area. Baluns are generally used to convert unbalanced input into two balanced outputs and have been widely used in planar microwave integrated circuits, such as balanced antennas, balanced mixers, etc. There are many types of balun (1{3) used in modern wireless communication systems. Among them, the Marchand-type and branch-line-type baluns are very popular for this purpose. The con- ventional Marchand balun is inherently composed of two quarter-wave length coupled transmission lines, and has a narrow bandwidth (4). Moreover, its performances depend on the even and odd mode impedances. To develop broadband performance, even and odd mode impedances are equally reduced to half by connecting two baluns in parallel, which have better performance at the expense of increased overall structure size. Therefore, spiral-shaped coupled lines are used to reduce the overall size. Although it is efiective to miniaturize size, it increases manufacturing complexity. The branch-line balun is another well-known design. The conventional branch-line balun is composed of two quarter-wavelength branches and two half-wavelength branches. It has simple structure and can provide low insertion loss. To reduce circuit size, the use of two open stubs in the middle of the two half-wavelength branches has been presented in (5). This study presents a balun structure based on conventional branch-line balun by introducing meander branches to minimize size. The proposed baluns not only have small size and wider impedance bandwidth advantages, but also have a ∞at coupling between ports 2 and 3 within a certain frequency, when compared with the conventional one. The designed balun has been successfully implemented and the experimental results have been presented. 2. ANTENNA DESIGN AND EXPERIMENTAL RESULTS Figure 1 is a conventional branch-line balun. It is composed of a pair of quarter-wavelength trans- mission lines (Branches 1 and 3) and a pair of half-wavelength transmission lines (Branches 2 and 4). The balun is presumed to flx the voltage and current associated with the outward traveling waves at the two output ports so that they are equal in magnitude and opposite in phase. The con- ventional branch-line balun can be analyzed using the odd and even mode decomposition method based on the circuit symmetry. The Sparameter can be expressed as (S) = ij p 2