Abstract
Artificial transmission lines are realized by a series of meandered-line inductors, parallel-plate capacitors, and interdigital capacitors, which belong to metamaterial transmission lines. An ameliorated artificial transmission line is proposed to realize a low characteristic impedance transmission line. A two-section half-wave balun at 900 MHz is designed, fabricated, and measured in this paper. The compact balun is based on conventional and ameliorated planar artificial transmission lines instead of microstrip transmission lines. The main advantage of the proposed balun is its size reduction, which occupies only about 10% of a conventional one. Measured results match well with theory and simulation. The balun features excellent amplitude and phase balance in microwave power combining and a reasonable bandwidth of the return loss as well.
Highlights
A balun is a device to act as a transformer to match an unbalanced circuit to a balanced one, or vice versa, which was first proposed by Marchand in 1944
Various balun configurations have been reported for applications in microwave circuits, such as coupled-line baluns, composite right/left-handed (CRLH) transmission line baluns, Wilkinson power divider baluns, three-line baluns, microstrip taper baluns, and lumped baluns [1,2,3,4,5]
With the rapid development of modern communication systems, the increasing demands for miniaturized components have been imposed on microwave circuits, as the size reduction in radio frequency (RF) circuit is an important part of the miniaturization and cost reduction of wireless communication systems
Summary
A balun is a device to act as a transformer to match an unbalanced circuit to a balanced one, or vice versa, which was first proposed by Marchand in 1944. We proposed a compact two-section halfwave balun at 900 MHz, which was designed with artificial transmission lines. Artificial transmission lines [6] have been proposed, and some microwave components with artificial transmission lines have been realized, such as branch-line couplers [7], power dividers [8], and antennas [9, 10]. The proposed design method adopts seven transmission line sections composed of the artificial transmission lines to replace the conventional transmission lines so as to miniaturize the circuit size of the balun as far as possible. The proposed balun demonstrates a very compact dimension of 28.9 mm × 26.8 mm, which is about 10% of the area of a conventional balun of this same topology without folded lines. The overall dimension of the proposed realization is approximately 0.13λg × 0.12λg
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