A broadband balun using metamaterial phase-shifting lines

A Broadband Balun Using Metamaterial Phase-Shifting Lines Marco A. Antoniades* and George V. Eleftheriades The Edward S. Rogers Sr. Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario, M5S 3G4, Canada Abstract A 3-port device that converts a single-ended input to a differential output over a large bandwidth is presented. The balun comprises a Wilkinson divider, followed by a+900 negative-refractive-index (NRI) metamaterial (MM) phase-shifting line along the top branch and a -90° NRI-MM phase-shifting line along the bottom branch. By independently adjusting the parameters of the two MM lines, the slopes of their phase responses can be matched and therefore a broadband differential output signal is achieved. In addition, the MM balun is compact, occupying only a fraction of the area that a conventional TL balun would occupy.

Introduction Materials that exhibit simultaneously negative electric permittivity e and permeability j, termed left-handed-materials (LHM), were first envisioned by Veselago[1] and were shown to exhibit a negative refractive index. The first volumetric structure exhibiting backward-wave propagation characteristics, and therefore a negative refractive index, was developed by Shelby et al.[2], using an array of split-ring resonators and thin wires. More recently, a planar NRI medium was realized by periodically loading a conventional TL with lumped-element series capacitors (CO) and shunt inductors (Lo) in a dual-TL (high-pass) configuration[3],[4]. Various devices that have been developed based on the TL-metamaterial structure include a leaky-wave antenna radiating its fundamental spatial harmonic[5], a metamaterial co-directional coupler that couples power backwards[6], and compact, broadband metamaterial phase-shifting lines[7]. Numerous other interesting applications can be found in[8]. Many balun designs are inherently narrowband due to the frequency dependence of the components used in their construction. In addition, balun designs that are broadband usually require transmission lines that are at least several wavelengths long[9],[10] or employ bulky ferrite cores in their construction[11], and are therefore not very compact. This work employs the one-dimensional MM phase-shifting lines presented in[7] to develop a compact balun realized in microstrip technology, ideal for feeding planar devices that require a broadband differential signal. Examples of such devices could include a printed bow-tie antenna or a series-fed dipole scanning array.

Design The proposed structure for the MM balun is shown in Fig. 1, and consists of a Wilkinson power divider, followed by a+90° MM phase-shifting line along the top branch and a -90° MM phase-shifting line along the bottom branch. The Wilkinson divider was designed to provide equal power split between the two output branches. The input feed line was designed with a characteristic impedance of Z0=50Q, while

the two X/4 branches

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