Design of a dual‐band microstrip branch‐line balun using T‐shaped coupled lines

Abstract

This article presents the development of a novel dual‐band branch line balun with design flexibility. The T‐shaped coupled line structure is constructed by two series short‐ended coupled lines and one open‐ended stub to obtain the dual‐band characteristics. The proposed dual‐band structure has the capability to design a conventional microstrip line with arbitrary impedances and electrical lengths at two different operating frequencies. In addition to this, different range of frequency ratios is obtained by varying the length of the stub. The required explicit design equations are derived analytically to compute the physical dimensions of the coupled lines and stub. Based on these equations, closely and widely spaced frequency bands useful for many wireless communication systems are devised. To prove better agility, three examples of dual‐band branch line baluns are designed. Finally, a prototype of dual‐band balun is demonstrated for GSM and WiMAX applications at 0.9 and 3.5 GHz, respectively. Good agreement is found between the theoretical predictions and experimental results. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:1197–1202, 2017

Miniaturized Twelve-Stubbed Microstrip Balun With Twelfth Higher Order Harmonic Suppression and Improved Bandwidths

“…Several planar balun designs have been reported [2][3][4][5][6][7][8][9][10][11]. Marchand balun [2], using coupled lines, is difficult to realize and further requires compensation to match the even-and odd-mode phase velocities.…”

“…The balun based on Wilkinson topology [3] requires a very large area and additional circuitry. In contrast, the branch-line balun (BLB) design [4][5][6][7][8][9][10][11], based on modified rat race coupler, entirely composed of transmission line sections, is simple and easy to implement, hence being the choice in wide attempts like dual-band operation [4,5] and bandwidth enhancement [6,7]. However, the circuit size of BLB is large due to the transmission line topology composed of half-wavelength lines; moreover, BLB exhibits harmonic responses, which interfere with other circuitries in system level.…”