An asymmetrical coupled-line circuit is proposed to design planar microstrip balun, which has the advantages of compact structure and complex source to complex load impedance transformation. This balun consists of three pairs of coupled lines and two tapped transmission-line stubs. Based on the traditional even-odd mode technique and ABCD parameters, closed-form mathematical equations for circuit electrical parameters are obtained. To demonstrate our design theory, a practical microstrip balun is designed, simulated and measured. The results show that the return loss is larger than 25 dB, the insertion loss S21 (S31) 3.15 dB (3.129 dB), and the output phase difference -180.22˚ at the operating frequency. Good agreements between the simulated and measured results verify our design theory.
2. Ang, K. S. and I. D. Robertson, "Analysis and design of impedance-transforming planar Marchand baluns," IEEE Trans. Microw. Theory Tech., Vol. 49, No. 2, 402–-406, Feb. 2001.
3. Leong, Y. C., K. S. Ang, and C. H. Lee, "A derivation of a class of 3-port baluns from symmetrical 4-port networks," IEEE MTT-S Int. Microwave Symp. Dig., Vol. 2, 1165-1168, Seattle, WA, United States, Jun. 2002.
4. Park, M.-J. and B. Lee, "Stubbed branch line balun," IEEE Microw. Wireless Compon. Lett., Vol. 17, No. 3, 169-171, Mar. 2007.
5. Zhang, H., Y. Peng, and H. Xin, "A tapped stepped-impedance balun with dual-band operations," IEEE Antennas Wirel. Propag. Lett., Vol. 7, 119-122, 2008.
6. Li, J.-L., S.-W. Qu, and Q. Xue, "Miniaturised branch-line balun with bandwidth enhancement," Electron. Lett., Vol. 43, No. 17, 931-932, Aug. 2007.
7. Ahn, H.-R. and B. Kim, "Toward integrated circuit size reduction," IEEE Microwave Mag., Vol. 9, No. 1, 65-75, Feb. 2008.