本論文提出雙頻帶密置天線陣列解耦合系統之創新設計。此架構乃利用新型合成傳輸線結合兩種解耦合技術實現。利用該新穎設計，可同時滿足密置陣列之雙頻帶訊號解耦合與阻抗匹配，將可有效提升小型化多重輸入多重輸出系統之傳輸效能。
該雙頻帶解耦合網路之高、低頻帶，乃分別利用並聯電抗元件與180度分合波器實現，由於其雙模電路操作之特殊需求，本論文乃設計兩款新型合成傳輸線。其50歐姆合成傳輸線於低頻帶可等效為傳統微帶線，但於高頻帶則近似於完美開路。而35歐姆合成傳輸線於低頻帶亦可等效為傳統微帶線，而於高頻帶則具有50歐姆之特徵阻抗。利用合成傳輸線所設計之雙模態180度分合波器，可於低頻帶保有傳統電氣響應；而操作於高頻帶時，則等效為兩相互隔離之直通傳輸線。
本論文利用兩款雙頻帶密置天線陣列充分驗證此構想之可行性，其模擬與實驗結果十分吻合，針對其單一激發埠之低輻射效率問題，本論文乃分析其電流分佈並提出解決方案。

A novel decoupling system for closely spaced dual-band antenna array is proposed in this thesis. To simultaneously achieve dual-band decoupling and impedance matching, new synthesized transmission lines are proposed and discussed. The new architecture is capable of effectively improving the transmission efficiency of a multiple-input-multiple-output (MIMO) system with compact circuit size.
The lower and upper bands of the proposed dual-band decoupling system are realized by the 180-degree hybrid coupler and shunt reactance elements, respectively. To fulfill this distinct dual-mode property, two novel synthesized transmission lines are proposed. The 50-ohm synthesized line can be equivalent to its conventional counterpart in the lower band, but introduces a transmission zero with nearly open-circuited input impedance in the upper band. Similarly, the proposed 35-ohm synthesized line functions as a conventional 35-ohm microstrip line in the lower band, but features a 50-ohm equivalent characteristic impedance in the upper band. By utilizing the proposed synthesized lines, a dual-mode 180-degree hybrid coupler is developed. The dual-mode hybrid coupler shows comparable electrical performances with its conventional counterpart when operating in the lower band. In the upper band, due to the perfectly open-circuited responses of the synthesized lines, it can be equivalent to a pair of isolated transmission lines with direct-thru transmission properties.
Two unique closely-spaced dual-band antenna arrays are developed to validate the signal decoupling capability of the proposed new architecture. The simulation and measurement agree well with each other. By analyzing the excited currents on the antenna elements, the low radiation efficiency issue in the lower band is discussed and successfully resolved.

[1] C. Volmer, J. Weber, R. Stephan, K. Blau and M. A. Hein, “An eigen-analysis of compact antenna arrays and its application to port decoupling,” IEEE Trans. Antenna Propagat., vol. 56, no. 2, pp. 360–370, Feb. 2008.

[2] J. C. Coetzee and Y. Yu, “Port decoupling for small arrays by means of an eigenmode feed network,” IEEE Trans. Antenna Propagat., vol. 56, no. 6, pp. 1587–1593, Jun. 2008.

[3] L. K. Yeung and Y. E. Wang, “Mode-based beamforming arrays for miniaturized platforms,” IEEE Trans. Microwave Theory Tech., vol. 57, no. 1, pp. 45–52, Jan. 2009.

[4] T.-I Lee and Y. E. Wang, “Mode-based information channels in closely coupled dipole pairs,” IEEE Trans. Antenna Propagat., vol. 56, no. 12, pp. 3804–3811, Dec. 2008.

[5] J. C. Coetzee and Y. Yu, “New modal feed network for a compact monopole array with isolated ports,” IEEE Trans. Antenna Propagat., vol. 56, no. 12, pp. 3872–3875, Dec. 2008.

[6] S.-C. Chen, Y.-S. Wang and S.-J. Chung, “A decoupling technique for increasing the port isolation between two strongly coupled antennas,” IEEE Trans. Antenna Propagat., vol. 56, no. 12, pp. 3650–3658, Dec. 2008.

[7] H. J. Chaloupka, X. Wang and J. C. Coetzee, “A superdirective 3-element array for adaptive beamforming,” Microw. Opt. Technol. Lett., vol. 36, no. 6, pp. 425–430, Mar. 2003.

[8] P. T. Chua and J. C. Coetzee, “Microstrip decoupling networks for low-order multi-port arrays with reduced element spacing,” Microw. Opt. Technol. Lett., vol. 46, no. 6, pp. 592–597, Sep. 2005.

[9] H. J. Chaloupka, X. Wang and J. C. Coetzee, “Performance enhancement of smart antennas with reduced element spacing,” in Proc. IEEE Conf. Wireless Communications and Networking, Mar. 2003, vol. 1, pp. 425–430.

[10] J. Weber, C. Volmer, K. Blau, R. Stephan and M. A. Hein, “Miniaturized antenna arrays using decoupling networks with realistic elements,” IEEE Trans. Microwave Theory Tech., vol. 54, no. 6, pp. 2733–2740, Jun. 2006.

[11] S. H. Kim and J. H. Jang, “Folded monopole LC-loaded antenna and its polarization reconfigurability,” Microw. Opt. Technol. Lett., vol. 53, no. 6, pp. 1197–1201, Jun. 2011.

[12] H.-M. Chen and Y.-F. Lin, “Printed monopole antenna for 2.4/5.2 GHz dual-band operation,” in Proc. IEEE AP-S Int. Symp., Jun. 2003, vol. 3, pp. 60–63.

[13] D. M. Pozar, Microwave Engineering, 3rd ed. Wiley, 2005.

[14] C.-W. Wang, T.-G. Ma and C.-F. Yang, “A new planar artificial transmission line and its applications to miniaturized Butler matrix,” IEEE Trans. Microwave Theory Tech., vol. 55, no. 12, pp. 2792-2801, Dec. 2007.

[15] 蔡志偉，「新型雙頻帶信號回溯陣列天線之研究」，碩士論文，國立台灣科技大學，民國99年。

[16] W. R. Eisenstadt and Y. Eo, “S-parameter-based IC interconnect transmission line characterization,” IEEE Tans. Comp., Hybrids, Manufact. Technol., vol. 15, no. 4, pp. 483-490, Aug. 1992.

[17] W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, 2nd ed. Wiley, 1998.

[18] L. K. Yeung, “Compact linear antenna arrays for MIMO applications,” in Proc. IEEE APMC, Dec. 2010, pp. 754–757.

[19] S. Dossche, J. Rodriguez, L. Jofre, S. Blanch and J. Romeu, “Decoupling of a two-element switched dual band patch antenna for optimum MIMO capacity,” Proc. Int. Symp. on Antennas and Propagations, Albuquerque, USA, Jul. 2006, pp. 325–328

[20] C.-H. Lai, W.-S. Chung, C.-H. Wu and T.-G. Ma, “A study of mutual coupling reduction with two closely-spaced dual-band fork-shape monopoles,” ISAP, 2011.