The Butler matrix is a traditional beam-forming network used in wireless communications due to its well-known advantages such as high gain, larger coverage area, low profile and low cost. Due to increased demands in wireless communications for high data throughput, matrix with a wide bandwidth is highly desirable. However, there is room for the design complexity and size of the matrix to be reduced. Moreover, matrix performance can be further improved by reducing side-lobe levels and providing flexible phase differences at the output ports. The most critical component of matrix is the coupler. For higher order matrix, differential phase shifters will also have a significant impact. Therefore, this thesis presents different approaches to improve these components to complete the above challenges.
A coupler with arbitrary coupling coefficient can be used to reduce the matrix’s effect on the antenna array side lobe level (SLL), which will enhance the overall system’s signal-to-noise ratio (SNR). However, complexity as well as size of the matrix will be increased by using multiple couplers with arbitrary coupling values. This problem can be overcome by using a coupling coefficient reconfigurable coupler. In some smart antenna systems, the matrix is cascaded with amplifiers to increase the overall gain. Due to this cascading of matrix with non-linear devices, higher order harmonic signals are generated that cause deterioration in the system’s performance. This problem can be resolved by designing a coupler with harmonic suppression functionality. A wideband branch line coupler (BLC) having both coupling coefficient reconfigurability and harmonic suppression functionality is presented here. The proposed coupler methodology is divided in to two parts.
In the first part, a novel broadband BLC structure is proposed that enables a wide (3 dB to 15 dB) range of coupling coefficients. The proposed structure is also able to achieve different coupling levels by the tuning of only one design parameter. This allows the proposed structure to be used as a coupling coefficient reconfigurable coupler. To validate the proposed structure, a variety of wideband BLCs operating at 3.5 GHz with different coupling coefficients are designed, fabricated and measured. Measurements show that couplers with 6 dB, 9 dB and 12 dB coupling coefficients provide 49%, 49% and 57% fractional bandwidths, respectively.
In the second part, a coupling coefficient reconfigurable wideband branch line coupler with harmonic suppression is proposed. Closed form equations and parametric studies are provided to facilitate the practical design. To validate the proposed structure and the corresponding design methodology, a BLC operating at 3 GHz is designed, fabricated and measured. The circuit works at four different states providing coupling coefficients of 4 dB, 6 dB, 8 dB and 10 dB. Measurements of the couplers gave 40% (2.3 - 3.5 GHz) fractional bandwidth with a 1 dB amplitude imbalance, 5° phase stability and 15 dB harmonic suppression up to 7.5 GHz.
Apart from coupling coefficient reconfigurable and harmonic suppression, coupler having arbitrary phase difference is another possible solution in the design of improved Butler matrix. Using arbitrary phase difference couplers will provide a relatively flexible phase at the output port of the matrix. Due to this feature, the range of radiation beam angles resulting from the matrix can be extended. A wideband BLC capable of achieving an arbitrary phase difference is presented here. This is achieved by inserting a phase compensation network (PCN) directly in each of the series branch lines, which effectively modifies the electrical lengths. A prototype 60°/-120° coupler is fabricated and experimentally characterized to achieve a 0.5-dB magnitude imbalance, 5° phase stability, return loss better than -15 dB and isolation better than -14 dB from 2.3 GHz to 3.5 GHz.
A differential phase shifter based on meta-materials is designed for higher order Butler matrix. The reference lines have identical dimensions and are of equal lengths with the main lines. This significantly reduces the complexity and size of the network. The proposed structure covers a wide bandwidth by aligning the main line phase slope without degradation in return and insertion losses. A design optimization methodology is presented for wideband DPSs in the 0° - 90° range. For demonstration, a six-way DPS is designed and fabricated. Measurements show that the phase shifters have less than 5° differential phase stability and a better than 10 dB return loss, with insertion loss less than 1.24 dB from 1.6 GHz to 3.2 GHz.
Finally, this thesis concludes and describes the future work that could further improve the Butler matrix as well as other multi-way systems.