Design of Miniaturised Micro-Strip Based Dual-Band Branch-Line Coupler Using Folded Stubs

Abstract

Modern day communication system poses large number of requirements, such as wide bandwidth, multiple band of operations, compact size etc. in front of us. Branch-line couplers, a very important passive device in communication systems, need to be at par with these requirements. The objective of this thesis is to design a branch-line coupler which has small size and operates on dual band. Here, a dual band branch-line coupler is proposed. Firstly, a dual band coupler is obtained and then the design is modified to reduce its size. To achieve dual band, stubs are incorporated in the conventional quarter wave-length line. Depending upon the electrical length of the stub, and on the fact whether it is open circuited or short circuited, it can be either capacitive or inductive. Here, the quarter wave-length lines are replaced by equivalent T-shaped lines. So, the proposed coupler has four stubs, one stub centrally tapped at each line of the branch-line coupler. The design formulas are obtained by using ABCD matrix. To reduce size of the coupler, the stubs can be folded inwards. But here, the length of stubs turns out to be quite long. So, right angled bends are introduced in the stubs. Each stub is incorporated with four right angled bends. These bends create discontinuities, which cause the generation of unwanted capacitive and inductive reactance in the design. To compensate for this extra reactance generated, mitring of bends is done. The corners of the bends are adequately cut out to balance the capacitive reactance as the corners lead to charge accumulation, and hence, give rise to capacitance in the design. The two couplers are designed and simulated on CST Microwave Studio. The performance of the two couplers is analysed and compared. There is a difference in the frequency response of two designs due to presence of extra inductance and capacitance (due to discontinuities) in the modified design. Here, almost 90% reduction in size is achieved with better operating bandwidth and better performance characteristics.