Design of Miniaturized and Broadband Forward-Wave Directional Couplers Using Artificial Microstrip Coupled-Line
Date Issued
2012
Date
2012
Author(s)
Hsu, Sen-Kuei
Abstract
This dissertation focuses on the planar forward-wave directional coupler (FWDC) design by using the concept of periodic structure. Compact sizes and broadband performances are two goals achieved in this thesis. In the beginning, a three-layered FWDC with periodic mushroom-shaped ground plane is developed for the purpose of size reduction. The electrical length of odd-mode is kept linearly dependent on frequency, but the even-mode electrical length is dramatically enlarged due to the capacitive enhancement. The phase difference between odd- and even-mode is significantly increased because the even-mode phase delay is getting higher. Besides the large phase difference, the characteristic impedances of these two modes can be designed to be 50 Ω simultaneously for the impedance matching without using additional matching networks. The proposed coupler is designed with the length of 1.26 λg as an example. Compared to the conventional microstrip forward coupler, the size reduction of the proposed structure is more than 80% due to the slow-wave effect of the even-mode.
Next, a two-layered FWDC with periodic defected ground structure (DGS) is proposed to reduce both the cost and coupler size. The odd-mode electrical length of the proposed structure is linearly dependent on frequency, but the even-mode electrical length is enhanced due to the inductive loading. The phase difference is also enlarged because the even-mode phase delay is getting higher. Input impedances of these two modes can be perfectly matched by adjusting the electrical lengths. For example, 0-dB and 3-dB coupler with periodic DGS are designed with the length of 0.55 λg and 0.275 λg, respectively. Compared to the microstrip forward coupler, the size of the proposed structure reduces by more than 92%.
The aforementioned works concentrate on the design of size reduction. Finally, a novel forward coupler is developed with periodic H-shaped structure for broadband applications. The broadband impedance matching for even- and odd-modes can be achieved and the phase difference between these two modes can be maintained as constant within a wide frequency range. For example, 0-dB and 3-dB couplers are designed with the fractional bandwidth (FBW) 64% and 44%, respectively. The lengths of them are 1.93 λg and 0.96 λg , respectively. Compared to the conventional works, the proposed H-shaped forward couplers have wider operational bandwidth with the compromise on the size.
Subjects
forward-wave directional coupler
phase difference
slow-wave factor
fractional bandwidth
Type
thesis
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