I. INTRODUCTION
For commercial communication applications, the US Federal Communications Commission (FCC) allocated the ultra-wideband (UWB) spectrum, ranging from 3.1 to 10.6 GHz. According to FCC, UWB device is defined as any device, which has a fractional bandwidth greater than 0.25, or occupies 1.5 GHz or more of the spectrum [1]. Directional couplers are an integral part of several components in the UWB microwave commercial communication subsystems that include amplifiers, antenna array network, and balanced mixers [Reference Abbosh and Bialkowski2]. A multilayer UWB directional coupler is realized using slot coupling method in [Reference Abbosh and Bialkowski2, Reference Moscoso-Martir, Wanguemert-Perez, Molina-Fernandez and Marquez-Segura3] with transmission lines (ports) in different layers. Further in [Reference Moscoso-Martir, Wanguemert-Perez, Molina-Fernandez and Marquez-Segura3], the fabrication complexity is increased, since it requires alignment of printed circuit boards (PCBs) with precision pins and linking with nylon screws. In [Reference Chen and Chang4], ultra broad band operation has been realized using multi-section hybrids with varying substrate thickness and additional special dielectric blocks. In [Reference Abbosh5], UWB is realized using coupled microstrip structures and a lumped capacitor.
The prime focus of the proposed work is to design a coupler that operates over a major portion of the UWB band, using a new simple transmission line technology, without the usage of multi-layered configuration, via holes or lumped components. Certain techniques have been proposed in literature to achieve wideband response in couplers within a single layer. In [Reference Arriola, Lee and Kim6], wide band characteristics is obtained by connecting four open circuited quarter wavelength coupled lines at each feed of the conventional branch-line coupler. However, the transition between microstrip and coupled lines requires the installation of post and screws on the circuit board for mechanical stability.
Multi-section branch-line (BL) couplers are another interesting choice for achieving wideband response. Impedance bandwidth (10 dB return loss) ranging from 1.2 to 3.8 GHz is obtained using four sections of branch-line coupler in [Reference Tang, Chen, Lin and Wu7]. Bandwidth ranging from 1 to 3.8 GHz and 0.9–4.4 GHz is obtained using 6 and 10 sections of BL couplers in [Reference Tang, Tseng and Hsu8]. In both these works, the high impedance of the vertical arms is realized using defective ground structures (DGS). In [Reference Chiu and Xue9], phase inverters and modified ports have been used in a two section coupler to obtain wide bandwidth equivalent to that in four sections. However, phase inverters require two vertical via connecting the signal and ground lines. Wideband dual layer substrate integrated waveguide (SIW) is responsible for providing the wide bandwidth in [Reference Ali, Aubert, Fonseca and Coccetti10]. In [Reference Chun and Hong11], bandwidth ranging from 1.2 to 2.9 GHz is obtained using three sections of BL couplers and open stubs. In [Reference Lee and Lee12], bandwidth is enhanced by placing quarter wave transformers at each feed of a conventional single section BL coupler.
In the proposed work, a method of multi-stage impedance feed is employed to enhance the bandwidth of a three section coupler and to achieve wideband performance, covering a major portion of the UWB range and offering an effective bandwidth of over 6 GHz. In addition, a technique to improve coupling throughout the operating band by placing the ring shaped DGS elements in the series arm is implemented. The proposed single layer prototype covers the entire UWB range, with improved percentage bandwidth to number of sections ratio compared with [Reference Tang, Chen, Lin and Wu7, Reference Tang, Tseng and Hsu8, Reference Chun and Hong11].
The design and description of the coupler has been presented in Section II. The results and discussion about the results have been provided in Section III. Section IV concludes the paper.
II. BL COUPLER DESIGN
The geometry of the proposed UWB BL coupler is shown in Fig. 1. The overall dimension of the coupler is 50 × 16 mm2. The prototype is designed on a 0.8 mm thick FR-4 substrate with ε r = 4.3 and tan δ = 0.025. The ring shaped slot structures are introduced in the ground plane as shown in Fig. 1. 50 Ω coaxial connectors are used to feed the coupler. The lengths l 1 and l 2 (Fig. 1) have been modeled and optimized corresponding to λ g /4 at the center frequency of the UWB range. w 1 and w 1 ’ are optimized to obtain enhanced bandwidth.
Fig. 1. Geometry of the proposed UWB coupler with slotted ground plane. w 1 = w 1′ = 0.2 mm, w 2 = 2.8 mm, w 3 = 1 mm, w 4 = 2.1 mm, l 1 = 4.4 mm, l 2 = 7.2 mm, l 3 = 8.14 mm, l 4 = 4.8 mm, outer ring radius (R) = 0.8 mm, inner ring radius (r) = 0.6 mm.
A study on percentage bandwidth and circuit area variation for different number of coupler sections has been performed. The observations from the study have been presented in Fig. 2.
Fig. 2. Percentage bandwidth and circuit area for different number of coupler sections.
From Fig. 2, it can be observed that till three sections, both percentage bandwidth and circuit area increases with increase in number of coupler sections. Percentage bandwidth above 90% is achieved for three sections itself. Further, it is observed that there is minimal increment in percentage bandwidth for the four section coupler. Also the circuit area is more for four sections. Hence, the number of coupler sections is chosen to be three.
Further enhancement is provided by the multi-stage impedance feed. An impedance stage is the section of a transmission line with different impedance of that of preceding transmission line section as shown in Fig 3.
Fig. 3. Bandwidth performance with variations in number of stages and impedance of the second stage while fixing first stage impedance. Legends: stage 1: 64 Ω – solid line with circular marking, stage 1: 50 Ω – dashed line with triangular marking, stage 1: 35 Ω – dash dotted line with crossed marking, no. of stages – solid line with square marking.
A study is performed to observe the effect of number of stages on the bandwidth performance and is depicted in Fig. 3. For a single stage feed a maximum bandwidth of 99% is observed when the impedance stage 1, is 50 Ω. For the two stage design, following the 50 Ω port, two impedance stages are incorporated (stage 1 and stage 2). Following a similar trend, maximum bandwidth of 129% is observed when the impedance of the three stages is fixed as 42, 53 and 64 Ω. The fractional bandwidth required for UWB operation is 109%. Since the two impedance stage itself provides sufficient bandwidth for UWB operation (Fig. 3), it is chosen for the final design.
For the double stage feed, a study is performed varying the impedance of stage 2, fixing the impedance of stage 1 as 35, 50 and 64 Ω, as shown in Fig. 3. Maximum bandwidth of 113% is observed when the impedance of stage 1 and stage 2 are fixed as 42 and 64 Ω, respectively, as depicted in the legends of Fig. 3.
Another study has been performed to choose the length of the impedance stage as observed in Fig. 4.
Fig. 4. Bandwidth performance with variations in length of the impedance stages.
The graph (Fig. 4) represents the bandwidth corresponding to variation in length of l 3, keeping l 4 constant as 5 mm, and the bandwidth corresponding to variation in length of l 4, keeping l 3 constant as 5 mm. Bandwidth of 115% is obtained at the point of intersection of both the curves, where the length of both the stages is nearly 5 mm. After final optimization, l 3 and l 4 have been chosen as 5 and 4.8 mm, respectively (corresponds to λg/6 at the center frequency).
With enhancement in bandwidth, a reduction in power transferred to the coupled port has been observed. The power division to the through port (port 2) and coupled port (port 3) can be related using the power division ratio (k) [Reference Wu, Zheng, Leung, Liu and Xue13].
$${k^2} = {\left \vert {\displaystyle{{{S_{21}}} \over {{S_{31}}}}} \right \vert ^2}.$$
The power division ratio can be related to the impedance of the series arm (Z 1) and impedance of the shunt arm (Z 2) as,
$${k^2} = {\left( {\displaystyle{{{Z_2}} \over {{Z_1}}}} \right)^2} - 1.$$
From the above two relations it can be inferred that the power transferred to the coupled port can be increased by increasing the impedance of the series arm. In [Reference Tang, Chen, Lin and Wu7, Reference Tang, Tseng and Hsu8], the impedance of the shunt arm is increased by placing rectangular defects beneath the shunt arm. In the proposed work, the impedance of the series arm is increased by placing ring shaped DGS.
Based on the results obtained by varying the outer ring radius of the ring shaped slots, optimal value of 0.8 mm has been chosen as the outer ring radius, since it provided improved matching and isolation along with a better amplitude stability performance. The inner ring radius reduces the aperture size of the DGS, in turn reducing the radiations caused due to it.
III. RESULTS AND DISCUSSION
The front and rear view of the fabricated prototype is portrayed in Fig. 5. The commercial software package, CST Microwave Studio is the simulation tool used for designing the proposed multi-section BL coupler. The measurements are done using Agilent E8363B Vector Network Analyzer. The simulated and measured S-Parameter magnitude characteristics of the proposed UWB coupler are plotted in Fig. 6. A close correspondence between the simulated and measured values can be observed from the graph. The simulated and measured amplitude and phase stability plots of the proposed UWB BL coupler is depicted in Fig. 7. Flat amplitude and phase is observed over a major portion of the band.
Fig. 5. Photograph of the fabricated prototype (a) front view (b) rear view.
Fig. 6. Simulated and measured S-Parameter magnitude characteristics of the UWB BL coupler. (a) Simulated and measured |S11| and |S21| in dB (b) Simulated and measured |S31| and |S41| in dB. UWB range is highlighted.
Fig. 7. Simulated and measured amplitude and phase stability plots of the proposed UWB multi-section BL coupler. UWB range is highlighted.
A performance comparison of the proposed model with several other multi-section BL couplers is presented in Table 1. From the table it is evident that the proposed coupler provides improved bandwidth performance with minimal number of sections.
Table 1. Performance comparison of proposed model with several other multi-section (MS) BL couplers reported in literature.
IV. CONCLUSION
The design of a tri-section UWB BL coupler deploying multi-stage impedance feed has been presented. Further, an analysis to observe the bandwidth performance with increase in number of stages of transformation is performed. The proposed coupler with two stage impedance feed offers an enhanced impedance bandwidth of 114.8%, covering the UWB range. It also offers an improved “bandwidth to number of sections ratio” of 38%. The prototype can be fabricated using the normal PCB manufacturing process without the requirement of multi-layer fabrication or via holes.
ACKNOWLEDGEMENT
This work was supported by the Anna Centenary Research Fellowship, Anna University, Chennai 600025, India.
Sangeetha Velan is currently working towards the Ph.D. degree at College of Engineering Guindy, Anna University, Chennai, India. Her research interests include microwave couplers and antenna design.
Malathi Kanagasabai was born in India. She completed her Ph.D. degree in microwave engineering from the Department of Electronics and Communication Engineering, College of Engineering Guindy, Anna University, Chennai. She is currently Associate Professor in the Department of Electronics and Communication Engineering, Anna University, Chennai. Her research interests include microwaves, transmission lines, antenna structures, and high speed systems.
