D. Packiaraj
1
, K. J. Vinoy
2
, M. Ramesh
1 1
and A.T. Kalghatgi
1
Central Research Laboratory, Bharat Electronics Limited, Bangalore, India
2
Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore, India dpackiaraj@bel.co.in
Abstract:
This paper presents analysis and design of multilayer ultra wide band (UWB) power splitter suitable for wireless communications. An UWB power splitter is designed in suspended substrate stripline medium. The quarter wave transformer in the conventional Wilkinson power divider is replaced by broadside coupled lines to achieve tight coupling for broadband operation. The
UWB power splitter is analyzed using circuit models of coupled lines and full wave simulator. Experimental results of 3dB power splitter designed using the proposed structure have been verified against the results from circuit simulation and full wave simulation. The return loss is better than 12 dB across the band 3.1GHz to 10.6GHz. Size of the power splitter is 30mm× 20mm×6.38mm.
Key Words: broadside, coupled line, splitter, suspended substrate stripline, UWB.
I I NTRODUCTION
Ultra wide band power splitters find numerous applications in various microwave modules such as detectors, modulators, mixers, phase shifters, antenna arrays. Conventional microstrip power splitter shown in
Fig. 1 is well known power divider for their easy design and realization. But they are limited by their narrow band operation(less than 50%) which limits the usage of such couplers in broad band communication systems.
Modern electronic warefare systems require broadband power splitters. Various topologies for the design of power dividers were reported in literature [1-
9]. Design of a broadband splitter is still a challenging problem. Broadband power splitters have been conventionally realized using multi-stage Wilkinson splitters or Lange couplers. Though Lange coupler is broadband in nature, the critical issue is the bonding between alternate arms of couplers which limits the frequency of operation. On the other hand, multi-stage power splitters are easy to design, but are lengthy and occupy more space.
In this paper, a compact UWB splitter capable of providing equal power division (3dB) over wide frequency band is designed in suspended substrate stripline (SSS) medium. Section 2 describes the design and analysis of a broadband splitter. In section 3, measured results of coupler are compared against full wave simulation results for the validation. Section 4 concludes this paper.
Fig. 1. Popular microstrip power splitter
II. DESIGN OF UWB POWER SPLITTER
The present paper uses SSS to design UWB power splitter over 3.1GHz to 10.6GHz (110% bandwidth). The proposed power splitter which provides equal power division over wide frequency band is shown in Fig. 2. Coupler consists of pair of broadside coupled lines for tight coupling and broadband operation. Broad side coupled lines are formed using top and bottom layer suspended substrate as shown in Fig. 2. The length of broad side coupled line is quarter wave long at the center frequency (6.85GHz) of the band. The inter connecting stub between port 2 and port 3 improves the matching at port 1. The line width W c is one of the parameters which control the coupling and bandwidth.
UWB splitter is analyzed based on the even and odd mode propagation characteristics of the broadside coupled lines. The coupling factor `K' for 3dB coupling is 0.708. The coupling factor can be determined from the even and odd mode impedances of the broadside coupled lines using
K

Z
Z oe oe


Z
Z oo oo
(1)

Coupled line sections of the power splitter are characterized using the following ABCD parameters,
A

D

Z oe cot
 e

Z oo cot
 o (2)
Z
1
B

2 j Z
2 oe

Z
2 oo

2 Z oe
Z oo
(cot
 e cot
 o
Z
1
 csc
 e csc
 o
) (3)
Fig. 4. Odd mode analysis. a) Top layer, b) Bottom layer
C

2
Z
1 j (4)
Z
1

Z oe csc
 e

Z oo csc
 o
. (5) where θ e
is even mode phase velocity and θ o
is odd mode phase velocity. Z oe
and Z oo
are even mode and odd mode impedances respectively. Z oe
and Z oo
extracted from EM simulator are 174 and 13 respectively. Broadside coupled line excitations used in EM simulation for even and odd modes are shown in Fig. 3 and Fig. 4 respectively. The power splitter is analyzed by dividing the structure using perfect magnetic conductor ( PMC) and perfect electric conductor(PEC) as shown in Fig. 5. Even mode analysis is done by using PMC. PEC is used for odd mode analysis.
Fig. 5. Analysis of power splitter
Overall performance of the power splitter is obtained using the following equations.
S
11

( Z ine
Z ine

Z
Z o ino
)( Z

Z
O
2
 ino
Z o
)
(6)
S
21

S
31
Z
( Z ine ine
Z
 o
Z o

)(
Z
Z ine oZ o ino

Z o
)
(7)
Z ine
and Z ino
are the input impedances of the even mode and odd mode equivalent circuits respectively.
Analytical calculations are performed in MATLAB and results are shown in Fig. 6 in comparison with EM simulations [10].
Fig. 1. Proposed SSS UWB power splitter
Fig. 3. Even mode analysis. a) Top layer, b) Bottom layer
Fig. 6. Circuit simulation of power splitter
Fig. 7 shows the simulation results of the 3dB power splitter in comparison with conventional power splitter.
As shown in Fig. 6, the bandwidth of the proposed splitter is from 3.1GHz to 10.6GHz (ultra wide band) whereas bandwidth of the conventional power splitter is from 4.8GHz 8.65GHz (50%). Power is equally divided

within 0.3dB. Return loss is better than 14dB over the band. The proposed power splitter can reject the out of band frequency components. The SSS power splitter offers broadband operation than conventional power splitter.
Physical parameters coupled line width W c
and ground plane spacing b in the structure have influence on bandwidth of the power splitter. In this paper W c
and b are chosen to be 3.2mm and 6.38mm respectively to achieve the required bandwidth of the filter
Table 1. Substrate parameters of SSS
Substrate thickness „h‟ 0.38mm
Substrate permittivity „ε r
‟
Ground plane spacing „b‟
2.17
6.38mm
Table 2. Physical parameters of Power splitter
Dimensions
W o
W c
L c
Values
8.4mm
3.2mm
6.8
Fig. 8. Photograph of assembled SSS power splitter
Fig. 9. Layers of PCB. a)Top layer, b)Bottom layer
Fig. 7. Simulation results of UWB power splitter
III. E XPERIMENTAL R ESULTS
Fig. 8 shows the photograph of assembled broadband power splitter. The filter housing consists of two half metallic air cavities and were fabricated in a workshop with the standard CNC machining process. Top and bottom layers of the PCB are shown in Fig. 9. PCB having power splitter circuit is suspended between air metallic cavities. Strip line launchers have been used in measurement. Fig. 10 shows the measured characteristics of the power splitter. It is evident from the measured results that the proposed coupler exhibits broadband frequency characteristics. The coupling is 3.1 ± 0.25dB over 3.1GHz to 10.6GHz frequency band. The return loss is better than 12dB. Length and width of the coupler are 25mm and 30mm respectively.
Fig. 10. Measured S parameters of power splitter
IV. CONCLUSIONS
SSS power splitter has been proposed in this paper based on broadside coupled lines for broadband applications.
The bandwidth was controlled through coupled lines. A
3dB splitter from 3.1GHz to 10.6GHz was designed in this paper. Experimental and simulation results were found to be satisfactorily matching. The coupled-line length and the width of the proposed power splitter structure are approximately λ g
/4 and λ g/
8 respectively.
This splitter is well suitable for multilayer integrated systems, broadband communication systems and antenna arrays

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BIO DATA OF AUTHORS
D. PACKIARAJ is currently a Research Staff at
Central Research Laboratory of Bharat Electronics
Limited, Banglaore. He received M. E degree in communication systems from National Institute of
Technology, Trichirappali in 2002. His area of interest includes microwave filters, antennas and
Phase shifters. He is a research student under Prof.
K. J. Vinoy in IISc, Bangalore.
K. J. VINOY is an Associate Professor in the
Department of ECE at the IISc, Bangalore. He received M. Tech degree from CUSAT, Cochin, and Ph.D. degree from the Pennsylvania State
University, USA in 1993, and 2002 respectively.
From 1994 to 1998 he worked at National AICX
Aerospace Laboratories, Bangalore, India. He was a research assistant at the Center for the
Engineering of Electronic and Acoustic Materials and Devices (CEEAMD) at the Pennsylvania State University, from
1999 to 2002. He continued there from 2002 to August 2003 for Post
Doctoral research. His research interests include several aspects of microwave engineering such as antennas, circuits, RF-MEMS and
Computational Electromagnetics. He has published over 120 papers in technical journals and conferences. His other publications include three books: Radar Absorbing Materials: From Theory to Design and
Characterization, RF MEMS and their Applications and Smart Material
Systems and MEMS: Design and Development Methodologies.
M. RAMESH was born in India in 1969. He received his B. E Degree from Andhra University
Visakhapatnam in 1990 and M.Tech degree from the
Indian Institute of Technology Kharagpur in 1993.
His areas of interest include Microwave circuits and antennas. He has published 30 research papers in various international journals.
Dr. A.T. KALGHATGI is currently Chief Scientist at Central Research Laboratory of Bharat Electronics
Ltd., Bangalore. He has more than 25 Years of
Experience in the field of RF & Microwave
Engineering. After Graduating from Mysore
University with BE in Electronics &
Communications, he completed his M Tech in
Microwave & Radar Engineering from IIT, Kharagpur and Ph.D., from Leeds University UK. He worked in Avionics Design Bureau of
HAL Hyderabad as Design Engineer responsible for Microwave frontends & IFF transreceiver. From 1988 to 1993 he was in the University of Leeds, UK for his Ph.D as a Commonwealth Research Scholar and later as a Post Doctoral Fellow. From 1994 he has been with Central
Research Lab of Bharat Electronics Ltd. His research interests are in
RF and Microwave system, Smart Antennas, RF MEMs & Secure
Communication Technologies. He has more than 60 research papers and a patent to his credit. He is recipient of R&D Award from BEL for significant contribution to its R&D programme, and the Society of
Defence Technologists (SODET) Gold Award for Technology
Development & Innovation for the year 2005-06. He is also recipient of
Raksha Mantri Award of Excellence in Innovation for the year 2007-08 as well as for the year 2008-09. He is a member of review committees of Government of India, Member of the Editorial Board of IET‟s
Microwave & Antennas research publications, Member of IEEE and
IET-UK. He is presently Chairman of local Network of the IET-UK at
Bangalore.