A REVIEW OF BRANCH LINE DIRECTIONAL
COUPLER WITH BROAD BANDWIDTH
Bhupesh Aneja
Assistant Professor
JSSATE, Noida(INDIA)
er_bhupesh@jssaten.ac.in
Ish Tyagi
Student
JSSATE ,Noida(INDIA)
ish_tyagi60@yahoo.com
ABSTRACT
In this paper a review of branch-line coupler is
proposed, that is able to operate at wider
frequency range around centre frequency . In
this novel coupler analytical readings are used
which allow us to fully exploit the design
flexibilty inherent in two arm branchline
couplers with open stubs and arc stubs which
avoids the low impedence lines at a centre
frequency of 2.6GHz with conventional
microstrip transmission lines.Moreover it is
manufactured on FR4 substrate with probable
miniaturized structure of a composite branch
line structure.This review covers the study of
branchline directional coupler & microstrip
transmission line.
KEYWORDS
Branch line coupler, microstrip, characteristic
impedence, design frequency, scattering
parameters, open stubs .
1.INTRODUCTION
The branch-line couplers are mostly used in
microwave and millimeter wave circuits. There
are lots of applications of 90 degree hybrid and
180 degree hybrid branch-line tight couplers,
Vaibhav Gautam
Student
JSSATE ,Noida(INDIA)
gautamvaibhav007@gmail.com
Shivank Shukla
Student
JSSATE ,Noida(INDIA)
shivankshukla09@gmail.com
such as 3-dB or 6-dB coupler in our modern
microwave and millimeter wave communication
systems [1].
Branch line couplers find various
applications in wireless communication
systems, such as the phase shifters, balanced
amplifiers and mixers, and antenna/array
feeding networks. It can only be operated in
the odd multiples of the fundamental band,
which prevents itself from the versatile
applications in dual-band or multi-band
systems [2]. Further, it avoids narrow line
gaps and need for bond wires.
The disadvantages of branch line couplers
are:
1. Inconvenient line impedance
2. Narrow operating bandwidth [3]
They offer equal magnitude and quadrature
phase outputs at the operating frequency band.
They have been used extensively in the design
of power dividers and image rejection mixers.
A hybrid branch line is a special case of
directional coupler with a coupling factor of 3dB
and phase difference of 90° in the outputs of the
through and couple arms. It has a property that
when all the ports are matched, the power
entering from one port will be divided into two
other ports and the fourth port is isolated.
1.1 Proposed Approach
Even the conventional branch line couplers
suffer from a too narrow bandwidth for many
applications, therefore in order to increase the
operating frequency range it becomes important
to find some realizable circuits. Some work has
been done on broad banding hybrid couplers in
the past. Based on the past work, a novel broad
banding procedure will be developed in the
following coupler structure using stubs. As for
the stubs, a structure consisting of two sections
of high and low impedance transmission lines is
presented in order to connect a low impedance
stub to a signal line [23].
Since the input impedance of the coupler is real
at the centre frequency f0, the reflection
coefficient S11(f0) is real also for a defined
mismatch at the centre frequency, the
corresponding input impedance Z1 can be
calculated [3]. The following figure shows a
conventional branch line directional coupler
with four ports. Here the port 1 is input port,
3dB output is obtained at port 2 and port 4 and
the port 3 is isolated.
2Ө1
Port 1
Port 2
ZZZZZZzzhh
2Ө2
Z2
Port 3
Port 4
Z1
Fig1: Composite branch line coupler with 3dB gain
where Ө1 = Ө2 = 450
where Ө1 is electrical length. If Z0 is
characteristic impedance, then
Z1 /Zo = ( 1+S11(fo))/(1-S11(fo))
-(1)
Now we can calculate the impedance level of
inner coupler. If Z2 is input impedance of shunt
transmission line
then,
Z1/Z2 = Z2/(Z0(2)1/2)
-(2)
1.2 Microstrip transmission line
Microstrip is a special type of electrical
transmission line which can be fabricated using
printed circuit board technology and is used to
convey microwave frequency signals .It consists
of a conducting strip separated from a ground
plane by a dielectric layer known as the
substrate. Microstrip is thus much less expensive
when compared to traditional waveguide
technology and is also well very lighter and
more compact.
The disadvantages of microstrip when compared
with waveguide are the generally lower power
handling capacity and higher losses. Also unlike
waveguide, a microstrip is not enclosed and is
therefore susceptible to cross-talk and
unintentional radiation.
Power dividers and directional couplers are
passive devices which are used in the field of
radio technology. They couple a defined
amount of the electromagnetic power in
a transmission line to a port enabling the signal
to be used in another circuit. An important
feature of directional couplers is that they only
couple power flowing in one direction. Power
entering the output port is coupled to the
isolated port but not to the coupled port.
Directional
couplers
are
most
frequently
constructed from two coupled transmission lines
set close enough together such that energy
passing through one is coupled to the other. This
technique is favored by the devices which
operate at microwave frequencies. However
lumped component devices are also possible at
lower
frequencies.
Also
at
microwave
frequencies, particularly for the higher bands,
waveguide designs can be used. Many of these
waveguide couplers correspond to one of the
conducting transmission line designs, but there
are also certain types that are unique to
waveguide.
Good approximation for phase velocity,
propagation
constant
and
characteristic
impedance can be obtained from static or quasi
static solutions. The phase velocity can be
expressed as
Vp = c/√ ∈e
- (3)
where c is speed of light, Ԑe is the effective
dielectric constant of the microstrip line.
Propagation constant can be expressed as,
β = k0√ ∈e
- (4)
where k0 is wave number when air is used as
dielectric. Since some of the field lines are in the
dielectric medium and some are in the air, the
effective dielectric constant satisfies the relation
1< ∈e< ∈r
- (5)
and is dependent on the substrate thickness d
and conductor width W. Here Ԑr is the relative
dielectric constant.
1.3 Formulas for effective dielectric
constant, characteristic impedance and
attenuation:
The effective dielectric constant(∈e) of a
microstrip line is given by
∈e = (∈r+1)/2 + ( ∈r-1)/2 *1/√(1+12d/W) -(6)
If the dimensions of the microstrip line are
known, then the characteristic impedance Z0
can be calculated as
Z0= {60/ ∈e log(8d/W+W/4d) }
for W/d≤1
Z0={120π/√∈e[W/d+1.393+0.667log(W/d+1.44)
]
for W/d ≥1}
-(7)
Z0 , ϴ=β *l
ZL
Z
Zin
Fig2: Transmission lines without load
Fig3: Transmission lines with load
1.4 For perfect mismatch in stubs
I4,1 = -10log( P4/P1) dB
Open circuit ZL= ∞ , Zin = -j Z0 cotβl
(8)
Closed circuit ZL = 0, Zin = j Z0 tanβl
(9)
- (11)
1.6 Isolation
Isolation of a directional coupler can be defined
as the difference in signal levels in dB between
the input port and the isolated port when the two
other ports are terminated by matched loads,
1.5 Coupling factor
The coupling factor represents the primary
property of a directional coupler. Coupling
factor is a negative quantity, it cannot
exceed 0dB for a passive device, and in practice
does not exceed −3 dB since more than this
would result in more power output from the
coupled port than power from the transmitted
port.
The coupling factor is given as:
C3,1 = 10 log ( P3/P1) dB
So Isolation is given by
Li2,1 = -10log(P2/P1) dB
1.7 Directivity
Directivity is directly related to isolation. It is
defined as:
D3,4 = -10log(P4/P3)
= -10log(P4/P1)+10log(P3/P1) dB
-(10)
where P1 is the input power at port 1 and P3 is
- (12)
- (13)
where P3 is the output power from the coupled
port and P4 is the power output from the isolated
the output power from the coupled port.
port.
The main line insertion loss from port 1 to port 2
The directivity should be as high as possible.
(P1 – P2) is:
Substrate FR4 = ∈r = 2.2
h
Fig 4: conventional BLC with substrate height h ,width w
w
2. ABOUT THE SIMULATION
SOFTWARE
window are specified from the board window.
These dimensions set the scale which is used to
draw the distributed components on the screen.
PUFF is a scattering parameter and layout
calculator for microwave circuits
2.4. The plot window- After an analysis has
been completed the plot window is used to list
Every element in the parts window is
assigned a part with its parameters.
the values of the scattering coefficients at design
frequency fd.
The two most important parameters of the
simulation
software
are
the
characteristic
S(dB)
impedance Z0 and the design frequency f0.To
calculate
the
scattering
parameters
the
characteristic impedance is used. The design
frequency is used to calculate the electrical
length of the transmission line. Different
commands are used to obtain the smith chart and
the frequency plot. The various types of window
which appears on screen while using this
software are,
F
c
y
2.1.The parts window -The program begins
in the parts window. The initial parts list is taken
from the setup file. The parts list can be edited
by using the arrow key, the backspace key, enter
key, insert key and delete key.
2.2 The layout window- This window is in
the upper portion of the screen. The square
represents the substrate, and the numbers on the
side represents connectors. Typing an arrow key
will draw the selected parts window in the
direction of the arrow. The change in the x and y
coordinates can be seen in the message box.
2.3 The board window- The relative
dimensions of the circuit board in the layout
Frequency(GHz)
Fig4: Frequency range of wide band
directional coupler
Table 1. Comparison Of Various Proposed Approaches
REPRESENTATIVE
SETUP
Bernd Mayer and Reinhard Branch line coupler with
Knochel[3]
improved design flexibility and
broad bandwidth
M. Y. O. Elhiwaris, S. K. A. Miniaturized Size Branch
Rahim, U. A. K. Okonkwo
Line Coupler Using Open
and N. M. Jizat[11]
Stubs
With
High
Low
Impedances
RESULTS
Open stubs can be used to
avoid low impedance lines .
The size reduction of the
proposed design is 64.21%
with comparable performance
as that of the conventional
branch line coupler.
Tamasi Moyra, Arabinda Roy, Design Of 10 dB Branch line Adding DGS introduces some
Susanta Kumar Parui, Santanu Coupler By Using DGS
inductance and increases the
Das[1]
effective electric length of the
microstrip line and provides
slow wave characteristics.
Tse Yu Chen , Pei Lin and
Ting[2]
Miniaturized
Branch
line
Coupler
with
Coupling
dependent dual frequency
Operation
Compared to the conventional
branch line coupler operating
at 0.9GHz, the proposed
coupler shows a size reduction
by 56%.
Iwata Sakagmi, Ryo Teraoka A Reduced Branch line Using
low
impedance
and Takatsugu Munehir.[23]
Coupler With Eight Stubs
transmission line for the stubs,
The proposed coupler reduced
to 25% in the ratio of area.
Hui-Yong Zeng, Guang-ming Miniaturization of Branch–
Wang, Zhong-wu Yu and Line Coupler Using Composite
Xiao-kuan Zhang[24]
Right/Left–Handed
Transmission Lines
A coupler without Lumped
Components can be fabricated
easily with a standard printed
circuit board process
Kimberley W. Eccleston, Compact Planar Microstripline
Member, IEEE, and Sebastian Branch-Line and Rat-Race
H. M. Ong, Student Member, Couplers
IEEE[12]
N. Zheng1, L. Zhou1 and W.-Y.
Yinz[13]
A Novel Dual-Band Shaped
Branch-Line Coupler With
Stepped-Impedance Stubs
K. P. Ray, V. Sevani, and S.
Kakatkar Sameer[18]
Compact Broadband GapCoupled
Rectangular
Micropstrip Antennas
The development and design
method for compact branchline and rat-race couplers can
be realized in microstripline
with one layer of metal
without additional lumped
elements.
A novel 2.4/5.2 GHz compact
dual-band branch-line coupler
using stepped-impedance stub
is demonstrated and analyzed
with closed form formulas
obtained
Multiple numbers of parasitic
elements formed by splitting a
single RMSA into numbers of
equal, smaller strips along the
width has been carried out
Abdulkadir Yilmaz 1, Emine Sonnet Modelling Simulation A
cascaded
two-section
Rumeysa Cetiner 1, and of Broadband Branchline broadband branchline coupler
Moamer Hasanovic [25]
Coupler
has been designed and
simulated using SONNET to
have low input VSWR and
high isolation over a wider
bandwidth. The results of the
simulation
provide
satisfactory results with a
deviation of 5% in the
frequency band.
3. CONCLUSION
The analysis in this paper shows that with
the help of varying parameters of shunt and
series transmission lines and by varying the
parameters of open or short circuit stubs
around the design frequency, the operating
bandwidth of the branch line coupler can be
increased efficiently.
Besides this, the performance of the branch
line coupler can be increased with the help
of different improved designs using PUFF
and DosBox . Further the open stubs help in
designing high impedance lines, which help
in miniaturization and enhanced output.
The plot obtained gives all the scattering
parameters implicitly on the specified design
parameters.
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