COUPLER, POWER DIVIDER AND CIRCULATOR IN V-BAND SUBSTRATE INTEGRATED WAVEGUIDE TECHNOLOGY

International Journal on Computational Sciences & Applications (IJCSA) Vol.3, No.6, December 2013

COUPLER, POWER DIVIDER AND CIRCULATOR IN VBAND SUBSTRATE INTEGRATED WAVEGUIDE
TECHNOLOGY
Bouchra Rahali and Mohammed Feham
STIC Laboratory University of Tlemcen, Algeria

ABSTRACT
In recent years substrate integrated waveguide technology (SIW) has been applied successfully to the
conception of planar compact components for the microwave and millimeter waves applications. In this
study, a V-band substrate integrated waveguide coupler, power divider and circulator are conceived and
optimized by Ansoft HFSS code. Thus, through this modeling, design considerations and results are
discussed and presented. Attractive features including compact size and planar form make these devices
structure easily integrated in planar circuits.

KEYWORDS
Rectangular Waveguide, Microwave Components, SIW, Power Divide, Circulator, Coupler, HFSS.

1. INTRODUCTION
The rectangular waveguide in the technology SIW (RSIW) [1] [2] has some interesting
characteristics in terms of easy integration while offering components to a high quality factor. A
large range of SIW components such as bends [3], filters [4], couplers [5], duplexers [6], sixports
[7], circulators [8] and phase shifters [9] has been proposed and studied at low cost, high quality,
relatively high power and integration with other microwave components in the same dielectric
substrate [1] [2].The (RSIW) (Figure 1) is fabricated using two rows of periodic metallic posts
which connect two higher and lower planes mass of dielectric substrate. The geometry and the
distribution of the electric field in (RSIW), illustrated in Figure 2 and 3 are similar to those of the
equivalent rectangular wave guide [2] [3]. In this paper the building blocks of many millimeterwave integrated circuits are designed and optimized in V-band SIW technology.

2. FUNDAMENTAL RSIW CHARACTERISTICS
Two rows of holes are drilled and metalized, making contact between the two metal planes of the
dielectric substrate, allowing propagation of all modes TEn0 [5]. The diameter d of holes stems, p
the spacing between the holes and
spacing between the two rows of holes are physical
parameters necessary for designing (RSIW) (Figure 1). Indeed its electrical behavior is similar to
that of a conventional rectangular waveguide filled with dielectric of width
[4]. The current
lines along the side walls of the RSIW is vertical, the fundamental mode TE10 can propagate
efficiently.

DOI:10.5121/ijcsa.2013.3607

63


Figure 1. Rectangular wave guide integrated into a substrate RSIW

Based on the work [2], empirical equations were derived for determining the width of the
equivalent rectangular wave guide, giving the same characteristics of the fundamental mode
propagating in the RSIW (Figure 2) having the same height and the same dielectric.
(1)
√

(2)
(3)

,

is the space wavelength.

The period p should be kept low to reduce leakage losses between adjacent cylinders. The choice
of subject is also the problem of losses.
We analyzed, the RSIW designed in V-band [50-75] GHz from a conventional waveguide [10]
whose characteristic parameters are described in Table 1. Following the same approach, cited
above [2], we deduce the parameters of the RSIW and the equivalent waveguide (Figure 2) Table
1.
Table 1

Classic wave guide
WR15, a=3.759mm,
b=1.88mm,
1

Equivalent wave guide
h=0.15mm,
=2.12mm

3.15

RSIW
h=0.15mm,
3.15,
d=0.2mm,p=0.4mm,
=2.24mm

Figure 2. Equivalent rectangular waveguide and RSIW

In this study, we have analyzed many structures with HFSS tool [11] based on the finite element
method (FEM). It should be noted that the formulas given by equations (1), (2) and (3) are
commonly used to obtain initial values
, optimized later by HFSS. It has allowed the layout
64


of the cartography of the electromagnetic field of the TE10 mode and scatter diagram. Figure 3
shows the similarity of the electromagnetic field distribution of TE10 mode guided in the guide
RSIW and its equivalent in waveguide technology.

(b)

(a)

Figure 3. Electric field distribution of the TE10 mode in the equivalent rectangular waveguide (a) and
RSIW (b) at the frequency f = 60GHz

Figure 4 also shows the consistency of the dispersion characteristics between these two equivalent
wave guides. It should be noted that this similarity propagation is valid for all modes TEn0.
Mode TE10 Equivalent wave guide
Mode TE20 Equivalent wave guide
Mode TE10 RSIW
Mode TE20 RSIW

3000

Beta (rd/m)

2500

2000

1500

1000

500

0
30

40

50

60

70

80

90

Frequency GHz

Figure 4. Dispersion characteristics

3. RSIW-MICROSTRIP TAPERED TRANSITION
The microstrip transition taper [12] employed to interconnect RSIW to the planar transmission
lines is used in this paper. There is a tapered section which is used to match the impedance
between a 50 Ω microstrip line and the RSIW. The 50 Ω microstrip line, in which the dominant
mode is quasi-TEM, can excite well the dominant mode TE10 of the RSIW, as their electric field
distributions are approximate in the profile of the structure.
Initial parameters
and
are determined from several formulas given [13], following by an
optimization using the HFSS [11] Table 2. Figure 5 shows the proposed configuration of two
back-to-back transitions of microstrip line to RSIW. Figures 6 and 7 show the results of the RSIW
analysis without transition and with a coplanar taper of dimensions
and
.

65

Table 2

LT

L

1.57mm
0.77mm
0.37mm
2.8mm

Figure 5. Electric field distribution of TE10 mode at f = 60 GHz in the matched RSIW
0

0

-5
-10

-10

IS11I
IS12I

-15

ISijI dB

ISijI dB

-20

-30

IS11I
IS12I

-40

-20
-25
-30
-35

-50

-40
-60
50

55

60

65

70

75

Frequency GHz

Figure 6. Transmission coefficients S21
and reflection S11 of the RSIW

50

55

60

65

70

75

Frequency GHz

Figure 7. Transmission coefficients S21 and
reflection S11 of the matched RSIW with taper

The results illustrated in figure 7, indicate that the reflection coefficient S11 remains below-15 dB
over 31.51% of the frequency band and the transmission coefficient S21 is around -0.88 dB across
the entire band. This concept [13] allows the design of a completely integrated planar circuit of
microstrip and waveguide on the same substrate without any mechanical assembly [14] [15].

4. DESIGN OF RSIW PASSIVE DEVICES
4.1. SIW Coupler
The development of modern communication systems, in microwave and millimeter waves domain,
requires high quality and high density integration circuits. The size and cost are two essential
requirements of these systems. This has stimulated the rapid development of many passive
compact [14] [15] and low cost components. Couplers [5] which have been widely used as key
components in many systems have been intensively studied for decades. The directional couplers
are passive devices used for routing, dividing and combining the signals in the microwave system.
In the antenna beam-forming networks, the directional coupler is generally an important element in
power dividing/combining networks, so great interest and effort have been directed to the
development of different types of directional couplers [16]. The RSIW directional couplers are
extensively investigated. Coupler’s configuration is illustrated in figure 8. The RSIW directional
coupler (-3 dB) is realized by two RSIW with a common wall on which an aperture is utilized to
66


realize the coupling between these two guides. The geometry of the coupler is determined [15]
based on an even/odd mode analysis, where the even mode is the TE10 and the odd mode is the
TE20. The phase difference ∆φ between the two modes is expressed by
∆φ = ( -

)

(4)

Where and
are the propagation constants of the TE10 and TE20 modes, respectively. In the
band of operation, the condition ∆φ = π/2 needs to be satisfied. Port 1 is the input port, port 2 is
named as the through port, port 3 is the coupling port and port 4 is designed as the isolated port.
The tapered transition between the 50Ω microstrip line and the RSIW is added to each port so that
we can integrate this component directly into a microstrip circuit. The matrix [S] of a symmetric
coupler (-3 dB), adapted to its access, is given by equation (5).

[ ]

√

[

]

(5)

Figure 8. RSIW coupler

The coupler parameters are finely tuned using three-dimensional (3D) electromagnetic (EM)
simulation under HFSS [11] to achieve wide-band performance. The final dimensions of the SIW
coupler [10] are presented in Table 3.
Table 3

L

5.93mm
1.498mm
0.227mm
3.138mm
0.313mm

Figures 9 and 10 present electric field distribution of the TE10 mode in the V-band and the
reflection coefficients S11, transmission coefficients S21, coupling and isolation coefficients S31, S41.
The results of this analysis [10] show clearly the directional coupler character in the band [50-75]
GHz, where we have the levels of reflection and isolation below -15 dB in more than 26.43% of
the bandwidth, and the insertion losses S21 and coupling S31 fluctuate between -3.59 dB ±0.5 dB.
These simulation results prove the good performance of this integrated structure.

67


Figure 9. Electric field distribution of TE10 mode for RSIW coupler at f=60 GHz
0

-10

-20

ISijI dB

-30

-40

IS11I
IS21I
IS31I
IS41I

-50

-60

-70
50

55

60

65

70

Frequency GHz

Figure 10. Frequency response of the RSIW directional coupler

4.2. SIW Power Divider
The power dividers [17] are passive microwave devices. There are mainly two types T and Y [14]
[15], which are commonly used to deliver copies of a signal in a system. This application focuses
on the three ports power dividers with equal power division ratio where the half power (-3 dB) of
an input signal is provided to each of the two output ports. The S matrix for a three ports network,
is shown in equation (6)

[ ]

[

]

(6)

The analyzed power divider (Figure 11), designed in the V- band [50-75] GHz, is based on three
identical RSIW [10] with the characteristic parameters reported in Table 1 and 2. Then Three
RSIW of length L=2.8mm are connected to form a T. An inductive metal cylinder of radius r and
position xp is added to this junction in order to minimize reflection losses at the input port. To
achieve this objective, an optimization of the radius r and the position xp of this disruptive element
is required. For this purpose, it is generally useful to fix the radius r to the corresponding available
practical value of diameter drills, and then change xp to reduce the reflection losses [10]. This will
be repeated until reflection losses, below -15 dB, reach acceptable limit. Then, a microstrip
transition is added to each port so that we can integrate this component directly into a microstrip
circuit. The input wave (port 1) is divided into two parts which output to port 2 and port 3,
respectively.

68


Figure 11. RSIW power divider

Figures 12 and 13 illustrate the distribution of the electric field of the TE10 mode in the band [5075] GHz [10] and the transmission coefficients S21, S31 and the reflection coefficient S11,
respectively. The figure 13 indicates that S11 is less than -15 dB between 50 GHz and 61 GHz,
which is more than 19.82% of the bandwidth. The optimal values of the inductive cylinder are r =
0.1mm, xp=1.02mm. Transmission coefficients S21 and S31 fluctuate between -3.65 dB and -3.76
dB, being very acceptable levels.

Figure 12. Electric field distribution of the TE10 mode
at f = 55GHz in the RSIW power divider with inductive cylinder

-5

IS11I
IS21I
IS31I

-10

ISijI dB

-15

-20

-25

-30

50

52

54

56

58

60

62

Frequency GHz

Figure 13. Parameters Sij in the RSIW power divider with inductive cylinder

4.3. SIW Circulator
Protection microwave sources are a very common and recommended to increase the reliability of
emitters practice. Several types of solutions are used to provide this kind of function. However, in
high power, the circulator waveguide technology [8] is still the best solution. Its topology of a
69


hexapole (Figure 14) having three access separate of 120° from each other, around a central body
of ferrite (nickel materials and lithium ferrite) [8] [9] , to which is applied a vertical magnetic field
conferring to the circulator the property of non-reciprocity. Indeed, when a transverse magnetic
field is applied, it creates in its central part an internal field such that incoming wave from port 1, 2
or 3 cannot out by the access 2, 3 or 1, respectively.
The two important variables for a circulator are insertion losses to be as low as possible (<-1 dB)
and good isolation (-30 dB). The ideal circulator is a suitable hexapole that would be able to direct
the energy to the next access, the third being isolated. Its ideal matrix [ ] would be as follows:
[ ]

[

]

(7)

Where φ is the phase shift associated with transmission of the access signal to the next access. The
non-symmetry of the matrix reflects clearly the non-reciprocity of the component. This nonreciprocity is the whole point of the device and explains that this function can be used in many
applications in telecommunications. This circulator was designed [10] by using cylindrical metal
rods, with the same parameters presented in Tables 1 and 2, but L = 2.8mm.
In this paper the saturation magnetization of ferrite material is [9] 4πMs = 5000 Gauss. Its relative
dielectric constant is 13.7 and a radius calculated by [8].
√

(8)

Where c and
are respectively the velocity of light in the free space and the operation frequency
[8]. The ferrite height is equal to the RSIW thickness [10], =0.5mm =0.15mm.
The RSIW circulator, presented in figure 14, has been simulated by HFSS [11]. Figure 15
illustrates the distribution of the electric field of the TE10 mode in the band [50-75] GHz. The
frequency response of this SIW circulator, the transmission coefficient S21, the reflection
coefficient S11 and the isolation coefficient S31 are reported through the figure 16. The analysis of
these results [10] shows that the reflection losses S11 below -15 dB occupy more than 6% of the
bandwidth, the insertion loss S21 is about -0.8 dB, while the maximum of the isolation S31 is
-36.32 dB. At frequency of 55 GHz, the two figures 14 and 15 confirm traffic property of this
device [14].

Figure 14. RSIW circulator
70


Figure 15. Electric field distribution of the TE10 mode
of the RSIW circulator at f = 55 GHz

0

-5

-10

ISijI dB

-15

-20

IS11I
IS31I
IS21I

-25

-30

-35
50

51

52

53

54

55

56

57

58

59

60

Frequency GHz

Figure 16. Parameters Sij of RSIW circulator

5. CONCLUSIONS
In the design of complex microwave and millimeter wave circuits the developed RSIW
components can serve as building blocks. Through this study we have investigated a V-band
substrate integrated waveguide passive components. We have presented a simple and fast method
to design three passive components based on RSIW using Ansoft HFSS code. Firstly, the V-band
coupler which is composed of two matched RSIW with a common wall on which an aperture is
used to realize the coupling. Secondly, the RSIW three ports power dividers with equal power
division ratios where half the power of an input signal is provided to each of the two output ports.
The HFSS code has also been applied to the analysis of the V-band RSIW circulator, suitable for
millimeter wave applications, has been designed; it can be integrated on the same substrate with
microstrip and other planar circuit. The simulation results have shown the good performance of
these integrated structures.

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.

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