-
Ultra-compact C
Reza Karimian
Department of Electrical Engineering
Iran University of Science and Technology
Tehran, Iran
[email protected]
Abstract— Design, simulation and fabrication of a compact
waveguide loop coupler have been reported in this paper. Design
coupler has little ripple in whole design band (less than 1dB)
and
high directivity (more than 16dB for whole band and more
than
18dB for 30% of C band). Fabrication results indicate the
simulation and measurement result are in a good agreement.
I. INTRODUCTION
Waveguide couplers have a vital rules in many microwave
circuits such as high power microwave network,
measurement of output power of a network and use it for
comparison in feedback loop [1-2] .
Waveguide couplers can be divided in three main part;
directional couplers [3-4], cross-guide couplers [5], and
loop
couplers [6-7]. Each of these couplers has some advantages
and disadvantages. Directional couplers have very good
directivity but the size of this type of couplers is very
big.
Cross-guide couplers have relative good directivity and
relative big size. Loop couplers have relative good
directivity
and compact size.
In this paper a compact loop coupler has been presented.
The proposed coupler has the size of only
322.15 47.55 70mm× × and coupling -40dB. Good directivity and
low ripple is the advantage of proposed coupler. To the
author knowledge the fabricated coupler is the smallest
coupler that has been reported until now.
A. Design formulation for flat coupling
Side view and front view of proposed coupler has been
depicted in Fig. 1 and Fig. 2 respectively. The proposed
coupler is consisting of two main parts, main waveguide and
a cavity that has been mounted on the waveguide.
The dominant mode (TE10) moves into the main
waveguide and going through the cavity via the cut gap on
top of the waveguide. As it has shown in Fig. 3 the dominant
mode that propagates through main waveguide is TE10, this
wave produce surface current on loop gap that has been cut
compact C-band waveguide loop
coupler
Department of Electrical Engineering
Iran University of Science and Technology
[email protected]
Homayoon Oraizi
Department of Electrical Engineering
Iran University of Sci
Tehran, Iran
[email protected]
and fabrication of a compact
waveguide loop coupler have been reported in this paper.
Design
coupler has little ripple in whole design band (less than 1dB)
and
ivity (more than 16dB for whole band and more than
18dB for 30% of C band). Fabrication results indicate the
simulation and measurement result are in a good agreement.
NTRODUCTION
Waveguide couplers have a vital rules in many microwave
igh power microwave network,
measurement of output power of a network and use it for
Waveguide couplers can be divided in three main part;
guide couplers [5], and loop
Each of these couplers has some advantages
and disadvantages. Directional couplers have very good
directivity but the size of this type of couplers is very
big.
guide couplers have relative good directivity and
elative good directivity
In this paper a compact loop coupler has been presented.
The proposed coupler has the size of only
40dB. Good directivity
ntage of proposed coupler. To the
author knowledge the fabricated coupler is the smallest
Side view and front view of proposed coupler has been
pectively. The proposed
coupler is consisting of two main parts, main waveguide and
a cavity that has been mounted on the waveguide.
The dominant mode (TE10) moves into the main
waveguide and going through the cavity via the cut gap on
As it has shown in Fig. 3 the dominant
mode that propagates through main waveguide is TE10, this
wave produce surface current on loop gap that has been cut
on top of the main waveguide and consequently this current
surface produce E-field and H
coupling consists of a narrow slot and cut
waveguide. The cavity is consisting of two output coaxial
(SMA connector) and a rectangular conductor that connect
the inner conductors of the SMA connectors.
The inner conductors and the rectangular conductor form a
coupling loop. The currents flowing on the conductor
Fig. 1. Side view of the proposed c
Fig. 2. Front view of the proposed coupler
band waveguide loop
Homayoon Oraizi
Department of Electrical Engineering
Iran University of Science and Technology
Tehran, Iran
[email protected]
on top of the main waveguide and consequently this current
field and H-field on cavity. The area of
coupling consists of a narrow slot and cut-off rectangular
waveguide. The cavity is consisting of two output coaxial
(SMA connector) and a rectangular conductor that connect
the inner conductors of the SMA connectors.
ctors and the rectangular conductor form a
coupling loop. The currents flowing on the conductor
Fig. 1. Side view of the proposed coupler
Fig. 2. Front view of the proposed coupler
-
generate fluxes with opposite directions by the Ampere’s
law.
By setting this loop coupler at the desired position, the
TM111 mode can be suppressed and TE011 mode excited in
one-port cavity. In other words the main waveguide is
coupled with the cavity through the narrow slot, the cut gap
on top of the main waveguide and the coupling loop.
B. Coupling theory of loop
Condon has given a most convenient theory of the
excitation of cavities by loop coupler [8]. In general, a
current
flowing in the coupled loop will excite all of the cavity
modes
in varying degree [9]. The resonator has a number of
resonant
frequencies nυ . An arbitrary field inside the cavity satisfied
the boundary conditions can be expanded in terms of the
resonant wave normal mode An( ρ ). It is assumed the loop is
small compared to the wave length, that the current
distribution in the loop is uniform in our theoretical
analysis.
So
( ). .n n nA ds curlA ds Mρ = =∫ ∫∫ (1) Mn is the flux through
the loop of the nth mode. For a unit
current, this is then equal to the mutual inductance between
the coupling loop and the nth mode. And the impedance of
the coupling loop can be calculated 2
0 02 2
2
2
( )
nin
n
nn
iZ R j L
Vi
c Q
Mπ υωυυπ
υ υ= + +
− +∑ (2)
where V is the volume of the cavity. It should be pointed
out that the electric field at the loop coupler is the
minimum
that corresponds to the point where the input impedance is
zero. Hence using (2) in fact shows the impedance of the
point about λ /4 away from the feed point. The distribution of
currents on the loop and the direction of the magnetic lines
of flux should obey Ampere's circuital law. In the other
place
the trend of magnetic lines will be governed by the boundary
condition of the cavity.
C. Approximation formula for flat coupling
An approximation theoretical and empirical formula for flat
coupling expressed in dB is:
Fig. 3. E-Field distribution at the frequency of 5 GHz
Fig. 4. Comparison between simulation and Measurement
for scattering parameters
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6-100
-80
-60
-40
-20
0
Frequency (GHz)
Sca
ttering P
aram
eter
s (d
B)
S21 Simulation
S21 Measurment
S11 Simulation
S11 Measurment
S31 Simulation
S31 Measurment
S41 Simulation
S41 Measurment
Fig. 5. Comparison between simulated and measured
coupling
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6
-40.6
-40.4
-40.2
-40
-39.8
Frequency (GHz)
Coupling (dB)
Simulation
Measurment
Fig. 6. Comparison between simulated and measured
directivity
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 610
20
30
40
50
60
Frequency (GHz)
Direc
tivity (dB)
Simulation
Measurment
Fig. 6. Comparison between simulated and measured
directivity
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 610
20
30
40
50
60
Frequency (GHz)
Direc
tivity (dB)
Simulation
Measurment
Fig. 7. coupling with different value of height h1
4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6-43
-42
-41
-40
-39
-38
-37
Frequency (GHz)
Coupling (dB)
h1=2 h1=2.33 h1=2.66 h1=3
-
2
1 2
10log( )
ab FC G E
h h P= + −
+
2 1
1 1 2
22 1 ln(1 )
h hmP
h h hπ
= + + + +
m- Width of rectangular conductor
h1- Height of the coupling loop
h2- thickness of rectangular conductor
327.3( )
4
htG
V= +
40log( )4
WE =
t- Thickness of slot (the gap that has been cut on top of
waveguide)
w- Width of the slot
h3- Height of rectangular cavity
V- Width of rectangular cavity
The flat characteristic of the coupling frequency occurs at
some length L1 of the slot. This length depends on the
thickness and width of the slot and type of rectangular
waveguide.
The purpose of this project is to design a 40 dB coupler for
C band. The broad dimension and narrow dimension of main
waveguide according to dominant cutoff frequency at 5 GHz
is 47.55mm�22.15
mm. This dimension is corresponding to
standard commercial waveguide WR187.
fixing optimum parameters of the proposed coupler for 40dB
coupling, following parameters has been achieved. These
parameters are listed in Table. I.
Table. I. Dimension of the proposed coupler (unite: mm)
a b W V
47.55 22.15 5.27 9
L1 L2 h1 h2
21.42 26.8 2.37 3.5
II. SIMULATION AND MEASURMENT RESULT
The comparison between simulated and measured
scattering parameters of the proposed coupler obtained by
using CST 2010 and the Agilent E8361C vector network
analyzer is shown in Fig. 4. Comparison between simulated
and measured coupling and directivity has been shown in Fig.
5 and Fig. 6. As it has clarified in Fig. 4 the simulation
and
measurement result are in a good agreement. A 40dB
coupling has been achieved in both simulated and measured
result. The ripple for coupling in whole band (40% band
width) is less than 0.7dB that is very good for microwave
usage. The comparison between simulated and measured
directivity that has been shown in Fig. 6 indicating that
the
result are very close to each other and the directivity is
more
than 16dB for whole band and is more than 20dB for 25% of
band that is very reasonable.
(3)
2 1 ln(1 )
(4)
(5)
(6)
Thickness of slot (the gap that has been cut on top of
The flat characteristic of the coupling frequency occurs at
of the slot. This length depends on the
thickness and width of the slot and type of rectangular
sign a 40 dB coupler for
C band. The broad dimension and narrow dimension of main
cutoff frequency at 5 GHz
. This dimension is corresponding to
standard commercial waveguide WR187. By calculating and
g optimum parameters of the proposed coupler for 40dB
coupling, following parameters has been achieved. These
. Dimension of the proposed coupler (unite: mm)
t m
1.62 4.5
h3
8
MENT RESULT
The comparison between simulated and measured
scattering parameters of the proposed coupler obtained by
using CST 2010 and the Agilent E8361C vector network
rison between simulated
and measured coupling and directivity has been shown in Fig.
As it has clarified in Fig. 4 the simulation and
measurement result are in a good agreement. A 40dB
coupling has been achieved in both simulated and measured
result. The ripple for coupling in whole band (40% band-
width) is less than 0.7dB that is very good for microwave
The comparison between simulated and measured
directivity that has been shown in Fig. 6 indicating that
the
ach other and the directivity is more
than 16dB for whole band and is more than 20dB for 25% of
As it is cleared from Fig. 4 the significant difference
between simulation and measurement result is related to S11.
The S11 that has achieved from simulation is better than
35dB and this is -21dB from measurement result. Due to use
of waveguide to coax transition that has been used for
measurement result this large difference between simulation
and measurement result is achieved.
The inner conductors and the rectangular conductor form a
coupling loop. In other words,
result of the height h1 changes. Fig. 7 shows the changes of
coupling with different value of height h
It can be seen that the coupling
increasing the size of h1.
One of the vital parameters in coupling is the amount of
power reflected by the coaxial connector. Comparison
Fig. 8. Simulation and measurement result for S31
4 4.2 4.4 4.6 4.8-35
-30
-25
-20
-15
Frequency (GHz)
S33 (dB)
Simulation
Measurment
Fig. 9. Simulation and measurement result for S21
4 4.2 4.4 4.6 4.8
-0.25
-0.2
-0.15
-0.1
-0.05
0
Frequency (GHz)
S21 (dB)
S21 Simulation
S21 Measurment
Fig. 10. Prototype of fabricated proposed coupler
As it is cleared from Fig. 4 the significant difference
between simulation and measurement result is related to S11.
has achieved from simulation is better than -
21dB from measurement result. Due to use
of waveguide to coax transition that has been used for
measurement result this large difference between simulation
and measurement result is achieved.
he inner conductors and the rectangular conductor form a
In other words, the coupling ring changes as a
changes. Fig. 7 shows the changes of
coupling with different value of height h1.
It can be seen that the coupling has been increase with
One of the vital parameters in coupling is the amount of
power reflected by the coaxial connector. Comparison
n and measurement result for S31
5 5.2 5.4 5.6 5.8 6
Frequency (GHz)
Measurment
Fig. 9. Simulation and measurement result for S21
5 5.2 5.4 5.6 5.8 6Frequency (GHz)
S21 Simulation
S21 Measurment
Fig. 10. Prototype of fabricated proposed coupler
-
between simulated and measured result of power reflected by
the coaxial connector has been shown in Fig. 8.
III. FABRICATION CONSIDERA
Fabricated proposed coupler has been shown in Fig.10 to
Fig. 12. The fabricated coupler has two essential parts; the
main waveguide and cavity and the other part is secondary
port that inner conductors have been c
rectangular conductor. The main waveguide cavity, and
flange are made integrated. This technique can reduce the
loss of coupler. The proposed coupler is made of with copper
Fig. 12. Conductor with SMA connectors
Fig. 11. Main part of proposed
conductor
between simulated and measured result of power reflected by
n in Fig. 8.
ABRICATION CONSIDERATION
Fabricated proposed coupler has been shown in Fig.10 to
The fabricated coupler has two essential parts; the
main waveguide and cavity and the other part is secondary
that inner conductors have been connected through
rectangular conductor. The main waveguide cavity, and
are made integrated. This technique can reduce the
loss of coupler. The proposed coupler is made of with copper
that is very low loss material. The comparison between
simulation and measurement result of ohmic loss has been
clarified in Fig. 9. The ohmic loss for measured result is
less
than 0.25 dB that is very excellent and it can be said that
ohmic loss of the proposed coupler is very low. This an
important factor in high powe
low loss coupler.
IV. C
Design and fabrication of compact waveguide loop coupler has
been presented in this paper. 40 dB coupling with ripple less than
0.7 dB are the properties of proposed coupler. Directivity of the
proposed coupler in 25% of Cthan 20 dB and more than 16 dB for
entire CGHz). The integrated of significant parts of coupler and
copper material for fabrication leads to low loss result in
measurement. It can be concluded the good candidate for
communication applications.
ACKNOWLEDGMENT
The authors sincerely appreciate help of Mr. Naderi and Tadayon
for their kind assistance in the design procedure.
REFERENCES
[1] T. Tanaka, “Ridged-shaped nTE10, TE20, and TE30 modes,” vol,
MTT 28, pp. 239-245, 1980.
[2] R. Levy, “Directional couplers,” in Advances in Microwave,
vol. 1, L. Young, Ed. New York, Academic Press, 1966.
[3] G. G. Gentili, L. Lucci, R. Nesti, G. Pelosi, and S.
Selleri, “ A novel design for a circular waveguide directional
coupler”, Microwave Theory Tech, Vol 57, N0,7 pp. 1840
[4] H. Y. Yee, “ Slotted waveguide directional coupler
characteristics IEEE Trans. Microwave Theory TechJul 1990.
[5] R. Collin, “ Filed Theory of Guided Waves,” Second Edition,
IEEE Press 1990, pp .499-523
[6] J. Kulinski, “Waveguide loop couplerwith flat coupling
characteristic,” Prace PIT, No 103, pp. 10-16 1984
[7] J. Kulinski, “ Waveguide loop couplers design,” Prace PIT,
No 107, pp. 30-37, 1987.
[8] Condon, E. U., “Forced oscillations in cavity resonators,”
Applied Physics, Vol. 12, pp. 129
[9] Jakes, Jr, W. C., “Analysis of couapplication to the design
of diode switch.” Theory and Tech, Vol. 14, pp. 189
Fig. 12. Conductor with SMA connectors
Main part of proposed coupler without
that is very low loss material. The comparison between
and measurement result of ohmic loss has been
clarified in Fig. 9. The ohmic loss for measured result is
less
than 0.25 dB that is very excellent and it can be said that
ohmic loss of the proposed coupler is very low. This an
important factor in high power microwave network to have a
CONCLUSION
Design and fabrication of compact waveguide loop coupler has
been presented in this paper. 40 dB coupling with ripple less than
0.7 dB are the properties of proposed coupler.
proposed coupler in 25% of C-band is more than 20 dB and more
than 16 dB for entire C-band (4 GHz- 6 GHz). The integrated of
significant parts of coupler and
material for fabrication leads to low loss result in
measurement. It can be concluded the fabricated coupler is good
candidate for communication applications.
CKNOWLEDGMENT
authors sincerely appreciate help of Mr. Naderi and Tadayon for
their kind assistance in the design and fabrication
EFERENCES
shaped narrow wall directional coupler using TE10, TE20, and
TE30 modes,” IEEE Trans. Microwave Theory Tech,
245, 1980.
R. Levy, “Directional couplers,” in Advances in Microwave, vol.
1, L. Young, Ed. New York, Academic Press, 1966.
ili, L. Lucci, R. Nesti, G. Pelosi, and S. Selleri, “ A novel
design for a circular waveguide directional coupler”, IEEE
Trans.
, Vol 57, N0,7 pp. 1840-1849, Jul 2009.
H. Y. Yee, “ Slotted waveguide directional coupler
characteristics ”, IEEE Trans. Microwave Theory Tech, Vol 38, N0,10
pp. 1497-1502,
R. Collin, “ Filed Theory of Guided Waves,” Second Edition,
IEEE
J. Kulinski, “Waveguide loop couplerwith flat coupling
characteristic,” 16 1984
J. Kulinski, “ Waveguide loop couplers design,” Prace PIT, No
107, pp.
Condon, E. U., “Forced oscillations in cavity resonators,”
Journal of , Vol. 12, pp. 129-132, Feb. 1941.
Jakes, Jr, W. C., “Analysis of coupling loops in waveguide and
application to the design of diode switch.” IEEE Trans. On
Microwave
, Vol. 14, pp. 189-200, Apr. 1996.
