International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 1021
ISSN 2229-5518
Optimized Broad Band Riblet Short-Slot Waveguide Coupler for X-Band Applications A. A. Sarhan, M. Tayarani, H. Oraizi, N. Ghadimi, I. Hamidi
Components.
—————————— ——————————
results.
Directional waveguide couplers are found in many microwave systems. They can be designed in diverse technologies and can take many configurations. This wide range of options is motivated by the diverse functions that they can carry out [1], [2]. For instance, one common application is power monitoring or power division/combination [3] in balanced amplifiers and mixers. They can also be found in beam-forming networks for multi-beam array antennas. In this context, they are used in Butler matrices [4]. Six-port Riblet couplers have been also designed in [5], with equal power division for the three output ports. From a CAD point of view, the structure is an H-plane structure (the E-plane case would be dual) and its analysis and design can be carried out very efficiently by general techniques such as the Finite Element Method (FEM) or by modal techniques such as the Boundary Integral-Resonant Mode Expansion (BI-RME) [6] or the Boundary Contour Mode-Matching method (BCMM) [7].
The most suitable configuration for a specific application is determined by the frequency of operation, bandwidth, insertion losses and power handling capabilities [1], [2]. In addition, the selected structure must be able to provide the desired coupling and its size must comply with the mass and volume restrictions of the system. From this last point
The physical structure of Riblet short-slot coupler Fig. 1 consists of two waveguides with a common sidewall. Coupling takes place in the region where part of the common wall has been removed. In this region, both the TE10 (even) and the TE20 (odd) mode are excited, and by proper design can be made to cause cancellation at the isolated port and addition at the coupled port. The width of the interaction region must generally be reduced to prevent propagation of the undesired TE30 mode. For a 3 dB coupler, the length of the coupling region must be greater than half wavelength at center frequency.
Fig. 1. Riblet short-slot coupler (RSC) [9].
The General Scattering Matrix (GSM) for a four port junction Fig. 2 is
of view, the short-slot Riblet coupler [8] presents very
S11
S12
S13
S14
S S S S
interesting properties, because of its compact size and less
S = 21 22 23 24
(1)
S31
S32
S33
S34
weight. It can be used to implement the hybrid junction
used in many microwave circuits, with high isolation and
low return loss. It is usually shorter than other types of
couplers such as the branch-line or the multiple-slot
configurations, which is a significant advantage for spatial applications. Another relevant feature is its simple manufacturing, since it can be implemented in E/H-plane configuration. This contribution is focused on H-plane type of couplers, showing optimized design using the CST2011 software for modeling and compared with the measured
S S S S
41 42 43 44
• Ph.D. Candidate, Dept. of Electrical Eng., Maleke Ashtar Univ. of Tech., Islamic
Republic of Iran. Email: alaaaldinn@Gmail.com
• Professor, Dept. of Electrical Eng., Iran Univ. of Sience and Tech. Islamic
Republic of Iran. Email h_oraizi@iust.ac.ir
• Assistant Professor, Dept. of Electrical Eng., Maleke Ashtar Univ. of Tech.,
Islamic Republic of Iran. Email: Ghadimi@Gmail.com
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International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 1022
ISSN 2229-5518
Fig. 2. Quadrature Hybrid Junction.
In a theoretical ideal quadrature hybrid junction, port 1 (P1) is isolated from port 4 (P4) and so are port 2 (P2) and port 3 (P3). Therefore, these elements in the matrix are
S14 = S41 = S23 = S32 = 0
( 2)
Fig. 3. (a) Simulated structure in CST, (b) Front profile, (c) Side profile.
TABLE 1
If all the ports are matched, the diagonal elements therefore
equal zero
OPTIMIZED DIMENSIONS OF RSC.
S11 = S22 = S33 = S44 = 0
(3)
The junction is reciprocal. This makes the rest of the matrix’s elements symmetrical, therefore they become
S12 = S21 = S24 = S42 = 1 2
S13 = S31 = S24 = S42 = − j 2
( 4)
The resulted matrix becomes
0 1 − j 0
S = =1
1 0 0
− j 5
The fabricated RSC Fig. 4 is measured by Network
2 − j
( )
0 0 1
Analyzer (NA) and waveguide calibration is used. Return
loss parameters are shown in Fig. 5. This figure shows a
0 − j
1 0
good agreement between simulated and measured S- parameters (<-23.5 dB) in the frequency range 8-10 GHz.
If port 1 (P1) is taken as input port, one can deduce that
Transmission parameters are shown in Fig. 6. It shows
output signals from port 2 (P2) and port 3 (P3) are with
equal amplitudes, but with 90o phase difference.
about
−3.2 ± 0.3 dB.
In practice, a capacitive dome (tuning screws) Fig. 3(a), (b) and (c) is placed in the coupling region in order to adjust the 900 phase difference (phase balance between modes TE10 and TE20 ). Nevertheless, the structure can be also designed without these elements [10], which may simplify the manufacturing. Full wave simulation and optimization are done using the CST2011. Optimized dimensions are shown in Table. 1.
Fig. 4. Photograph of the manufactured device.
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International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 1023
ISSN 2229-5518
Fig. 5. Simulated and measured Return Loss (RL (dB)) parameters.
Fig. 6. Simulated and Measured Transmission Parameters (TL (dB)).
Fig. 7. Simulated and Measured Isolation (dB).
Isolation is shown in Fig. 7. It shows that isolation is better than 25 dB (<-25 dB). Phase difference is shown in
Fig. 8. Simulated and Measured phase balance (Degrees).
RSC in X-band has been presented with bandwidth about 22.2 %. Its main properties and modeling has been described, with special emphasis on their electrical properties. The experimental results have been compared with the simulations results.
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[6] G. Conciauro, P. Arcioni, M. Bressan, L. Perregrini, “Wideband modeling of arbitrarily shaped H-plane waveguide components by the Boundary Integral-Resonant Mode Expansion method”, IEEE Tran. on Microwave Theory and Techniques, vol. 44, no. 7, pp. 1057-1066, July 1996.
[7] J. A. Ruiz-Cruz, J. M. Rebollar, “BCMM Analysis of short-slot waveguide couplers with an FFT algorithm”, Proc. IEEE 2003 APS Int. Symp., vol. 2, pp. 1193-1196, June 2003.
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Fig. 8, which shows a phase balance about
−900 ± 1.30.
[10] L.T. Hildebrand, “Results for a simplecompact narrow-wall directional coupler,”, IEEE Microwave and guided wave letters, vol. 10, no. 6, pp 231-232, June 2000.
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