Design of 3dB Directional Coupler for Ka-Band Input Multiplexer of Satellite Payload Applications

Pil-yong Lee1, Duck-ki Baek1, Jong-hee (Martin) Park2, Eun-seok Choi2

1 R&D Center, PILAS Co., Ltd., Yong-in City, Republic of Korea 2 R&D Center, QNION Co., Ltd., Dae-jeon City, Republic of Korea

Abstract – In this paper, we have studied that the easily

implementation of low insertion loss, good matching, and high isolation of 3dB directional coupler required in Ka band Input Multiplexer, in a type of Riblet Short Slot(RSC) using Coupling Matching Stub(CMS) and it successfully applied to Input Multiplexer (IMUX) for satellite applications.

Index Terms — Riblet Short Slot(RSC), Coupling Matching Stub(CMS), 3dB Directional Coupler, Waveguide, Satellite

1. Introduction

The 3dB coupler is used in a variety of applications in the system, and its type and shape are various as well. In this paper, we obtain result of design and production of Ka band 3dB directional coupler used for Input Multiplexer(IMUX) for satellite, and we applied it to IMUX to obtain the system’s required results. This 3dB directional coupler applied to this paper is based on the classic Short Slot topology [2] and combinations the Step Impedance Matching method [3] by adopting the Riblet Short Slot (RSC) topology [5]. Also, Coupling Matching Stub(CMS) was added to improve isolation and return Loss, named it CMS-RSC, and successfully manufactured and tested.

2. Design of Directional Coupler by CMS-RSC

As directional coupler basically consists of four ports, and typical Riblet Short Slot coupler has the shape shown in Fig 1.

Fig. 1. Riblet Short Slot Coupler [4].

φ is the electrical length of the slot and is 90° and the

relationship with L which is mechanical length of the slot is as following formula.

(1)

Here, λge and λgo each mean the even mode wavelength and

odd mode wavelength of coupler, the formulas can be referenced in Reference [3]. Step impedance matching requires at least one step in the RSC coupler and the larger

the number of steps, the wider the usable bandwidth can be secured. The Coupling Matching Stub, CMS is located at the center of coupler and has a vertical symmetrical structure. Fig. 2 shows the design parameters of the CMS-RCS schematically.

Fig. 2. Design Parameters of CMS-RSC.

The bandwidth of the coupler is set from 19.0GHz

~20.05GHz, which is the bandwidth of IMUX. The diameter of DCMS, which is the diameter of CMS, is 4mm, and the characteristics of the coupler can be easily secured according to the CMS size. Other parameter values are as follows, Unit[mm]: W1=11.8, W2=10.9, W1=11.1, W1=10.0, L1=9.1, L2=7.2, L3=4.0, L4=8.4. Also, CMS size=(H-HCMS)/2, which means the length of the actual CMS inserted into the waveguide.

3. Simulation and Measurement

Fig. 3 shows the analysis results for both simulation model and E-field of coupler. It can be ascertained that when E-field is branched in the slot, the CMS pulls a part of the energy toward the coupling port. The results of simulating the characteristics of the coupler each CMS size, and the manufacturing test results are summarized in TABLE 1. The coupler was made with CMS size=0.3mm. The reason is that the best characteristics such as Isolation and Return Loss are 0.4mm, but the amplitude of 0.3mm is most equal in the Thru port and Coupling port.

Fig. 3. Simulation Modeling and E-field.

CMS

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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TABLE I Simulation Result as CMS Size and Test Result

CMS size (mm)

Characteristics [dB] @ 19500[MHz] (‘-’ is omitted) Thru Coupling Return Loss Isolation

0.0 2.88 3.18 25.6 25.5

0.1 2.90 3.14 27.8 27.7 0.2 2.94 3.09 30.7 31.2 0.3 3.01 3.00 35.9 35.8 0.4 3.11 2.91 36.0 36.3

Test Result 3.10 3.03 30.0 29.3

Simulation uses CST2015 and Fig. 4 ~ Fig. 6 display the

S-parameter characteristics in graph from the simulation and test according to CMS size. The longer the CMS size is, the more power is transferred to the coupling side and the return loss and isolation characteristics are improved.

Fig. 4. Thru & Coupling characteristics for the simulation

and test.

Fig. 5. Return loss characteristics for the simulation and test.

Fig. 6. Isolation characteristics for the simulation and test.

Fig. 7. Photograph of the 3dB Directional Coupler.

The photograph of the fabricated 3dB directional coupler is shown in Fig. 7, the input and output waveguide is WR51, and the transition for connecting to the internal step impedance matching circuit is inserted to each ports. The surface treatment is based on the MIL-C-5541, Class 3 and the size is about 95×52×30.2mm.

4. Conclusion

The Ka band 3dB directional coupler by utilizing CMS-RCS, for application to IMUX for satellite, was successfully designed and manufactured, and it applied to IMUX for satisfying the performance demanded by the sub-system. Fig. 8 is a graphical representation of the manufacturing photograph and channel characteristics of an Engineering Model(EM) of Ka band IMUX with 8 channels in total. In the future, we will develop the Engineering Qualification Model(EQM) of a space qualified Ka-band IMUX and secure the reliability of Ka-band 3dB directional coupler for development quality and space environment.

Fig. 8. Photograph and Test Result of the Ka band IMUX

Acknowledgement

The research was supported by the Space Technology Development Program through the National Research Foundation of Korea(NRF) funded by the ministry of Science and ICT (Grant No. 2017M1A3A3A04014519).

References

[1] G. Matthaei, L. Young and E. Jones, “Microwave Filters, impedance-matching networks and coupling structures,” New York: McGraw- Hill, 1964, pp.775-842.

[2] D. M. Pozar, “Microwave engineering,”. 3rd ed. Wiley, 2005, pp.362-379.

[3] M. Mohammadi and F. H. Kashani, “Planar Eight Port Waveguide Mono-pulse Comparator,” Progress In Electromagnetics Research C., vol. 6, pp. 103-113, 2009.

[4] B. Hussain, “Short-Slot Hybrid Coupler in Gap Waveguides at 38 GHz,” in Proc. Master of Science Thesis in Wireless and Photonics Engineering 2011 by Chalmers University, pp. 1-62.

[5] A. A. Sarhan, “Optimized Broad Band Riblet Short-Slot Waveguide Coupler for X-Band Applications,” International Journal of Scientific & Engineering Research., vol. 4, pp. 1021-1023, 2013.

[6] M. Mohammadi and F. H. Kashani, “A Compact Broadband Riblet-Type Three-Way Power Divider in Rectangular Waveguide,” IEEE Microwave and Wireless Components Letters., vol. 27, pp. 141-143, 2017.

[7] J. Ding, “E-Plane Five-Port Two-Way Waveguide Power Divider/Combiner with High Amplitude and Phase Consistency,” Progress In Electromagnetics Research Letters., vol. 66, pp. 113-119, 2017.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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