Design of a W-band Orthomode Transducer

저자(Authors)

Jae-Ho Cha, Myung-Sook Jung, Bierng-Chearl Ahn

출처(Source)

한국정보기술학회논문지 14(3), 2016.3, 91-96 (6 pages)Journal of Korean Institute of Information Technology 14(3), 2016.3, 91-96 (6 pages)

발행처(Publisher)

한국정보기술학회Korean Institute of Information Technology

URL http://www.dbpia.co.kr/Article/NODE06644877

APA Style Jae-Ho Cha, Myung-Sook Jung, Bierng-Chearl Ahn (2016). Design of a W-band OrthomodeTransducer. 한국정보기술학회논문지, 14(3), 91-96.

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Journal of KIIT. Vol. 14, No. 3, pp. 91-96, Mar. 31, 2016. pISSN 1598-8619, eISSN 2093-7571 91

Design of a W-band Orthomode Transducer

Jae-Ho Cha*, Myung-Sook Jung**, and Bierng-Chearl Ahn***

Abstract

An orthomode transducer (OMT) is widely employed to launch two perpendicularly polarized waves in a waveguide for frequency reuse and for polarization diversity applications. In this paper, a circular waveguide orthomode transducer operating at W-band is designed, fabricated and tested. First, an OMT is designed by transforming a circular waveguide into a rectangular waveguide with a multi-step transformer to couple one polarization and by introducing a small longitudinal slot in the circular waveguide wall and impedance matching steps to couple the other orthogonal polarization. The designed structure is amenable to fabrication in two split blocks by precision milling operations. Next, the designed OMT is fabricated to required precision after many trials of milling operations. Tests of the fabricated OMT shows good performance: reflection coefficients of less than -20dB at all ports, insertion loss of less than 0.5dB and isolation of 30dB.

요 약

직교모드변환기 (OMT)는 주파수 재사용과 편파 다이버 등의 용도로 도파관 내부에 두 개의 직교하는 파를

진행시키기 위해 널리 사용된다. 본 논문에서는 W-대역에서 동작하는 원형도파관 OMT를 설계, 제작, 측정하

였다. 첫째로 한 개 편파를 결합시키기 위해 다수의 계단형 변환기를 사용하여 원형 도파관을 사각형 도파관

으로 변환하였으며 원형 도파관 벽에 종방향의 소형 슬롯과 임피던스 정합용 계단을 설치하여 나머지 직교하

는 편파성분을 결합시킴으로써 직교모드변환기를 설계하였다. 설계된 구조는 두 개의 분리된 블록으로 가공하

기에 적합하다. 둘째로 여러 번의 밀링작업을 반복하여 요구되는 정확도가 얻어지도록 OMT를 가공하였다. 제

작된 OMT를 측정한 결과 모든 포트에서 -20dB 이하의 반사계수, 0.5dB 이하의 삽입손실 및 30dB의 분리도

등 우수한 성능을 확인하였다.

Keywordsorthomode transducer (OMT), W-band, waveguide, precision machining, measurement

* Chungbuk National University ** Agency for Defense Development*** Chungbuk National University / President of Vector Systems Received: Feb. 29, 2016 Revised: Mar. 21, 2016 Accepted: Mar. 24, 2016

ž Corresponding Author: Bierng-Chearl Ahn Director of Applied Electromagnetics Lab., Chungbuk National University, Cheongdae-ro 1, Cheongju City, Chungbuk Province, 361-763, Korea Tel.: +82-43-261-3194, Email: bician@cbu.ac.kr

http://dx.doi.org/10.14801/jkiit.2016.14.3.91

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92 Design of a W-band Orthomode Transducer

I. Introduction

An orthomode transducer (OMT) is a device for launching or receiving two perpendicularly-polarized waves in a circular or square waveguide. It is employed often in communication systems[1], radars [2] and radio astronomy systems[3]. OMT's are used from the L-band[3] to the millimeter-wave[1] and the terahertz range[2]. OMT's have been actively investigated by domestic researchers[4]-[7] as well as by international research groups[1]-[3].

In the majority of existing works on the OMT, a square waveguide is employed for the common port. In [8][9], a circular waveguide OMT is presented where the side port is coupled via a small aperture while the straight port is transformed into a rectangular waveguide with a multi-step transformer. The OMT's presented in [8][9] do not work at high millimeter-wave frequencies so that the problem of precision fabrication has not been studied.

In this paper, a circular waveguide OMT operating at W-band is designed, fabricated and tested. The designed structure is amenable to split-block fabrication with high precision [10]. After an optimum design is obtained using a commercial software (CST's Microwave Studio), the proposed OMT is fabricated and its performance is measured. The following sections provide detailed descriptions of the proposed OMT.

II. Design

Fig. 1 shows the structure of the OMT proposed in this paper. The circular waveguide g1 has a vertically polarized wave at the port 1v and a horizontal one at the port 1h. The vertically polarized wave is transmitted to the port 3 of the rectangular waveguide g3, while the horizontally polarized wave is transmitted to the port 2 of the rectangular waveguide g2. Fig. 2 shows the proposed OMT seen from different directions.

The operating principles of an orthomode transducer are as follows. For the transmission of a vertically polarized wave from the circular waveguide g1 to the rectangular waveguide g3, the circular cross section is transformed into the rectangular shape by sections a, b, and c. The horizontally polarized wave is coupled to the waveguide g2 via the longitudinal slot d. Sections e and f serve as an impedance matching structure. The section c is chosen so that the horizontally polarized wave is cutoff. The coupling slot e suppresses the coupling of the vertically polarized wave since its width is far below cutoff for the vertically polarized wave.

The isolation between the coupled ports is ensured by making coupling apertures or slots geometrically perpendicular to each other. Multi-step impedance transformers are employed to smoothly transform a coupling aperture or a slot into an output waveguide.

The proposed OMT is designed in the following steps. First, a transition from the circular waveguide g1 to the rectangular waveguide g3 is designed with sections a, b and c. Next, sections c, d and e, and the waveguide g2 are added and the whole structure is optimized for low reflection and low loss.

Fig. 1. Structure of the proposed OMT

Fig. 2. Proposed OMT seen from different directions

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Journal of KIIT. Vol. 14, No. 3, pp. 91-96, Mar. 31, 2016. pISSN 1598-8619, eISSN 2093-7571 93

Fig. 3. Simulated reflection coefficients of the fabricated

OMT

Fig. 4. Simulated transmission coefficients of the fabricatedOMT

Table 1. Dimensions of the designed OMT (unit: mm)

Name Symbol Dimensions

Circular waveguide diameter a1 2.20

Broad wall of rectangular

waveguides g2 and g3a2 2.54

Narrow wall of rectangular

waveguides g2 and g3b1 1.27

Height of section a mt2 2.00

Length of section a ml2 1.00

Height of section b mt1 1.60

Length of section b ml1 0.93

Height of section b st1 1.22

Length of section c sl1 0.60

Width of section c sw1 2.05

Height of section d st2 0.44

Width of section d sw2 0.48

Length of section d sl2 1.61

Height of section e mt3 0.63

Width of section e mw1 0.64

Length of section e ml3 2.22

Height of section f mt4 0.70

Width of section f mw2 1.05

Length of section f ml4 2.54

Radius of the bend in

rectangular waveguide g2ba 5.40

Fig. 3 shows the reflection coefficient of the designed OMT, which is less then -20dB at all the ports. Fig. 4 shows the transmission coefficients between the orthogonally polarized waves, which is very low in each case. Table 1 shows the dimensions of the designed OMT.

III. Fabrication and Measurement

The designed OMT is fabricated using a precision milling machine with specially designed miniature drills in a split block form [10]. Fig. 5 shows the fabricated OMT. Due to tight tolerances required (±0.02mm), many trial matching operations have been carried out until satisfactory accuracies were obtained.

(a) Split blocks seen at the mating faces

(b) Split blocks seen at waveguide pots

(c) Assembled unit seen at waveguide ports

Fig. 5. Fabricated OMT

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94 Design of a W-band Orthomode Transducer

Fig. 6. Setup for the measurement of the fabricated OMT

(a) Vertically polarized wave at the inline port

(b) Horizontally polarized wave at the side port

Fig. 7. Measured reflection coefficients of the fabricated

OMT

Fig. 6 shows a setup for the measurement of the fabricated OMT. The HP8501C vector network analyzer with a 110GHz extension module has been used in the measurement, where a rectangular waveguide 90-degree twist, a circular-to-rectangular waveguide transition, a rectangular waveguide bend, and matched loads in the circular and rectangular waveguides are also employed.

(a) Vertically polarized wave

(b) Horizontally polarized wave

Fig. 8. Measured transmission coefficients of the fabricated

OMT

Fig. 7 shows the measured reflection coefficients of the fabricated OMT. The reflection coefficient of Fig. 7(a) is less than -22.5dB at the inline port (port 3 in Fig. 1) over 2% bandwidth centered at f0 , while it is less then -31dB in Fig. 7(b) at the side port (port 2 in Fig. 1). Ripples in the measured data are mostly due to mechanical mating inaccuracies at waveguide flanges.

Fig. 8 shows the transmission coefficients (equivalent to the insertion losses) of the fabricated OMT. Again ripples in the measured curves can be observed which are due to flange mating inaccuracies. The insertion loss from the inline port to the common port is about 0.15dB while it is about 0.50dB from the side port to the common port. The increased insertion loss at the side port is due to a longer waveguide run with a cut in the narrow wall caused by split-block fabrication.

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Fig. 9. Transmission coeffient between isolated ports of

the fabricated OMT

Fig. 9 shows the transmission coefficient (equivalent to the isolation) between the inline port and the side port. The fabricated OMT has an isolation of 30dB over 2% bandwidth. The measured isolation is not as high as predicted in Fig. 4 due to finite precision in fabrication.

Results in Fig. 7 - Fig. 9 show that the proposed OMT structure performs well even at the W-band frequencies where small tolerances in the fabrication may significantly degrade the OMT's performance.

IV. Conclusions

In this paper a circular waveguide OMT operating at W-band has been presented, which is amenable to precise split-block fabrication by numerically controlled milling operation. The fabricated OMT shows good impedance matching, low insertion loss and high isolation. The reduced isolation of the fabricated OMT is due to imperfections in the machining. The proposed OMT can be applied to communication and radar systems operating at W-band.

References

[1] O. A. Perverini, R. Tascone, G. Virone, A. Olivieri, and R. Orta, "Orthomode transducer for millimeter-wave correlation receivers", IEEE Trans. Microw. Theory Tech., Vol. 54, No. 5, pp. 2042- 2049, May 2006.

[2] C. A. Leal-Sevillano, K. B. Cooper, E. Decrossas,

R. J. Dengler, J. A. Ruiz-Cruz, J. R. Montejo- Garai, G. Chattopadhyay, and J. M. Rebollar, "Compact duplexing for a 680-GHz radar using a waveguide orthomode transducer", IEEE Trans. Microw. Theory Tech., Vol. 62, No. 11, pp. 2833-2842, Nov. 2014.

[3] G. Valente, G. Montisci, T. Pisanu, A. Navarrini, and P. Marongiu, "A compact L-band orthomode transducer for radio astronomical receivers at cryogenic temperature", IEEE Trans. Microw. Theory Tech., Vol. 63, No. 10, pp. 3218-3227, Oct. 2015.

[4] Y. M. Kim, Y. S. Jung, J. W. Noh, L. J. Zhang, and B. C. Ahn, "Design of 70/80GHz broadband orthomode transducer", J. Korean Inst. Inform. Tech., Vol. 13, No. 7, pp. 43-50, July 2015.

[5] J. K. Lee, B. Y. Chae, D. H. Park, and B. C. Ahn, "Design of a Ka-band orthmode transducer", J. Korean Inst. Electromag. Eng., Vol. 15, No. 1, pp. 110-118, Jan. 2004.

[6] J. W. Li, J. Y. Kim, H. W. Ko, D. H. Park, and B. C. Ahn, "Design of a narrow-band orthomode transducer operating at Ku-band", J. Korean Inst. Electromag. Eng., Vol. 15, No. 12, pp. 1123-1130, Dec. 2004.

[7] S. M. Hwang, Y. M. Kim, S. G. Lee, and B. C. Ahn, "Design of an orthomode transducer for use in multi-band antenna feeds", J. Korean Inst. Electromag. Eng., Vol. 13, No. 1, pp. 53-59, Jan. 2002.

[8] R. C. Gupta, K. K. Sood, and R. Jyoti, "Compact and high performance stepped truncated-circular waveguide branching ortho-mode transducer (STCWB-OMT)", Prog. Electromag. Res. Lett., Vol. 25, pp. 135-141, July 2011.

[9] A. O. Perov, L. A. Rud, and V. I. Tkachenko, "Orthomde transducers with a common circular waveguide", J. Comm. Tech. Electon., Vol. 52, No. 6, pp. 626-632, Aug. 2005.

[10] T. W. Crowe, P. H. Koh, W. L. Bishop, C. M. Mann, J. L. Hesler, R. M. Weikle II, P. A. D.

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96 Design of a W-band Orthomode Transducer

Wood, and D. Matheson, "Inexpensive receiver components for millimeter wave and submillimeter wavelengths", Proc. 8th Int. Symp. Space Terahertz Tech., pp. 377-384. Mar. 1997

Authors

Jae-Ho Cha

2007 : BS, Electrical Eng.,

Dankook University

2015 ~ Present : MS, Radio and

Comm. Eng., Chungbuk

National University

Research interests : Antennas,

design of RF circuit

Myung-Sook Jung

1999 : BS, Electrical Eng.,

Kyungpook National University

2001 : MS, Electrical Eng., Pohang

University of science and

technology

2001 ~ Present : Senior researcher

of Agency for Defense

Development

Research interests : Antennas, design of RF circuit,

radiometer system

Bierng-Chearl Ahn

1981. 2 : BS, Electrical Eng., Seoul

National University

1983. 2 : MS, Electrical Eng.,

Korea Advanced Inst. Sci. Tech.

1992. 12 : Ph. D., Electrical Eng.,

University of Mississippi.

1983 ~ 1986 : Researcher,

Goldstar Precision Co.

1992 ~ 1994 : Research Associate, Agency for Defense

Development

1995 ~ Present : Professor, Dept. of Radio and Comm.

Eng., Chungbuk National University

2010 ~ Present : President of Vector Systems

Research interests : Antennas, applied EM

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