Millimeter Wave In-line Coaxial-to-Rectangular Waveguide Transition

Meenakshi Durga, Seema Tomar, Sushil Singh, Lalit Suthar Defence Electronics Application Laboratory, DRDO

Dehradun, India durgamnu88@gmail.com

Abstract— The paper deals with the design of an in-line coaxial to rectangular waveguide transition at millimeter wave frequency band. The design characteristics include low insertion loss, easy integration, simple geometry and ease of fabrication in a cost effective manner. The design simulation and optimization is based upon CST Microwave Studio.

Keywords- Millimeter Wave; in-line; coaxial to waveguide; transition; ridge

I. INTRODUCTION

At Millimeter Wave frequencies, RF circuits usually employ waveguides as its size is no longer a consideration and its losses are low. Filters, diplexers and antennas often use waveguides in conjunction with micro-strip and coaxial lines [1]. Therefore a coaxial to waveguide adapter forms an important part in the design and/or testing of microwave circuits. Such transitions have been widely designed for ridge waveguides as ridge waveguide is generally used for wide frequency band and has smaller equivalent impedance which is easier to match with the coaxial and micro-strip line [2]. The traditional design of a coaxial to rectangular waveguide transition consists of a probe entering normal to the H-plane of a waveguide. The frequency and bandwidth of the transition depends on the shape and size of the probe [3]. The design of this transition is based on mode-matching techniques [4]. Such a design is difficult to integrate when multiple systems are being connected as the coaxial connector is at 90 degrees to waveguide. Therefore an in-line coaxial to rectangular waveguide adapter is preferable. In this paper, the design of an inline coaxial-to-rectangular waveguide transition for 36 GHz to 40 GHz frequency range is described.

II. DESIGN CONCEPT

Fig. 1(a) and Fig. 1(b) show the side view and top view of a typical design of an inline coaxial to rectangular waveguide as described by the author of [5]. The first region in the rectangular waveguide to coaxial line is known as rectax [5] where the centre pin of the coaxial line is inserted. This region is followed by two or more steps in the rectangular

waveguide. For the ease of fabrication, the rectax is replaced by a step and step width is kept same for all the steps in the proposed design as shown in Fig. 2 [6]. A gap from the wall of the waveguide to the wall of the first step is optimized according to the length of the center pin of COTS available coaxial bead.

.

Figure 1. Typical design: (a) Side view, (b) Top view of in-line coaxial to rectangular waveguide transition

Figure 2. Modified design: (a) Side view, (b) Top view of in-line coaxial to rectangular waveguide transition

1(b)

1(a)

Rectax

Step Step

Vacuum

Vacuum

Vacuum

L0 L1 L2 L3

H1 H2 H3

2(a)

2(b)

Step1 Step2 Step3

T

L

Vacuum

Vacuum

Vacuum

978-1-4577-1099-5/11/$26.00 ©2011 IEEE

Figure 3. Simulated S-parameters for the proposed in-line coaxial to waveguide transition

Design of any transition is based on two key factors [6]. The first factor is mode conversion from dominant mode of input transmission medium to that of output transmission medium whereas the second factor is impedance matching which is required to achieve low insertion loss. In coaxial to rectangular waveguide transition, TEM mode in coaxial line is to be converted to TE10 mode in waveguide and an impedance transformation is required to match the impedances between the two. The characteristic impedance of rectangular waveguide is a function of frequency. Therefore a multi-step matching transformer is needed to match the 50 ohm coaxial line impedance to the rectangular waveguide impedance over the desired frequency range [7].

In the design, the parameter L should be of the order of wavelength of the desired frequency range. The gap L0 has to be optimized with precision as it is one of the critical parameters which define mode conversion. The length (L1, L2, L3) and the height (H1, H2, H3) of the ridges are optimized for matching impedance so that the wave sees a gradual transition of impedance from centre pin of coaxial connector to waveguide. As the width T increases, the return loss over the desired range improves and the minima of the curve shifts towards lower frequency. There is an optimum value beyond which no further gain in return loss can be achieved. From the simulations it was observed that this value is one-fourth of the order of wavelength.

III. SIMULATION RESULTS

The design has been simulated using CST Microwave Studio [8]. The simulated results show that the insertion loss from 35 GHz to 41 GHz is less than 0.5 dB and return loss is better

than 10 dB. Bandwidth of 4 GHz (10%) in Ka-band has been achieved using this transition.

Figure 4. 3-D view of proposed in-line coaxial to rectangular waveguide

transition

Figure 5. Sensitivity anlysis with tolerance limits to ± 40 micron for the proposed transition.

IV. EASE OF FABRICATION

The main advantage of the design is that it can be fabricated as a single piece metallic structure where the internal dimensions are calculated according to the available manufacturing capabilities. The 3-D view is shown in Fig. 4. The corner radius of waveguide incorporated in the design is to remove the need of any Electric Discharge Machining (EDM) operations. Sensitivity analysis results are shown in Fig. 5. The design is insensitive to fabrication accuracies within ± 40 micron.

V. CONCLUSION

The paper describes a unique design approach of coaxial to waveguide transition at Millimeter Wave frequencies. The design has simple geometry, low insertion loss, cost effective, easy to integrate and fabricate.

ACKNOWLEDGMENT

The authors express their sincere thanks to Director DEAL, Group Director (MMW Systems), Associate Group Director (MMW System) for permission to publish the paper.

REFERENCES [1] Peter Delmotte, “Waveguide-Coaxial Line Transitions,” , ON4CDQ

Belgian Microwave Roundtable, 2001. [2] Nie Rui-xing, Li En, Guo Gao-feng and Wang Yi, “Simulation and

Design of 18-40GHz Ridge Waveguide to Coaxial Transition, “IEEE International Conference on Microwave Technology and Computational Electromagnetics.” pp. 183-185, 2011.

[3] “Introduction to transmission lines and waveguides“, www.techlearner.com/Apps/TransandGuides.pdf.

[4] R. B. Keam, and A. G.. Williamson, “Broadband design of coaxial line/rectangular waveguide probe transition,” IEE Proc. Microwaves, Antennas and Propagation, vol. 141, no. 1, pp. 53-58, 1994.

[5] Ralph Levy , Louis W.Hendrick ,“ Analysis and Synthesis of In-Line Coaxial to Waveguide Adapters,” Microwave Symposium Digest, IEEE MTT-S International, pp. 809-811, 2002.

[6] Yang Zhou, En Li, Gao-Feng Guo, Tao Yang, Lin-Sheng Liu, “Design of Millimeter Wave Wideband Transition From Double-ridge Waveguide to Coaxial Line,” Journal of Infrared, Millimeter and Terahertz Waves, vol 32, no.1, pp. 26-33, 2010.

[7] Wang Yi, En Li, Gaofeng Guo, and Ruixing Nie, “An X-band Coaxial-to-Rectangular Waveguide Transition,” IEEE International Conference on Microwave Technology and Computational Electromagnetics (ICMTCE) , pp. 129-131, 2011.

[8] CST Software and User’s Manual.