bethe hole coupler

International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013

ISSN: 2231-5381 http://www.ijettjournal.org Page 676

Design of 20 dB Bethe-hole Waveguide Coupler Mehul P Vanvi#1, Ved Vyas Dwivedi*2, Jaymin K Bhalani#3

#Communication System Engineering, Charusat University At & Po:Changa-388421, Dist-Anand,India

*C U Shah College of Engineering & Technology Wadhvan-363030, Dist-Surendranagar, India

Abstract: Design of a X-band WR-90 based 20 dB Bethe-hole Waveguide Coupler is chosen as main coupler Operating at 9GHz frequency. Theoretical design of this coupler has been done to find out the aperture position, radius of hole and Directivity. Theoretical results are verified with simulation result. ANSOFTs High Frequency Structure Simulation(HFSS) has been used for simulation. Results of simulation and the theoretical results are compared in this paper. Calculated results and simulation results are in close approximation. Although the Directivity is more sensitive function of frequency this comparison helps in fabrication of the coupler. Keywords: C-coupling coefficient, D- directivity.

I. INTRODUCTION A directional coupler of four-port passive device

commonly used for coupling a known fraction of the microwave power to a port( coupled port) in the auxiliary line while flowing from the input port to the output port in the main line. The remaining port is an ideally isolated port and matched terminated. Bethe-hole coupler is one of the type of Waveguide directional coupler in which two waveguide coupled together through one hole in common broad wall as shown in fig (1).

Figure (1) Bethe-hole Coupler

II. DESIGN We have considered bethe-hole coupler of S aperture couple two parallel waveguides. Here in Design used WR-90 type waveguide which is used for X-band application. An aperture excites forward and reverse travelling waves with different amplitudes.The structure of bethe-hole coupler is symmetric with respect to all four ports. Basic definition of coupling and directivity giving by the following formula,

10

20 log AC dBA

(1)

10

10

20log AD dBA

(2)

Where, 10A and 10A

is the amplitude of forward and reverse travelling wave is

10 10 1010 10

1 1A E Jdv H MdvP P

22 2 20

0 2 2 210 10

sin sin cosmejwA s s sP a Z a a a

(3)

10 10 1010 10

1 1A E Jdv H MdvP P

2

2 2 200 2 2 2

10 10

sin sin cosmejwA s s sP a Z a a a

(4)

Where P10= a*b /z10 normalized power constant Note from (4&5)that the amplitude of the wave exited towards port 4 () is generally different from that excited toward port 3( 10A

) so

we can cancel the power delivered to port 4 by setting 10A =0.

If we assume that the aperture is round, then gives the

polarizabilities as 3

023er

and 3

043mr

, where r0

is the radius of the aperture. Then from (4) we obtain the following condition to find the position of aperture s,



0

2 2 2 2 20 0

2sin4 2

sa k a a

(5)

Design a Bethe-hole coupler of type shown in figure(1) for X-band waveguide WR-90 operating at 9Ghz,with a coupling of 20 dB. We have the following constant,a = 2.286cm, b =1.016cm,

0 =c / f = 3.333cm, k0= 2 / 0 = 1.886cm

-1, = 2 / g = 1.29cm-1,Z10 = k00 / = 550.9 ohm, P10= a*b / Z10=4.215 10-3 cm2/ohm,

Then (5) can be used to find the aperture position s, from (1) get aperture radius and for directivity equation (2) is used. For solving all these equation we get s=0.969cm, r0=0.415cm and directivity D= -62.95dB.

III. SIMULATION

Using the above result simulation model has been created in ANSOFTs HFSS-11. Waveguide WR-90 of 0.127cm wall thickness is chosen as main waveguide. Coupler has been designed for operating frequency of 9GHz over a 7-11GHz band. The common wall of main waveguide and auxiliary waveguide is also chosen as 0.127cm.

The structure has been simulated in HFSS by using driven model. Figure (2) shows the HFSS model of simulation. In this model we have taken WR-90 waveguide with Inside dimensions 2.2861.016 cm and Outside dimensions 2.5401.270cm. In which outside waveguide are taken as copper material and inside boxes are taken as vacuum. Also a hole in the center of the waveguide is take as vacuum material. The length of guide is in multiple of 0 . So for better result we get 5 time multiple of 0 .so we take 16.65cm the length of waveguide. Which have four wave port with two excitation mode in this design.

Figure (2) HFSS model of coupler

Also far field pattern of the E-filed is shown in figure(3) where we can see vector pattern for loss of some E-field.

Figure(3) E-field pattern

Here in below figure result is shown. Where,S31 is coupling coefficient and S41 is directivity. By Observing that coupling varies by less than 1dB over the band. The directivity is very large (>60dB)at the design frequency, but decreases 5-10 dB at the band edges.

Figure (4) S-parameter result of coupler in HFSS

IV. COMPARISON

In this case the radius of hole is taken same as calculated. The simulated results of HFSS is close to designed value. Table 1 shows the comparison of coupling coefficient and directivity of coupler with HFSS and calculated at 9GHz.



CODE Coupling coefficient

Directivity

HFSS -25.69dB -64.33dB

Calculated -21.62dB -62.95dB

Table 1: Comparative values of coupling coefficient and directivity at

9GHz.

V. CONCLUSION X-band 20dB Bethe-hole waveguide coupler is designed. Result of design calculation and simulation are closely matching. The theoretical calculations helped in engineering drawing of this coupler is ready and it is in the process of fabrication.

ACKNOWLEDGMENT The author is thankful to Dr. Niraj Shah, and Prof. Brijesh

Shah, for their support and encouragement during the research Endeavour. We would like to thank V. T. Patel Department of Electronics and Communication, CHARUSAT University, India, for cooperation in the research work.

REFERENCES [1] Yashwant D. Wanmode, Deodatta Baxy, and Purushottam

Shrivastava.Design Of 10 dB S-Band Multihole Waveguide Coupler. Pulse High Power Microwave Section, Raja Ramanna Center for Advanced Technology, INDORE,INDIA.

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[4] Amin M.Abbosh and Marek E. Bialkowski. Design of Compact Directional Couplers for UWB Applications. IEEE Transaction on Microwave Theory and Techniques , Vol. 55, No. 2, February 2007

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[8] Devid H Whittum. Single-Hole Directional Coupler for W-Band ARDB Technical Note 119.

[9] Microwave encyclopedia, Retrieved 22 July,2010 from World Wide Web: http://www.microwaves101.com/encyclopedia/coupled_line_couplers.cfm

bethe hole coupler

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