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Wideband Non Linear 3 dB Hybrid Coupler for X-Band Switched Beam Antenna Yuli K. Ningsih, M. Asvial, and Eko T. Rahardjo Abstract— This paper presents a new wideband 3 dB hybrid coupler for X-band. The proposed wideband 3 dB hybrid coupler has been designed by using non linear arm with exponential impedance taper to achieve a good performance. The proposed hybrid coupler achieved an impedance bandwidth over 22% with coupling coefficient -3.3 dB, isolation coefficient of below -25 dB and reflection coefficient of below -20 dB. The result shows a good performance. To implement the new hybrid coupler, a switched beam antenna is designed. The propose antenna can produce two radiation patterns that can be changed to different beam for reduce the existing interference. Switched beam antenna using a non linear 3 dB hybrid coupler has been design and realization in a single layer substrate, that it has a wideband characteristic. The simulation and measurement results show that the antenna can generate two beams at 20° and 330° with a beamwidth of around 40° at 9 GHz. The return loss of the switched beam antenna can be achieved below -18 dB over a 20% bandwidth with a center frequency at 9 GHz. Index Terms—3 dB hybrid coupler, microstrip, exponential impedance taper

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1 Introduction Recently, switched beam antenna system has been widely used in numerous applications, such as in communication system, satellite system and modern multifunction radar. This is because of the ability of the switched beam antenna to reduce the existing interference and to improve the quality of transmission [1][2], in addition to increase gain and diversity [3]. A switched beam system consists of a beamswitching network and antenna array. A simple design of beam-switching uses a 3 dB hybrid coupler. The 3 dB hybrid coupler is a directional coupler with a 90° phase difference in the output of the through and coupled arms [4]. Several methods have been reported in reference [5-6], concerning how to achieve the switched-beam antenna using the 3 dB hybrid coupler. The switched beam antennas are often made of microstrip form. In reference [5], a flat antenna consists of a trapezoidal conducting cavity with an aperture and fed through a 3 dB hybrid coupler was proposed, but the measurement result didn’t agree with the simulation result. This shows that the ————————————————

• Y.K. Ningsih is with Department of Electrical Engineering, Universitas Indonesia, Depok, 16424 • M. Asvial is with Department of Electrical Engineering, Universitas Indonesia, Depok, 16424 • E,T. Rahardjo is with Department of Electrical Engineering, Universitas Indonesia, Depok, 16424

magnetic current element model is not sufficient to accurately model the radiation pattern. A flat beam switching antenna using two 3 dB-couplers and two pairs of magnetic line current array was proposed in [6], however the bandwidth has a narrowband characteristic. For the bandwidth enhancement of the 3dB hybrid coupler, several designs were published in [7]-[10]. Multilayer hybrid coupler can achieve a one–octave bandwidth in [7]. In reference [8], design and realization of hybrid coupler in extra high frequency on bi-layer microstrip structure was reported. This design can achieve a wideband characteristic. The disadvantages of these designs are large size and bulk. A compact 3 dB coupler in an N-section, tandem connected structure was reported in [9]. The design resulted in a wide bandwidth up to 42%. Another design, two elliptically shaped microstrip lines which are broadside coupled through an elliptically shaped slot, is used in [10]. This design is used in a UWB coupler with high return loss and isolation. However, these designs require a more complex design. This paper proposes a new and simpler design 3 dB hybrid coupler for a switched beam antenna. The proposed of 3 dB hybrid coupler uses non linear impedance at the series arm to obtain a wideband characteristic for the X-band. The non linear arm has characteristic impedance that varies continuously in a smooth fashion from the impedance of one line to that of the other line. It is more effective for the thick substrates

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with low dielectric constant values, where the required reflection and the isolation are plotted against frequency for different value of impedance in the series arm (Zos), as line widths are too wide. shown in Fig. 2. The reflection and the isolation coefficients decrease as Zos decreases. The optimal 2 Linear 3 dB Hybrid Coupler Fig.1. shows the structure of the linear 3 dB hybrid characteristic impedance at the series arm can be achieved coupler. This component is symmetrical and has the when the lowest value of the reflection and the isolation following properties: if port 1 (P1) is fed, then the signal coefficients are obtained. travels to port 4 (P4) and port 3 (P3) is consequently coupled, while port 2 (P2) is isolated. If port 2 (P2) is fed, then the signal travels to port 3 (P3) and port 4 (P4) is consequently coupled, while port 1 (P1) is isolated.

Z0 2

Fig.2a. Reflection coefficient (S11) characteristic as a function of impedance value at the series arm for linear 3 dB hybrid coupler

Z0 2 Fig. 1. The linear structure 3 dB hybrid coupler An ideal 3 dB hybrid coupler is designed to have zero reflection power and splits the input power in port 1 into equal powers in port 3 and 4. Thus a 3 dB hybrid coupler serves as a power divider. The performance of 3 dB hybrid coupler is measured by three parameters [11], the coupling, C = 10 log (P1/P3) = 10 log (P1/P4) = -3 dB, with the phase difference between two outputs is 90°, the reflection, the reflection = 10 log (P1/P1) and the isolation, I = 10 log (P1/P2). The standard 3 dB hybrid coupler characteristic impedance 35 ohms is use in the series arm. In fact, in order to realize a 35 ohm impedance condition, the microstrip line must be 8 mm wide. This width is occupy a large area and larger than λ/4 at 9 GHz, so that practically it is difficult to realized. To solve this problem, higher impedance at the series arm will be utilized. It will make the quarter wave line have more sufficient spacing and a smaller microstrip line width The different impedance conditions in the series arm (Zos ) have been investigated both in reflection coefficient and isolation coefficient response. The impedance values of the design are 50.6 Ω, 52.5 Ω, 54.5 Ω, and 56.7 Ω. The

Fig.2b. Isolation coefficient (S12) characteristic as a function of impedance value at the series arm for linear 3 dB hybrid coupler The best compromise between reflection coefficient and isolation coefficient is obtained for Zos = 50.6 Ω. Electrical lengths of the series arm were maintained at a quarter wavelength. The microstrip widths corresponding to the impedances are shown in Table 1. With this condition, the microstrip line width at the series arm is smaller than the linear design [4].

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Table 1 Microstrip Width of the Linear and proposed design

Normal Proposed

Series Zos(Ω) 35 50.6

Arm W(mm) 8 4.75

Shunt Zos(Ω) 50 50

Arm W(mm) 5 5

Γi =

Z 1 L − j 2 βz d e (ln 2 ) dz ∫ 0 2 dz Z1

(7)

for the input reflection coefficient (Гi), where L is the total discrete steps length. So if is known, Гi can be found as a function of frequency. The exponential taper is one of

varies exponentially, the impedance tapers. Hence matching condition between two impedances, dan along the line can be calculated using the following of Non Linier Arm 3 dB Microstrip Hybrid [13]:

3 Coupler

(8) An ideal 3 dB hybrid coupler is designed to have zero reflection power and an equal output power (3 dB). One method to have a minimum reflection was using Substituting (8) into (7) gives the following equation: multisection impedance with an exponential impedance ln Z 2 / Z 1 L − j 2 βz taper [4][11]. The coupled equations are given by (1) and Γi = e dz 0 2L (2) [12].

∫

(1)

Γi =

lnZ 2 / Z 1 − jβL sin β L e 2 βL

(9)

(2) Schematic of the proposed non linier 3 dB hybrid where is the amplitude of forward wave, is coupler is shown in Fig 3. the amplitude of reflected wave, β (z) is the propagation constant and c (z) is the coupling coefficient between forward and reflected wave.

(3) 1/2

(4)

Where ko is the free space propagation constant, λo is the free space wavelength, a(z) is the broad wall dimension as a function of z and is the characteristic impedance. To determine the wideband characteristic the series arm is designed using a number of discrete steps. The step Fig.3. Geometry structure of a new 3 dB hybrid coupler change d in impedance at z produces a differential design with non linear arm with exponential impedance taper at the series arm reflection coefficient (dГ): d Гo =

Z 1 d (ln 2 )dz 2 Z1

)

for the output reflection coefficient (Гo)

dΓi = e − j 2 βz

Z 1 d (ln 2 ) dz 2 dz Z1

(6)

The quarter-wave transformer in the normal 3 dB hybrid coupler is replaced by an exponentially tapered. Since the discrete step provides a consistent impedance transformation across all frequencies [14] and the exponentially-tapered lines have the advantage of lower internal reflection and shorter line length compared to linear taper. The input port is coupled with two output port by two quarter wavelength long section at the series

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arm. Each branch comprises an exponentially-tapered frequency is depicted in Fig.6. The figure shows that a microstrip line which transforms the impedance from 50 reflection coefficient of 22.5 dB is achieved at 9 GHz. ohms to 50.6 ohms and 50.6 ohms to 50 ohms at the output port. This impedance transformation has been designed across a discrete steps length L = 6.75 mm. This length is a quarter wavelength long sections. Fig. 4 shows variation of impedance with an exponential impedance taper, by considering it to be made up from a number of section lines with differential length, dz, and therefore the impedance changes by differential amounts d from section to section. Z (z )

Fig. 6. Reflection coefficient of novel 3 dB hybrid coupler The amplitude response and the phase response of the coupling factors at the desired bandwidth are depicted in Fig. 4. A matching section with exponential impedance Fig. 7. The amplitude response varies between -2.7 dB and -4.3 dB in the entire 8-10 GHz band. The amplitude taper response at 9 GHz for the direct port and the coupled port Simulation results for the new design of hybrid is -3.3 dB as shown in Fig. 7(a). The result has shown the coupler are depicted in Fig. 5 to Fig. 7. The optimum equal power splits with a center frequency of 9 GHz. ground plane dimension is 22 mm x 24 mm. The isolation factor can be defined as the difference in signal level between the input port and the isolated port. The isolation should be as high as possible. S12,S21,S34 and S43 are the isolation coefficients. Fig.5 shows the isolation coefficient of the new 3 dB hybrid coupler. The isolation coefficient takes the value of -25 dB at 9GHz.

Fig. 7a. Coupling factor of non linear 3 dB hybrid coupler – amplitude response As expected, the ideal value of phase difference between the direct port and the coupled port is 90°. Fig. Fig. 5. Isolation coefficient of non linear 3 dB hybrid 7(b) shows the phase response of the new 3 dB coupler. It can be observed that the phase difference between the coupler S11, S22, S33, S44 are the reflection coefficients. The direct port and the coupled port is very close to the ideal coupler is designed to maintain the reflection coefficient value of 90° for the frequency range 8-10 GHz. at an acceptable level. The reflection coefficient vs

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Fig. 8. Switched beam antenna using non linear 3 dB Fig. 7b. Coupling factor of non linear 3 dB hybrid coupler hybrid coupler with exponential impedance taper –phase response

4 Design and fabrication of wideband switched beam array antenna using 3 dB Hybrid Coupler Non Linear Arm To implement of the proposed wideband coupler, a switched beam antenna consists of a linear array antenna with rectangular shape are designed. This design uses a substrate with a thickness of 1.57 mm (0.049λ) and dielectric constant (εr) of 2.2. A switched beam antenna is designed and realized at frequency of 9 GHz. Initially, the effective dimension of patch antenna is achieved when the length is 10 mm and the width is 12.8 mm. Then, 2 elements linear array is designed with an initial inter element spacing of 0.5λ. This element spacing is efficient for the effective dimension which doesn’t result in overlap between patches. The ground plane dimension of a novel wideband switched beam array antenna is 45 mm x 46 mm. Careful design and optimization procedure are performed to obtain accuracy and an optimal performance between ports. Fig.8 shows the fabrication of switched-beam antenna. The antenna can produce two radiation patterns that can be changed to different beam, corresponding to previous port assigning. The switched beam array antenna proposed here has the advantage of low cost, small volume and easy fabrication. The antenna is designed to maintain return loss factors at an acceptable level. Return loss characteristic of the proposed antenna is less than -10 dB as shown in Fig. 9. The simulated bandwidth with RL of 10 dB is around 2 GHz (8 –10 GHz). The measured return loss of the antenna is in good agreement with the return loss results obtained from the simulation.

. Fig. 9a. Return loss characteristic of switched beam array antenna (simulation and measurement) - when port 1 is switched on

Fig. 9b. Return loss characteristic of switched beam array antenna (simulation and measurement) - when port 2 is switched on The simulation and measurement results show that the switched-beam antenna has two beams with different

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radiation pattern and can be switched according to the desired target. Fig. 10 shows the E-Plane radiation pattern of antenna when one of port is switched on at frequency of 9 GHz. Fig. 10(a) shows the radiation pattern if port 1 (P1) is switched on, where it can generate a beam of 20°. However, as shown in Fig 10(b), if port 2 (P2) is switched on, it can generate a beam of 330°. The half power beam width of the switched beam antenna covers around 40°. The results show that the 3 dB hybrid coupler can function as a power divider that distributes the current proportionally based on the hybrid arm.

5 Conclusion Non Linear 3 dB hybrid coupler has been designed and simulated. The result of non linear 3 dB hybrid coupler shows a wideband characteristic with exponential impedance taper. The result shows, hybrid coupler is able to cover more than 22% of 8-10 GHz, with coupling coefficient -3.3 dB, isolation coefficient of below -25 dB and reflection coefficient of below - 20 dB. The new design of non linear 3 dB hybrid coupler has been applied for a simple switched beam antenna. The results show a wideband characteristic over 20 % (8–10 GHz). The switched beam antenna has two beams at 20° and 330° with beam width around 40°, respectively at frequency of 9 GHz. It also shows that the simulation and measurement are in a good agreement with each other.

References [1]

Fig. 10a. E-Plane directivity radiation pattern of the proposed antenna (dB) - when port 1 is switched on at frequency of 9 GHz

Fig.10b. E-Plane directivity radiation pattern of the proposed antenna (dB) - when port 2 is switched on at frequency of 9 GHz

T.A. Denidni, and T.E. Libar , ”Wide Band Four Port Butler Matrix for Switched Multibeam Antenna Array,” Proc. Personal, Indoor, and Mobile Radio Communication (PIMRC ’03), pp. 2461-2464, doi:10.1109/PIMRC.2003.1259161. [2] E. Siachalou, E. Vafiadis, S.S. Goudos, T. Samaras, C.S. Koukourlis, and S. Panas, ”On The Design of Switched Beam Wideband Base Station”, IEEE Antennas and Propagation Magazine, vol. 46, no.1, pp. 158-167, 2004, doi: 10.1109/MAP.2004.1296180. [3] P.S. Hall, and S.J. Vetterlein, ”Review of Radio Frequency Beamforming Technique for Scanned and Multibeam Antennas”, IEE Proc.HMicrowave,Antennas and Propagation, vol. 137,no.5, pp. 293-303,1990. [4] D.M. Pozar, Microwave Engineering, John Wiley&Sons, 2nd ed, pp. 169-174, New York, 1998. [5] N. Kuga, and H. Arai,” A Flat four Beam Switched Array Antenna”, IEEE Transaction on Antennas and Propagation, vol. 44, no.9, pp. 1227-1230,1996. [6] H. Arai,” Beam Switched Flat Antennas for Modern Communication,”, Proc. EMTS ’07, 2007, available at http://ursi.org/B/EMTS-2007/05-61/6-Arai-094.pdf [7] S. Banba, and H. Ogawa, “Multi-layer MMIC Directional Couplers Using Thin Dielectric Layers,” IEEE Transaction on Microwave Theory and Techniques, vol. 43, no. 6, pp. 1270 – 1275, 1995, doi: 10.1109/22.390182. [8] J. Sebastien, and G.Y. Delisle, ”Microstrip EHF Butler Matrix Design and Realization,” ETRI Journal, vol. 27 ,no. 6, pp. 788 - 797, 2005.

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Yuli K. Ningsih received the Ir. degree from the Universitas Indonesia, Jakarta in 1992, the MS degree from the Trisakti University at Jakarta, Indonesia and now student of the PhD degree with the Antenna propagation and Microwave Research Group (AMRG), Universitas Indonesia, all in electrical engineering. She joined Electrical Engineering Department Trisakti University,Jakarta since 1996 as a teaching assistant. Her research interests include antenna engineering, wave propagation, microwave circuits. She is student member of IEEE Antenna and Propagation Society (AP-S).

communication networks, radio resource management and genetic algorithms applications. He has some publications in several international journals and conferences. He is a member of the IEEE, the IEE, UK and the AIAA (American Institute of Aeronautics and Astronautics). Eko Tjipto Rahardjo, PhD. received the Ir.Degree from the Universitas Indonesia, Jakarta in 1981, the MS degree from the University of Hawaii at Manoa, Honolulu in 1987, and the PhD. degree from the Saitama University, Urawa in 1996, all in electrical engineering. He joined to the Department of Electrical Engineering Universitas Indonesia since 1982 as a teaching assistant. Since 2005 he has been appointed as Professor in electrical engineering. He has been Head of Electrical Engineering Department, Universitas Indonesia (2004 –2008). He was Executive Director of the Quality Undergraduate Education (QUE) in the Department of Electrical Engineering Universitas Indonesia (1999 – 2004), and was Head of Telecommunication Laboratory Universitas Indonesia (1997 – 2004). Since 2003, he has been Director of the Center for Information and Communication Engineering Research (CICER), Universitas Indonesia as well as Antenna propagation and Microwave Research Group (AMRG) leader. His research interests include antenna engineering, wave propagation, microwave circuits and communication system and regulations. He has been published and presented more than a hundred research papers both national and international journals and symposiums. He has been recipient of the Indonesian government scholarship through MUCIA (1984 –1987); the Hitachi Scholarship (1992 – 1996); and the Young Researcher's Award from the Universitas Indonesia (1996), 2nd best UI Research Award in Science and Technology (2009) and 2nd best UI Lecturer Award (2010). Dr. Rahardjo is members of IEEE Antenna and Propagation Society (AP-S), IEEE Microwave Theory and Technique Society (MTT-S), IEICE Japan and IECI Indonesia. He is now serving as chairman of IEEE Joint Chapter MTT-S/AP-S, IEEE Indonesia Section

Muhamad Asvial received the Ir. degree in Electrical Engineering from Electrical Engineering Department, Universitas Indonesia (1993), MSc. degree from Keio University, Japan (1998) and PhD. Degree from University of Surrey, UK (2003). He joined Electrical Engineering Department, Universitas Indonesia in 1993. His research interests include mobile cellular and satellite

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