Ka-Band SIW-fed Slot Array Antenna

H. S. Farahani, B. Rezaee, R. A. Sadeghzadeh Department of Electrical Engineering K. N. Toosi University of Technology

Tehran, Iran. sarbandifarahani@ee.kntu.ac.ir

Abstract—This paper describes the design of a 30 GHz slot array antenna fed by substrate integrated waveguides (SIWs) targeting in Ka-band applications. The proposed antenna consists of a 4*8 slot arrays on the broad wall of a corporate SIW-based feeding network which bring the advantages of low profile, low loss, cost efficient, easy fabrication and low cross polarization. The simulation results extracted from full wave software HFSS and CST are in good agreement and verify the design procedure by achieving bandwidth, high directivity, side lobe level (SLL), and 3-dB beamwidth of 1.12 GHz, 18.7 dB, -12.4 dB, and 24 degree, respectively.

Index Terms—High gain; Low profile; SIW feeding network; Slot array antenna;

I. INTRODUCTION

Owing to the advantages of conventional waveguide-fed slot array antenna such as high gain, narrow-beam and shaped-beam radiation pattern, high power handling, and mechanical strength, they have been successfully used for numerous applications in communication and radar systems [1, 2]. The standing (resonant) and travelling (non-resonant) wave fed arrays of longitudinally arranged slots on the broad wall of rectangular waveguides are two commonly used configurations. However, the rectangular waveguide-fed slot arrays are bulky, expensive to fabricate and need very accurate manufacturing particularly for resonant and large arrays where the slots have to be located in the peaks of the standing wave. Resonant arrays have a narrow bandwidth due to the use of a half-wavelength rectangular slots even though this drawback can be mitigated by employing elliptical and fractal slots [2, 3]. The design procedure of the slot array antennas is mainly based on the formulas provided by Elliot [4-7] by considering mutual coupling between the radiating elements. Nevertheless, several approaches have been proposed to reduce the mutual coupling such as electromagnetic band-gap (EBG) and metamaterial structures [8].

In contrast, the substrate integrated waveguide (SIW) technology has been introduced and applied for many application in the last decade [9-16], exhibiting the advantages of both conventional hollow rectangular waveguide and microstrip technology. Some attractive benefits of SIW structures are low-profile, low cost of manufacture, PCB-based and also possibility of integration with planar devices, higher power handling capacity and lower dissipation loss than microstrip structures, particularly for high frequency applications. SIW technology has been already used in the design of slot array antennas [11] by corporate microstrip

feeding network which suffers from dissipation loss and surface waves reducing in radiation efficiency of the antenna.

To overcome the mentioned drawbacks of the previous works, SIW-fed slot array antennas is proposed. The main objective is to feed in-phase and equal amplitude signals into the arrays. Corporate and coupling configuration types are commonly used feeding networks of the slot arrays which the corporate one is employed in this work as well. In this paper, the corporate feeding network and slot arrays are integrated on a single layer of SIWs which take the advantages of size, weight and cost reduction as well as increasing the antenna efficiency than similar structure demonstrated in [11]. In this study, slot array antenna fed by SIW T- and Y-junction power dividers is presented and designed which in turn mitigate drawbacks of the structures presented in previous works. Moreover, several Ka-band SIW-fed lot array configurations including rectangular and elliptical slots are simulated. The full wave software CST and HFSS have been adopted to verify and clarify the validity and performance of the proposed structures.

II. SIW AND ARRAY ANTENNA DESIGN

SIW is a well-known transmission line appearing in a substrate which is generally surrounded by top and bottom copper clad plates and the parallel arrays of metallic via-holes in both sides along the signal propagation direction, as shown in Fig. 1. The SIW width and via-holes period and diameter are chosen so that the dominant TE10 mode is propagated and located between the via-holes as well. It can be also seen in Fig. 1 that due to the measurement and integration purposes SIW is excited through a microstrip to SIW tapered line transition which in turn provide good in-band impedance matching [16].

Figure 1. Schematic view of a SIW slot array antenna (with marked design parameters in mm).

After choosing an appropriate feeding network, the slot length, offset from central line of broad wall, element spacing, and the last slot distance from end-wall are the main design

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parameters of resonant array configurations for achieving desired impedance matching and array radiation pattern characteristics such as main lobe directivity and side lobe levels (SLLs). Hence, the SIW and resonant slot array parameters in order to use at 30 GHz operating frequency are calculated base on the formula described in [11].

Fig. 2 shows return loss and radiation pattern of the simulated 1*4 slot array antenna. The results exhibit the bandwidth, directivity and SLLs of 2.43 GHz and 9.83 dB, -11.7 dB, respectively.

(a)

(b)

Figure 2. 1*4 SIW-fed slot array antenna: (a) Retrun loss and (b) E-field radiation pattern.

For the design of 4*4 and 4*8 antenna arrays arrangements, a corporate SIW feeding network including both Y- and T-junction power divider are used in this study. Fig. 3 (a) and (b) illustrate both the power dividers to be used and their related scattering parameters. The proposed 1*4 feeding network by combining the Y- and T-junctions into a unit element with a bandwidth of 2.5 GHz, is depicted in Fig.3 (c). The optimum design parameters of the proposed feeding network are also labeled in Fig.3.

The substrate with a dielectric constant of 2.1 (Roger RT/ Duroid 5880) and thickness of 0.508 mm were considered for all of the structures simulated in this work.

(a)

(b)

(c)

Figure 3. Schematic view and S-parameters: (a) Y-junction (b) T-junction, SIW power dividers and (c) 1*4 feeding network.

It was demonstrated in [2] that elliptical slots can provide broadband resonant arrays in conventional waveguides. However, it has been investigated in this work that by keeping the same radiation pattern for a 4*4 slot array configuration, not only there is no significant bandwidth improvement in the elliptical SIW-fed slot arrays compared to the rectangular ones but also it causes some reduction in the bandwidth, as shown in Fig. 4 (a) and (b). As revealed in Fig. 4, the bandwidth (S11<-10 dB) of about 1.1 GHz and directivity of 16.5 dB are achieved by using just 4*4 rectangular slot array configuration.

(a)

(b)

Figure 4. 4*4 SIW-fed rectangular and elliptical slot array antennas: (a) radiation pattern and (b) Return Loss(up:rect./down:elliptic.).

Fig. 5 (a) exhibits the structure of the proposed 4*8 SIW-fed slot array antenna which has been designed at 30 GHz operating frequency.

(a)

(b)

Figure 5. Propoed 4*8 SIW-fed slot array antenna: (a) prepective view and return loss (b) E-field radiation patterns (Phi= 0o and 90 o @ 30 GHz)

using HFSS and CST.

As can be seen, to verify the validity of the design, two full wave software HFSS and CST have been adopted for the

simulation which are based on the various EM computational methods, namely, FEM and FIT, respectively. It can be revealed that the results are in good agreement and the S11<-10 dB and E-plane directivity are about 1.12 GHz and 18.7 dB, respectively. It is also observed that the proposed antenna has 3-dB beamwidth and SLLs of 24-degree and -12.4 dB, respectively.

III. ACKNOWLEGEMWNT

Authors would like to thank Faculty of Microelectronic for the financial support.

IV. CONCLUSION

In this paper, several configurations of Ka-band SIW-fed slot array antenna have been investigated. At first, a 1*4 corporate SIW feeding network integrated of Y- and T-junction power divider operating in 30 GHz was designed and simulated in order to provide a proper feeding network intended for the design of 4*4 and 4*8 antenna arrays. As a comparison, the performance of rectangular and elliptical SIW-fed slot array antennas were studied from which the results were almost the same. Finally, a 4*8 SIW-fed rectangular slot array antenna is proposed. To verify its performance, two various EM computational methods were adopted. As a consequence, the results agree well with together which in turn exhibit the validity of the work.

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