Suspended 60 GHz Phased Array Antenna WithHigh Efficiency

Kaveh Keshtkaran and Nima GhalichechianElectroscience Laboratory

Department of Electrical and Computer EngineeringThe Ohio State University, Columbus, OH, USA

E-mail: keshtkaran.2@osu.edu, ghalichechian.1@osu.edu

Abstract—This paper presents 5×5 aperture coupled mi-crostrip patch phased array antenna with suspended radiating el-ements operating at 60 GHz. A novel microfabrication techniqueenables separation of radiating elements from a lossy substrate.It was found that suspended elements improve efficiency andscanning range without sacrificing gain of the array by decreasingtrapped energy at air dielectric boundary (surface waves). Thearray has a bandwidth of 7.5 GHz (12.5%) and capable ofscanning ±60◦ in both E and H planes. Total array efficiencyis calculated to be >90%. Simulation results show 18.6 dB gainwith -12 dB sidelobe level at boresight. Furthermore, array gainis 15 dB and 14.5 dB when scanning ±60◦ in E and H-planes,respectively.

Keywords—Suspended, MEMS, Phased array, Large Scanning.

I. INTRODUCTION

The 60 GHz band has received much attention due tothe its unique properties such as high data rate transmissionand 7.5 GHz bandwidth allocated by FCC, which offersimportant benefits for secure multi-gigabit communication. Asa result of high absorption by oxygen molecules (O2) at 60GHz, large amount of wave energy dissipates in atmospherewhich lower the detection range. This behavior make 60 GHzband attractive for secure short range communication suchas wireless personal-area networks (WPANS)[1][2] as well assatellite cross-link communication.

Microstrip antennas are commonly used for satellite, aircraftand defense applications due to the planar and lightweightstructure [3]. However, when miniaturized on a substrate formillimeter wave application, they suffer from low efficiencydue to excitation of higher-order-mode surface waves at theboundary of air dielectric as well as conductive and dielectriclosses [4]. It is known that increasing dielectric thicknessand permittivity cause excitation of higher-order-mode surfacewaves in the antenna substrate that increases coupling betweenarray elements and reduces antenna efficiency [5][6]. In scan-ning arrays, as the scanning angle increases, large amount ofenergy is trapped at surface (blind spot) because of higher-order-mode surface waves excitation [5]. This non-radiatingenergy at the surface, reduces array’s overall efficiency.

One way to reduce the surface waves and improve theefficiency is to implement air cavity in the substrate or Elec-tromagnetic Band-Gap (EBG) structures to lower the effective

dielectric constant [7–9]. There has been intensive researchregarding cavity fabrication to modify the gain and bandwidth.For example, Yeap el at. [10] showed that partial substrateremoval in an LTCC substrate reduces the surface waves,dielectric loss and improves gain of the antenna. Althoughthe effect of decreasing effective dielectric constant has beenstudied, little attention has been paid to fabricating new typesof structures. This paper presents monolithic fabrication andsimulation results for aperture coupled microstrip antenna(ACMPA) [11] with suspended patch. Relative dielectric con-stant of conventional substrates for high frequency applicationsare ranged between 2 to 12. In contrast, by suspendingthe patch antenna with aperture coupled feeding, effectivedielectric constant of patch substrate is reduced to 1. Thisincreases the efficiency of the antenna by reducing surfacewaves and dielectric loss. Furthermore, this antenna array isable to scan more efficiently than conventional patch arrayantennas. Unlike the previous work that implements suspendedACMPA on a single PCB-based antenna [12], this workenables monolithic fabrication of high efficiency phased arrayon a silicon or silica substrate.

II. FABRICATION

5×5 ACMPA antenna array involves fabricating suspendedpatches over a fused silica substrate with low dielectric lossat millimeter wave band. For the array, 1.65×1.65 mm2

suspended square patches are positioned 300 µm above thecenter of 1550×155 µm2 slots on the ground plane. Tostart, ground plane with slots and feed lines with 700-µm-width are fabricated on the two sides of the 300-µm silicasubstrate(tanδ ' 0.0002)[13]. A 300-µm-thick photoresist(SU-8) layer is spin coated on the wafer followed by UVexposure for transferring the mask pattern (circular posts)to the SU-8 layers. SU-8 is great choice for high frequencyMEMS applications due to known electrical properties [14] aswell as ability to form thick high resolution structures. Silicondioxide layer film is deposited on the photoresist followed bygold deposition. The gold and dielectric layers are etched toform 1.65×1.65 mm2 square patches and membranes. Waferis immersed in developer solution to remove unexposed SU-8regions and form posts to hold suspended patches. For testingpurposes, a spacer with 0.65 mm thickness is used to place

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a conducting surface below the transmission line to eliminatethe back radiation.

III. RESULTS

Fig.1 shows 3D view of 5×5 ACMPA phased array withelement spacing of λ/2. The antenna array was designed tooperate efficiently at 60 GHz with more than 60◦ scanningand 7.5 GHz of bandwidth. The impedance of antenna array

Fig. 1. 3D schematic of a suspended 5×5 Phased array aperture coupledmicrostrip patch antenna. Microstrip transmission line is not shown above.

was optimized using ANSYS-HFSS to improve matching andscanning angle. Refection magnitude of the antenna array isdemonstrated in Fig.2. As is shown, the -10 dB bandwidth ofarray antenna is found to be 12.5%. High bandwidth is result ofa low permittivity dielectric substrate for patch antenna arraywhich is essential to reduce surface waves at higher scanningangles.

Fig.3 a and b, respectively, show obtained realized gain fordifferent scanning angles for E and h-planes. It can be ob-served that this antenna array has significantly large scanningrange of ±60◦ for a patch array at 60 GHz. The first sidelobehas magnitude of -12 dB with respect to the first beam.Although, at higher scanning angle sidelobe level is reducedto -9 dB, the array is cable to scan ±60◦ without sacrificing

Fig. 2. Reflection coefficient (S11) vs frequency for the array.

gain owing to its unique suspended radiating elements. ForE-plane, the array has 18.6 dB gain at boresight and 15 dBat ±60◦. For H-pane gain at highest scanning angle is 14.5dB. Furthermore, it is found that array efficiency is more than90% at 60 GHz. The fabrication and measurement results willbe presented at the conference.

Fig. 3. Realized gain at different scanning angles for (a) E-Plane and (b)H-Plane.

IV. CONCLUSION

We have presented simulation result and fabrication modelfor 60 GHz high efficiency aperture coupled phased arrayantenna with suspended radiating elements with steering ca-pability of ±60◦. Sidelobe level is as low as -12 dB atboresight and more than -9 dB at highest scanning angle. Arrayefficiency is found to be >90% at 60 GHz. The primary resultsshow feasibility of using suspended patches in phased arrayantenna to improve efficiency and scanning volume.

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