A 60 GHz 8x8 Planar Array Antenna with Corporate Feed Network using Meandered Probe

Fed Patch in LTCC Technology

Tyler Reid and Satish K. Sharma Department of Electrical and Computer Engineering

San Diego State University 5500 Campanile Drive, San Diego, CA, 92182-1309, USA

Email: treid@sdsu.edu, and ssharma@mail.sdsu.edu

Abstract—We present a 60 GHz wideband rectangular patch antenna array (8x8) with corporate feed network using meandered probe fed patch in low temperature co-fired ceramic (LTCC) technology. Each radiating element is composed of a rectangular patch excited by a meandered probe. This array is being fabricated at Kyocera and will be tested for both impedance matching and radiation patterns in the millimeter wave (mmWave) mini-compact range (MCR) of the Antenna and Microwave Lab (AML) at San Diego State University.

Keywords—5G Technology, LTCC; Antenna Array; Meandered Probe Fed Patch.

I. INTRODUCTION

This paper consists of the design and simulation of a rectangular patch antenna fed with a meandered probe to increase the impedance bandwidth to cover the 5G band centered at 60GHz. While similar antenna designs have been done in Duroid for single elements as in [1], there are applications where environmental considerations drive the material set to require a more robust solution such as that provided by LTCC. In this paper, an LTCC technology with a relative dielectric constant of 7.15 is used as this demonstrates the feasibility of the design to work with a high dielectric constant. Lower dielectric constant materials are available as used in [2] and [3], however the higher dielectric constant chosen for the proposed antenna is both to demonstrate performance with the higher dielectric constant as well as to take advantage of the reduction in size. First a single antenna is simulated and optimized then a 2x2 array was created using the corporate feed structure. After that, the design is re-optimized to account for mutual coupling. Next, it is repeated for a case of 4x4 and then 8x8 array antennas. The 8x8 array fits in a square of 0.7 by 0.7 inch. At center frequency, the simulated gain is 19.6dBi. Over the band, a total antenna efficiency of 48% to 56% is seen. All simulations were performed using Ansys HFSS. During the conference, additional simulation and measurement results will be presented.

II. ANTENNA DESIGN AND RESULTS

A. Feed Network Design

To reduce the losses in the feed network, rather than using 50 Ohm divided to 100 Ohm followed by quarter wave matching sections to convert back to 50 Ohm, the matching feedline is first matched to a 25 Ohm line with a quarter wave transformer with an impedance of 35.4 Ohm. Another reason not to use 100 Ohm lines is the line widths are too narrow to be manufacturable with a substrate thickness of 0.016 inches. Even the 50 Ohm line width of 0.003 inches is at the edge of manufacturing limits. A thicker substrate can be used at lower frequencies to allow for wider lines, however in the case of this design, thicker substrates would introduce additional substrate modes to the structure. The T-junction showing the 50 to 25 Ohm transition as well as the division is shown in Figure 1.

Figure 1 T-Junction Power Divider.

B. Antenna Element Design and Simulation

The patch itself is designed to be manufactured on the top of 4 layers of 0.004 inch each, with the meandered probe formed of printed traces and vias. Two layers of 0.008 inches

2018 11th Global Symposium on Millimeter Waves (GSMM)

978-1-5386-4584-0/18/$31.00 ©2018 IEEE

are used for the strip line and a third layer of 0.008 inches is placed between the meandered probe feed and the strip line. One advantage of LTCC over other advanced ceramic materials is the high conductivity metals such as copper and gold that can be used for the internal traces and vias. The metal conductivity used for the simulations of this paper was 1.6E7 Siemens/meter. This value is less than pure gold because of the adders and methods used in printing gold in LTCC. The stack up for the single antenna with the transition to 50 Ohm strip line is shown in Figure 2. Four layers of 0.004inches are sued for the meandered probe, while three layers of 0.008inches are used for the transition and stripline. The ground plane is located between the fourth and fifth layers from the top, .016inches below the patch. All layers are made using a dielectric constant of 7.15.

Figure 3 shows that matching bandwidth is from 58 GHz to 63 GHz which accounts to 8.3% bandwidth considering S11 = -10 dB criteria. Similarly, figure 4 shows that peak gain is 3.72 dBi and peak cross-polarization is 28.7dB. Back lobes are not computed although present.

Figure 2 Meandered Probe Fed Rectangular Patch Antenna.

Figure 3 Reflection Coefficient Magnitude of Single Antenna

Figure 4 Co- and Cross-Polarization at 60 GHz.

C. Antenna Array Design and Simulation

To minimize grating lobes, antenna elements should be spaced λ/2 or less. At 60GHz in air λ/2 is 0.094 inches. In practice reducing spacing by 5-10% avoids pattern deterioration due to mutual coupling effects [4]. Taking the conservative 10%, gives an element spacing of 0.0846 inches which was used for this array design. The spacing for the corporate feed is tightest at the point near the elements being fed, so the divider length is reduced from full quarter wave for the 25 Ohm and 50 Ohm sections. The transformer is kept at quarter wave to maintain the center frequency. The 2x2 subarray was optimized with the corporate feed to adjust for mutual coupling. The array and feed are shown in Figure 5 using the same stackup as figure 1. The stripline is contained in the bottom two 8 mil layers.

The reflection coefficient magnitude is better than 10dB throughout the matching bandwidth from 55 GHz to 65 GHz (Figure 6). Radiation patterns at 60 GHz (Figure 7) have broadside co-polarization gain of 10 dBi and peak cross-polarization of 30 dB. Back lobes are not computed although present.

Figure 3 2x2 Sub-array geometry.

Figure 4 Reflection Coefficient Magnitude versus frequency.

2018 11th Global Symposium on Millimeter Waves (GSMM)

Figure 5 Radiation patterns at 60 GHz of the 2x2 subarray.

To extend the pattern to 4x4 and then 8x8, the same layout

is used as the 2x2 with a step and repeat pattern for the meandered probe fed patches and the corporate feed structure is mirrored and extended to connect all the patches. As the length of the feed network increases the losses due to the feed structure reduce the efficiency, while the added elements increase the gain helping to overcome the losses. The 64-element array is shown in Figure 8 with dx and dy spacing of elements at .00846inches. The overall size of the array is 0.6768inches in both the x and y directions.

Figure 9 shows the peak gain at 60GHz to be 19.6dBi with a peak cross-polarization as 39dB. The radiation efficiency of the sixty-four-element array varies from 48-54% over the entire band from 55 to 65GHz. The impedance matching exceeds the -10dB requirement except for a small band from 63.6 to 64.4GHz.

Figure 6 Corporate feed network based 8x8 planar array geometry with Meandered Probe Fed Patches in LTCC technology.

Figure 7 Co- and cross-polarization radiation patterns of the 8x8 Array.

Figure 8 Total antenna efficiency versus frequency.

Figure 9 Reflection Coefficient Magnitude versus frequency.

III. CONCLUSION

A 64-element array of meandered probe fed patch antennas in LTCC technology was designed and simulated. The performance of the array gives a peak gain of 19.6 dBi at center frequency. The antenna efficiency ranges from 48% to 54%, which includes both dielectric and conductor losses. For 5G

2018 11th Global Symposium on Millimeter Waves (GSMM)

applications in the 60 GHz band, this LTCC based antenna array performs adequately from 55 to 65GHz.

REFERENCES

[1] Q. W. Lin, H. Wong, X. Y. Zhang, and H. W. Lai, “Printed meandering

probe-fed circularly polarized patch antenna with wide bandwidth,” IEEE Antennas Wireless Propag. Lett., vol. 13, pp. 654–657

[2] M. Sun, Y. X. Guo, M. F. Karim, L. C. Ong, "Linearly polarized and circularly polarized arrays in LTCC technology for 60GHz radios", IEEE APSURSI Symp. Dig., pp. 1-4, July. 2010.

[3] P. F. Sun, T. Liu, J. Zhang and L. P. Huang, "Integration of a 60 GHz packaged LTCC grid array antenna with an amplifier," 2017 IEEE Electrical Design of Advanced Packaging and Systems Symposium (EDAPS), Haining, 2017, pp. 1-3.

[4] Knittel, G. H., A. Hessel, and A. A. Oliner, ‘‘Element Pattern Nulls in Phased Arrays and Their Relation to Guided Waves,’’ IEEE Proc., Vol. 56, No. 11, November 1968, pp. 1822–1836

2018 11th Global Symposium on Millimeter Waves (GSMM)