Microstrip Patch Antenna Array At 3.8 Ghz For Wimax And Uav Applicationss

Microstrip Patch Antenna Array at 3.8 GHz for WiMax and UAV Applications Hassan Sajjad, Waleed Tariq Sethi, Khan Zeb, Adnan Mairaj Electrical Engineering Department, King Saud University, Riyadh, Saudi Arabia hs [email protected], [email protected] Abstract—This paper presents the design of a rectangular microstrip line-fed patch antenna array with a centre frequency of 3.8 GHz for WiMAX and Unmanned Air Vehicle (UAV) applications. A single element, 1x2 and 2x2 microstrip rectangular patch antennas were designed and simulated in Computer Simulation Tool (CST) Microwave Studio environment. The results of designed antennas were compared in terms of Return Loss (S11 parameters), bandwidth, directivity, gain and radiation pattern. Compared to traditional microstrip antennas the proposed array structure achieved a gain and directivity of 13.2 dB and 13.5 dBi respectively. The antenna was fabricated using Rogers Duroid RT5880 substrate with a dielectric constant r of 2.2 and a thickness of 1.574 mm respectively. The array antennas were measured in the laboratory using Vector Network Analyser (VNA) and the results show good agreement with the array antenna simulation.

Keywords– Microstrip 2×2 array, 3.8 GHz, WiMax, UAV, Rogers RT-5880 Substrate I.

I NTRODUCTION

With the increase in data rates and a trend of miniature electronic circuits for wireless digital applications, the antennas required for these applications should be light weight, easily mountable and have a broad bandwidth [1]-[3]. These requirements can be met by using microstrip antennas and patch arrays. Balanis [4] states that an antenna should be low profile, simple and inexpensive to fabricate and it should be easy to mount on planar and non-planar surfaces. When the type of patch to be used for an application is chosen, the dimensions should be carefully analysed. A small change in any dimension can cause a noticeable change in the results e.g., the frequency, impedance matching, bandwidth, directivity and gain etc. Microstrip single element antenna has advantages but it also has several disadvantages, such as low efficiency, narrow bandwidth, low gain and directivity. These disadvantages can be overcome by using multiple patch elements in different configurations called patch arrays. Unmanned Aerial vehicle (UAV) has gained an immense popularity among researchers due to its surveillance, reconnaissance and sensing applications. The earliest UAV was used in 1916 [5] during World War I for military applications but now it has found its importance in RADAR applications as well. The antennas used in UAV should be low profile, compact and directional. This paper shows a single element, 1 × 2 and 2 × 2 rectangular microstrip patch array antennas with edge feeding method and quarter wave transformer for impedance matching. All the antennas are centred at 3.8 GHz which can be used for UAV and WiMax applications. However, we need a more directive beam for the UAV which would require

the arrays to be bigger, for example an array of the order 10 × 10. As visible from the simulation results the patch array antenna outperforms the single element antenna in terms of gain, directivity and bandwidth. The paper is organized as described. Section II explains the antenna design, synthesis and measurements. Section III shall discuss the results and section IV shows the conclusion. II.

A NTENNA D ESIGN

The goal of designing a microstrip antenna at 3.8 GHz was to improve gain and directivity for the UAV application. There are various important steps in designing microstrip antennas. The most important one is choosing the right kind of substrate. Different substrates can be used to fabricate an antenna with a good response. The dielectric constants of the substrates lie in the range of 2.2 ≤ r ≤ 12. Usually the substrate chosen for the antenna design is thick and has a dielectric constant on the lower end. This provides better performance in contrast to thin substrates with higher dielectric constants [4]. FR-4 and Rogers 5880 was available for fabrication, since FR-4 has higher losses and has higher dielectric constant, Rogers 5880 was chosen for fabrication and simulation purpose. Once the single element antenna was designed and simulated then the array configuration performance evaluation was carried out. Table I shows the specifications for the rectangular single patch antenna. TABLE I.

D ESIGN SPECIFICATIONS FOR S INGLE E LEMENT Substrate

Rogers-5880

Center Frequency,

3.8 GHz

Copper Thickness

0.035 mm

Substrate Height

1.574 mm

Loss Tangent

0.0009

Dielectric Constant

2.2

A. Single Element Antenna Design This section describes the design of a rectangular single element patch antenna with quarter wave transform for impedance matching. The most important design features of the patch are its width (W ), length (L), width of transmission line and the length of the feeding line. These specifications are dependent on each other as well as the frequency of operation. Different equations given in [4] and [7] were used to calculate the patch dimensions i.e., W and L: The patch was fed by a 50 Ω discreet port. Quarter wave transformer network was used for impedance matching of the feed line,

patch and the connector. The single element rectangular patch design in shown in Fig. 1.

Fig. 1.

Single element rectangular patch design Fig. 2.

Corporate feed network for 2 × 2 array

A 50 Ω surface mount adapter (SMA) connector was used to connect to the feed line. The feed line will be fed to the patch through a quarter-wave transformer matching network. Table II shows the dimensions for the feed line impedance matching.





















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TABLE II.





F EEDLINE DIMENSIONS















Impedance (Ω)

Width (mm)

Length (mm)

50

4.84

28.8

70

2.83

29.2 (QWT)

100

1.40

29.7








































B. Rectangular Patch Array Design After designing and simulating a single patch, 2 × 2 array was designed and fabricated. The design uses corporate feeding network for matching the impedance. Corporate fed arrays are more efficient than single fed arrays. In the former case design, feed of each element can be controlled which helps in beam steering and scanning in case of phased arrays. Another important phenomenon is mutual coupling which occurs between antenna elements and the transmission line. It is important to take into account mutual coupling and internal reflections in an array [6]. For better radiation and efficient power transfer the spacing between antenna elements is very important. In this paper, the centre to centre distance between the patches is set to λ/2. Width and length of different branches (50 Ω, 70 Ω, 100 Ω) of the transmission line are tabulated in table II, where QWT is the quarter wavelength transform length. Fig. 2 is a rough sketch of the corporate feed network of 2 × 2 array. III.













































Fig. 3.

Return loss of single element rectangular patch antenna

Fig. 4.

Bandwidth of 1 × 2 array

Fig. 5.

Bandwidth of 2 × 2 array

R ESULTS AND D ISCUSSION

The return loss of a single element microstrip patch antenna, both measured and simulated, is shown in Fig. 3. The simulation result gives a return loss of −18.2 dB at an operating frequency of 3.8 GHz while the measurement result gives a return loss of −14.22 dB at 3.82 GHz. Minor flaws during the fabrication process leads to a shift of the operating frequency of measurement result. Fig. 4 and 5 show the bandwidth for both 1 × 2 and 2 × 2 array antenna. The bandwidth of single element antenna is

about 2.8% (3.7617−3.8683 GHz). Meanwhile the bandwidth of 1 × 2 and 2 × 2 patch array antenna is 1.63% (3.7727 − 3.8349 GHz) and 1.84% (3.7693 − 3.8413 GHz) respectively. There is a percentage decrease of about 0.92% which rises due to coupling loss and needs better element spacing optimization.

Fig. 6.

Simulated radiation pattern of a single element Fig. 8.

Measured polar plot for 2 × 2 array

Fig. 9.

Fabricated 2 × 2 microstrip rectangular patch array antenna

TABLE III.

G AIN AND D IRECTIVITY FOR THE SIMULATED A NTENNAS Gain Directivity

Single Element 7.02 dB 8.19 dB

IV.

Fig. 7.

Simulated polar plot for 2 × 2 array

Fig. 6 shows the simulated radiation pattern of a single microstrip patch antenna along the z-axis. The achieved directivity of a single patch antenna was 8.19 dB and gain of 7.02 dB. Compared to [8] the simulation of 1 × 2 patch array antenna showed a directivity and gain of 9.83 dB and 9.39 dB, respectively. Fig. 7 shows the improved directivity and gain of 13.5 dB and 13.2 dB respectively. The measured polar plot in Fig. 8 had a decrease of 1 dB in the directivity and gain due to fabrication and connector losses. Table III shows the comparison of gain and directivity of the simulated antennas. Comparing the results conclude that the array design generates intense radiation at the centre and achieves more directivity and gain. The fabricated antenna is shown in Fig. 9

1×2 9.39 dB 9.83 dB

2×2 13.2 dB 13.5 dB

C ONCLUSION

A rectangular microstrip patch antenna at 3.8 GHz for Wimax and UAV applications was designed and tested. The single microstrip antennas performance was then improved in terms of directivity and gain by comparing it with 1 × 2 and 2 × 2 array structures. The array antennas outperformed the single antenna in terms of directivity, and gain. However the sidelobes level was too high (> −10 dB) to be used as conformal antennas. The final 2 × 2 array antenna design was then fabricated and the performance was then compared with the simulated array antenna. Overall, the performance of the array antenna met the desired requirement in terms of return loss. The simulation return loss was equal to −18.2 dB at the centre frequency of 3.8 GHz. The maximum directivity and gain achieved for 2 × 2 array antenna was 13.5 dB and 13.2 dB respectively. R EFERENCES [1] P. Pigin, Emerging mobile WiMax antenna technologies, IET Communication Engineer, October/ November 2006. [2] C.Y Pan, T. S Horng, W. S Chen and C.H Huang, Dual wideband Printed Monopole Antenna for WLAN/WiMax Applications, IEEE Antenna and Wireless Propagation letters, vol 6, pp. 149-151, 2007.

[3] Y. Yu, Y. Lee, S. Lee and J. Choi, A compact internal antenna for wireless USB Dongle application, in Proc. ICAT 2009, pp. 1084-1086. [4] C. A.Balanis, Antenna Theory, 2nd. Edition ed. Arizona State University: John Wiley & Sons,Inc., 1997. pp. 722-723. [5] Taylor, J. W. R., Jane’s Pocket Book of Remotely Piloted Vehicles, Collier Books, New York, 1977. [6] H.Y.D. Yang, Miniaturized printed wire antenna for wireless communications, IEEE Antennas Wireless Propag. Lett., vol. 4, pp. 358-361, 2005. [7] D. M.Pozar, Microwave Engineering, 3rd Edition ed. University of Massachusetts at Amherst: John Wiley & Sons,Inc. [8] Shah, M. Melaka, Ayer Keroh, Rose, M.R.C., Kadir, M.F.A., Misman, D., Aziz, M.Z.A.A., Suaidi, M.K., Dual polarization inset-fed microstrip patch antenna, Applied Electromagnetics, APACE 2007. Asia-Pacific Conference, pp. 1-6, 4-6 Dec. 2007.