A 16×16 Ka Band Aperture-Coupled Microstrip Planar Array Yi-Chun Lilia Liu and Yuanxun Ethan Wang Department of Electrical Engineering, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA Email: [email protected] I. Introduction Microstrip patch antennas are popular in telecommunication and radar applications due to its light weight and low profile configurations that can be made planar or conformal and fed using a variety of methods. A compact microstrip antenna is also suitable as a radiating element for antenna arrays. When considering the different feeding techniques available, both the microstrip feed line and probe possess inherent asymmetries that generate higher order modes, which can result in cross-polarized radiation. As a result, the noncontacting aperture-coupling method, as first introduced by Pozar [1], is of great interest, since the arrangement allows for independent optimization of the feed line and the radiating element. Similar to other types of microstrip antennas, aperture-coupled elements lend themselves well to arrays using either series or corporate feed networks. In addition, the use of two substrates allows for optimal array performance and eliminates the competition for surface space between the radiation elements and the feed network. The common ground plane separating the two substrate layers also shields the antenna half-space from spurious radiation emitted by the feed lines and active devices. Moreover, since the radiating elements and the feed network are not in direct contact, critical problems that occur at millimeter-wave frequencies, such as large probe self-reactances, can be avoided. This paper presents a corporate-fed 16×16 aperture-coupled microstrip antenna array operating at 35 GHz. The design consists of two stacked substrate layers separated by a common ground plane. Fig. 1 shows the basic geometry of a single-element, aperture-coupled microstrip antenna. The microstrip antenna elements are printed on the upper antenna substrate, whereas the feed network is printed on the lower feed substrate. Electrically small, rectangular apertures in the ground plane couple the antenna elements to the feed network. II. Analysis of the Aperture-Coupled Antenna Analysis of the aperture-coupled microstrip antenna is complicated by the presence of two dielectric layers and the coupling aperture, but still generally easier to model in a rigorous manner than regular line- or probe-fed antennas, since the antenna current near the feed point is less singular. Pozar’s initial study of the aperture-coupled antenna utilized a simplified cavity-type model of the

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patch antenna. The objective was to deduce the appropriate coupling mechanism and the aperture placement to obtain maximum coupling. Results from the simplified coupling theory showed that the patch should be placed directly over the center of a thin rectangular aperture to maximize the coupling coefficient. The aperture-coupled microstrip antenna has been extensively analyzed since its initial debut in 1985. Consequently, useful data showing the effects of various design parameters, such as aperture size and position on input impedance, have been reported and verified. Results from the modal expansion technique show that resonant resistance always increases with the increase in aperture length. The resonant frequency, however, decreases due to the loading effect produced by the aperture as in accordance with the transmission line model. These design data were used as a set of guidelines for the design of the initial single element aperture-coupled microstrip antenna. III. Design and Construction of the Array A single aperture-coupled microstrip antenna was designed to operate at 35GHz using a low dielectric constant (εr1=2.2) antenna substrate with thickness t1 = 20 mil and a high dielectric constant (εr2 =10.2) feed substrate with thickness t2 = 10 mil. The lower dielectric constant for the antenna substrate was chosen to promote radiation, while the higher dielectric constant for the feed substrate was chosen to enhance the binding of the fields to the feed line. Once the operating frequency was established, the dimensions of the patch were readily calculated to be 0.239×0.160 cm. In order to produce design curves, the input impedance over a range of frequencies close to the operating frequency for a given aperture length was first plotted on the Smith chart. Repeating the process while maintaining the stub length for various aperture lengths produced a family of curves from which values of resonant resistances and their corresponding resonant frequencies could be determined. Optimum matching was achieved for Lap=0.124 cm and Lstub=0.0425 cm. The aperture width was chosen to be Wap=0.0156 cm, slightly larger than one tenth of the aperture length as suggested by Pozar [1]. The simulated input reflection coefficient for the single element design shown in Fig. 2(a) revealed an impedance bandwidth of around 6.5%. With the design of the single antenna element complete, the 2×2 elemental subarray was then designed using a corporate-fed network. Elemental spacings of 0.74λ0 and 0.56λ0 in the x- and y-directions, respectively, were maintained so as not to exceed the maximum area size of 1.27×0.9525 cm, while creating a fair compromise between antenna gain and sidelobe levels. . The internal feed network consists of 50 Ω microstrip transmission lines with quarter-wavelength transformers used to match the T-junctions and a 50 Ω primary feeding line. After considering the effects of various bend designs, the corner bend was chosen for its simplicity and reasonable simulation results. The

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feed network displayed a relatively high impedance bandwidth, sufficient insertion losses at the four ports in the range of − 6 dB , and approximately equal phase delay. Fig. 2(b) shows the simulated return loss for the 2×2 elemental subarray. The simulated results indicate an impedance bandwidth of almost 7%, with a center frequency close to 35 GHz. By duplicating the 2×2 elemental sub-array, larger arrays with higher gain consisting of 16 and 64 elements were easily constructed. Simulation results for the 4×4 elemental sub-arrays are shown in Fig. 2(c). Having obtained proper impedance matching in the larger arrays, the final 16×16 element array was constructed by duplicating the 8×8 elemental sub-array. IV. Fabrication and Measured Results Fig. 3 shows the manufactured 16×16 element array. The measured return loss of the array as shown in Fig. 4 indicates an impedance bandwidth of almost 8%, with a center frequency close to 35 GHz. Both the E- and H-plane co-polarization patterns are presented in Fig. 5. As expected, the 16×16 element array produced a pencil beam with the backward and side lobe levels suppressed mostly below -20 dB in the E- and H-planes. The gain of the 16×16 element array was measured to be around 21.7 dB with respect to a reference horn antenna. An expected 5 dB radiation efficiency loss is contributed by the connector and ohmic loss of the transmission line. V. Conclusion The design and performance a 16×16 Ka band aperture-coupled microstrip planar array have been presented. The array has been manufactured and measured in terms of its return loss, radiation pattern, and gain. Good results for both return loss and radiation characteristics of the array have been achieved. A reasonable gain of almost 22 dB was obtained at the center frequency of 35 GHz. References: [1] Pozar, D.M., “Microstrip Antenna Aperture-Coupled to a Microstrip Line,” Electron. Lett., Vol. 21, 1985, pp 49-50. [2] Sullivan, P. L., and D. H. Schaubert, “Analysis of an Aperture-Coupled Microstrip Antenna,” IEEE Trans. Antennas and Propagation, Vol. AP-34, 1986, pp. 977-984. [3] Oostlander, R., et al., “Aperture Coupled Microstrip Antenna Element Design,” Electron. Lett., Vol 26, 1990, pp. 224-225. [4] Blaauw, C., et al., “Large Bandwidth Aperture-Coupled Microstrip Antenna,” Electron. Lett., Vol. 26, 1990, pp. 1293-1294. [5] Song, H. J., and M. E. Bialkowski, “Ku-Band 16 × 16 Planar Array with Aperture-Coupled Microstrip-Patch Elements,” IEEE Trans. Antennas and Propagation, Vol. 40, 1998, pp. 25-29.

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[6] Garg, R., et al., Microstrip Antenna Design Handbook, Artech House, Norwood, MA, 2001.

b t1

Antenna Substrate, εr1

a

(a)

Ground Plane Lap

t2

Wap

Lstub

Feed Substrate, εr2

(b)

Wfeed

Fig. 1. The basic geometry of the aperture-coupled microstrip antenna. Fig. 3. The manufactured 16×16 element array (a) the antenna substrate and (b) the feed substrate. (a)

(b) Fig. 4. The measured return loss for the 16×16 element array.

(c)

Fig. 2. The simulated return loss for (a) single patch antenna, (b) 2×2 array, and (c) 4×4 array.

Fig. 5. The normalized E- and Hplane co-polarization patterns.

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