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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
Printed Frequency Beam-Scanning Antenna With Flat Gain and Low Sidelobe Levels Lan Cui, Wen Wu, Senior Member, IEEE, and Da-Gang Fang, Fellow, IEEE
Abstract—A frequency beam-scanning antenna with backfire-to-endfire beam-steering capability is proposed and investigated. This antenna is with printed planar structure that consists of a low-loss slow-wave printed meander line based on the even-mode bilateral broadside-coupled suspended microstrip line (BSML). The propagation properties of the meander-line unit and its principles for frequency-scanning applications are studied extensively. This printed antenna achieves the measured beam scanning of 27.5 to 46 with the frequency sensitivity of 43.24 /GHz and a maximum gain of 15.5 dBi. The antenna exhibits flat gain of more than 13 dBi in the whole frequency range from 8.9 to 10.6 GHz. The sidelobe levels are near 20 dB. The design flexibility of this kind of antenna is shown. Index Terms—Broadside-coupled suspended microstrip line (BSML), frequency-scanning antenna, printed planar structure, slow-wave meander line.
I. INTRODUCTION
T
HE FREQUENCY-SCANNING antenna is a kind of traveling-wave antenna that supports an efficient and economical way to realize beam-scanning capabilities through tuning the frequency [1]–[3]. In the design of frequency-scanning antennas, the range of scan angle, the required frequency bandwidth, and the loss in the traveling-wave structure of the antenna should be considered. Actually, the loss is very critical to the antenna gain. The conventional way used in designing beam-scanning antennas is based on the effect of phase shifting induced by the slow-wave structures in the waveguide [2]. The waveguide structure exhibits very low loss. However, it is heavy and difficult to integrate with other planar components. A frequency-scanning microstrip antenna with a slow-wave structure was first proposed in [1]. However, the gain and efficiency are quite low due to the losses in the microstrip line. Planar printed leaky-wave antennas (LWAs) have been intensively investigated due to their low profile, low cost, ease of fabrication, and beam-scanning capabilities from the backfire to the endfire [4]–[7]. They are based on the periodic structures and composite right/left-handed (CRLH) transmission structures. They offer a simple way for frequency scanning by simply feeding a Manuscript received November 27, 2012; revised February 01, 2013 and February 15, 2013; accepted February 19, 2013. Date of publication February 22, 2013; date of current version March 15, 2013. The authors are with the Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing 210094, China (e-mail:
[email protected];
[email protected];
[email protected]. cn). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2248696
leaky mode on the traveling-wave structure, thus avoiding more complex and expensive feed networks. In a recent paper [8], a beam-steerable slot array is presented using a CRLH waveguide as the feeding structure. Due to the independent design of the feeding structures and the radiating elements in that paper, an engineering approach has been adopted for the synthesis to achieve an antenna design fulfilling real specifications. As an alternative to the structure proposed in [8], a frequencyscanning antenna with printed planar structure is proposed. This structure consists of a low-loss slow-wave printed meander line. The meander line is based on the even-mode bilateral broadsidecoupled suspended microstrip line (BSML), which has been proven to exhibit a much lower loss than that of a microstrip line [9]. The design example is a 16-slot array fed by the slowwave meander-line units. The design flexibility of the antenna on the range of scanning angle, frequency sensitivity, and aperture distribution is presented. This flexibility comes from the possible separate design of the slow-wave feed structure and the radiating elements. The antenna exhibits flat gain and low sidelobe levels (SLLs) in the whole scanning range with less frequency resources. II. ANTENNA DESIGN The configuration of the proposed antenna is shown in Fig. 1(a). The antenna consists of three dielectric substrates, and the entire structure is based on the BSML. As shown in Fig. 1(b), by symmetrically printing the metal strips on both sides of the middle substrate and biasing equi-voltage to both of them, the even mode of BSML can be excited. Most of the currents concentrate on the top surfaces of the two metal strips, and the electromagnetic fields are mostly distributed in the air region between the substrate and the metal plates. Thus, air of height is employed as transmission medium in the structure. The meander-line unit for phase shifting is symmetrically printed on both sides of substrate Rogers 5880 ( ) in the middle. Power is coupled to mm, the bilateral printing feedline on the same substrate, which is used to feed the equal radiating slots spaced apart by a distance of , as shown in Fig. 1(c) and (d). Thus, meander-line units, power dividers, and feedlines are all symmetrically printed on both sides of the middle substrate. For clear presentation, in Fig. 1(c), the printed patterns are placed a distance away from Rogers 5880. The slots are etched on the inner metal surface of mm, the upper Rogers 4003 ( ). The inner metal surfaces of the upper and lower substrates Rogers 4003 are both employed as grounds as shown in Fig. 1(b). The host transmission structure is terminated in a matched load at the end.
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Fig. 2. Insertion loss comparison among different units.
designing the meander-line units, where determines the position of transmission zeros in the frequency response and contributes to the coupling strength and insertion loss according to the coupled-line theory. Therefore, the number of bending times of slow-wave units is a very important parameter in the design. For example, is fixed at 14.4 mm, and the electric length is fixed at at 9.5 GHz, and the losses of meander-line units with different are compared in Fig. 2. It is found that when , the lowest loss is obtained (0.10 dB) at 9.5 GHz, and the unit exhibits very low loss in a wide frequency range in X-band. The loss differences are obvious in Fig. 2, showing that careful selection of the bending times is very important. B. Beam-Scanning Mechanism
Fig. 1. Configuration of the proposed frequency-scanning antenna. (a) Overall antenna prototype. (b) 3-D view of the BSML. (c) Single element. (d) Feedline specifications.
If is the actual length of a slow-wave meander-line unit in the antenna array, is the distance between the two adjacent radiating elements, and is the guided wavelength in the meander line. The space phase shift between two elements and the interelement phase shift in the array can be expressed respectively as
A. Design of Slow-Wave Meander-Line Unit
(2)
In a conventional frequency-scanning antenna, the slow-wave unit employed for phase shifting is very important, especially for its losses. The even-mode BSML, as the structure shown in Fig. 1(b), may reduce the transmission losses significantly compared to a microstrip line and suspended substrate stripline [9]. Due to the low-loss characteristics, BSML is chosen to design the slow-wave structure between array elements in this letter. The longitudinal size of the element, , is given by
(3)
(1) where is the coupled-line space and is the width of the line as shown in Fig. 1(d). The number stands for the number of bending times of the meander-line unit, e.g., in Fig. 1(d). is chosen to avoid grating lobes. The electric length is determined by the scanning angle and working bandwidth. A different leads to a different coupled-line space and horizontal size . The two parameters are very important in
when (4) which leads to the beam-scanning angle (5) is a positive integer corresponding to a broadside where beam. If we rewrite (4) as follows: (6) where
and , the beam angle can be expressed as (7)
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
Fig. 3. Dispersion diagram of the proposed meander-line unit.
in (6) is the equivalent phase constant in the slow-wave structure. If the equal constant phase of the antenna elements is produced by the periodic structures, and the phase constant is a function of , then the beam can be scanned by varying the frequency. Take the meander-line unit shown in Fig. 1(d) as an example ( mm). The structure is still designed at 9.5 GHz, and mm . To achieve the scanning angle from to 45 in the band from 8.5 to 10.5 GHz, the integer is chosen to be 3. Then other parameters of the meander-line unit can be obtained easily as described in Section II-A. Fig. 3 plots the dispersion diagram of of the unit and the curve, , which follows the periodic extension of from . As shown in Fig. 3, the intersection points A and B indicate the backfire and endfire directions and point C indicates the broadside direction. Thus, the extended curve is like the dispersion curve in the dominant mode of CRLH structures, which has LH mode , RH mode , and the transition frequency in a balance case [10]. Using (6) and (7), we can see that the beam can be scanned from to 40 within only 2-GHz bandwidth. The frequency sensitivity is 57 /GHz. We also note that is only based on the slow-wave transmission structure without radiating elements. Thus, if the parameter is properly chosen, then the scanning beam is single with no multibeam problem [3], which should be considered in the design of LWAs operating in high-order space harmonics. Designers can easily adjust the frequency sensitivity and transmission losses of the proposed slow-wave structures by properly choosing specific design parameters. In the general design, and are first determined by the operating frequency, scanning angle, and bandwidth. Then, and are properly chosen for the meander line to exhibit low loss. C. Antenna Design Example As an example, we design a frequency-scanning antenna with the following specifications: a frequency range of 8.9–10.6 GHz, a scanning range of 30 to 45 , and an SLL of 20 dB. In the design, the 25-dB Taylor aperture distribution is applied. The parameters are chosen as: an integer in (6) of 3, mm, mm, (three bending times), mm, and mm. The simulated insertion loss of one meander-line unit is around 0.10 dB in the working bandwidth. Then, initial different coupling distances in
Fig. 4. Photograph of the fabricated antenna. (a) Printed structures and slots. (b) Overall antenna.
Fig. 5. Measured and simulated
-parameters.
every coupled power divider can be given easily by simple simulation according to the Taylor aperture distribution. The dimensions of the radiating slots fed by the even-mode BSML are: width mm, length mm, and feedline width mm, as shown in Fig. 1(d). Then, slight modification has been done in the full-wave simulation by using CST Microwave Studio. The fabricated antenna is shown in Fig. 4. Fig. 4(a) presents the printed meander line and feed structures on the middle substrate and the radiating slots etched on the upper substrate. The overall structure is shown in Fig. 4(b). To improve the flatness of the substrate, many Teflon pins are placed among the three substrates. In order to bias equi-voltage to the printing structures on both sides of the middle substrate, via are placed on the transmission structures in fabrication. III. EXPERIMENTAL RESULTS Fig. 5 presents the measured and simulated -parameters. The observed return loss and insertion loss are quite low, indicating a good matching and a good radiation performance. There is a maximum discrepancy of 10 dB in . However, when it is converted to the sum of radiation and ohmic losses discrepancy, it is only 0.71 dB. Fig. 6 shows the measured and simulated radiation patterns at different frequencies. The radiation pattern at 9.2 GHz is given by Fig. 6(a), where the beam angle is 14.5 . Fig. 6(b) displays the radiation at the broadside at 9.5 GHz. Fig. 6(c) presents the
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frequency-scanning antenna based on the low-loss even-mode BSML. IV. CONCLUSION
Fig. 6. Normalized measured and simulated radiation patterns at different frequencies. (a) 9.2 GHz. (b) 9.5 GHz. (c) 10 GHz.
A novel printed frequency-scanning antenna has been presented employing a low-loss, slow-wave, printed meander-line unit that is based on an even-mode bilateral BSML. The dispersion diagram, beam-scanning properties, and radiation characteristics have been discussed. This antenna scans from the backfire to the endfire in a narrow bandwidth resource with a frequency sensitivity of 43.24 /GHz. A low sidelobe level (20 dB) is achieved by easily controlling the aperture distribution. The increase of the frequency-scanning sensitivity can be realized by using larger . The tradeoff of the antenna size, loss, available frequency resources, and the scanning range is easy to achieve through a different choice of . In addition, the radiating element could be arbitrarily chosen without changing the slow-wave meander line. The slow-wave structure and the radiating elements can be designed independently, thus increasing the design flexibility. The antenna performances are relatively stable in the whole scanning range. The proposed structure is based on the printed planar technology that is easily integrated with other devices and components. REFERENCES
Fig. 7. Gain and the simulated radiation efficiency.
radiation pattern at 10 GHz. The beam angle switches to 24 . The slight beam shift between the simulated and measured results is due to the measurement error. The range of the scanning angle is 27.5 46 with the frequency sensitivity of 43.24 /GHz in the bandwidth of 8.9–10.6 GHz. The SLLs at most of the frequencies are near 20 dB. Fig. 7 shows the antenna gain response. The simulated radiation efficiency is also plotted in the figure. The measured maximum gain is 15.5 dBi, and the maximum efficiency is 85%. The maximum discrepancy between the simulated and measured gains is 1.5 dB. The measured gain is more than 13 dBi in the whole frequency range with the variation of less than 2.5 dB. The antenna gain is much higher than that of the frequency-scanning antennas based on microstrip meander line, in which maximum gain is 12.5 dBi of a 1 8 array and is only 14.2 dBi of a 1 32 array due to the high losses in the microstrip meander line [11]. The results validate the design for
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