Research Article  Open Access
Min Wang, DaoYu Wang, Wen Wu, DaGang Fang, "SingleLayer, DualPort, DualBand, and OrthogonalCircularly Polarized Microstrip Antenna Array with Low Frequency Ratio", Wireless Communications and Mobile Computing, vol. 2018, Article ID 5391245, 10 pages, 2018. https://doi.org/10.1155/2018/5391245
SingleLayer, DualPort, DualBand, and OrthogonalCircularly Polarized Microstrip Antenna Array with Low Frequency Ratio
Abstract
A singlelayer, dualport, dualband, and dual circularly polarized (CP) microstrip array is designed for satellite communication in this paper. The operating frequencies are 8.2 and 8.6 GHz with a very low ratio of 1.05. First, a rectangular patch element is fed through microstrip lines at two orthogonal edges to excite two orthogonal dominant modes of TM_{01} and TM_{10}. The very low frequency ratio can be realized with high polarization isolations. Then, a 2by2 dualband dualCP subarray is constructed by two independent sets of sequentially rotated (SR) feed structures. An 8by8 array is designed on the singlelayer thin substrate. Finally, by utilizing onetofour power dividers and semirigid coaxial cables, a 16by16 array is developed to achieve higher gain. Measured results show that the 16by16 array has 15 dB return loss (RL) bandwidths of 4.81% and 6.75% and 3 dB axial ratio (AR) bandwidths of 2.84% and 1.57% in the lower and the upper bands, respectively. Isolations of 18.6 dB and 19.4 dB and peak gains of 25.1 dBic and 25.6 dBic are obtained at 8.2 and 8.6 GHz, respectively.
1. Introduction
In satellite communication systems, dualband antennas are usually required for the uplink and downlink operating at different frequencies. Orthogonal polarization is much preferable to improve the isolation of separate transmitreceive channels, especially for dualband antennas with a low frequency ratio. Circular polarization is the better choice than linear polarization because of the advantages of insensitivity to antenna orientations, elimination of the signal Faraday rotation effect caused by the ionosphere, and resistance to bad weather conditions. Various antenna types can be used to address the sharedaperture dualband dualcircular polarized (CP) problems. Planar antennas, such as printed dipoles, slots and microstrip patches, become more favorite candidates attributing to their low profiles.
Traditionally, dualband duallinear or circular polarized antennas tend to adopt a multilayer and stackedpatch structure [1–5]. Separate elements are placed on different layers to achieve a dualband dualpolarized design flexibly. Thus, appropriate frequency ratios can be easily realized [1, 2]. Agile feed networks are designed to form larger arrays with high gain and efficiency [3, 4], improved frequency response [2], and wide bandwidth [5]. The only drawback is that the fabrication process of multilayer antennas is rather difficult and costly.
Several singlelayer antenna elements have been proposed to achieve a dualband and dualCP radiation. Cross slots with unequal armlengths can be loaded on patch antenna [6] or annularslot [7, 8] to achieve dualband and dualCP antennas with a singlelayer configuration. Nevertheless, the bidirectional radiation property of these slot antennas limits their applications for satellite. In this context, a dualband CP planar monopole antenna is presented in [9] by combining an “L”shaped strip and a “C”shaped strip. Besides, a circular patch with eight curved slots and a diskloaded coaxial probe is presented to achieve the dualband dual circularly polarized pattern [10]. Yet, their omnidirectional radiation is not desirable. It should be pointed out that there is also a singlelayer design that can achieve dualband and dualCP directional pattern [11]. However, the gain of abovementioned antennas is relatively low. Moreover, their coaxial probe feeding scheme increases the difficulty to form a larger array that is much desirable in satellite communications.
Only a few works have been carried out on dualband dualpolarized antenna arrays with a singlelayer substrate [12–14]. In [12], two disparate patches are connected directly to construct a dualband orthogonalCP element fed by microstrip line, which can be extended to a larger array easily. A square patch loaded by four stubs is proposed in [13], where two pairs of orthogonal modes, that is, the TM_{10}/TM_{01} and TM_{30}/TM_{03}, are excited simultaneously. In these two cases, the dualband orthogonalCP microstrip array has been implemented on a singlelayer substrate, but the low frequency ratio is difficult to achieve. The realized ratios of two center frequencies are 1.44 and 1.42, respectively, which are rather high for some particular satellite communication applications. In [14], a low frequency ratio of 1.14 is achieved by exciting TM_{10} and TM_{01} modes of a rectangular patch, while it is orthogonal linear polarized (LP). In addition, these configurations tend to have only a single port, which is not suitable for systems with separate transmitreceive antennas.
In this paper, a dualport, dualband, and dualCP microstrip array on a singlelayer substrate is presented. First, by exciting two orthogonal dominant modes of TM_{01} and TM_{10}, a rectangular patch is adopted to realize a very low frequency ratio, while radiating the orthogonalLP waves. For a specific satellite communication system, the element is designed at 8.2 GHz and 8.6 GHz with a ratio of 1.05. Then, the sequentially rotated (SR) feeding scheme is utilized to construct a dualport, dualband, and dualCP array with improved impedance and axial ratio (AR) bandwidth. For demonstration, a 2by2 subarray is constructed by two independent sets of sequentially feed network. Afterwards, an 8by8 array is proposed on the singlelayer thin substrate. Finally, by utilizing the onetofour power dividers and semirigid coaxial cables, a 16by16 array is successfully developed with high gain of more than 25 dBic. Measured results indicate that the two antenna arrays exhibit ideal radiation patterns and good isolations of better than 18 dB between two ports.
2. Design Concept
The specifications of the antenna to be designed are shown in Table 1. It is dualport, dualband, and orthogonalCP at 8.2 and 8.6 GHz. The targeted gains of 25 dBic are not very high for singleband microstrip antenna arrays, which can be achieved with about 100 elements. However, they must be met for a dualport sharedaperture array as well as specified frequencies and polarization. The bandwidths of 15 dB return loss (RL) and 3 dB axial ratio are 80 MHz in both bands. The fractional bandwidths are about 1%.

The frequency ratio is only 1.05. It is convenient to construct a sharedaperture array with equal element spacing in two bands, but it causes restrictions on the designs of the feed networks and sharedaperture elements (or subarray). For such a low frequency ratio, it is difficult to implement a diplexer with desirable isolations between two bands, so that two independent feed networks are required for dualport designs. Sharedaperture elements (or subarray) can be designed with singlelayer or multilayer configuration. For the former, four appropriate modes should be excited simultaneously in one patch to implement orthogonalCP in dual bands; however, it is difficult for the frequency ratio of 1.05. For the latter, the overlapped patches on two layers are required, but strong coupling significantly increases the design complexity. Moreover, the fabrication of multilayer antennas is rather difficult and costly. Therefore, we choose to design with a singlelayer configuration.
After a preliminary investigation, two candidates of the sharedaperture array on a singlelayer substrate were investigated: (1) interlaced array consisting of two independent CP arrays in two bands; (2) sharedaperture array with sequentialrotated CP subarrays consisting of LP elements.
The first configuration is formed by using two independent interlaced arrays consisting of CP patch elements and feed networks. It is difficult to interlace two independent corporatefed arrays with appropriate element spacing and without crossing in a single layer. Seriesfed arrays are relatively easy; however, they suffer from very low bandwidth and tilted beam at frequencies off the center.
The second configuration of sharedaperture array adopts the sequentially rotated CP subarray as a unit cell. The SR technique was first proposed in [15], which substantially improves the bandwidth and polarization purity of CP arrays in spite of using the narrow band elements [16, 17]. Either LP or CP elements can be adopted to construct a SR array; however, LP rectangular patch has two orthogonal dominant modes which can realize dualband radiation with very small frequency ratio as well as desired isolation. The following design is based on this configuration.
The patch with edges and of unequal length is adopted and shown in Figure 1. Two orthogonal dominant modes TM_{01} and TM_{10} are excited by two microstrip lines at the center of the orthogonal edges and for 8.2 and 8.6 GHz, respectively.
A basic SR subarray has its elements arranged in a 2by2 square or rectangular grid configuration with element angular orientation and feed phase arranged in either 0°, 90°, 0°, 90° or 0°, 90°, 180°, 270° fashion. In the latter arrangement, the axial ratio bandwidth of the array can be increased substantially [17]. Either parallel feed or serial feed can be used for a SR array [18]. Here, both of them are utilized to construct a dualband dualCP subarray with dual ports based on the dualband dualLP elements.
The 2by2 subarray is shown in Figure 2. Both the element angular orientation and feed phase are arranged in the 0°, 90°, 180°, 270° fashion. A serial feed network is placed at the array center and connected to to form righthand circularly polarization (RHCP) for the lower band. Meanwhile, a parallel feed structure is placed outside the subarray and connected to to form lefthand circularly polarization (LHCP).
The proposed 2by2 subarray possesses all the desired features: being singlelayer, dualport, dualband, dualcircular polarization and with a low frequency ratio. This subarray is taken as the unit cell to form an 8by8 array by using two independent sets of parallel feed networks. The crossover of the feed lines in the two sets of the feed networks can be avoided through careful arrangement. By utilizing onetofour power dividers and semirigid coaxial cables, a 16by16 array is further developed. The performances of this design in various stages of development are provided below in detail.
3. DualBand DualLP Element
As illustrated in Figure 1, a dualband dualLP rectangular patch is designed on a singlelayer substrate of Rogers RT/duroid 5880 with the relative permittivity of 2.2 and a thickness of 0.787 mm. The dimensions of edges and are 11.65 mm and 11.04 mm, corresponding to the frequencies = 8.2 GHz and = 8.6 GHz, respectively. Two orthogonal dominant modes TM_{01} and TM_{10} are excited by two microstrip lines at a pair of orthogonal edgecenters. In each feed line, a dualsection transformer consisting of two quarterwavelength segments with impedance of 160 ohms and 100 ohms is introduced to match the patch to the feed line of 150 ohms. The line widths of the impedance transformers , , , and are 0.2 mm, 0.8 mm, 0.2 mm, and 0.6 mm, respectively. Simulated parameters of the element are shown in Figure 3. We can see that it matches well at two center frequencies, respectively. The corresponding 15 dB return loss bandwidths are 0.97% (8.16–8.24 GHz) and 1.05% (8.56–8.65 GHz). The isolations of two ports are better than 30 dB.
In this design, the frequency ratio is approximately equal to the ratio between the two orthogonal edges and of the patch. Because of the narrowband property and good polarization purity, two operating frequencies with a very low frequency ratio can be achieved with good isolations.
4. DualBand DualCP Subarray
4.1. Subarray Structure
As indicated in Figure 2, a 2by2 subarray is constructed by SR dualband orthogonalLP patches. All the elements match microstrip lines with characteristic impedance Z_{0}. The topological structure in Figure 2 is useful where dualband, dualport, and dualcircular polarization are required.
A serial feed network is placed at the array center and connected to to form RHCP for the lower band. It is modified from the one in Ref. [18]. The curved segments Z_{2} and Z_{3} perform as transitions rather than as quarterwavelength impedance transformers. Only onequarterwavelength impedance transformer Z_{1} is used to match the subarray to impedance Z_{0}. Therefore, the network can be more compact to be accommodated in an array with small element spacing. To meet the feed phase requirements, 90° phase shifts are achieved by stretching the length of corners along with the arcs.
A corporate network is placed outside the subarray and connected to to form LHCP. It is a combination of three 3 dB Tjunction power dividers, in which a quarterwavelength impedance transformer Z_{4} is used. Impedance values of the transitions and impedance transformers are shown in Table 2, which ensure equal power required for each element. The 90° phase shifts are achieved by adding two 90° and one 180° microstrip segments.

4.2. Element Spacing Selection
For a conventional microstrip patch array, maximum directivity is obtained when the element spacing is in the range of 0.8–0.9, where denotes the wavelength in free space [19]. However, for a SR CP array with LP elements, gain bandwidth broadens for reduced spacing [20]. Therefore, element spacing of the proposed array should be chosen as small as possible. To accommodate the patch and feed networks, is chosen as 22 mm, about 0.6 and 0.63, where and are the free space wavelengths of = 8.2 GHz and = 8.6 GHz, respectively.
4.3. Simulated Results
Simulated parameters of the subarray are shown in Figure 4. It is seen that desired matching at two ports is obtained for both frequency bands. The 15 dB RL bandwidths are 3.92% (8.00–8.32 GHz) and 3.15% (8.43–8.7 GHz) in the lower and upper bands, respectively. The bandwidths are obviously broadened. The isolations between two ports are 20 dB and 16 dB at 8.2 GHz and 8.6 GHz, respectively. We can see that the isolation performance deteriorates obviously. This is the result of the broadening of the bandwidth and the coupling between the feeder and the patch. However, the insertion of a bandpass filter in an independent feed network can easily increase the isolation of up to 20 dB.
Simulated ARs versus frequency are shown in Figure 5. The 3 dB AR bandwidths are 3.23% (8.075–8.34 GHz) and 1.28% (8.57–8.68 GHz) in the lower and upper bands, respectively.
Gain patterns are shown in Figure 6. Maximum gains of 10.5 dBic and 11.29 dBic are obtained at 8.2 GHz and 8.6 GHz, respectively. It is seen that the patterns are much symmetrical and the cross polarization levels are below −20 dB in two principle planes. However, high crosspolarized lobes appear in the = 45° plane. The high diagonal lobes can be significantly reduced when the sequential array is placed in a larger array environment that will be shown in Section 5.
(a)
(b)
(c)
(d)
For SR arrays, the gain bandwidth is dependent on the elements’ polarization properties. The array factor (AF) of the 4element subarrays with different polarized elements is investigated in [21]. It shows that, for an SR array with LP elements, the AF is generally independent of frequency, but the maximum is 3 dB lower than an array with CP elements. That is the reason why the gains of the propose dualband array are about 3 dB lower than those of a conventional 4element microstrip array, and the gain bandwidth is mainly determined by the minimum of the impedance and AR bandwidth.
5. 8by8 DualBand DualCP Array
5.1. 8by8 Array Structure
Considering the 2by2 subarray as a dualport unit, an 8by8 array on a singlelayer substrate is designed and fabricated. As shown in Figure 7, a hard plastic plate with a thickness of 4 mm is set on the back. Two independent sets of parallel feed networks are used to feed 16 subarrays. It is seen that the designed layout can avoid the crossover of feed lines in the two sets of the feed networks.
To accommodate the networks and reduce the coupling between feed lines, the spacing is chosen as = 22 mm, = 28 mm, = 26 mm, and = 32 mm. The feed networks should be adjusted carefully for appropriate amplitudes and phases. The array dimension is 220 mm × 220 mm.
5.2. Simulated and Measured Results
parameters and ARs are optimized at two center frequencies, and the optimal values are shown in Table 3. Good impedance matching, moderate isolations, and desired CP radiation are achieved simultaneously. Measured parameters with desired performances are shown in Figure 8. The 15 dB RL bandwidths are 5.3% (7.91–8.34 GHz) and 7% (8.23–8.83 GHz) in the lower and upper bands, respectively. The isolations between two ports are 22.4 dB and 18.5 dB at 8.2 GHz and 8.6 GHz, respectively. We found that the impedance bandwidths are further broadened, which can be attributed to the increase of loss and coupling in the array. However, the isolations do not deteriorate further, even though two band overlap to some extent. It can be explained by the measured ARs shown in Figure 9. The 3 dB AR bandwidths are 3.23% (8.02–8.22 GHz) and 1.39% (8.54–8.66 GHz) in the lower and upper bands, respectively, which are close to those of the subarray. The minimum ARs of 0.45 dB and 0.4 dB are obtained, respectively, at 8.1 GHz and 8.6 GHz. This is ideal for CP performance to ensure good isolation. It is also found that the frequency deviation of the minimum AR is very small, which is due to the error in simulation and manufacturing process. At 8.2 GHz, the AR of 2.48 dB is acceptable.

Patterns at 8.1 GHz and 8.6 GHz are simulated and measured. Normalized patterns in the = 0° plane are shown in Figure 10. Measured and simulated results maintain good consistency. The patterns are very symmetric and have very low cross polarization level in the main beam. Halfpower beamwidths of 9.1° and 8.8°, and side lobes of −11 dB and −10 dB are obtained at 8.1 GHz and 8.6 GHz, respectively. Normalized patterns in the = 45° plane at 8.1 GHz are also given in Figure 11. It can be seen that the cross polarization reduces to −11.3 dB and is far away from the main beam. This is because of the small spacing of the subarray and the averaging effects of the large array.
(a)
(b)
The measured gains of 20.1 dBic and 20.5 dBic are obtained at 8.2 GHz and 8.6 GHz, respectively. While simulated gains are 21.3 dBic and 21.9 dBic. The deviations are mainly attributed to the fabrication tolerances and the dielectric and conductive loss.
5.3. Discussion on Array Configuration
In the above discussion, two independent sets of parallel feed networks are used to extend the subarray to the 8by8 array. The SR feed networks can be adopted again to further broaden the 15 dB RL and 3 dB AR bandwidth [18]. In that case, however, the gain bandwidth will decrease. In addition, the complexity of array arrangement will also increase dramatically for the dualband sharedaperture antenna array.
6. 16by16 DualBand DualCP Array
To achieve higher gains, a 16by16 array is designed. Because of the rapid increase of loss, microstrip line is not the advisable choice to expand the feed network. A onetofour power divider (PD) and semirigid coaxial cables are used to set up the 16by16 array, which are shown in Figures 12 and 13, respectively. The PD consists of four symmetric quarterwavelength impedance transformers to achieve complete impedance matching. Two independent PDs are designed for upper and lower bands, respectively. Measured return losses at the input ports are better than 20 dB over the operating bands. Insertion losses between input and output ports are less than 6.5 dB. Amplitude and phase deviations between four ways are less than 0.15 dB and 2°, respectively. The semirigid coaxial cables have a low insertion loss of about 0.1 dB and a small phase deviation less than 2°. Therefore, desired balance of amplitude and phase can be achieved. The total insertion loss, including the PD, the coaxial cables, and SMA connectors, is about 1 dB.
The fabricated 16by16 array is shown in Figure 14. It is seen from Figure 14(a) that the array consists of four 8by8 arrays with spacing values and of 32 mm, about 0.87 and 0.92. The array has a size of 424 × 424 mm^{2} and is fixed on a hard plastic plate with a thickness of 4 mm. Figure 14(b) shows the connections between the 8by8 subarray ports connected by the onetofour power dividers and semirigid coaxial cables.
(a)
(b)
Measured parameters are shown in Figure 15. The impedance bandwidths are 4.81% (7.91–8.30 GHz) and 6.75% (8.30–8.88 GHz) for 15 dB RL in the lower and upper bands, respectively. At 8.2 GHz and 8.6 GHz, the isolations between two ports are 18.6 dB and 19.4 dB, respectively. Measured ARs are shown in Figure 16. The 3 dB AR bandwidths are 2.84% (7.99–8.22 GHz) and 1.57% (8.52–8.66 GHz) in the lower and upper bands, respectively, which are close to those of the subarray and the 8by8 array. Minimum ARs of 0.4 dB and 0.6 dB are obtained at 8.1 GHz and 8.6 GHz, respectively. The small deviation of the minimum AR frequency in the lower band is in accordance with the deviation of the 8by8 array. At 8.2 GHz, the AR of 2.8 dB is acceptable.
Patterns are measured at 8.1 GHz and 8.6 GHz. Normalized patterns in the = 0° plane are shown in Figure 17. The patterns are very symmetrical and have very low cross polarization levels in the main beam. Halfpower beamwidths of 4.4° and 4.1°, and side lobes of −11.7 dB and −11 dB are obtained at 8.1 GHz and 8.6 GHz, respectively. Normalized patterns in the = 45° plane at 8.1 GHz are also given in Figure 18. It can be seen that the cross polarization level is decreased to −15.9 dB and far away from the main beam. This demonstrates that better cross polarization performance can be obtained for larger arrays.
(a)
(b)
Measured gains of 25.1 dBic and 25.6 dBic are obtained at 8.2 GHz and 8.6 GHz, respectively, which are in accordance with the measured results of the 8by8 array and the connection networks.
7. Conclusion
In this paper, a singlelayer, dualport, dualband, and dualCP microstrip antenna array is proposed. The frequencies of 8.2 GHz and 8.6 GHz with a ratio of 1.05 have been realized by adopting a dualband orthogonalLP rectangular patch as the elements. Two independent sets of SR feed networks are utilized to combine the dualband, dualLP elements into a 2by2 dualband, dualCP subarray. This subarray is taken as the unit cell and the 8by8 array is successfully designed on the singlelayer substrate. Furthermore, with the help of the onetofour power dividers and semirigid coaxial cables, a 16by16 array is also developed to achieve higher gains. Measured results of the 8by8 and 16by16 arrays show that good CP performance, impedance, and AR bandwidths have been obtained. Measured gains more than 25 dBic have been achieved for the 16by16 array. Isolations between two ports can be improved up to 20 dB conveniently by inserting bandpass filters into the independent feed networks.
The proposed array has advantages of the singlelayer and dualport structure and dualband dualpolarized performance. Therefore, it would be a good candidate for satellite communication systems, especially for dualband applications with a very low frequency ratio.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors would like to acknowledge National Natural Science Foundation of China for funding this study under the General Program entitled “Pattern Synthesis on the Conical Beam Antennas for the Applications of VehicleBorne Satellite Communication and MissileBorne Detection System” (Project Code 61771242).
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Copyright © 2018 Min Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.