Dual-band orthogonal polarised shared aperture array with shared-elements

In this paper, an orthogonal polarised dielectric resonator antenna (DRA) which can operate at S- and C-band, has been designed as “shared-element” for forming a dual-band shared aperture array. Fur- thermore, in this shared aperture array, the S-band array contains four shared-elements, and the C-band array contains four shared-elements and twelve C-band DRA elements. Moreover, a phase compensation power divider is employed to help offset the radiation performance dif- ferences between the two types of antenna elements when forming the C-band array. A prototype array was fabricated and measured, showing that the antenna can cover the band of 2.4–2.63 GHz and 6.9–7.1 GHz, respectively. The isolation between bands is higher than 30 dB. The cross-polarisation levels are less than -30 dB in both bands.


Introduction:
In modern communication and radar systems, the dualband shared aperture array is considered to be a good candidate for its multifunction and small size [1,2].
Several shared-aperture arrays have been reported, and the typical configurations are based on perforated technique [2][3][4], interlaced technique [4][5][6], and multi-band antenna elements [7][8][9]. In general, the shared-aperture arrays based on multi-band antenna elements realise the reuse of antenna itself, which leads to higher aperture efficiency. However, in a shared-aperture array based on multi-band antenna, the physical element spacing is the same at the different bands, but the electric length is different. Thus the frequency ratio (FR) of the higher band to the lower band is theoretically limited to smaller than two to avoid grating lobes at the higher band [9].
In this paper, a dual-band shared aperture array with an FR of 1:2.8 based on dual-band shared DRA elements has been designed. We evenly replaced some of the conventional C-band DRA array elements with shared elements to ensure that element spacing of both bands is at the proper electrical length. Moreover, we used phase compensation technology to offset the difference in phase patterns between them for excellent array radiation characteristic.
Design and analysis of the array element: The dual-band shared DRA element is working at the fundamental HEM 11δ mode with probe fed in S-band and HEM 133 mode with slot coupled in C-band. The detail information is shown in Figure 1a, and the electric field distribution of HEM 133 mode is shown in Figure 2. Figure 3a shows that the antenna covers the frequency bands of 2.4-2.63 GHz and 6.9-7.1 GHz with reflection coefficient below −10 dB.
The C-band DRA is working at HEM 11δ mode with slot coupled, and the detailed information is shown in Fig. 1b. The C-band antenna covers the frequency bands of 6.55-7.55 GHz with reflection coefficient below −10 dB as Figure 3b shows, which completely covers the working frequency band of the shared elements at C-band.   Since the C-band array is composed of two types of elements, it is essential to analyse their radiation performance separately, and the comparison chart is shown in Figure 4. The Figure 4a shows that both two types of antennas have symmetrical patterns with maximum radiation direction in +z direction. However, Figure 4b shows that in +z direction within ±50 degrees, the radiation phase of the shared element is about 138 degrees ahead. Based on the above analysis, we should use a 138-degrees phase shifter to delay the radiation phase of shared element when forming the C-band array by two types of element.
Array configuration: The proposed antenna operates in S-band (2.4-2.63 GHz) and C-band (6.9-7.1 GHz), and the FR is about 1:2.8, so a shared element will replace every three elements of C-band array. In order to avoid mutual coupling and appearing grating lobe, element spacing is selected as 36 mm (0.84λ 0 at 7 GHz) at C-band and 108 mm (0.9λ 0 at 2.5 GHz) at S-band. The S-band array employs an anti-phase feed network to reduce the cross-polarisation suffered by probe fed, and a parallel feed network with 138 degrees phase shift function is designed to match the C-band array. The configuration and some detailed information are shown in Figure 5.
Results and discussion: In order to validate the design, the proposed array is manufactured and tested. The fabricated antenna and the experimental environment is shown in Figure 6, and the simulated and measured data is shown in Figures 7-9.
The simulated/measured reflection coefficient and isolation data are shown in Figure 7. In S-band, the measured reflection coefficient below   −10 dB from 2.4 to 2.63 GHz is achieved; however, the measured reflection coefficient is wider than the simulated data. It may be caused by fabrication errors and the insertion losses of the SMA connectors. In C-band, because two types of DRA elements with different bandwidth are employed, the analysis and calculation of reflection coefficient are complex. However, both simulated and measured results cover the bandwidth of the shared element, i.e. 6.9-7.1 GHz. For both bands, the band isolation between two ports is better than 30 dB.  The simulated and measured radiation patterns are shown in Figures 8  and 9. In both two bands, the simulated and measured co-polarisation patterns show good agreement, and the measured cross-polarisation is better than −30 dB in the main lobe. However, the measured crosspolarisation is worse than simulated data in both two band. The possible causes are analysed as follows. In S-band, the feed probes are cut and welded manually, the length error is unavoidable, and the symmetrical characteristic of the array is affected, and which influence the polarisation performance. Moreover, the elements are pasted on GND by glue manually, and the position errors of DRA array will occur inevitably, which will influence the polarisation performance too. Especially in Cband, the small size elements are more sensitive to position error.
Conclusion: In this paper, an S/C shared aperture array with a FR of 1:2.8 based on dual-band shared DRA elements and phase compensation power divider is presented. Meanwhile, the problem that the FR of a dual-band shared aperture array based on dual-band element is no more than two is solved. The proposed antenna array is fabricated and measured, covering the frequency bands of 2.4-2.63 GHz and 6.9-7.1 GHz with the reflection coefficient below −10 dB, the isolations between bands are over 30 dB, the cross-polarisation level for both bands is lower than −30 dB in the main lobe. The proposed antenna has many advantages such as high aperture efficiency, high integration, and low cost and this technology has broad prospects in shared aperture array applications.