Circularly polarized phased array antenna elements and antenna arrays

Abstract

This invention discloses a circularly polarized phased array antenna element and antenna array, belonging to the field of antenna technology. The circularly polarized phased array antenna element includes: a first substrate layer, a second substrate layer, a fourth substrate layer, a fifth substrate layer, and a fourth metal ground layer arranged sequentially; parasitic patches, radiating patches, and bridge striplines are respectively disposed on the first, second, and fourth substrate layers; the bridge stripline is electrically connected to the radiating patch through an L-shaped feed probe, and the bridge stripline can generate two signals with a 90-degree phase difference. The antenna array includes several array elements arranged in a rectangular array; each array element includes four of the above-mentioned circularly polarized phased array antenna elements, arranged in a ring array. This invention expands the antenna bandwidth and optimizes the antenna's broadband performance, while reducing manufacturing costs.

Description

Technical Field

[0001] This invention relates to the field of antenna technology, specifically to a circularly polarized phased array antenna element and antenna array. Background Technology

[0002] With the rapid development of satellite communication technology, the performance requirements of circularly polarized antennas, as key components of the system, are increasing. In applications such as spaceborne, airborne, and shipborne systems, antenna design faces multiple challenges: First, space and cost constraints require antennas to have wide-angle scanning, broadband, and low-profile characteristics; second, to overcome transmission problems such as rain attenuation and multipath effects, circular polarization has become a basic requirement; in addition, physical size and weight constraints have prompted low-cost phased array antennas to gradually replace traditional parabolic antennas.

[0003] Single-point feeding and multi-point feeding are two important feeding methods in antenna design. While single-point feeding offers a simpler structure and a smaller feeding network compared to dual-wire feeding, reducing system complexity and PCB design difficulty, it places higher demands on antenna manufacturing precision to ensure a 90-degree phase difference for dual-wire circular polarization. Furthermore, it has lower fault tolerance compared to multi-point feeding, which allows for flexible adjustment using phase shifters. Additionally, the axial ratio performance of single-point feeding becomes unstable due to manufacturing precision errors; even a 0.1mm error can cause the axial ratio to exceed the limit. The additional current path in single-point feeding increases ohmic loss, especially in the millimeter-wave band, where high conductor loss directly reduces overall radiation efficiency. As described in CN115621748A, the single-point fed antenna requires the first segment of the coupling slot to be 1 / 4 of the dielectric wavelength, the second segment to be 1 / 2 of the dielectric wavelength, and the difference between L3 and L4 to be 1 / 2 of the dielectric wavelength. The resulting millimeter-wave phased array antenna has a relative bandwidth of 11.5% (26.3 GHz) below the reflection coefficient S11 of -10 dB, and a relative bandwidth of 10% (26 GHz) below the axial ratio of -3 dB in the maximum radiation direction. It can be seen that this antenna is not very effective in the Ku band, and its performance in the Ka band is also mediocre. Furthermore, even with the scheme described in CN115621748A, the antenna efficiency of a single-point fed antenna in the Ku band is not as good as that of a multi-point fed antenna.

[0004] In planar antenna design, microstrip antennas are an ideal choice due to their advantages such as low profile, ease of fabrication, and low cost. However, their inherent narrow bandwidth (typically about 1% to 10%) limits their application range. Currently, the main techniques for widening the bandwidth include L-shaped feeding, increasing the profile height, or introducing multi-resonant structures. However, these methods often come at the cost of sacrificing other performance aspects or increasing size. Summary of the Invention

[0005] The purpose of this invention is to provide a circularly polarized phased array antenna element and antenna array to solve the defects of limited bandwidth and high manufacturing difficulty when circularly polarized antennas are used to form wide-bandwidth angle scanning phased arrays.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0007] A circularly polarized phased array antenna element includes: a first substrate layer, a second substrate layer, a fourth substrate layer, a fifth substrate layer and a fourth metal ground layer arranged sequentially; parasitic patches, radiating patches and bridge striplines are respectively disposed on the first substrate layer, the second substrate layer and the fourth substrate layer.

[0008] The bridge stripline is electrically connected to the radiating patch via an L-shaped feed probe, and the bridge stripline can generate two signals with a 90-degree phase difference.

[0009] Furthermore, the aforementioned radiating patch includes a circular patch and two trapezoidal gradient feed lines; four trapezoidal slots are evenly spaced on the circular patch, and the two trapezoidal gradient feed lines extend into the two trapezoidal slots respectively, without contacting the circular patch.

[0010] Furthermore, the two trapezoidal gradient feeders extend into two adjacent trapezoidal slots.

[0011] Furthermore, the parasitic patch includes a concentrically arranged circular metal radiating patch and an annular metal strip, the annular metal strip being located outside the circular metal radiating patch and having a gap between the annular metal strip and the circular metal radiating patch.

[0012] Furthermore, the aforementioned circularly polarized phased array antenna unit also includes a metal shielding wall that penetrates the second substrate layer, the third substrate layer, and the fourth substrate layer, and has the same height as the L-shaped feed probe.

[0013] Furthermore, a hollow support layer is provided between the first substrate layer and the second substrate layer.

[0014] Furthermore, the hollow space of the aforementioned support layer is the same as the space enclosed by the metal shielding wall.

[0015] Furthermore, the transverse width of the aforementioned bridge strip is 1 / 4 of the dielectric wavelength.

[0016] Furthermore, the above-mentioned bridge stripline has two fourth metal through holes symmetrically arranged about the center line, and the direction of the line connecting the two fourth metal through holes is perpendicular to the setting direction of the two L-shaped feed probes.

[0017] An antenna array includes several array elements arranged in a rectangular array; each array element includes four of the aforementioned circularly polarized phased array antenna elements, which are arranged in a ring array.

[0018] The present invention has the following beneficial effects:

[0019] (1) This invention expands the antenna bandwidth. On the one hand, it adopts an L-shaped feeding form, with the feed line gradually changing horizontally on the trapezoidal feed line, and the metal radiating circular patch is gradually slotted. The L-shaped probe feeding structure introduces an inductive effect in the vertical part, while the feed probe part introduces a capacitive effect between itself, the patch, and the ground plane. This unique LC resonance mechanism excites a new resonance point through the mutual cancellation of capacitive and inductive effects, thereby expanding the bandwidth. On the other hand, a larger profile height is selected to reduce the antenna quality factor Q value, further optimizing the antenna's broadband performance.

[0020] (2) The present invention adds a parasitic patch, which introduces an additional resonant point through electromagnetic coupling, forming a multi-peak response with the resonant frequency of the main radiator, thereby merging and expanding the bandwidth. Furthermore, both the antenna radiator and the parasitic patch have central symmetry, which effectively reduces the cross-polarization level.

[0021] (3) This invention effectively improves the impedance matching between the feeding structure and the radiating structure through a new coupling feeding structure, overcomes the shortcomings of the narrow bandwidth (10%) of traditional patch antennas, improves the antenna impedance bandwidth, and finally achieves a relative bandwidth of 30.2%, thereby improving the functionality of the antenna and reducing the price and space costs. Attached Figure Description

[0022] Figure 1 This is an exploded view of the circularly polarized phased array antenna element provided in Embodiment 1 of the present invention.

[0023] Figure 2 This is a schematic diagram of the structure of the parasitic patch provided in Embodiment 1 of the present invention;

[0024] Figure 3 This is a schematic diagram of the structure of the radiation patch provided in Embodiment 1 of the present invention;

[0025] Figure 4 This is a schematic diagram of the structure of the first metallic stratum provided in Embodiment 1 of the present invention;

[0026] Figure 5 This is a schematic diagram of the structure of the bridge stripline provided in Embodiment 1 of the present invention;

[0027] Figure 6 This is a schematic diagram of the structure of the fourth metallic stratum provided in Embodiment 1 of the present invention;

[0028] Figure 7 The reflection coefficient curve of the circularly polarized phased array antenna element provided in Embodiment 1 of the present invention;

[0029] Figure 8The axial ratio curve of the circularly polarized phased array antenna element provided in Embodiment 1 of the present invention;

[0030] Figure 9 The gain curve of the circularly polarized phased array antenna element provided in Embodiment 1 of the present invention;

[0031] Figure 10 This is a schematic diagram of the structure of the antenna array (4x4) provided in Embodiment 2 of the present invention;

[0032] Figure 11 The beam scanning curve pattern of the 4x4 array of circularly polarized phased array antenna elements provided in Embodiment 2 of the present invention;

[0033] Figure 12 The radiation pattern of the 4x4 array of circularly polarized phased array antenna elements provided in Embodiment 2 of the present invention;

[0034] Figure 13 The axial ratio pattern of the 4x4 array of circularly polarized phased array antenna elements provided in Embodiment 2 of the present invention.

[0035] In the diagram: 1: First substrate layer; 2: Support layer; 3: Second substrate layer; 4: First prepreg layer; 5: Third substrate layer; 6: Second prepreg layer; 7: Fourth substrate layer; 8: Third prepreg layer; 9: Fifth substrate layer; 10: Metal shielding wall; 11: Circular metal radiating patch; 12: Circular metal strip; 13: Circular patch; 14: Trapezoidal gradient feed line; 15: First metal ground layer; 16: Bridge strip; 17: Second metal... 18: Third metal formation; 19: Fourth metal formation; 20: L-shaped feed probe; 21: Quasi-coaxial outer conductor; 22: Quasi-coaxial inner conductor; 23: Trapezoidal slot; 161: Fourth metal via; 191: Center signal line; 192: Circular window; 193: Rectangular window; 194: Circular portion; 195: Rectangular portion; 196: Third metal via; 211: Second metal via; 221: First metal via. Detailed Implementation

[0036] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0037] Example 1:

[0038] like Figure 1As shown, this embodiment provides a circularly polarized phased array antenna element, including: a first substrate layer 1, a second substrate layer 3, a fourth substrate layer 7, a fifth substrate layer 9, and a fourth metal ground layer 19 arranged sequentially. Parasitic patches, radiating patches, and bridge striplines 16 are respectively disposed on the first substrate layer 1, the second substrate layer 3, and the fourth substrate layer 7. The bridge striplines 16 are electrically connected to the radiating patches through L-shaped feed probes 20. The bridge striplines 16 can generate two signals with a phase difference of 90 degrees. The fourth metal ground layer 19 is provided with two center signal lines 191.

[0039] In this embodiment, the circularly polarized phased array antenna unit further includes a support layer 2, a first semi-cured layer 4, a first metal ground layer 15, a third substrate layer 5, a second semi-cured layer 6, a second metal ground layer 17, a third semi-cured layer 8, and a third metal ground layer 18. The first substrate layer 1, support layer 2, second substrate layer 3, first semi-cured layer 4, first metal ground layer 15, third substrate layer 5, second semi-cured layer 6, fourth substrate layer 7, second metal ground layer 17, third semi-cured layer 8, third metal ground layer 18, fifth substrate layer 9, and fourth metal ground layer 19 are arranged sequentially.

[0040] Preferably, the support layer 2 is thermally bonded to the first substrate layer 1 and the second substrate layer 3 respectively. The support layer 2 is made of FR4 board. By using inexpensive FR4 material and thermal bonding technology, the expensive low-loss board is replaced as the support structure, reducing the number of PCB lamination layers. This allows the antenna to meet performance requirements while significantly reducing processing costs.

[0041] like Figure 2 As shown, the parasitic patch includes a circular metal radiating patch 11 and a circular metal strip 12. The circular metal radiating patch 11 and the circular metal strip 12 are concentrically arranged, and their centers coincide with the geometric center of the antenna element. The circular metal strip 12 is located outside the circular metal radiating patch 11, and there is a gap between the circular metal strip 12 and the circular metal radiating patch 11. Preferably, the radius of the circular metal strip 12 is 1 / 4 of the dielectric wavelength. Through the arc design of the parasitic patch (circular metal radiating patch 11 and circular metal strip 12), electromagnetic coupling is achieved, introducing additional resonant points, forming a multi-peak response with the resonant frequency of the antenna's main radiator, thereby combining and expanding the bandwidth and improving the relative bandwidth ratio.

[0042] like Figure 3As shown, the radiating patch includes a circular patch 13 and two trapezoidal gradient feed lines 14. The center of the circular patch 13 coincides with the geometric center of the antenna element. Four trapezoidal slots 23 are evenly spaced along the edge of the circular patch 13. The upper base of each trapezoidal slot 23 is located inside the circular patch 13, and the lower base is located at the edge of the circular patch 13. The two trapezoidal gradient feed lines 14 extend into the two trapezoidal slots 23 respectively, without contacting the circular patch 13.

[0043] Preferably, the two trapezoidal gradient feed lines 14 extend into the two adjacent trapezoidal slots 23 respectively.

[0044] like Figure 5 As shown, the bridge stripline 16 includes two input terminals and two output terminals. The trace length of the bridge stripline 16 controls the input signals at the two output terminals to have a 90° phase difference, thereby realizing the synthesis of circularly polarized electromagnetic waves through two-port linear polarization. Preferably, the lateral width of the bridge stripline 16 is 1 / 4 of the dielectric wavelength.

[0045] The two output terminals of the bridge stripline 16 correspond one-to-one with the two trapezoidal gradient feed lines 14. The two output terminals of the bridge stripline 16 pass through the fourth substrate layer 7, the second prepreg layer 6, the corresponding output terminal of the bridge stripline 16, the third substrate layer 5, the first metal ground layer 15, the first prepreg layer 4, the second substrate layer 3, and the corresponding trapezoidal gradient feed line 14 in sequence through the L-shaped feed probe 20. The two input terminals of the bridge stripline 16 correspond one-to-one with the two center signal lines 191. The two input terminals of the bridge stripline 16 pass through the corresponding input terminal of the bridge stripline 16, the second prepreg layer 6, the fourth substrate layer 7, the second metal ground layer 17, the third metal ground layer 18, the fifth substrate layer 9, and the corresponding center signal line 191 in sequence through the coaxial inner conductor 22. A plurality of coaxial outer conductors 21 arranged in a ring are provided on the outer side of the coaxial inner conductor 22. The coaxial outer conductors 21 pass through the fourth substrate layer 7, the second metal ground layer 17, the third semi-cured layer 8, the third metal ground layer 18, the fifth substrate layer 9 and the fourth metal ground layer 19 in sequence.

[0046] Both the hole through which the coaxial inner conductor 22 passes and the hole through which the coaxial outer conductor 21 passes are metal vias, namely a first metal via 221 and a second metal via 211, respectively. There are five second metal vias 211, which serve as metal isolation. In this embodiment, the first metal via 221 sequentially passes through the second prepreg layer 6, the fourth substrate layer 7, the second metal ground layer 17, the third metal ground layer 18, and the fifth substrate layer 9. The second metal via 211 sequentially passes through the second prepreg layer 6, the fourth substrate layer 7, the second metal ground layer 17, the third metal ground layer 18, the fifth substrate layer 9, and the fourth metal ground layer 19.

[0047] The inside of the bridge stripline 16 is provided with two fourth metal through holes 161 that are symmetrical about the center of the antenna element, serving as shielding holes for the bridge stripline 16. The fourth metal through holes 161 are at the same height as the coaxial outer conductor 21, and the line connecting the two fourth metal through holes 161 is perpendicular to the setting direction of the two L-shaped feed probes 20.

[0048] like Figure 6 As shown, the fourth metal layer 19 has two sets of interconnected circular windows 192 and rectangular windows 193, each set of circular windows 192 and rectangular windows 193 corresponding to a center signal line 191. The center signal line 191 includes a circular portion 194 and a rectangular portion 195. The circular portion 194 is located inside the circular window 192, and the rectangular portion 195 is connected to the circular portion 194 and extends into the rectangular window 193. There are gaps between the circular portion 194 and the rectangular portion 195 and the circular window 192 and the rectangular window 193. The center signal line 191 can be directly bonded to the fifth substrate layer 9 for fixation. A coaxial inner conductor 22 passes through the circular portion 194 and is connected to the circular portion 194. Second metal through-holes 211 are distributed on the outside of the circular windows 192.

[0049] In this embodiment, a third metal through hole 196 is provided on both sides of the rectangular window 193, and the third metal through hole 196 penetrates the fourth metal ground layer 19 and the fifth substrate layer 9.

[0050] like Figure 1 and Figure 4 As shown, a metal shielding wall 10 is provided on the outside of the second substrate layer 3, the first semi-cured layer 4, the first metal ground layer 15, the third substrate layer 5, the second semi-cured layer 6 and the fourth substrate layer 7, and the height of the metal shielding wall 10 is consistent with the height of the L-shaped feed probe 20.

[0051] like Figure 1 As shown, the support layer 2 is hollow and uses air as the dielectric to reduce dielectric loss and improve radiation efficiency. When viewed from the top of the antenna element downwards, the projections of the circular metal radiating patch 11, the circular metal strip 12, the circular patch 13, the trapezoidal gradient feed line 14, the bridge strip line 16, and the center signal line 191 are all located within the cavity of the support layer 2.

[0052] The signal transmission path inside the antenna unit provided in this embodiment includes the following multi-stage conversion:

[0053] First, the chip and the antenna unit are transmitted to the antenna unit through the grounded coplanar waveguide (GCPW) formed by the circular window 192 and the rectangular window 193 of the fourth metal ground layer 19 and the center signal line 191. The signal is formed by a coaxial-like coaxial outer conductor 21 and a coaxial-like inner conductor 22 to achieve efficient lateral-to-longitudinal conversion.

[0054] Subsequently, the signal is transmitted through the vertical structure to the bridge stripline 16, and the two sets of output signals have a 90° phase difference due to the difference in the length of the bridge traces;

[0055] Finally, the signal is transmitted vertically through the L-shaped feed probe 20 to the trapezoidal tapered feed line 14, thereby achieving coupled feeding of the radiator. The radiator then couples the signal to the circular metal radiating patch 11 and the annular metal strip 12 on the first substrate layer 1, coupling the radio frequency signal to the parasitic patch. The circular metal radiating patch 11 and the annular metal strip 12 radiate electromagnetic signals outward.

[0056] The entire signal transmission link described above achieves low-loss, high-precision phase control signal transmission from the chip to the antenna.

[0057] Figure 7 This is a graph showing the reflection coefficient of the broadband dual-circularly polarized phased array antenna element based on L-type feed in this embodiment. The graph shows the variation of the antenna element's reflection coefficient with frequency. In the graph, the horizontal axis represents frequency (GHz); the vertical axis represents the reflection coefficient value (dB).

[0058] Depend on Figure 7 The coefficient values ​​of the S(1,1) and S(2,1) curves show that, within the range of 10.7GHz to 14.5GHz, i.e. the Ku band, the reflection coefficient S(1,1) < -15dB and the port isolation S(2,1) < -12dB are basically achieved, and the antenna impedance matching and cross isolation performance are good.

[0059] Figure 8 This is a graph showing the axial ratio of the broadband dual-circularly polarized phased array antenna element based on L-type feeding in this embodiment. The graph shows the axial ratio of the antenna element as a function of frequency. In the graph, the horizontal axis represents frequency (GHz); the vertical axis represents the axial ratio value (dB). Figure 8 As can be seen from the axial ratio values ​​of the curves, the axial ratio bandwidth of antenna elements with an axial ratio less than 2 is greater than 37%, indicating that the dual circular polarization performance of the antenna element is good.

[0060] Figure 9 This is a gain curve diagram of the broadband dual-circularly polarized phased array antenna element based on L-type feed in this embodiment. The figure shows the gain of the antenna element as a function of frequency. In the figure, the horizontal axis represents frequency (GHz); the vertical axis represents the gain value (dB). As can be seen from the gain curve values ​​shown in the figure, the antenna element basically achieves a gain greater than 6dB in the Ku band.

[0061] Example 2:

[0062] This embodiment provides an antenna array, including a plurality of array elements arranged in a rectangular array; the array elements include four broadband dual-circularly polarized phased array antenna elements based on L-type feeding as described in Embodiment 1, and the four antenna elements are arranged in a ring array. Figure 10 As shown, the antenna array in this embodiment includes 4 array elements, that is, the antenna array is a 4x4 array of antenna elements.

[0063] Figure 11 This figure shows the beamwidth scanning curve of the 4x4 array of broadband dual-circularly polarized phased array antenna elements based on L-type feed in this embodiment. In the figure, the horizontal axis represents the scanning angle, i.e., the angle between the scanning direction and the positive Z-axis in the antenna coordinate system; the vertical axis represents the antenna gain amplitude, in dB. The figure also shows the phase angles... The radiation direction curves on 11 surfaces, combined with the axial ratio pattern, show that the scanning angle range of the phased array elements reaches [a certain value]. .

[0064] Figure 12 This is the radiation pattern of a 4x4 array of broadband dual-circularly polarized phased array antenna elements based on L-type feed in this embodiment. In the figure, the horizontal axis represents the scanning angle, i.e., the angle between the scanning direction and the positive Z-axis in the antenna coordinate system; the vertical axis represents the antenna gain amplitude, in dB. The figure also shows the phase angles... Radiation patterns of the main polarization (LHCP) and cross polarization (RHCP) on four surfaces.

[0065] As shown in the figure, the 3dB beamwidth and Two sides and The two surfaces are basically overlapping, and the 3dB beamwidths of the four surfaces are not much different. The radiation pattern has good rotational symmetry, and the polarization performance of the antenna array is good within the 3dB beam range.

[0066] Figure 13 This figure shows the axial ratio radiation pattern of the 4x4 array of broadband dual-circularly polarized phased array antenna elements based on L-type feed in this embodiment. In the figure, the horizontal axis represents the scanning angle, i.e., the angle between the scanning direction and the positive Z-axis in the antenna coordinate system; the vertical axis represents the antenna axial ratio, in dB. The figure also shows the phase angles... Axis ratio pattern on four planes.

[0067] As can be seen from the curve in the figure, the antenna array in The 3dB axial ratio scanning angle is nearly 170 degrees, in The 3dB axial ratio scanning angle reaches nearly 200 degrees, and the coupling between array elements is low, meeting the performance requirements.

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A circularly polarized phased array antenna element, characterized in that, include: A first substrate layer (1), a second substrate layer (3), a fourth substrate layer (7), a fifth substrate layer (9), and a fourth metal ground layer (19) are sequentially arranged; parasitic patches, radiating patches, and bridge strip lines (16) are respectively laid on the first substrate layer (1), the second substrate layer (3), and the fourth substrate layer (7). The bridge stripline (16) is electrically connected to the radiation patch via an L-shaped feed probe (20), and the bridge stripline (16) can generate two signals with a phase difference of 90 degrees. The radiation patch includes a circular patch (13) and two trapezoidal gradient feed lines (14); the two trapezoidal gradient feed lines (14) extend into two adjacent trapezoidal slots (23) respectively, and do not contact the circular patch (13).

2. The circularly polarized phased array antenna element according to claim 1, characterized in that, The parasitic patch includes a concentric circular metal radiation patch (11) and an annular metal strip (12), the annular metal strip (12) being located outside the circular metal radiation patch (11) and having a gap between the annular metal strip (12) and the circular metal radiation patch (11).

3. The circularly polarized phased array antenna element according to claim 2, characterized in that, The radius of the annular metal strip (12) is 1 / 4 of the dielectric wavelength.

4. The circularly polarized phased array antenna element according to claim 1, characterized in that, It also includes a metal shielding wall (10) that penetrates the second substrate layer (3), the third substrate layer (5) and the fourth substrate layer (7) and has the same height as the L-shaped feed probe (20).

5. The circularly polarized phased array antenna element according to claim 4, characterized in that, A hollow support layer (2) is provided between the first substrate layer (1) and the second substrate layer (3).

6. The circularly polarized phased array antenna element according to claim 5, characterized in that, The hollow space of the support layer (2) is the same as the enclosed space of the metal shielding wall (10).

7. The circularly polarized phased array antenna element according to claim 1, characterized in that, The transverse width of the bridge stripline (16) is 1 / 4 of the dielectric wavelength.

8. The circularly polarized phased array antenna element according to claim 1, characterized in that, The electrical bridge strip (16) has two fourth metal through holes (161) symmetrically arranged about the center line, and the line connecting the two fourth metal through holes (161) is perpendicular to the arrangement direction of the two L-shaped power supply probes (20).

9. The circularly polarized phased array antenna element according to any one of claims 1 to 8, characterized in that, The bridge strip (16) is a two-stage bridge.

10. An antenna array, characterized in that, It includes several array elements arranged in a rectangular array; the array element includes four circularly polarized phased array antenna elements as described in any one of claims 1 to 9, and the four antenna elements are arranged in a ring array.

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