MICROSTRIP ANTENNA DEVICE WITH CENTER-FED ANTENNA ARRAYS

Abstract

The present disclosure relates to a microstrip antenna device, which may comprise a center-fed antenna array. Further, the present disclosure provides a radar device, which comprises the antenna device, and a method for fabricating the antenna device. The antenna device comprises a substrate with top and bottom surface, a two-dimensional first and second conductive structure, which are arranged adjacent to each other on the top surface, and a two-dimensional third conductive structure, arranged on the bottom surface and providing an electric ground plane. The first conductive structure comprises a first array of antennas and a first feed network, and the second conductive structure comprises a second array of antennas and a second feed network. Further, a slot line is formed in the third conductive structure, for feeding a signal to the first feed network and to the second feed network.

Claims

1. A microstrip antenna device (100) comprising: a substrate (101) having a top surface (101a) and a bottom surface (101b), a two-dimensional first conductive structure (102a) and a two-dimensional second conductive structure (102b), arranged adjacent to each other on the top surface (101a) of the substrate (101), and a two-dimensional third conductive structure (102c), arranged on the bottom surface (101b) of the substrate (101) and providing an electric ground plane, wherein the first conductive structure (102a) comprises a first array of antennas (104a) and a first feed network (105a), each of the antennas in the first array (104a) being connected to the first feed network (105a), the second conductive structure (102b) comprises a second array of antennas (104b) and a second feed network (105b), each of the antennas in the second array (104b)being connected to the second feed network (105b), wherein a slot line (106) is formed in the third conductive structure (102c), for feeding a signal to the first feed network (105a) and to the second feed network (105b).

2. The microstrip antenna device (100) according to claim 1, wherein: the first feed network (105a) and the second feed network (105b) are electromagnetically coupled to the slot line (106) in a coupling region (300).

3. The microstrip antenna device (100) according to claim 2, wherein: the coupling region (300) is located between the first and the second array of antennas (104a, 104b).

4. The microstrip antenna device (100) according to claim 2, wherein: the coupling region (300) is located centrally between the first and the second array of antennas (104a, 104b).

5. The microstrip antenna device (100) according to claim 2, wherein: the slot line (300) is coupled to the first feed network (105a) and the second feed network (105b), in the coupling region (300), via a coupling structure (401a, 401b, 402), and the coupling structure (401a, 401b, 402) comprises a coupling portion (402) of the slot line (106), a coupling portion (401a) of the first feed network (105a) and a coupling portion (401b) of the second feed network (105b).

6. The microstrip antenna device (100) according to claim 5, wherein: the coupling structure (401a, 401b, 402) comprises a cross-over coupler.

7. The microstrip antenna device (100) according to claim 5, wherein: the coupling portion (402) of the slot line (106) comprises an end portion of the slot line (106), said end portion having a circular shape, the coupling structure (401a) of the first feed network (105a) comprises an end portion of the first feed network (105a), said end portion terminating in a first curved stub, and the coupling structure (401b) of the second feed network (105b) comprises an end portion of the second feed network (105b), said end portion terminating in a second curved stub, wherein the curved stubs and the end portion have the same curvature.

8. The microstrip antenna device (100) according to claim 7, wherein the first curved stub and the second curved stub are located above an inner region of the circular shape of the end portion of the slot line (106).

9. The microstrip antenna device (100) according to claim 1, wherein: the first feed network (105a) comprises a primary feed line and a plurality of secondary feed lines, the primary feed line passing through a central region of the first array of antennas (104a), and the secondary feed lines branching off from the primary feed line at different branch-off points, and wherein: the second feed network (105b) comprises a primary feed line and a plurality of secondary feed lines, the primary feed line passing through a central region of the second array of antennas (104b), and the secondary feed lines branching off from the primary feed line at different branch-off points.

10. The microstrip antenna device (100) according to claim 1, wherein: the first and the second array of antennas (104a, 104b) are symmetric to each other with respect to a symmetry axis.

11. The microstrip antenna device (100) according to claim 10, wherein: the coupling structure (401a, 401b, 402) is configured to introduce a 180° phase shift between the signal in the first feed network (105a) and the signal in the second feed network (105b).

12. The microstrip antenna device (100) according to claim 10, wherein: the slot line (106) extends along the symmetry axis.

13. The microstrip antenna device (100) according to claim 1, wherein: the first conductive structure (102a) and the second conductive structure (102b) are separated from each other by a distance in a range of 0.7-0.85 times a wavelength of operation of the microstrip antenna device (100).

14. The microstrip antenna device (100) according to claim 1, wherein the first array of antennas (104a) and the second array of antennas (104b) are both spatially periodic in a direction orthogonal to the slot line (106), with a spatial period in a range of 0.5-0.65 times the wavelength of operation of the microstrip antenna device (100).

15. The microstrip antenna device (100) according to claim 1, wherein: the antennas (104a) of the first array are arranged in a first lattice and the antennas (104b) of the second array are arranged in a second lattice.

16. The microstrip antenna device (100) according to claim 15, wherein each of the first array and the second array is a microstrip combline antenna array.

17. A radar device comprising a microstrip antenna device (100) according to claim 1.

18. A method (1100) for producing a microstrip antenna device (100), the method (1100) comprising: forming (1101) a two-dimensional first conductive structure (102a) and a two-dimensional second conductive structure (102b) adjacent to each other on a top surface (101a) of a substrate (101), the first conductive structure (102a) comprising a first array of antennas (104a) and a first feed network (105a), the second conductive structure (102b) comprising a second array of antennas (104b) and a second feed network (105b), each of the antennas (104a) in the first array being connected to the first feed network (105a), each of the antennas (104b) in the second array being connected to the second feed network (105b), and forming (1102) a two-dimensional third conductive structure (102c) on a bottom surface (101b) of the substrate (101) in order to provide an electric ground plane, wherein forming the two-dimensional third conductive structure (102c) comprises: forming (1103) a conductive layer on the bottom surface (101b) of the substrate (101), and forming (1104) a slot line (106) in the conductive layer, the slot line (106) being suitable for feeding a signal to the first feed network (105a) and to the second feed network (105b).

19. The method (1100) of claim 18, wherein forming (1103) the slot line (106) comprises removing conductive material from the conductive layer along a line.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0045] The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

[0046] FIG. 1 shows a microstrip antenna device according to an embodiment of the invention.

[0047] FIG. 2 shows the microstrip antenna device of FIG. 1 according to an embodiment of the invention.

[0048] FIG. 3 shows a microstrip antenna device according to an embodiment of the invention.

[0049] FIG. 4 shows a hybrid slot line/microstrip cross-over power splitter of a microstrip antenna device according to an embodiment of the invention.

[0050] FIG. 5 shows typical S-parameters of the slot line-to-microstrip transition, as used in a microstrip antenna device according to an embodiment of the invention.

[0051] FIG. 6 shows configurations used for performance comparison in: (a) a microstrip antenna device with an edge-fed array, (b) a microstrip antenna device with a symmetric center-fed full-microstrip array, (c) a microstrip antenna device with a symmetric center-fed antenna array, with ground integrated slot line feeding network, according to an embodiment of the invention.

[0052] FIG. 7 shows normalized radiation patterns of the microstrip antenna device comprising the edge-fed array at different frequencies for: (a) azimuth, (b) elevation.

[0053] FIG. 8 shows normalized radiation patterns of the microstrip antenna device comprising the center-fed full-microstrip array at different frequencies for: (a) azimuth, (b) elevation.

[0054] FIG. 9 shows normalized radiation patterns of the microstrip antenna device comprising the symmetric center-fed antenna array, with ground integrated slot line feeding network, e according to an embodiment of the invention at different frequencies for: (a) azimuth, (b) elevation.

[0055] FIG. 10 shows co-polar to cross-polar ratios in the main beam of microstrip antenna devices comprising different arrays in: (a) azimuth plane, (b) elevation plane

[0056] FIG. 11 shows a method for producing a microstrip antenna device, according to an embodiment of the invention.

[0057] FIG. 12 shows an example of a microstrip antenna device with an edge-fed array.

[0058] FIG. 13 shows an example of a microstrip antenna device with an interleaved center/edge-fed combline array.

DETAILED DESCRIPTION OF EMBODIMENTS

[0059] FIG. 1 and FIG. 2 shows a microstrip antenna device 100 according to an embodiment of the invention. FIG. 1 shows a top view of the microstrip antenna device 100 and FIG. 2(a) shows a side view/cross-section of the microstrip antenna device 100, and FIG. 2(b) shows a portion of the microstrip antenna device 100 in the top view.

[0060] The microstrip antenna device 100 comprises a substrate 101 having a top surface 101a and a bottom surface 101b. The substrate 101 may be an electrically isolating substrate, for instance, a dielectric. Further, the antenna device 100 comprises a two-dimensional first conductive structure 102a and a two-dimensional second conductive structure 102b, arranged adjacent to each other on the top surface 101a of the substrate 101. The first and the second conductive structure 102a, 102b may comprise metal, and may be formed by metallization of the top surface 101a of the substrate 101. The antenna device 100 further comprises a two-dimensional third conductive structure 102c, arranged on the bottom surface 101b of the substrate 101 and providing an electric ground plane. Also the third conductive structure 102c may comprise a metal, and may be formed by metallization of the bottom surface 101b of the substrate 101.

[0061] The first conductive structure 102a comprises a first array of antennas 104a and a first feed network 105a. Each of the antennas in the first array 104a is connected to the first feed network 105a. The second conductive structure 102b comprises a second array of antennas 104b and a second feed network 105b. Each of the antennas in the second array 104b is connected to the second feed network 105b. Thus, the microstrip antenna device 100 may comprise an antenna array comprising the first array of antennas 104a and the second array of antennas 104b, and may comprise a feed network of this antenna array comprising the first feed network 105a and the second feed network 105b.

[0062] Further, a slot line 106 is formed in the third conductive structure 102c, for feeding a signal to the first feed network 105 and to the second feed network 106. Thus, the third conductive structure 102c serves multiple functions: it serves as a ground plane of the microstrip antenna device 100; it serves to feed the signal into the first and second feed network 105a, 105b; and it may provide terminals to, for example, an integrated distribution line or external array feeder.

[0063] According to the above, the present disclosure provides, in the microstrip antenna device 100, a feeding architecture for the antenna array comprising the first and second array of antennas 104a, 104b, which employs the ground-integrated slot line 106. This enables flexibility in choosing where the antenna array is fed, e.g., allows central feeding of the antenna array comprising the first and second arrays of antennas 104a, 104b. In particular, this is achieved with low perturbation to the radiation section of the antenna array.

[0064] FIG. 3 shows the antenna device 100 according to an embodiment of the invention, which builds on the embodiment shown in FIGS. 1 and 2. The microstrip antenna device 100 is a practical, exemplary implementation of the proposed slot line 106 feeding scheme.

[0065] In particular, as an example, an N×M microstrip antenna array is considered, which is provided on top of the substrate 101. In particular, the first and second array of antennas 104a, 104b, which form the microstrip antenna array, are both spatially periodic in a direction orthogonal to the slot line 106. More particularly, the first and second array of antennas 104a, 104b each comprise a plurality of columns of antennas, in total N columns. Each column of antennas is formed by a plurality of antennas—in this example each column includes M antennas—which are arranged one after the other along the column direction, wherein the M antennas are interconnected by secondary feed lines. In this respect, the first feed network 105a and the second feed network 105b each comprise a primary feed line and a plurality of secondary feed lines (N/2 per feed network), wherein the primary feed lines pass through a central region of the first or second array of antennas 104a, 104b, respectively, and the secondary feed lines branch off from the respective primary feed line at different branch of points.

[0066] The ground plane-integrated slot line 106 distribution circuit allows bringing the signal from an external source to the antenna array, for example, to an inner region of the antenna array. In particular, the slot line 106 may be electromagnetically coupled to the feeding networks 105a, 105b in a coupling region 300. The coupling region 300 may be located between the first array of antennas 104a and the second array of antennas 104a, particularly, it may be located centrally between these arrays 104a, 104b, more particularly, it may be in a center of the antenna array formed by the first and second array of antennas 104a, 104b. As shown in FIG. 3, the first and the second array of antennas 104a, 104b may be symmetric to each other with respect to a symmetry axis, wherein the slot line 106 may extend along this symmetry axis.

[0067] The energy transfer between the microstrip antenna array comprising the first and second array of antennas 104a, 104b and the slot line 106 may be implemented by means of a proper slot line-to-microstrip transition at the coupling region 300 (e.g., balanced or single-ended, e.g., depending on the array geometries). In particular, the energy (particularly the above-mentioned signal) can be coupled in the coupling region 300 from the slot line 106 to the first and second feeding networks 105a, 105b, and vice versa, e.g., by means of proper slot line-to-microstrip splitters (i.e. differential crossover power splitters for fully-symmetric arrays, or single-ended slot line-to-microstrip transitions for non-symmetric geometries).

[0068] Interdistances Δx.sub.1 among the columns of antennas may be chosen to avoid the formation of strong grating lobes in the azimuth radiation pattern. The central interdistance Δx.sub.2 between the first array of antennas 104a and the second array of antennas 104b, can be slightly different from Δx.sub.1 to permit a sufficient clearance between the vertical slot line in the ground metallization (defined as “backbone” feeding line) and the open stubs of the central columns. In any case, also Δx.sub.2 may be small enough to avoid non-negligible degradation of the Side Lobe Level on the azimuthal cut. For instance, Δx.sub.1 may be 0.5-0.65 times the wavelength of operation of the microstrip antenna device 100. Δx.sub.2 may be 0.7-0.85, in particular, 0.76-0.82, more particularly 0.79-0.80, times the wavelength of operation of the microstrip antenna device 100.

[0069] The advantage of separating the main feeding (the slot line 106) from the radiating section (the first and second array of antennas 104a, 104b) on two different layers of the substrate 101 (the top surface 101a and bottom surface 101b) leads to a clear reduction of radiation loss from the feed itself. In addition, the balanced feed of a fully symmetric antenna array, which is enabled by a central feed, induces a significant reduction of cross-polarized radiation, sensitivity to manufacturing tolerance and temperature variations, and frequency dependency of the radiation characteristics. The slot line backbone can be easily connected to any external component (e.g., transmit/receive chipset modules), for instance, by a further standard slot line/microstrip transition (in the second coupling region; e.g., realized by a second cross-over coupler), which can be arbitrarily implemented on the top surface 101a or bottom surface 101b of the substrate 101 stack-up.

[0070] The connection between the different transmission line types, which are located on two opposite faces of the substrate 101, can be implemented without the need of vertical interconnects. Indeed, the energy transfer between the conductive structures 102a, 102b, 102c on opposite surfaces of the substrate 101, is possible thanks to a reactive coupling mechanism, which is controlled by means of a central slot line-to-microstrip transition.

[0071] An example, which is described in the following as proof of concept for demonstrating the effects achieved by embodiments of the invention, is a microstrip antenna device 100 comprising a fully symmetric microstrip “combline” array (composed of the first and second array of antennas 104a, 104b). The horizontal and vertical amplitude taperings of the microstrip array elements (antennas) and currents are optimized to guarantee a sufficiently low level of the side lobes in the radiation pattern.

[0072] FIG. 4 shows a cross-over power splitter, which may be used in microstrip antenna devices 100 according to embodiments of the invention, as balanced stripline-to-microstrip transition to provide a central feed to the antenna array, and transform a signal from slot line mode into microstrip mode, and vice-versa. The cross-over coupler comprises a coupling structure 401. The coupling structure 401 comprises a coupling portion 402 of the slot line 106, which may comprise an end portion of the slot line 106, and may have a circular shape as shown in FIG. 4. The coupling structure 401 may further comprise a coupling portion 401a of the first feed network 105a, which may comprise an end portion of the first feed network 105a, which may terminate in a first curved stub. The coupling structure 401 may also comprise a coupling portion 401b of the second feed network 105b, which may comprise and end portion of the second feed network 105b, which may terminate in a second curved stub. The first and the second curved stub—as shown in FIG. 4—may have the same curvature as the end portion of the slot line having the circular shape.

[0073] As a consequence, only a very simple stack-up is required (e.g., only one dielectric substrate 101a with two metallizations on top surface 101a and bottom surface 101b) with clear advantages in terms of easiness of manufacturing and cost-effectiveness. For the case at hand, the fully balanced radiating circuitry requires that the output currents, which are derived from the cross-over splitter, provide a broadband phase difference of 180° with each other, and zero amplitude unbalance. Such a behavior is easily obtained by the inversion of the electric field lines at the slot line Tee junction of the cross-over splitter. Typically, the relative impedance bandwidth of the slot line/microstrip hybrid cross-over splitter is pretty large (in the order of 50%). Typical S-parameters in amplitude and phase of the 180° cross-over power splitter are shown in FIG. 5

[0074] Examples of radiation performances and numerical comparison are now described in the following.

[0075] To this end, the configurations shown in FIG. 6 are considered: (a) a microstrip antenna device 1200 (similar to FIG. 2) comprising a 15×9 edge-fed comb line array implemented in full microstrip technology with equidistant columns at Δx.sub.1=0.65λ0; (b) a microstrip antenna device 600 comprising a 16×8 center-fed symmetric comb line array with full microstrip feed having column interdistances Δx.sub.1=0.65λ.sub.0 and Δx.sub.2=0.8λ.sub.0; (c) a microstrip antenna device 100 comprising a 16×8 center-fed combline array with hybrid microstrip/slot line feed, according to an embodiment of the invention. The interdistances among columns of antennas are also Δx.sub.1=0.65λ.sub.0 and Δx.sub.2=0.8λ.sub.0. λ.sub.0 is the wavelength of operation of the respective microstrip antenna device. All the configurations operate in the same frequency band [f.sub.min, f.sub.max] with central frequency f.sub.0. All the configurations use the same type of insulating substrate with double lamination of 17.5 μm-thick electrodeposited copper, a relative dielectric constant ε.sub.r=3, loss tangent tan δ=0.001, and substrate thickness h=0.127 mm. The center-fed array in full-microstrip technology (FIG. 6(b)) would use a similar architecture as the microstrip/slot line hybrid array (FIG. 6(c)) with just a replacement of all slot line-based components (vertical backbone slot line and cross-over splitter ring) with their microstrip counterparts.

[0076] The results described below demonstrate that the ground-integrated slot line feed according to embodiments of the invention provides way better radiation performance than fully coplanar microstrip either side or central feeding.

[0077] Radiation patterns in FIG. 7, FIG. 8, and FIG. 9 show the behavioral comparison of the three configurations at different frequencies f.sub.min, f.sub.0, and f.sub.max within a total relative bandwidth of 1.3%. FIGS. 7(a) and (b) show the azimuth and elevation normalized radiation patterns of the edge-fed array (as in FIG. 6(a)). As expected, the edge-feed suffers from an intrinsic beam squint of about 2° from f.sub.min to f.sub.max. In wideband radar applications, the relative bandwidth might reach about 6.5%, hence the total frequency-dependent beam squint could be in the order of 10°.

[0078] FIGS. 8(a) and (b) show the azimuth and elevation normalized radiation patterns of the center-fed full-microstrip array (as in FIG. 6(b)). Even though the beam squint is completely eliminated, it suffers from strong and asymmetric spurious radiation of the backbone microstrip feeding line. Moreover, the asymmetric microstrip cross-over divider has different coupling levels towards its neighboring left-hand column with respect to the closest column on its right-hand side. Those effects lead to a significant asymmetries of the radiated pattern and degradation of the side lobe suppression level.

[0079] FIGS. 9(a) and (b) show the azimuth and elevation normalized radiation patterns of microstrip antenna devices 100 according to embodiments of the present invention, namely the center-fed hybrid slot line/microstrip array (as in FIG. 6(c)). As expected, the beam squint is completely eliminated across the frequency bandwidth, and radiation from the backbone slot line is much weaker compared to the full-microstrip configuration. The slot line is weakly coupled to the central columns, therefore both azimuth and elevation patterns are much more balanced than the full-microstrip architecture.

[0080] As for the co-polar to cross-polar ratio (X-pol ratio), it is clear that the edge-fed array (as in FIG. 6(a)) provides the worst performances, as is demonstrated in FIG. 10 The azimuth performances of edge-fed and full microstrip center-fed array (as in FIG. 6(b)) are similar, whilst the architecture of the microstrip antenna device 100 according to an embodiment of the present invention (as in FIG. 6(c)) provides an average improvement of more than 15 dB. In the elevation plane, the differences are much more evident: the edge-fed array suffers from the unbalanced feed and radiation from the vertical microstrip lines of each column, therefore the X-pol ratio does not exceed 19 dB. The full-microstrip center-fed array, thanks to the balanced feed at each column, allows an improvement up to about 30 dB. Nevertheless, it still suffers from the second order radiation effect from the backbone microstrip line, which conducts a vertical current that radiates a spurious cross-polarized field. The best X-pol ratio in the elevation plane is provided by the balanced architecture with ground-integrated slot line 106 according to the embodiments of the present invention. Indeed, thanks to both the low spurious radiation from the slot line and the advantage of the balanced feeding, the X-pol ratio in elevation reaches values above 60 dB.

[0081] The proposed embodiments of the invention, compared to a multi-layer approach to feed the center of the array with ground-shielded feeding line (spurious coupling to the radiating elements and unwanted radiation from the feed), minimizes the number of manufacturing steps. Indeed, it helps reducing the stack-up complexity from four to two layers, leading to a considerable reduction in mass production costs and misalignment tolerances between multiple layers.

[0082] In this respect, FIG. 11 shows a method 1100 for manufacturing the microstrip antenna device 100 according to an embodiment of the invention. The method 1100 comprises a step 1101 of forming a two-dimensional first conductive structure 102a and a two-dimensional second conductive structure 102b adjacent to each other on a top surface 101a of a substrate 100. The first conductive structure 102a comprises a first array of antennas 104a and a first feed network 105a. The second conductive structure 102b comprises a second array of antennas 104b and a second feed network 105b, wherein each of the antennas in the first array 104a is connected to the first feed network 105a, and each of the antennas in the second array 104b is connected to the second feed network 105b.

[0083] The method 1100 further comprises a step 1102 of forming a two-dimensional third conductive structure 102c on a bottom surface 101b of the substrate 101, in order to provide an electric ground plane. The forming 1102 of the two-dimensional third conductive structure comprises a step 1103 of forming a conductive layer on the bottom surface 101b of the substrate 101, and a step 1104 of forming a slot line 106 in the conductive layer, the slot line 106 being suitable for feeding a signal to the first feed network 105a and to the second feed network 105b.

[0084] To summarize, the advantages of the embodiments of the present invention are: [0085] The embedded slot line 106 reduces the spurious radiation and interference towards the arrays of antennas 104a, 104b. [0086] Enhanced radiative performances of the arrays of antennas 104a, 104 that use such ground-integrated slot line central feeding (no frequency dependence of the main beam direction, symmetric side lobes, low cross-polarization). [0087] The slot line feeding also enables an easier and effective design of a fully symmetric microstrip antenna array (comprising first and second array of antennas 104a, 104b) with central feeding, leading to an improved robustness to tolerance variations (i.e. etching tolerance, substrate height, etc.). [0088] The stack-up is maintained cost-effective and simple, since the hybrid microstrip/slot line structure do only require one dielectric substrate to avoid unwanted cross-coupling between feeding line and array elements

[0089] The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.