Patent Description:
Multiple Input-Multiple Output (MIMO) radars usually exploit, at each physical Receive (Rx) or Transmit (Tx) channel, microstrip antenna devices including microstrip series-fed antenna arrays with an edge input feed. Such antenna arrays are simple to manufacture and relatively cost-effective. There are two different types of series-fed antenna arrays: travelling wave and standing wave. The first type uses a termination load that inhibits backward reflections, whilst the latter type does not use terminations, hence a standing wave across the array rises as effect of the combination of direct and reflected wave. In both approaches, feeding currents between the series antennas radiate a relatively high cross-polarized component of the electromagnetic field. In addition, both approaches suffer from a frequency-dependent direction of the main beam. Other issues are related to a strong dependency of the radiation and input impedance characteristics on temperature variations, etching, and dielectric tolerances, causing a lowering of the yield in mass production.

Edge-fed antenna arrays typically exploit a number of open stubs that populate a single column. The columns are conductively connected at one edge to a parallel microstrip feeding network. The latter is a sequence of Tee-junctions, which end up to a common input microstrip feed. Referring to <FIG>, there is shown a microstrip antenna device including a typical edge-fed comb-line array. The microstrip antenna device comprises an insulating substrate with a conductive ground plane adhered to the undersurface thereof, and comprises a pattern of etched printed circuitry on the obverse, major surface of the substrate.

The microstrip antenna device of <FIG>, which is provided as an example, radiates a horizontally polarized electric field as main polarization. The number of antennas and columns of antennas could be either even or odd, depending on the desired radiation properties. A microstrip feed at the center of the array is extremely difficult to be implemented, due to the lack of room. Further, it is also inefficient, due to unwanted radiation from the feed, and strong coupling to the central antennas of the array. In particular, the approach of using a microstrip edge feed makes the design, manufacturing, and integration to external components relatively easy and also the prototyping is cost-effective. However, the approach also leads to an unwanted frequency-dependent direction of the main beam, due to a variation of the relative phase associated to the feeding currents at each pair of elements.

A configuration that tried to solve these issues of a pure edge feed, introduced a microstrip antenna device with a hybrid edge/center array excitation and interleaved microstrip transmission lines. An arrangement of an interleaved comb antenna with double feeding was exploited by means of long microstrip lines, which are conductively connected to the center and sides of the array, as illustrated in <FIG>. However, the very long microstrip feeding lines still introduce some problems, which are typical of microstrip line networks, especially at K-band and above. In particular, there are intrinsic high losses, due to the long microstrip lines and strong cross-coupling among parallel microstrip branches, aside from a non-negligible spurious radiation that distorts the final radiated pattern from the array and increases the radiated cross-polarized component.

A further attempt to solve the problem of the unbalanced edge-feed configuration proposed a microstrip antenna device with a double interdigitated configuration of unbalanced comb array groups, in order to obtain an overall balancing effect of the radiation pattern, and to reduce tolerance to temperature and manufacturing variations. However, even though this approach achieves a simple and co-planar feeding (i.e., only one insulating substrate is required), two or more inputs are required, thus making interconnection to external components much more challenging and the overall feeding network highly lossy.

<CIT> discloses forming on the front surface of a dielectric substrate, a micro strip line L1 for power supply, and including an impedance converting circuit for converting impedance from characteristic impedance of the micro strip line L1 for power supply to characteristic impedance of a slot line L2. On a rear face of the dielectric substrate, the slot line L2 is formed so that it intersects perpendicularly with the micro strip line L1 for power supply substantially.

<CIT> discloses a series-feed array antenna including: a plurality of radiation units including a plurality of radiation devices which are arranged in a set interval, and transmit and receive signals; and a feeding unit supplying a current to each of the radiation units.

<CIT> discloses a planar array antenna comprising a powered antenna element and an adjacent passive element which are microstrip-line type ones and disposed on one principal surface of a dielectric substrate; and a feeding system for feeding high frequency power to the powered antenna element.

<CIT> discloses a switchable <NUM> DEG /<NUM> DEG phase shifter having a slotted line structure which is arranged at one side of a substrate and has an input line (SL1) and two slotted line paths (SL2, SL3) which branch off at the end of said input line (SL1), and having a stripline (ST2), which is coupled to the slotted line paths (SL1, SL2).

<NPL>, discloses a <NUM> ×<NUM> element microstrip comb-line antenna array for <NUM> automotive radar.

In view of the above-mentioned problems and disadvantages, embodiments of the present invention aim to improve the conventional microstrip antenna devices. An objective is to provide a microstrip antenna device with a better radiation performance, in particular, wherein an unwanted frequency-dependent direction of the main beam is suppressed. To this end, an improved feeding configuration is desired. In particular, a goal is to achieve a more balanced feeding of the antenna array of the microstrip antenna device. Ideally, a solution enabling center feeding of the antenna array is desired. In addition, the feeding configuration in the microstrip antenna device should show low losses. A fabrication of the microstrip antenna device should also be of low complexity. Further, a dependency of the radiation and input impedance characteristics on temperature variations, etching, and dielectric tolerances, should be small, in order to enable high yield in mass production of the microstrip antenna device.

The objective is achieved by the embodiments of the invention as described in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

A first aspect of the invention provide a microstrip antenna device comprising: a substrate having a top surface and a bottom surface, a two-dimensional first conductive structure and a two-dimensional second conductive structure, arranged adjacent to each other on the top surface of the substrate, and a two-dimensional third conductive structure, arranged on the bottom surface of the substrate and providing an electric ground plane, wherein the first conductive structure comprises a first array of antennas and a first feed network, each of the antennas in the first array being connected to the first feed network, the second conductive structure comprises a second array of antennas and a second feed network, each of the antennas in the second array being connected to the second feed network, wherein 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, wherein the antennas of the first array are arranged in a first lattice and the antennas of the second array are arranged in a second lattice, and wherein the microstrip antenna device is characterized in that each of the first array and the second array is a microstrip comb-line antenna array.

Together, the first array of antennas and the second array of antennas may form an antenna array of the microstrip antenna device of the first aspect. The slot line is able to feed this antenna array. Due to the slot line feeding, radiation characteristics of the microstrip antenna device are improved. In particular, a more balanced feeding of the antenna array becomes possible. As a consequence, an unwanted frequency-dependent direction of the main beam of the antenna device can be suppressed. The microstrip antenna device of the first aspect also shows low losses, is easy to manufacture, and thus may enjoy high yield in mass production.

In an implementation form of the first aspect, the first feed network and the second feed network are electromagnetically coupled to the slot line in a coupling region.

Due to the slot line arrangement, the location of the coupling region with respect to the antenna array comprising the first array of antennas and the second array of antennas can be flexibly selected. This enables various designs of the microstrip antenna device, and enables reducing the unwanted frequency-dependent direction of the main beam.

In an implementation form of the first aspect, the coupling region is located between the first and the second array of antennas.

Thus, the antenna array comprising the first and the second array of antennas can be fed in a balanced manner.

In an implementation form of the first aspect, the coupling region is located centrally between the first and the second array of antennas.

Thus, a center-fed antenna array, comprising the first and second array of antennas, can be realized.

In an implementation form of the first aspect, the slot line is coupled to the first feed network and the second feed network, in the coupling region, via a coupling structure, and the coupling structure comprises a coupling portion of the slot line, a coupling portion of the first feed network and a coupling portion of the second feed network.

In an implementation form of the first aspect, the coupling structure comprises a cross-over coupler.

In an implementation form of the first aspect, the coupling portion of the slot line comprises an end portion of the slot line, said end portion having a circular shape, the coupling structure of the first feed network comprises an end portion of the first feed network, said end portion terminating in a first curved stub, and the coupling structure of the second feed network comprises an end portion of the second feed network, said end portion terminating in a second curved stub, wherein the curved stubs and the end portion have the same curvature.

In an implementation form of the first aspect, 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.

The coupling structure of the above implementation forms achieves very good electromagnetic coupling between the slot line and the feeding networks, and provides advantages in terms of easiness of manufacturing and cost-effectiveness. The coupling structure also enables a fully balanced radiating circuitry, and thus improved radiation characteristics of the microstrip antenna device.

In an implementation form of the first aspect, the first feed network 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, and the secondary feed lines branching off from the primary feed line at different branch-off points, and wherein the second feed network 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, and the secondary feed lines branching off from the primary feed line at different branch-off points.

In an implementation form of the first aspect, the coupling region is a first coupling region and the microstrip antenna device further comprises: a transmission line arranged on the top surface of the substrate, wherein the transmission line is connectable to a feeder for the microstrip antenna device; and the transmission line is electromagnetically coupled to the slot line in a second coupling region, in particular via a second coupling structure.

Thus, the slot line may provide a terminal to an integrated distribution line or external array feeder.

In an implementation form of the first aspect, the first and the second array of antennas are symmetric to each other with respect to a symmetry axis.

Thus, a symmetric antenna array comprising the first and second array of antennas can be provided.

In an implementation form of the first aspect, the coupling structure is configured to introduce a <NUM>° phase shift between the signal in the first feed network and the signal in the second feed network.

This enables a fully balanced radiation characteristic of the antenna array comprising the first and second array of antennas.

In an implementation form of the first aspect, the slot line extends along the symmetry axis.

In an implementation form of the first aspect, the first conductive structure and the second conductive structure are separated from each other by a distance in a range of <NUM>-<NUM> times a wavelength of operation of the microstrip antenna device, in particular <NUM>-<NUM> times the wavelength of operation.

In an implementation form of the first aspect, the first array of antennas and the second array of antennas are both spatially periodic in a direction orthogonal to the slot line, with a spatial period in a range of <NUM>-<NUM> times the wavelength of operation of the microstrip antenna device.

Below <NUM> times the wavelength, the excessively close proximity between antennas on the same conductor would increase the mutual coupling, leading to a loss in the radiation and aperture efficiencies and, thus, to reduced gain.

As each of the first array and the second array is a microstrip comb-line antenna array, also a comb-line antenna array comprising the first array and the second array may be formed.

A second aspect of the present disclosure provides a radar device comprising a microstrip antenna device according to the first aspect or any implementation form thereof.

The radar device may comprise a radar transmitter, a radar received, or a radar transceiver. The radar device enjoys the above-described advantages of the microstrip antenna device of the first aspect.

A third aspect of the present disclosure provides method for producing a microstrip antenna device, the method comprising: forming a two-dimensional first conductive structure and a two-dimensional second conductive structure adjacent to each other on a top surface of a substrate, the first conductive structure comprising a first array of antennas and a first feed network, the second conductive structure comprising a second array of antennas and a second feed network, each of the antennas in the first array being connected to the first feed network, each of the antennas in the second array being connected to the second feed network, and forming a two-dimensional third conductive structure on a bottom surface of the substrate in order to provide an electric ground plane, wherein forming the two-dimensional third conductive structure comprises: forming a conductive layer on the bottom surface of the substrate, and forming a slot line in the conductive layer, the slot line being suitable for feeding a signal to the first feed network and to the second feed network, wherein the antennas of the first array are arranged in a first lattice and the antennas of the second array are arranged in a second lattice, and wherein the method is characterized in that each of the first array and the second array is a microstrip comb-line antenna array.

The conductive layer may be a metal layer. The metal layer may be formed by metallizing the bottom surface or part of the bottom surface. The microstrip antenna device is easy to fabricate with the method of the third aspect.

In summary, the aspects and implementation forms (embodiments of the invention) challenge the above-described problems of the edge-feed, by exploiting a different, e.g. central, excitation approach with very weak perturbation. The proposed feeding configuration using the slot line provides the benefit of a symmetrization of the antenna array's radiative performance on both azimuth and elevation cuts.

The embodiments of the invention may apply, in general, both to a fully-symmetrical architecture of the first and second array (horizontal and vertical planes) and to first and second arrays with partially symmetric geometry (only vertical plane). The embodiments of the invention rely on integrating the slot line feeder directly underneath the radiating section (first and second array) and into the arrays' ground plane. The embedded slot line feed does not introduce any relevant perturbations to the performances of the first and second arrays of antennas, and thus the antenna array formed by these arrays, compared to long microstrip co-planar solutions, and maintains an overall very simple stack-up.

<FIG> and <FIG> shows a microstrip antenna device <NUM> according to an embodiment of the invention. <FIG> shows a top view of the microstrip antenna device <NUM> and <FIG> shows a side view/cross-section of the microstrip antenna device <NUM>, and <FIG> shows a portion of the microstrip antenna device <NUM> in the top view.

The microstrip antenna device <NUM> comprises a substrate <NUM> having a top surface 101a and a bottom surface 101b. The substrate <NUM> may be an electrically isolating substrate, for instance, a dielectric. Further, the antenna device <NUM> 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 <NUM>. 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 <NUM>. The antenna device <NUM> further comprises a two-dimensional third conductive structure 102c, arranged on the bottom surface 101b of the substrate <NUM> and providing an electric ground plane. Also the third conductive structure 102c may comprise a metal, and may be sformed by metallization of the bottom surface 101b of the substrate <NUM>.

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 <NUM> 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.

Further, a slot line <NUM> is formed in the third conductive structure 102c, for feeding a signal to the first feed network <NUM> and to the second feed network <NUM>. Thus, the third conductive structure 102c serves multiple functions: it serves as a ground plane of the microstrip antenna device <NUM>; 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.

According to the above, the present disclosure provides, in the microstrip antenna device <NUM>, a feeding architecture for the antenna array comprising the first and second array of antennas 104a, 104b, which employs the ground-integrated slot line <NUM>. 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.

<FIG> shows the antenna device <NUM> according to an embodiment of the invention, which builds on the embodiment shown in <FIG> and <FIG>. The microstrip antenna device <NUM> is a practical, exemplary implementation of the proposed slot line <NUM> feeding scheme.

In particular, as an example, an NxM microstrip antenna array is considered, which is provided on top of the substrate <NUM>. 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 <NUM>. 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/<NUM> 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.

The ground plane-integrated slot line <NUM> 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 <NUM> may be electromagnetically coupled to the feeding networks 105a, 105b in a coupling region <NUM>. The coupling region <NUM> 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>, 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 <NUM> may extend along this symmetry axis.

The energy transfer between the microstrip antenna array comprising the first and second array of antennas 104a, 104b and the slot line <NUM> may be implemented by means of a proper slot line-to-microstrip transition at the coupling region <NUM> (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 <NUM> from the slot line <NUM> 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).

Interdistances Δx<NUM> 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<NUM> between the first array of antennas 104a and the second array of antennas 104b, can be slightly different from Δx<NUM> 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<NUM> may be small enough to avoid non-negligible degradation of the Side Lobe Level on the azimuthal cut. For instance, Δx<NUM> may be <NUM>-<NUM> times the wavelength of operation of the microstrip antenna device <NUM>. Δx<NUM> may be <NUM>-<NUM>, in particular, <NUM>-<NUM>, more particularly <NUM>-<NUM>, times the wavelength of operation of the microstrip antenna device <NUM>.

The advantage of separating the main feeding (the slot line <NUM>) from the radiating section (the first and second array of antennas 104a, 104b) on two different layers of the substrate <NUM> (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 <NUM> stack-up.

The connection between the different transmission line types, which are located on two opposite faces of the substrate <NUM>, 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 <NUM>, is possible thanks to a reactive coupling mechanism, which is controlled by means of a central slot line-to-microstrip transition.

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 <NUM> comprising a fully symmetric microstrip "comb-line" 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.

<FIG> shows a cross-over power splitter, which may be used in microstrip antenna devices <NUM> 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 <NUM>. The coupling structure <NUM> comprises a coupling portion <NUM> of the slot line <NUM>, which may comprise an end portion of the slot line <NUM>, and may have a circular shape as shown in <FIG>. The coupling structure <NUM> 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 <NUM> 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> - may have the same curvature as the end portion of the slot line having the circular shape.

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 <NUM>° 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 <NUM>%). Typical S-parameters in amplitude and phase of the <NUM>° cross-over power splitter are shown in <FIG>.

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

To this end, the configurations shown in <FIG> are considered: (a) a microstrip antenna device <NUM> (similar to <FIG>) comprising a 15x9 edge-fed comb-line array implemented in full microstrip technology with equidistant columns at Δx<NUM> = <NUM>λ0; (b) a microstrip antenna device <NUM> comprising a 16x8 center-fed symmetric comb-line array with full microstrip feed having column interdistances Δx<NUM> = <NUM>λ<NUM> and Δx<NUM> = <NUM>λ<NUM>; (c) a microstrip antenna device <NUM> comprising a 16x8 center-fed comb-line array with hybrid microstrip/slot line feed, according to an embodiment of the invention. The interdistances among columns of antennas are also Δx<NUM> = <NUM>λ<NUM> and Δx<NUM> = <NUM>λ<NUM>. λ<NUM> is the wavelength of operation of the respective microstrip antenna device. All the configurations operate in the same frequency band [fmin, fmax] with central frequency f<NUM>. All the configurations use the same type of insulating substrate with double lamination of <NUM>-thick electrodeposited copper, a relative dielectric constant εr = <NUM>, loss tangent tanδ = <NUM>, and substrate thickness h = <NUM>. The center-fed array in full-microstrip technology (<FIG>) would use a similar architecture as the microstrip/slot line hybrid array (<FIG>) with just a replacement of all slot line-based components (vertical backbone slot line and cross-over splitter ring) with their microstrip counterparts.

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.

Radiation patterns in <FIG>, <FIG>, and <FIG> show the behavioral comparison of the three configurations at different frequencies fmin, f<NUM>, and fmax within a total relative bandwidth of <NUM>%. <FIG> show the azimuth and elevation normalized radiation patterns of the edge-fed array (as in <FIG>). As expected, the edge-feed suffers from an intrinsic beam squint of about <NUM>° from fmin to fmax. In wideband radar applications, the relative bandwidth might reach about <NUM>%, hence the total frequency-dependent beam squint could be in the order of <NUM>°.

<FIG> show the azimuth and elevation normalized radiation patterns of the center-fed full-microstrip array (as in <FIG>). 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.

<FIG> show the azimuth and elevation normalized radiation patterns of microstrip antenna devices <NUM> according to embodiments of the present invention, namely the center-fed hybrid slot line/microstrip array (as in <FIG>). 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.

As for the co-polar to cross-polar ratio (X-pol ratio), it is clear that the edge-fed array (as in <FIG>) provides the worst performances, as is demonstrated in <FIG> The azimuth performances of edge-fed and full microstrip center-fed array (as in <FIG>) are similar, whilst the architecture of the microstrip antenna device <NUM> according to an embodiment of the present invention (as in <FIG>) provides an average improvement of more than <NUM> 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 <NUM> dB. The full-microstrip center-fed array, thanks to the balanced feed at each column, allows an improvement up to about <NUM> 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 <NUM> 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 <NUM> dB.

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.

In this respect, <FIG> shows a method <NUM> for manufacturing the microstrip antenna device <NUM> according to an embodiment of the invention. The method <NUM> comprises a step <NUM> 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 <NUM>. 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.

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

To summarize, the advantages of the embodiments of the present invention are:.

Claim 1:
A microstrip antenna device (<NUM>) comprising:
a substrate (<NUM>) 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 (<NUM>), and
a two-dimensional third conductive structure (102c), arranged on the bottom surface (101b) of the substrate (<NUM>) 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 (<NUM>) 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),
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, and
wherein the microstrip antenna device (<NUM>) is characterized in that each of the first array and the second array is a microstrip comb-line antenna array.