WO2019170541A1

WO2019170541A1 - Extreme scanning focal-plane arrays using a double-reflector concept with uniform array illumination

Info

Publication number: WO2019170541A1
Application number: PCT/EP2019/055153
Authority: WO
Inventors: Aleksei DUBOK; Ali AL-RAWI; Adrianus B. Smolders
Original assignee: Technische Universiteit Eindhoven
Priority date: 2018-03-05
Filing date: 2019-03-01
Publication date: 2019-09-12

Abstract

A double reflector focal plane array antenna has a main reflector [702]; a sub-reflector [704]; and a focal plane array [706] comprising millimeter wave sensor elements arranged in a plane. The sub-reflector [704] has a surface curvature configured to create a bifocal distribution for on-axis incidence and focused distribution for oblique incidence with a lateral displacement across a common focal plane of the array [706]. Preferably, the surface curvature of the sub-reflector [704] has a discontinuity in a scanning plane, which may be realized by two parabolic reflectors joined at a surface curvature discontinuity, where the two parabolic reflectors have focal points in the focal plane of the array separated by the lateral displacement.

Description

EXTREME SCANNING FOCAL-PLANE ARRAYS USING A DOUBLE-REFLECTOR CONCEPT WITH UNIFORM ARRAY

ILLUMINATION

FIELD OF THE INVENTION

The present invention relates generally to wireless radio frequency communications. More specifically, it relates to millimeter wave (30-300 GHz) reflector antenna designs.

BACKGROUND OF THE INVENTION

Future millimeter-wave 5G (fifth generation) and beyond-sG wireless communication systems require new antenna concepts. One of these concepts is the focal-plane array (FPA), or staring array, which has an array of detector elements located at the focal plane of an imaging system. FPAs combine the benefits of phased-arrays and traditional reflector-based solutions, providing electronic beam-steering, high performance and low cost. FPAs can be used in Point-to-Point (PtP) connections used as backhaul in wireless communications. Other applications include radio astronomy, Ka-band satellite communication, and low-cost Ka-band (30-40 GHz) multi-function radars. However, a number of problems limit a wide introduction of these FPA systems. First of all, the field-of-view (FoV) for electronic beam steering is limited by a significant beam deviation in the focal plane, and as a result, relatively large arrays are required in the focal plane. This leads to high costs and complexity of the FPA system.

Secondly, the small number of simultaneously active array elements in traditional FPA systems limits the multiple-beam capability and the maximum effective isotropic radiated power (EIRP) that can be achieved.

The existing technology for wide scanning is limited by single reflector shaping or by use of cylindrical reflectors. The use of focal plane arrays (FPAs) presented in the literature is limited to narrow scanning and combination of arrays with typical well-known reflectors.

So far the wide scanning problem with FPAs has been overcome by increasing the size of the array. This leads to high cost, high power consumption and system complexity. The problem of increasing the EIRP has been overcome by increasing the size of the reflector or by using expensive high-power amplifiers. Both of these options are very costly and could bring extra problems like construction bulkiness or heat dissipation issues.

The main competing products for FPAs with wide scanning capability are traditional phased-arrays. In phased-array systems, the main limitation is the very high cost and extreme power consumption due to the need to use many active array elements, typically 1000-10000 elements. High cost and high power consumption are only acceptable in military radar applications.

Next to this, traditional reflector antenna solutions with a single feed are also used in existing applications. The traditional reflectors with a single feed have a very limited functionality. Multi-beam capability and the achievement of a wide scan range are not possible.

SUMMARY OF THE INVENTION

A millimeter wave focal plane array reflector system is provided. The double-reflector system has a sub-reflector surface that includes a surface curvature discontinuity so that incident electromagnetic waves have a double interaction with the sub-reflector. The reflector surfaces are designed to achieve an optimal performance within a predetermined scanning range. For example, the reflector system can achieve wide beam scanning within the angular range of ±20° in the azimuth plane using a very compact array, provide a wide frequency bandwidth in the range of 20-40 GHz, provide a wide illumination area of the array to maximize the EIRP using silicon-based integrated circuits (ICs), minimize the beam deviation in the focal plane region during scanning within the range of ±20° in the azimuth plane and this antenna system achieves a close to linear phase distribution in the array region. The approach allows the use of low-cost and highly-integrated silicon based ICs.

In one aspect, the invention provides a double reflector focal plane array antenna comprising: a main reflector; a sub-reflector; and a focal plane array comprising millimeter wave sensor elements arranged in a plane; wherein the main reflector, the sub-reflector, and the focal plane array are aligned in a double reflector configuration; characterized in that the sub-reflector has a surface curvature configured to create a bifocal distribution for on-axis incidence and focused distribution for oblique incidence with a lateral displacement across a common focal plane; the focal plane array is positioned in the common focal plane.

Preferably, the main reflector, the sub-reflector, and the focal plane array are aligned in an offset double reflector configuration. Preferably, the surface curvature has a discontinuity in a scanning plane. Preferably, the sub-reflector comprises two parabolic reflectors joined at a surface curvature discontinuity, wherein the two parabolic reflectors have focal points in the focal plane of the array separated by the lateral displacement. The double reflector focal plane array antenna may also include a processor receiving individual amplitude and phase values of each of the millimeter wave sensor elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. lA-B show ray tracing for two angles of incidence upon a traditional symmetrical double-parabolic reflector with a main reflector, sub-reflector, and sensor array, all positioned coaxially.

Fig. 2 shows ray tracing for a double reflector configuration with a complex sub reflector having a bifocal illumination of the array, where the sub-reflector has a discontinuity in its surface curvature, according to an embodiment of the invention. Fig. 3 shows ray tracing for an angle of incidence equal to half a maximum scan angle for a sub-reflector with a discontinuity, according to an embodiment of the invention.

Fig. 4 shows ray tracing for an angle of incidence larger than half of the maximum scanning capability for a sub-reflector with a discontinuity, according to an embodiment of the invention.

Fig. 5 is a graph of amplitude vs aperture radius illustrating scan performance of a double reflector system over the scan range from o to 20 degrees using a 0.8 m main reflector and sub-reflector with 10 cm discontinuity, according to an embodiment of the invention. Fig. 6 illustrates broadside operation showing a split in a bifocal distribution of a sub-reflector having a discontinuity and bifocal distribution.

Fig. 7A and Fig. 7B show ray tracing with two angles of incidence for an offset double reflector antenna design with complex bifocal sub-reflector with a surface discontinuity, according to an embodiment of the invention.

Fig. 8 is a graph of amplitude vs. aperture radius for different incident angles for the antenna system shown in Fig. 7A and Fig. 7B.

Fig. 9A and Fig. 9B are two cross-sectional views of an offset double reflector system with complex bifocal sub-reflector, according to an embodiment of the invention. Fig. 10A and Fig. 10B are two cross-sectional views of a sub-reflector of an offset double reflector system, according to an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide a focal plane array millimeter wave antenna system. A double-reflector configuration has a main reflector and bifocal sub reflector whose surfaces are shaped to achieve an optimal performance (e.g., illumination in the focal plane) of focal plane arrays (FPAs) within a defined scanning range. In this way a very large field-of-view (FoV) is obtained. The bifocal FPA system with reflector shaping reduces the size of the required phased-array feed (PAF) used for wide-angle scanning and at the same time to increase the number of simultaneously active array elements. In this way, the electronics of the PAF can be realized with silicon semiconductor technologies, since only a limited output power per element is required. The sub-reflector includes surface curvature discontinuities to realize the bifocal behaviour. These discontinuities allow greater scanning range without increasing array size.

As shown in Fig. lA and Fig. lB, a traditional symmetrical double-parabolic reflector has a main reflector 100, sub-reflector 102, and sensor array 104, all positioned coaxially on a common axis 106. The two figures demonstrate the beam tracing based on geometrical optic (GO) for two different angles of incidence. This design has a limited scan range. The beam deviation of the incident rays after interaction with the sub-reflector becomes larger than after interaction with the main reflector. As a result, FPAs with a linear size comparable to the main reflector are required even for scan ranges of only a few degrees.

In order to solve this issue, embodiments of the present invention use complex sub reflectors that provide a bifocal illumination of the array. As illustrated in the embodiment of Fig. 2, a main feature of these sub-reflectors is a discontinuity 200 in the curvature of the sub-reflector surface in the scanning plane. During scanning, the beams 202 from the main reflector 204 deviate from the axis of symmetry 206. The bifocal sub-reflector 208 compensates this deviation and directs focussed beams to the array 210 in such a way that the overall deviation is minimized or fully compensated. Due to the bifocal property of the sub-reflector, a significantly larger scanning range is obtained without requiring a larger array.

For certain specific angles of incidence, the design fully compensates beam deviation and illuminates the central region of the array. The value of this angle of incidence is preferably half of the maximum scanning capability. This value could have a small variation as seen in Fig. 5 and Fig. 8, where the angles of incidence with fully compensated beam deviation are equal to 7.5 - 8 degrees. The variation is due to the fact that reflector has been optimized not only for the smallest beam deviation during scanning but for amplitude and phase linearity along the array. If optimization has been done only for the smallest array (minimization of beam deviation only), the angle of incidence where the design fully compensates beam deviation and illuminates the central region of the array will be exactly equal to half of the maximum scanning capability. Beam tracing based on geometrical optics (GO) between the sub-reflector 300 with discontinuity 304 and the array 302 is illustrated in Fig. 3 for an angle of incidence equal to half the maximum scan angle. These focusing properties can be tuned by choosing the discontinuity value in the sub-reflector configuration. Preferably, the discontinuity value is proportional to the required scan range. For large scan angles we have more deviated beams from the main reflector which could be compensated if the sub-reflector surface is shifted along the scan plane. Instead of physically shifting the sub-reflector, we introduce the discontinuity in mathematical function which describes the sub-reflector surface.

Fig. 4 shows beam tracing between the sub-reflector 400 with discontinuity 404 and the array 402 for angles of incidence larger than half of the maximum scanning capability. In this case, the beam deviates to the edge of the array 402 and defines the maximum size of the array.

For angles of incidence less than half of the maximum scanning capability, the beam will deviate to the opposite side of the array. However, when the scan angle is reversed, the beam will start again to deflect to the center of the array. This fact allows the sub reflector with discontinuity to provide a wide scanning range using only small number of array elements.

An example of the scan performance of such a system over the scan range from o to 20 degrees using a 0.8 m main reflector and sub-reflector with 10 cm discontinuity is presented in the graph of Fig. 5. This figure shows electric-field cuts in the array plane of the complex offset reflector with sub-reflector discontinuity based on physical optics (PO) simulation for various scan angles, f = 30 GHz. In case of scanning in the range from -20 to o degrees, we will have a similar performance by mirroring along the horizontal-axis.

For broadside operation, as illustrated in Fig. 6, the operation region of the array 602 will be split in a bifocal distribution. The illumination regions are deviating less from the center of the array than in case of maximum scanning (Fig. 5). To increase the number of active elements, we can displace the array towards the sub-reflector 6oo. Nevertheless, this would decrease the scanning capability and would require the adjustment of the sub-reflector discontinuity 604. To summarize, the bifocal distribution by itself leads to the increasing ratio of active array elements, and it is a significant additional advantage of this configuration.

The antenna systems of the present invention may be used in 4G/sG/beyond 5G point- to-point wireless communication (20-90 GHz) and mobile base stations (30 GHz). In addition, they have applications to satellite communication, where the wide FoV and multi-beam capability provide multiple access to the several satellites simultaneously. Applications also include multi-function radars, for example in Ka-band (30-40 GHz), where the high EIRP and wide FoV enables a highly efficient radar system. In radio astronomy applications, the reflector system combined with array can provide a high sensitivity with wide FoV. These factors are crucial for the next generations of radio astronomy antennas.

For example, in one embodiment of the invention, the beam is able to scan +/- 20 degrees azimuth and +/- 1.5 degrees elevation with an array size 50% less compared with traditional parabolic reflector or about 6% of the phased array size with the same gain. The reflector type is Off-set Fed with gain of 40 dBi. Linear phase distribution in the array region is used for optical beamforming.

Embodiments of the invention employ wide angle beam steering. Multiple shaped reflecting surfaces are used to control the amplitude and direction of incident electromagnetic waves. More specifically, the multiple reflecting surfaces are shaped in such a way to combine the reflector antenna high gain properties and the scan capabilities of antenna arrays. With such synthesized surfaces, the number of active elements in antenna arrays is reduced significantly and therefore a low-cost system can be designed. In contrast, in a conventional system, an antenna array of more than 4000 active element are required to produce gain of 40 dBi. Increased gain can be achieved by using a conventional reflector antenna configuration, but scanning is only limited to few degrees. Embodiments of the invention use synthesized and shaped reflecting surfaces to combine the high gain of a reflector antenna and the scan capabilities of phased array. The number of active antenna elements of the array systems used in this configuration is less than 6% of conventional 4odBi array antennas.

Fig. 7A and Fig. 7B show two views of an offset double-reflector antenna design with bifocal secondary reflector, according to an embodiment of the invention. Incident beams 700 reflect from main reflector 702 and are directed to complex sub-reflector 704 which has a surface discontinuity between reflector regions with different focal points in the plane of the array 706.

The surface of main reflector 702 is shaped in such a way to spread the incident waves from different directions on the designated spots on the surface of sub-reflector 704. The surface of sub-reflector 704 is shaped in such a way to spread power and direction of the incident waves across a planar surface that contains antenna array 706. The power is distributed across the focal plane antenna array 706 in a manner similar to a bifocal distribution for waves that are incident on axis. For the waves with oblique incidence, focusing effects of the shaped surfaces convert bifocal distribution into a focus distribution with a small lateral displacement across the planar array.

In a traditional high gain reflector antenna or double reflector antenna, the field is distributed to what is known as the focal field. Radiation pattern scanning is obtained by displacing the focal field. This design, however, has limited scanning capabilities. In addition, in these traditional systems the array antenna number is proportional to gain. Full radiation pattern scanning is obtained by active phased antenna array. The main drawback is very large number of antenna elements are required for high gain applications. In contrast with these traditional designs, embodiments of the present invention have reflecting surfaces to produce a bifocal distribution for on axis incidence and focused distribution for pattern scanning. The number of active focal plane array elements that are needed is less than 10% of that for the total active array antennas. The total number of antenna elements is 50% less compare with the traditional parabolic reflector.

Table 1

Table 1 compares properties of the traditional prime-focus reflector system with the complex double offset reflector design of the present invention. Advantageously, the complex double offset reflector design uses the whole array efficiently during operation within whole scanning range.

The surfaces of the offset double reflector model are designed using a 3-dimentional offset reflecting surfaces model, which is described in relation to the two views of an offset double reflector configuration shown in Fig. 9A and Fig. 9B. Main reflector surface can be expressed in terms of a second-order polynomial:

where A[j are polynomial coefficients and ZQ is the shift of the polynomial function along the z-axis. Note that the vertical offset zb between the array and reflector should be at least a few wavelengths in order to avoid blockage and diffraction from the edges of the reflector and array. Mutual reflections between the reflector and array are also avoided in this case. In addition, a proper choice of the array position z_c avoids multiple reflections between the main reflector, sub-reflector and array. According to this method, the reflector discretization along the y-axis is given by yi=yi,-,y_N, yi=-D/ 2, YN=D/2.

And along the z-axis we now have: Zi=Z!,...,ZjV, Z_l=Z₀ff, ZN=Z₁+D.

The determination of the reflected waves from the main reflector is done in a similar was as for a center-fed reflector, based on Snell’s law.

Solving for the intersection of the reflected wave from the main reflector and the sub- reflector provides the interaction points on the sub-reflector surface: x_ysi and y_Si (in the xy-plane), x_ZSi and z_s; (in the xz-plane). The reflected wave from the surface of the sub reflector is defined in a similar way as the reflection from the main reflector. The position of the array is close to the main reflector, as illustrated in Fig. 9B. To obtain wide-scanning properties, we have applied additional design features to the sub-reflector geometry, as illustrated in Fig. 10A and Fig. 10B. Note that in the ray tracing diagrams of the present application, the angle of incidence does not always appear equal to the angle of reflection because the figures illustrate only the idea of discontinuity. The actual required value of discontinuity (y_s in equations) for a reflector with scanning range ±20 degree is 10 cm and would be hardly visible in the figures if they were geometrically accurate. The figures show a much bigger discontinuity value (up to 1 m) for purposes of illustration in order to highlight this feature.

The shaped sub-reflector with two different discontinuities in the xy and xz-planes is defined in terms of a second-order polynomial:

where F_s is the sub-reflector position along the x-axis and By are sub-reflector polynomial coefficients, z_s0 is the shift of the polynomial function along the z-axis, b the angle of rotation of the whole system in the xz-plane, Y (Fig. 9B) is the feeding angle of the array, z_s is a shift in the polynomial function of the sub-reflector along the z-coordinate and y_s a shift along the y-coordinate.

Most accurately the discontinuity value is defined based on equations above, where y_s is a discontinuity value of a sub-reflector in the scanning plane, and the required scan range. In other words, the discontinuity value is defined by the shape of the reflectors (its size and focal length, bigger reflectors will require proportionally bigger discontinuity, as well as bigger focal distance) and the required scan range (wider scanning range wider discontinuity). As a rule of thumb, on practice, the value of discontinuity could be chosen based on the angle of incidence where the design fully compensates beam deviation and illuminates the central region of the array, which is approximately equal to half of the maximum scanning capability. We have applied a cost function defined as follows to optimize the offset double- reflector system. The cost function includes: Estimation of the array illumination pattern

Minimization of the beam deviation during scanning

^■ Minimization of the phase nonlinearity between elements across the array

The cost function is optimized by variation of polynomial coefficients of main and sub reflector within required scan range and reflectors manufacturing limitations. For example, the total cost function can be represented as

where K _sh, ^_Amp, kp_h are weighting coefficients, which will be determined empirically. Costp_h can be estimated as the standard deviation for the phase path. Cost_Amp can be defined as a standard deviation based on the desired distribution. Cost_& can be defined as the standard deviation of the reflected waves in the array plane, determined by calculation of the standard deviation of the field distributions in case of normal incidence verse the scan case.

The obtained total cost function can be used to estimate how a specific shape of the reflector is suitable for the illumination of the array for broadside or in case of scanning. By varying the

polynomial coefficients, the optimization algorithm provides the most optimal configuration of the reflector according to the GO approach.

A direct relation is thus provided between the shape of the reflectors and the total cost function. The shape of the reflectors can be expressed in terms of polynomial coefficients including design features like reflector shape discontinuities and the shifts of the polynomial function along the coordinates axis. The total cost function is presented as a combination of different optimization goals. By varying the ratio between the cost functions of the amplitude distribution, the phase linearity along the array and the deviation of the field in the array plane during scanning, it is possible to optimize the reflectors for a particular application. These techniques may be used for a symmetrical center-fed single reflector and for a complex offset double-reflector FPA. In addition, it can be applied to various other kinds of reflectors. For example, a double reflector system may have a discontinuity in the sub-reflector (as described above), the main reflector, or both. These designs may be in symmetric or offset configurations. For wide-scanning applications, the sub reflector discontinuity is preferred.

Claims

1. A double reflector focal plane array antenna comprising:

a main reflector; a sub-reflector; and a focal plane array comprising millimeter wave sensor elements arranged in a plane;

wherein the main reflector, the sub-reflector, and the focal plane array are aligned in a double reflector configuration;

characterized in that

the sub-reflector has a surface curvature configured to create a bifocal distribution for on-axis incidence and focused distribution for oblique incidence with a lateral displacement across a common focal plane;

the focal plane array is positioned in the common focal plane.

2. The double reflector focal plane array antenna of claim 1 wherein the main reflector, the sub-reflector, and the focal plane array are aligned in an offset double reflector configuration.

3. The double reflector focal plane array antenna of claim 1 wherein the surface curvature has a discontinuity in a scanning plane.

4. The double reflector focal plane array antenna of claim 1 wherein the sub

reflector comprises two parabolic reflectors joined at a surface curvature discontinuity, wherein the two parabolic reflectors have focal points in the focal plane of the array separated by the lateral displacement.

5. The double reflector focal plane array antenna of claim 1 further comprising a processor receiving individual amplitude and phase values of each of the millimeter wave sensor elements.

6. A method of wireless communication comprising

receiving millimeter wave signals by a double reflector focal plane array antenna

according to claim 1.