WO2018096307A1

WO2018096307A1 - A frequency scanned array antenna

Info

Publication number: WO2018096307A1
Application number: PCT/GB2017/000169
Authority: WO
Inventors: David Hayes; Richard Brooke Keeton
Original assignee: Plasma Antennas Limited
Priority date: 2016-11-28
Filing date: 2017-11-17
Publication date: 2018-05-31
Also published as: GB201620123D0; GB201719141D0; GB2559009A

Abstract

A frequency scanned array antenna comprising: (a) an electromagnetic feed (10); (b) a parallel plate structure having a reconfigurable reflector (11), the reconfigurable reflector (11) being composed of selectable elements that are either reflective, transmissive or absorptive, the selectable elements being plasma devices incorporating PiN diodes which when excited form a plasma between the parallel plates, and the reconfigurable reflector (11) being illuminated by the electromagnetic feed (10); and (c) a radial array (8) which is fed by the reconfigurable reflector (11) and which comprises regularly spaced elements which allow a beam launched from the electromagnetic feed (10) to be frequency scanned with a changing frequency, and the frequency scanned antenna array being such that the frequency scanning enables electronic beam steering in two orthogonal directions.

Description

A FREQUENCY SCANNED ARRAY ANTENNA

Field of the Invention

This invention relates to an antenna and, more especially, this invention relates to a frequency scanned array antenna.

Description of Prior Art

It is well known that a two-dimensional array of selectable electromagnetic feeds can be used in conjunction with a microwave-optic device such for example as a lens or reflector, in order to generate a beam pointing in a desired azimuth and elevation direction. It is also well known that parallel plate antenna structures are able to guide an electromagnetic wave. Furthermore, as disclosed in USA Patent No. 7,271 ,683, directed selected beam antennas are able to be produced by the use of controllable reflective structures between the parallel plates of a parallel plate antenna structure, in conjunction with electronically controlled switched reflective devices positioned between the parallel plates. The electronically controlled switched reflective devices may be, for example, a reflector composed of plasma PiN diodes or micro-actuators acting as controllable RD switches.

USA Patent Nos. 4,689,629 and 6,124,833 disclose the use of apertures that, when combined with centre fed antenna structures create broadly planar wave fronts that propagate normal to the aperture. More specifically, USA Patent No. 4,689,629 employs a centre fed dielectric disc with printed radiating elements. A radially propagating surface wave is generated at a central launch point in the disc. As the surface wave progresses radially outwardly, a proportion of its energy is transmitted away from the disc by the radiating elements. Provided the radiated elements are spaced and orientated appropriately for the required polarisation, the radiating elements add coherently in the far field, and the result is a fixed directed beam which is of maximum amplitude and which is normal to the disc. USA Patent No. 6,124,833 is similar in operation except that it uses a parallel plate construction having radial slots. A radially propagating transverse electromagnetic (TEM) wave is launched centrally into the parallel plate in the parallel plate disc construction. As the TEM wave progresses outwards, a proportion of its energy is transmitted through the slots, with the slots acting as radiating elements. Provided orthogonal radial pairs of radiating elements are spaced and orientated appropriately for the required polarisation, the radiating elements add coherently in the far field, and the result is a directed beam which is of maximum amplitude and which is normal to the parallel plate disc construction.

The structures disclosed in USA Patent Nos. 4,689,629 and 6,124,833 provide compact printed circuit board planar structures which are alternatives to fixed beam parabolic dishes. The compact printed circuit board planar structures are efficient in that they exhibit low loss. As a consequence, the structures are mainly used for satellite broadcast reception where a low cost antenna construction is a critical consideration.

The known antenna constructions disclosed above are fixed beam radial designs which are not able to perform two dimensional scanning. This limits the use of the known antenna constructions.

Brief Description of the Invention

It is an aim of the present invention to reduce the above mentioned problem.

Accordingly, in one non-limiting embodiment of the present invention there is provided a frequency scanned array antenna comprising:

(a) an electromagnetic feed;

(b) a parallel plate structure having a reconfigurable reflector, the reconfigurable reflector being composed of selectable elements that are either reflective, transmissive or absorptive, the selectable elements being plasma devices incorporating PiN diodes which when excited form a plasma between the parallel plates, and the reconfigurable reflector being illuminated by the electromagnetic feed; and

(c) a radial array which is fed by the reconfigurable reflector and which comprises regularly spaced elements which allow a beam launched from the electromagnetic feed to be frequency scanned with a changing frequency, and the frequency scanned antenna array being such that the combination of reflector reconfiguration and frequency scanning enables electronic beam steering in two orthogonal directions.

The frequency scanned array antenna of the present invention is advantageous in that the electronic scanning may be 2D scanning. Scanning may be via the electromagnetic feed which may be enclosed by the reconfigurable reflector. Azimuthal steering is able to be achieved using the reconfigurable reflector. Elevational steering is able to be achieved using the frequency scanning.

The frequency scanned array antenna of the present invention may advantageously be used multiply in a backhaul system. The frequency scanned array antenna may alternatively be used in mobile or fixed access systems for point to multi-point operation, and where the antenna apertures are tuned to allow for beam orientation and bandwidth requirements. The frequency scanned array antenna may alternatively be used to produce fan beams suitable for multiple-input multiple-output (MIMO) operation. The frequency scanned array antenna may alternatively be used in vehicular radar systems to facilitate automatic guidance or collision avoidance.

The frequency scanned array antenna may be one in which the reconfigurable reflector is fed from either the centre or the circumference of the radial array. The frequency scanned array antenna may be one in which the radial array comprises a parallel plate structure which is able to support a transverse electromagnetic wave, and in which the radial array has a regular radial pattern of slots. Alternatively, the frequency scanned antenna array may be one in which the radial array comprises a dielectric material with a ground plane supporting a surface electromagnetic wave, and in which the radial array has a regular radial pattern of metal patch elements. In both embodiments, the regularly spaced elements may be oriented to provide a linear to circular polarisation.

The frequency scanned antenna array of the present invention may be configured to operate in isolation.

Alternatively, the frequency scanned antenna array of the present invention may be configured to operate in conjunction with microwave-optical means which is reflecting or refracting. In this case, the frequency scanned antenna array may comprise the said means. The said means may be a reflector structure or radio frequency lenses.

The frequency scanned antenna array may include a rotationally symmetric microwave-optic component which is placed above the radial array, and which is for providing 360° azimuth coverage when a central axis of the radial array is pointed at zenith. The rotational symmetric microwave- optic component may comprise a lens or a mirror. Other types of rotationally symmetric microwave-optic components may be employed.

The frequency scanned array antenna may be configured to operate within the microwave, millimetre wave or tetrahertz bands. The frequency scanned array antenna may be configured to operate in other bands if desired.

Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 is a plan view of a surface wave antenna of the type disclosed in USA Patent No. 4,689,629;

Figure 2 is a plan view of a radial line slot antenna of the type disclosed in USA Patent No. 6,121 ,833;

Figure 3 shows reconfigurable reflector feeds to a radial line slot array;

Figure 4 is a cross-section showing combined selectable electromagnetic feed and an electromagnetic reflector in a common layer;

Figure 5a shows a selected reconfigurable elliptical reflector having double layers of electromagnetic switches;

Figure 5b shows a selected reconfigurable elliptical reflector having radial control tracks;

Figures 6a and 6b show an electromagnetic field between parallel plates for a selected reflector, with Figure 6a showing an off state, and Figure 6b showing an on state;

Figure 7 shows sector illumination allowing frequency scanning in a selected radial direction; Figure 8 shows a family of frequency scanned beams in an azimuth direction;

Figure 9 shows a frequency scanned slotted dielectric waveguide;

Figure 10 shows a set of frequency scanned beams in the azimuth direction at 61 - 67GHz;

Figure 11 shows a typical inward radial scan, and illustrates beam broadening;

Figure 12 illustrates centre beam broadening with frequency;

Figure 13 is a perspective view of a line of equal spaced radial elements;

Figure 14 is a plan view of a dual parallel plate ray trace illustrating the radial array being fed circumferentially;

Figure 15 shows two frequency scanned contour plots, with the left contour plot being at 52GHz and the right hand contour plot being at 62GHz;

Figure 6 shows 54 - 64GHz in 1GHz steps;

Figure 17 is a perspective view showing a frequency scanned radial array feeding a conical reflector;

Figure 18 shows azimuth and elevation plots for different frequencies, with the left hand plot being for 63GHz and the right hand plot being for 52 GHz;

Figure 19 shows a set of frequency scanned elevation beams;

Figure 20a shows an omni-directional beam structure where the reconfigurable reflector is inactive; Figure 20b shows an omni-directional beam obtained from the structure shown in Figure 20a;

Figure 21 is an omni-directional contour plot at zero elevation angle;

Figure 22 shows multiple focal points obtained by using a piecewise devisable reconfigurable reflector;

Figure 23 shows two non-overlapping mirrors, two overlapping mirrors, and three mirrors;

Figure 24 shows the effect of changing astart ;

Figure 25 illustrates elevational frequency scanning using a bi-conic antenna configuration;

Figure 26 shows a two-way opposing frequency link;

Figure 27 shows an autonomous drive vehicle having corner mounted radar sensors; and

Figure 28 shows an autonomous drive vehicle having a central mounted radar sensor.

Detailed Description of Preferred Embodiments

Figure 1 shows a surface wave antenna as disclosed in USA Patent No. 4,689,629. The USA patent employed a centre fed dielectric disc with printed radiating elements as shown in Figure 1. In this configuration, a radially propagating surface wave is generated at the central launch point. As the surface wave progresses radially outwards, a proportion of its energy is transmitted away from the disc by the radiating elements. Providing the radiating elements are spaced and orientated appropriately for the required polarisation, the radiating elements add coherently in the far field, and the result is a fixed directed beam which is of maximum amplitude and which is normal to the disc.

Figure 2 shows a radial line slot antenna as disclosed in USA Patent No. 6,124, 833. The radial line slot antenna operates similar to the surface wave antenna shown in Figure 1 , except that in the radial line slot antenna a parallel plate disc construction is employed with radial slots. In this configuration, a radially propagating TEM wave is launched centrally into the parallel plate disc construction. As the TEM wave progresses outwards, a proportion of its energy is transmitted through the slots, which act as radiating elements. Providing orthogonal radial pairs of radiating elements are spaced and orientated appropriately for the required polarisation, they add coherently in the far field, and the result is a directed beam which is of maximum amplitude and which is normal to the parallel plate disc construction.

Referring now to Figure 3, the invention relates to extending the fixed beam radial designs of the above patents, in order to perform 2D scanning. The invention does this by using a centralised launch enclosed by a reconfigurable reflector. An example of the frequency scanned antenna array of the present invention is shown with reference to Figure 3 which shows an example of a singular angular position of reconfigurable reflector. In Figure 3, there is shown a parallel plate disc with radially arranged slots 8 being fed via a smaller parallel plate disc 9 with a central electromagnetic feed 10 and a reconfigurable reflector 11. To perform a scan for one position of the reconfigurable reflector, the frequency is scanned.

Figure 4 shows a combined feed 13 within a parallel plate structure 14 having a dielectric filling 15. The combined feed 13 is fed from an RF source 12 and an active reflector 16. The RF source 12 and the active reflector 16 are both shown in their active On' state. In this state, the majority of the signal 17 is launched into a parallel plate 1 , with only a small signal 18 being reflected back at the RF source 12. In reality, a single active feed might be enclosed within many active reflector elements which collectively form a 2D reflector 1 , as discussed in later paragraphs. The wave 20 reflects off the reflector 16 to generate a high intensity wave travelling away from the reconfigurable reflector 1 1. A small amount of energy 21 travels through the reflector 11.

Figure 5 shows elliptical reflector shapes 24, centre fed from a centre feed 26. An active reflector is highlighted at 25 in the left-hand depiction 23. Radial control tracks 28 are shown in the right-hand depiction 27.

The electromagnetic field intensities are illustrated in Figure 6, for reflector 29 in Figure 6a, and for active reflectors 31 in Figure 6b. For the centralised launch 16 with inactive reflector 29, the electromagnetic field is radially symmetric. When the elliptical reflector 31 is active, a second focus of the ellipse is close to the circumference of the reflector as illustrated by the enclosed field pattern shown in Figure 6b. As can be seen from Figure 7, a broad angular sector 33 of the disk 8 is illuminated. Under these circumstances, the phase progression between the slots in the disk 8 is set according to the frequency of illumination, and hence the beam scans radially with changing frequency.

The effect is illustrated in Figure 8 for a frequency scan between 60.5GHz and 61.5GHz. The reconfigurable reflector points both left and right. A high dielectric material (ε,=10.2) is between the parallel plates. It is noted that the same effect as frequency scanning can be achieved by varying this dielectric constant to control the speed of propagation of the TEM wave between the parallel plates, for example by varying a DC bias voltage to a liquid crystal dielectric.

Referring to Figure 9, there is shown a slotted parallel plate waveguide 14. The phase difference Φ between two adjacent slots that are a distance D apart is:

Φ=2π D/Ve=2nDA_e where f is the frequency, v_e is the velocity of propagation, and _e is the wavelength in the dielectric.

For a beam with angular excursion ± Φ, the change in wavelength Δλ is:

where L_e is electrical length between the slots within the dielectric region 15 (n.b. D>L_e). If Φ=45°, a 30% bandwidth is required for D/L_e=5, and a 7% bandwidth is required for D/L_e=20.

Stated as a function of dielectric, that is:

where v the refractive index of the dielectric material (i.e. z ) in the slotted waveguide. Realistically, the refractive index of the dielectric v may range from 2^1/2 to 12 ^/2.

Figure 10 illustrates the effect of frequency scanning over a range 61 - 67GHz for a centre fed circular array. In this example, the diameter of the circular array is 20cm. The centre beam utilises the whole array and the reconfigurable reflector is off. Due the utilisation of a whole aperture, the gain of the centre beam is ~3dB greater than its first neighbour. The first neighbour is also fed at 61 GHz but has the reconfigurable reflector switched on. Activating the reflector causes the beam to scan due to the suppression of the outward going wave behind the reconfigurable reflector, and it is no longer balanced with a wave going in the opposite direction. The remaining frequency scanned beams gradually reduce in gain due to the scanned beams becoming broader (i.e. banana shaped when viewed as azimuth and elevation contour plots) in the plane orthogonal to the direction of scan. Scanning in the opposing radial direction is achieved by generating a mirror image of the reconfigurable reflector. The change in beam shape with scan angle is illustrated as azimuth and elevation contour plots in Figure 1 1. In Figure 1 1 , the beam is scanned in azimuth between -20° and 0°. Under these circumstances, the azimuth beamwidth remains around 14° while the elevation beamwidth decreases from 28° to 8°. At the central position, the beamwidth decreases to 9° in both azimuth and elevation planes. For this illustration, the directivity (i.e. gain based only on beamwidth, and no resistive losses) varies between 20 and 27dBi with scan angle.

With the reconfigurable reflector switched off, Figure 12 shows the effects of changing the frequency to achieve beam broadening. In this example, the beamwidth changes from 9° to 13° for a frequency scan of 61 to 66.5GHz. At the top of the band, it will be noted that the beam is beginning to degenerate with a null starting to appear at the beam centre, with an associated reduction in bore-sight directivity.

The contour plots shown in Figures 1 1 and 12 also illustrate how the reconfigurable reflector in effect extracts a segment of the broadened beam to form a characteristic banana shape. The effect becomes more pronounced at greater scan angles.

Figure 13 shows a wrap-around design where an array face 34 is fed from the circumference of the dish rather than from its centre. This is achieved by employ a two layer parallel plate structure with the reconfigurable reflector mechanism positioned at the centre of a layer 1 and a slotted array in a layer 2. A U-turn occurs at the circumference of the two parallel plate disks. The electromagnetic radiation leaving the disk is illustrated as a central radial line of rays 35 of varying intensity. The rays 35 of highest intensity are the lightest.

Figure 14 shows how the rays can be focused at the centre of a disk 36 by a reconfigurable reflector which is positioned in a first (lower) layer, (not shown). It will be noted that the greatest intensity of the rays is around the central line 35.

Figure 15 shows azimuth scans to 45° at 54GHz (left plot) and 20° at 62GHz (right plot). It will be noted that elevation beam is quite broad, -36°, due to only a fraction of the elevation aperture being used. However, the whole azimuth aperture is used resulting in narrow azimuth beamwidths.

As is illustrated in Figure 16, a broader radial scan for a frequency excursion of 54 - 64GHz in GHz steps can be achieved for less overall variation in directivity.

In comparison to the centre fed approach, the circumferentially fed approach offers a wider radial aperture resulting in narrower radial beams, and a more consistent tangential beam width. However, this is at the expense of a reduced aperture utilisation of ~30%, which is not as favourable as the centre fed approach that has an aperture utilisation of -45%.

As shown in Figure 17, the planar, frequency scanned, radial array antenna can be configured, by the incorporation of a total passive conical secondary reflector, to perform a full 360° azimuthal scan, with (a) frequency scanning in elevation, and (b) reconfigurable primary reflector scanning in azimuth. A conical secondary reflector 38 is placed directly above a circular radial array 34 and in this way provides a segmented cylindrical aperture for the new antenna configuration which is essentially cylindrical in its form. In effect, the conical secondary reflector performs a transformation, or mapping, of an upward travelling planar wave-front formed over a circular segment 39, into a radially outward travelling planar wavefront formed over a rectangular cylindrical sector. Due to radial symmetry of the antenna, any frequency scan that is performed, is translated from the horizontal to the vertical plane, so providing an elevation scan capability. By electronically 'rotating' the reconfigurable reflector, the antenna may also be scanned in the azimuthal plane.

Referring to Figure 18, there are shown azimuth and elevation plots for two different frequencies, i.e. 63GHz left plot and 52GHz right plot. It can be seen that for a 10cm diameter antenna very good directivity patterns with a wide angle elevation scan capability are achievable.

Figure 9 typifies an elevation scan for an arbitrary azimuthal pointing direction, for a 10cm diameter antenna. A frequency excursion of 50 - 70GHz achieves a -45° to 22.5° elevation scan for a parallel plate dielectric constant of 10. Between 63GHz and 68GHz, a -25° to 20° scan is achieved for less than a 2dB maximum gain variation. By choosing the antenna design parameters carefully, gain can be balanced against atmospheric absorption around the 60GHz unlicensed band.

The omni-directional performance (i.e. reconfigurable reflector off) is illustrated in Figure 20 at 0° elevation for a 10cm diameter antenna. Not surprisingly, the directivity for the omni beam case reduces by ~15dB relative to the narrow azimuth beam case, due to widening in the azimuth from 10° to 360°, a factor of 36x, (or 15dB). The omni-directivity variation shown in Figure 20 (right directivity plot) and Figure 21 (contour plot) of around 2.5dB with azimuth angle in the directivity plots is due to computation error in the ray trace's far field calculation.

By changing the shape of the reconfigurable reflector 43 in the piecewise way shown in Figure 22, the antenna beam patterns may be adjusted to maximise gain and reduce sidelobes. In Figure 22, multiple focal points 44 are achieved using a piecewise devisable reconfigurable reflector. It will be noted that a small proportion of rays do not interact with the reflector. These direct rays 45 give rise to lower level electromagnetic fields that affect the sidelobe structure of the directivity patterns and achievable beamwidths.

The parametric equation for a sector of an ellipse is:

F (a ,b, a ) = (a .cos (a) - (a²-b²)⁰⁵ ,b.sin (a)) for all astan <a<a_end

By way of an example only, it is mentioned that a piecewise reconfigurable reflector with spatially distributed foci can be generated by rotations about the first focus of each piecewise section. To illustrate this type of configuration, a set of splayed reconfigurable reflectors are shown in Figure 23. Figure 23, viewing left to right, shows two non-overlapping mirrors 46, 47, two overlapping mirrors, and three mirrors. Here, half the splay angle parameter 'β' is a key controlling variable in displacing the second foci of each piecewise linear elliptical section about a centre line 48. Figure 24 illustrates varying ctstart in order to improve the sidelobe levels which are seen to be quite significant, for example >10dB improvement.

The use of the alternate (or 'Z') angle theorem allows frequency scanned antennas to be used in communications links, see Figure 25. In this scheme, a conical reflector and an inverted conical reflector are used as separate Rx and Tx apertures. This allows a frequency scanned beam to scanned by equal angles upwards and downwards respectively. This ensures the intersection of the Rx and Tx beams at the same frequency, provided both antennas have a parallel axes, i.e. their cylindrical axes point at zenith.

In this same way, two-way links may be created as shown in Figure 26, or multi-hop relay systems.

By reducing the vertical aperture, for a given centre frequency, the resulting beam broadens and either the achievable instantaneous bandwidth increases or the requirement for absolute verticality of the central axis reduces. These type of system trade-offs may be seen to be important for mobile or fixed networks where the access point has less gain than the base station.

By injecting signals at various stepped frequencies, multiple fan beams can be created which are potentially suitable for massive MIMO. On transmit, each beam is associated with a different frequency band and modulation to each beam's centre frequency can, for example, be introduced into the antenna by a suitable channelized radio. On receive, each beam is associated with a different frequency band, and modulation from each beam's centre frequency can, for example, be demodulated by a suitable channelized radio. Typically the signals received on each beam will be digitized on receive to perform the massive IMO processing. In high multipath environments, each stacked beam will be highly de-correlated. Massive MIMO operation may be extended by changing the polarisation of the stack and varying the shape of the reconfigurable reflector.

Antennas may be stacked and connected together to act as retro- directing arrays or active echo-enhancers if a wide-band amplifier is added between the frequency scanned antennas. By changing the polarisation of the upper and lower stacked arrays, cross polar jamming may be realised

An example of a potential radar application of a planar frequency scanned array is shown in Figure 27. In Figure 27, there are shown four such antennas mounted at the four corners of an autonomous drive vehicle to provide high resolution 30 imaging around the vehicle. The azimuthal coverage is achieved using a reconfigurable reflector and the elevation coverage via frequency scan (as for example in Figure 3). The radar may be used for collision avoidance, blind-spot monitoring/warning, and inter-car communications/traffic management.

Figure 28 shows an alternative roof mounted radar device for an autonomous vehicle based on the antenna system shown in Figure 17. The radar device is able to offer a low cost alternative to a centrally positioned LIDAR (light detection and ranging) device, and is also able to allow both omni-directional beams 59 and highly focussed beams of controllable beamwidth 58 in the azimuth plane, with elevation scanning of controllable beamwidth to detect overhead hazards such for example as overhanging trees or arched tunnel entries.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention. The invention also extends to the individual components mentioned and/or shown above, taken singly or in any combination.

Claims

1. A frequency scanned array antenna comprising:

(a) an electromagnetic feed;

(c) a radial array which is fed by the reconfigurable reflector and which comprises regularly spaced elements which allow a beam launched from the electromagnetic feed to be frequency scanned with a changing frequency, and the frequency scanned antenna array being such that the frequency scanning enables electronic beam steering in two orthogonal directions.

2. A frequency scanned antenna array according to claim 1 in which the reconfigurable reflector is fed from either the centre or the circumference of the radial array.

3. A frequency scanned array antenna according to claim 1 or claim 2 in which the radial array comprises a parallel plate structure which is able to support a travelling electromagnetic wave, and in which the radial array has a regular pattern of slots.

4. A frequency scanned array antenna according to claim 1 or claim 2 in which the radial array comprises a dielectric material with a ground plane supporting a surface wave, and in which the radial array has a regular radial pattern of metal patch elements.

5. A frequency scanned array antenna according to claim 3 or claim 4 in which the regularly spaced elements are orientated to provide a linear to circular polarisation.

6. A frequency scanned array antenna according to any one of the preceding claims and configured to operate in isolation.

7. A frequency scanned array antenna according to any one of claims 1 - 5 and comprising microwave-optical means.

8. A frequency scanned array antenna according to claim 7 in which the means is a reflector structure or radio frequency lenses.

9. A frequency scanned array antenna according to any one of the preceding claims and including a rotationally symmetric microwave-optic component which is placed above the radial array, and which is for providing 306° azimuth coverage when a central axis of the radial array is pointed at zenith.

10. A frequency scanned array antenna according to claim 9 in which the rotationally symmetric microwave-optic component comprises a lens or a mirror.

11. A frequency scanned array antenna according to any one of the preceding claims in which the frequency scanned array antenna is configured to operate within microwave, millimetre wave or tetra hertz bands.