DIRECT AIR-TO-GROUND ANTENNA SYSTEMS FOR AIRCRAFT

Abstract

An antenna system for an aircraft, comprising one or more antennas configured and arranged on the aircraft to provide a downlink rate of at least one Gbps, the one or more antennas permitting a base cell tower placement with a diameter of at least 60 km, the one or more antennas being configured and arranged to support multiple data streams simultaneously, the one or more antennas supporting multiple polarizations, the one or more antennas having a high gain over most of a hemisphere around the one or more antennas of −85°≤θ≤85°, 0°≤φ≤360°.

Claims

1. An antenna system for a vehicle, comprising: one or more antennas configured and arranged on the vehicle to provide a downlink rate from a plurality of spaced apart cell towers of at least one Gbps, the one or more antennas communicating with one or more base cell towers at the same time, the antenna system being configured and arranged to support multiple data streams simultaneously, the antenna system supporting multiple polarizations, the one or more antennas having a high gain over most of a hemisphere around 360° of azimuth and around 170° elevation.

2. The antenna system according to claim 1, wherein the vehicle is an aircraft.

3. The antenna system according to claim 1, wherein at least one of the antennas comprise: a folded monopole extending from a body of the vehicle, which body is configured and arranged to act as a groundplane, a reflector positioned to one side of the folded monopole and also extending from the body of the vehicle, a set of one or more directors positioned opposite the one side of the folded monopole where the reflector is positioned, wherein the one or more directors, the folded monopole and the reflector form a linearly aligned array.

4. The antenna system according to claim 1, wherein the aircraft has a longitudinal axis, further comprising: a first of the one or more antennas mounted on a first portion of the aircraft, a second of the one or more antennas mounted on a second portion of the aircraft that is displaced along the longitudinal axis from the first portion, wherein although various components of the aircraft may shadow transmissions in certain areas from either one of the first or second antennas, the placement of the other of the first and second antennas will provide unshadowed transmissions to those certain areas.

5. The antenna system according to claim 4, wherein the first antenna is mounted forward of main wings of the aircraft and the second antenna is mounted rearward of the main wings.

6. The antenna system according to claim 1, wherein the aircraft has a longitudinal axis and a vertical center plane, a first of the one or more antennas mounted on a first lower portion of a fuselage of the aircraft positioned to one side of the vertical center plane, a second of the one or more antennas mounted on a second lower portion of the aircraft fuselage that is displaced to an opposite side of the vertical plane from the first portion.

7. The antenna system according to claim 6, wherein the first antenna is mounted at an angle of 15° on the other side of the vertical center plane of the aircraft.

8. An antenna for a vehicle comprising: a folded monopole extending from a body of the vehicle, which body is configured and arranged to act as a groundplane, a reflector positioned to one side of the folded monopole and also extending from the body of the vehicle, a set of one or more directors positioned opposite the one side of the folded monopole where the reflector is positioned, wherein the one or more directors, the folded monopole and the reflector form a linearly aligned array.

9. The antenna according to claim 8, wherein a second reflector is positioned to one side of the folded monopole, at an angle relative to the position of the first reflector, and a second set of one or more directors is positioned opposite the one side of the folded monopole where the second reflector is positioned, wherein the second set of one or more directors, the folded monopole and the second reflector form a linearly aligned array arranged at the angle relative to the first linearly aligned array formed by the first set of directors, the folded monopole and the first reflector.

10. The antenna according to claim 9, wherein at least one RF switch is provided to connect either the first or second reflector and either the first or second set of one or more directors to the groundplane or to leave them in an open circuit.

11. The antenna according to claim 8, wherein the vehicle is an aircraft and the body is a fuselage of the aircraft.

12. The antenna system according to claim 11, wherein the aircraft has a longitudinal axis and a vertical center plane, the antenna being a first antenna mounted on a first lower portion of a fuselage of the aircraft positioned to one side of the vertical center plane, a second antenna, constructed equivalent to the first antenna, mounted on a second lower portion of the aircraft fuselage that is displaced to an opposite side of the vertical plane from the first portion.

13. An antenna system for a vehicle having a body comprising: a plurality of crossed dipole antennas arranged in an array and mounted to the body of the vehicle, configured and arranged to provide two orthogonal polarizations and supporting two spatial data streams, with a gain of at least +10 dB, two switched Yagi antenna arrays mounted to the body of the vehicle, configured and arranged to provide a third orthogonal polarization, and with each Yagi antenna array providing 180° azimuth coverage, with more than a +5 dB gain at an 5° elevation, and with each Yagi antenna array providing one data stream.

14. The antenna system according to claim 13, further comprising: a parasitic element configured and arranged relative to each of the dipole antennas to improve the bandwidth of the antenna system.

15. The antenna system according to claim 14, wherein the parasitic element is printed onto a printed circuit board.

16. The antenna system according to claim 13, wherein the vehicle is an aircraft and the body is a fuselage of the aircraft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] In the figures, the same reference signs are used for elements, components or aspects that are the same or at least similar. It is noted that there follows a detailed description of an embodiment that is merely illustrative and not restrictive. In the claims, the word “comprising” or “having” does not exclude other elements and the indefinite article “a” or “an” does not exclude more than one. The fact alone that certain features are mentioned in various dependent claims does not restrict the subject matter of the invention. Combinations of these features can also be advantageously used. The figures are not to be understood as true to scale but are only of a schematic and illustrative character. In the figures

[0075] FIG. 1 shows a prior art DA2G flight geometry,

[0076] FIG. 2 shows a prior art Yagi-Uda Antenna,

[0077] FIG. 3 shows a prior art simple dipole over ground,

[0078] FIG. 4 shows a prior art crossed dipole over ground,

[0079] FIG. 5 shows a prior art placement of aircraft antennas on an aircraft,

[0080] FIG. 6 shows a typical prior art Yagi-Uda antenna,

[0081] FIG. 7 shows schematically a modified Yagi-Uda antenna of the present invention,

[0082] FIG. 8 shows an embodiment of an RF switch usable in the present invention,

[0083] FIG. 9 shows the radiation pattern performance of the modified Yagi-Uda antenna of the present invention in one plane,

[0084] FIG. 10 shows the radiation pattern performance of the modified Yagi-Uda antenna of the present invention in a perspective view,

[0085] FIG. 11 shows the complex impedance performance of the modified Yagi-Uda antenna of the present invention on a Smith Chart,

[0086] FIG. 12 shows a switched beam modified Yagi-Uda antenna of the present invention in a plan view,

[0087] FIG. 13 shows a switched beam modified Yagi-Uda antenna of the present invention in a perspective view,

[0088] FIG. 14 shows the selectable directivity of the beam of the modified Yagi-Uda antenna of the present invention in one plane,

[0089] FIG. 15 shows an embodiment of two arrays of the modified Yagi-Uda antenna and a 2×2 crossed dipole array in an antenna system of the present invention,

[0090] FIG. 16 shows a crossed dipole antenna array of the present invention,

[0091] FIG. 17 shows a choice of antenna location in accordance with the present invention,

[0092] FIG. 18 shows a schematic system architecture for the present invention,

[0093] FIG. 19 shows a choice of antenna location in accordance with the present invention,

[0094] FIG. 20 shows schematically how each array supports multiple base-stations,

[0095] FIG. 21 illustrates a combination of antennas into an antenna system of the present invention,

[0096] FIG. 22 shows an example 3D radiation pattern from the 2×2 crossed dipole array of the antenna system of the present invention.

[0097] FIG. 23 shows schematically the system architecture for the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0098] Although the present invention can be used in a wide variety of vehicles and other moving apparatus, an embodiment of the invention is disclosed in the context of an antenna system for use in an aircraft. In such an embodiment, the present invention covers an antenna design with sufficient bandwidth to support 5G bands, has one omnidirectional radiation pattern, beam selectable patterns and has three orthogonal polarizations to support several spatial streams. A main strength of the antenna design is that it has high directivity at low elevation angles which is necessary for a high-throughput air-to-ground radio link. Furthermore, the antenna is largely passive containing only PIN diode semiconductor devices and no amplifier and phase changers that you would find in an active phased array, thus in an extreme environment, such as that outside an aircraft flying at high altitude, it is extremely reliable. Reliability is very important in aeronautics as the time taken to find and make repairs to aircraft systems relates directly to lost profits.

[0099] An object of the invention is to provide an antenna system which can provide a high data throughput typically one Gbps as a downlink, allowing a cell size for ground antennas of a nominally 60-80 km diameter, with aircraft altitudes of 10,000 ft to 40,000 ft (3 km to 12 km). The antenna system is to support multiple data streams, to support multiple polarizations, and to have a high gain over most of the hemisphere (−85°<θ≤85°, 0°≤φ≤360°, where θ is a polar angle in a spherical coordinate system and φ is an azimuthal angle).

[0100] An embodiment of the present invention provides a modified Yagi-Uda antenna in which the magnitude of the e-field is zero along the centerline. A typical prior art Yagi-Uda antenna 120 is shown in FIG. 6, it comprises a half wavelength folded dipole. Along the dotted centerline 139, the E-field voltage is always zero (this is why in a practical antenna, the elements 122, 124, 126, can be galvanically connected to a metal boom for mechanical rigidity without disrupting the performance). For the present invention, half of the typical antenna configuration is disused, as shown in FIG. 7, and instead, is replaced with a conductive groundplane 140 along the centerline. The typical folded dipole is thus converted into a folded monopole 142 (with an impedance of about 150Ω). The aircraft fuselage is used as the groundplane 140 and thus, the antenna forms an end-fired array. See FIG. 7. The length of the driven folded monopole 142 being a quarter wavelength, the reflector 144 is slightly longer, and the directors 146 are slightly shorter. The directors and reflector can be connected to ground via RF switches 147, such as that shown in FIG. 8.

[0101] As with the Yagi-Uda antenna, either a monopole (impedance 37Ω) or folded-monopole (impedance 146Ω) configuration could be used. A folded-monopole configuration is shown in FIG. 7 where the far-end 148 of the fed element is galvanically connected to the groundplane 140. The reflector 144 and two directors 146 are also connected to ground (via the RF switches 147).

[0102] The position and the length of the elements (directors 146, reflector 144 and folded monopole 142) of the antenna 150 can be modified to affect the radiation pattern and the impedance. If the desired impedance were to be 50Ω (typically found in commercial radio applications as coaxial cables and test equipment are readily available with this characteristic impedance), a folded-monopole embodiment would be preferred. The reason being that the lengths and positions of the reflector and directors can be carefully chosen to provide reasonable fractional bandwidth (approaching 10%) and impedance close to 50Ω. For example, the elements can be modified to achieve an impedance of 50Ω and a very high gain at θ=85° of +5.6 dBi. See FIGS. 9, 10 and 11. FIG. 11 shows the impedance successfully optimized to 50Ω, and in FIG. 10, the 3D radiation pattern being essentially broad beamwidth in azimuth, a null perpendicular to the groundplane (as with all monopoles and dipoles) and essentially, high directivity along the axis of the array especially at low angles of elevation. This embodiment shows a maximum directivity of over 8 dB. This type of antenna is often referred to as an end-fire antenna.

[0103] In FIG. 9, it is shown that at an elevation of only 5°, the antenna has a directivity of over +5 dB.

[0104] A switched beam modified Yagi-Uda array may be achieved by adding a number of additional directors/reflectors and using RF switches to enable/disable the appropriate directors and reflectors. The folded monopole being common to all beams, the switches only switch in and out the parasitic elements.

[0105] In FIG. 12, the sets of directors 146 and reflectors 144 are mounted at angles with respect to each other while sharing the same driven element 142. In FIG. 12, there are three sets of elements, the driven element 142 in the center, three different reflectors 144 below, and three sets of two directors 146 above. This arrangement is shown in a perspective view in FIG. 13.

[0106] In summary, there can be provided three separate 4-element (in this embodiment) antennas 150 which share a common driven element 142. The idea is that RF switches 147 can be used, preferably PIN diode semiconductor high-speed switches, to select one of these antennas in turn and disable the others.

[0107] By placing a PIN diode switch (or other suitable device) 147 between all of the parasitic elements and ground, and applying suitable bias currents to the PIN diodes, a desired direction of the beam can be achieved. FIG. 14 shows 2D directivity patterns of an embodiment with three beams at 60 degree offsets. This shows that it is possible to select the angle of maximum directivity of the antenna in 60° increments.

[0108] The RF signal is always applied to the folded monopole 142. The RF switches 147 are used to either connect the directors 146 and reflector 144 to the groundplane 140, or leave them open circuit. So, the switches are used to direct the beam, not route the signal. In this way, the beam can be directed in different directions. See FIG. 14.

[0109] The advantages of using a traditional Yagi-Uda dipole antenna modified into an end fire folded monopole antenna are:

[0110] This allows the antenna to be used on a groundplane 140 (aircraft fuselage);

[0111] The antenna can be optimized for an impedance of 50Ω;

[0112] Excellent directivity is provided, with a good back/front ratio;

[0113] A high gain is achieved at low elevation angles;

[0114] A 110° beamwidth can be obtained with two directors 146 in that the number of directors determines the beamwidth;

[0115] Three sectors (using three directors 146) easily covers 180° azimuth with no inter-beam dropout;

[0116] Four to six sectors (four to six directors 146) would cover a 360° azimuth.

[0117] This antenna architecture provides a switched beam with high directivity at low elevation angles at very low cost as it employs simple PIN diode semiconductors. It provides far superior directivity than a flat-panel phased-array at low levels of elevation. This antenna architecture also has many use-cases including an airborne antenna for an air-to-ground communications system where high directivity is necessary to achieve the required signal-to noise ratio for distance ground terminals which subtend very low angles of elevation. An antenna with a higher directivity will generally require less transmit power (hence better DC efficiency) to achieve the same radiated power, or for the same transmit power, will result in higher signal to noise ratios thus increasing the data throughput of a digital communications system.

[0118] An embodiment of the invention may use two independent arrays 152, 154, one pointing forward and one pointing aft. Since these arrays have a high isolation from each other, they could support two independent data streams. See FIG. 15.

[0119] In an embodiment, the present invention proposes to use a crossed dipole antenna array 156, such as a two-by-two array. See FIG. 16. This arrangement could include a parasitic element 157 to improve bandwidth. Between the dipole and the parasitic element 157, there is a space which could be filled with air. The parasitic element 157 could also be printed onto a printed circuit board 158 and foam 159 could be provided in the circuit board.

[0120] By using a number of such crossed dipole antenna elements in an array, all of the elements of each polarization could be fed in-phase which would increase directivity of the antenna.

[0121] The antenna or antennas and antenna arrays can be selectively positioned on the aircraft to improve coverage and signal transmission to the various ground antenna.

[0122] For example, although the antennas are located on the fuselage 160 of the aircraft 162, on aircraft having engines located under the wings of the aircraft, the engines, with their relatively large diameters, cause some shadowing of the signals at some angles relative to the placement of the antenna, whether near the front of the aircraft, or near the rear. See FIG. 17.

[0123] In an embodiment, the present invention provides for the use of two antenna assemblies, one forward 164 and one aft 166, which then avoids the problem of shadowing since one antenna will provide an uninterrupted signal transmission in the shadow region of the other antenna assembly, and vice versa.

[0124] As shown in FIG. 18, the architecture for the aircraft antenna system 168 may be designed to minimize losses. For example, the RF head 170 may be positioned close to the antenna 172 to keep coaxial cable lengths 174 short. Long fiber optic cables 176 may be used to link the RF heads 170 to the main electronics 178 which typically are housed in the electronics bay located under the cockpit. Such an arrangement may be used on a two-antenna assembly system, without incurring undesirable losses.

[0125] As mentioned previously, in known aircraft antenna system placements, the antenna systems are typically mounted on the centerline of the bottom of the aircraft. While this placement provides for wide and symmetric coverage, there is a loss of coverage when the aircraft executes a roll maneuver, such as during a turn. In that instance, as seen in FIG. 5, when the aircraft is at a height of 10,000 feet (3 km), with a roll angle of 15°, the effective cell radius is reduced to 11 km because part of the cell is obscured by the fuselage. With a cell diameter of 30 km, which is desired to reduce the capital expenditure necessary to achieve full cell coverage, there can be a loss of transmission coverage during such roll maneuvers.

[0126] A solution to this problem is provided by an embodiment of the present invention by placing two antennas 180, 182 on the fuselage 160, each at an antenna install angle of, for example, 15° from the centerline 184 of the aircraft 162. See FIG. 19. This will assure that as an aircraft rolls, at least one of the antenna systems will have a direct view of the ground cell antenna at all times. The antenna install angle may be varied by +/−10°.

[0127] With multiple antenna systems on the aircraft, multiple streams of data may be transmitted simultaneously via different antenna systems. Such an arrangement may be useful during hand-over from one cell tower 102 to another. See FIG. 20.

[0128] As shown in FIG. 21, by combining the various concepts of the present invention, with the use of one crossed 2×2 array 186, which provides two orthogonal polarizations and supporting two spatial streams there is a gain of >10 dB. With the use, also, of two switched modified Yagi arrays 188, 190, a third orthogonal polarization may be obtained, each providing a 180° azimuth coverage and more than +5 dB gain at 5° elevation, and each providing one spatial stream. With this arrangement there is achieved a >5 dB gain over −85°≤θ≤85°, 0°≤φ≤360°, thus making a high data throughput system of typically one Gbps 5G air to ground system possible.

[0129] In the center of the antenna system is a horizontally polarized section 186 comprising at least one pair of cross-dipoles, the more crossed dipole sections, if fed in-phase, results in more directivity. For example, a 2×2 array of crossed dipoles will provide about 12 dB of directivity, an example 3D radiation pattern is shown in FIG. 22. Crossed dipoles are have orthogonal polarizations and could provide two independent data streams thus doubling the throughput that a single dipole could provide.

[0130] Additionally, there is at least one vertically polarized section 188, 190 (in this embodiment there are two). Thus, the complete antenna system has three orthogonal polarizations. The vertically polarized sections 188, 190 are comprised of the modified Yagi-Uda sections. They provide high directivity at low angles of elevation which is required to look forwards to independently provide the next handover and backwards to provide another traffic data-stream.

[0131] A typical system architecture outline is shown in FIG. 23. It comprises an antenna 192 with orthogonal horizontally polarized crossed dipole sections (H1 & H2) and two vertically polarized sections one pointing forward (V1), one aft (V2). Thus, the antenna has four ports. Each of these ports are connected to an independent radio section 194 often described as nTmR where n and m are the number of independent transmit and receive channels respectively.

[0132] Software, usually within the baseband unit 196, will arbitrate and control which streams are optimum to provide the highest data throughput and to manage handovers to the next tower in the chain.

[0133] So, this antenna 192 provides three orthogonal polarizations, is multi-section, has good low-angle performance and potentially provides several simultaneous data-streams to maximize data throughput on moving platforms such as aircraft.

[0134] The new antenna system has three orthogonal polarizations which helps with multiple streams to increase data throughput.

[0135] The antenna system has combined several different antennas with different radiation patterns which will insure both high gain at low angles needed for direct air to ground communications and when flying over the cell tower, in that flight paths are not fixed, however, the cell tower locations are.

[0136] The antenna system of the present invention further implements a folded monopole with several switched directors and reflectors which provides a low-cost solution to achieving beam steering to improve coverage, it is more power efficient and requires less weight.

[0137] Multiple antenna assemblies may be mounted on the aircraft to avoid aircraft roll shading and engine shading.

[0138] Multiple antennas within each assembly are able to communicate independently with several cell towers which increases data throughput and helps with cellular handovers.

[0139] The use of an end-fire ½ Yagi folded monopole antenna allows the antenna system to overcome low-angle gain problems.

[0140] Further, the antenna system of the present invention is small, easy to install and presents very low drag and has a low weight.

[0141] While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.