GB2546309A

GB2546309A - An Antenna

Info

Publication number: GB2546309A
Application number: GB1600771.8A
Authority: GB
Inventors: Naylon John; Gears Trevor
Original assignee: Cambridge Broadband Networks Ltd
Current assignee: Cambridge Broadband Networks Ltd
Priority date: 2016-01-15
Filing date: 2016-01-15
Publication date: 2017-07-19
Anticipated expiration: 2036-01-15
Also published as: GB201600771D0; GB2546309B

Abstract

An antenna comprising a dielectric element 20, 21 between a radiating element 14, such as a feed horn, and a reflector 12, where the dielectric element has shape or thickness properties which cause a difference in the wave profile along the radiation propagation path when comparing an azimuth plane and an elevation plane. The dielectric element 20, 21 allows for in-situ modification of the antenna post installation in order to adjust the beam width between horizontal and vertical directions. The reflector 12 may be parabolic or truncated. The dielectric plate 20, 21 may be flat, orthogonal to the propagation path, have a dielectric constant between 2.3 and 3 and have uniform or variable thickness, which may be related to a multiple of half the operating wavelength. There may be a plurality of dielectric elements 20, 21 with laterally separating gaps, which may be moved allowing the gap size to vary. Alternatively, the dielectric elements 20, 21 may be stacked in the propagation path with a variable gap between them or in direct contact. The antenna may be used at microwave frequencies.

Description

An Antenna

The present invention relates to an antenna, and particularly, but not exclusively, to a sector antenna.

An antenna is a transducer between a guided wave and a radiated wave, or vice versa. Antennas are well known in the field of communications.

The radiated wave from an antenna is characterized by the antenna's radiation pattern or profile. The pattern or profile can be depicted as a graphical representation of the radiation properties of the antenna as a function of space. Such radiation patterns or profiles are generally represented as a polar or Cartesian graphical representation similar to that depicted in Figure 1 with X, Y & Z coordinates and θ, φ to define angular orientation in that reference frame.

The term “azimuth” refers to the “horizontal" radiation pattern of an antenna, i.e. as seen from above, whilst the term “elevation” commonly refers to the “vertical" radiation pattern, i.e. as seen from the side. It will be appreciated that when used to describe antenna patterns or profiles, these terms assume that the antenna is mounted (or measured) in the orientation in which it will be used.

For example, in Figure 1, the x-y plane (Θ = 90 deg) is the azimuth plane. The azimuth plane pattern is measured when the measurement is made traversing the entire x-y plane around the antenna under test. The elevation plane is then a plane orthogonal to the x-y plane, say the y-z plane (φ = 90 deg). The elevation plane pattern is made traversing the entire y-z plane around the antenna under test.

Antennas may be omnidirectional or directional depending on the required use and other operational factors. An omnidirectional antenna is an antenna that has a non-directional pattern (circular pattern) in a given plane with a directional pattern in any orthogonal plane. Examples of omnidirectional antennas are dipoles and collinear antennas.

In contrast to omnidirectional antennas, a directional antenna is one that radiates its pattern more effectively in one (or more) direction than others, e g. a sector antenna (fan-beam antenna). Directional antennas are configured to radiate their energy out in a particular direction for coverage in a particular environment or location, as well as point-to-point communication links. Such antennas can be patch antennas, dishes, horns or other varieties as is well known.

Sector antennas are designed to provide segmented coverage over a selected (sector) area; sector antennas deliver a wider beam width or profile than point-to-point parabolic antennas. Sector antennas are typically used for ISM, WLL and MMDS band communications utilizing spread spectrum data streams for wireless connections between LAN base stations, wireless Internet, subscriber networks, PCS and other point-to-multipoint communications. Some common horizontal beam widths or profiles utilized include 60, 90, 120, and 180 degrees.

Antennas can be used with a wide range of frequency bands so for example 2.4, 3.5, & 5.8 GHz frequency bands as necessary for a network or a link with the antenna generally optimised for that expected band of operation.

Typically with a sector antenna the radiation patterns in the azimuth plane and elevation plane respectively will be configured differently as described above, whereby the radiation pattern is wide in the azimuth plane but narrower in the elevation plane. As will be appreciated by a person skilled in the art, it is possible for sector antennas to realise high gains by compressing the radiation pattern in the elevation plane. In a broad sense directional antennas are more focussed compared to sector antennas; occasionally it would be advantageous to be able to easily switch from a more directional antenna to a more sector-like antenna, and vice versa.

The size and shape of the reflector generally determines the performance of these antennas. Many sector antennas have reflector shapes that are somewhat flat with some ridges or other features along the edges.

The current preferred technology for sector antennas is to use a form of pyramidal horn 200 or parabolic horn 400 as shown in Figures 2a and 2b, respectively, whereby the horizontal beam profile is shaped by extensions 4a, 4b, respectively, to the vertical sides of the horn assembly from a feed path 201, 401 respectively, whereby the extensions 4a, 4b are referred to as flares. It will be appreciated that the construction of each flare 4a, 4b is different for horizontal and vertical polarization, whereby the electromagnetic radiation emitted by each horn assembly is linearly polarized and therefore on-site polarization diversity is only possible by changing whole antennas and structures, which can be costly.

Furthermore, the vertical aperture, i.e. the gap between the flares, has significant depth, i.e. the distance between a plane bounded by the flares and the beginning of the horn, e.g. where the pyramidal structure meets the rectangular tube in Figure 2a, and this makes it unsuitable for integrating within a slim-line electronics module.

Furthermore still, sector antennas are generally bulky products and are not easily mountable in position.

Alternatively, sector antennas may be provided using parabolic reflectors by modifying the reflector such that the reflector is truncated in either the horizontal or vertical axis. For example, if the reflector is truncated in the horizontal axis but parabolic in the vertical axis then the reflected radiation will be wider in the azimuth plane and narrower in the elevation plane. Modifying a parabolic reflector structure itself and in-situ to provide a sector beam radiation presents significant engineering challenges along with costs.

It is an aim of some embodiments of the invention to address at least one of the disadvantages discussed above.

According to the invention there is provided an antenna comprising a radiating element operable to radiate an electromagnetic wave, and a reflector operable to direct at least a portion of the electromagnetic wave in a wave profile in a direction along a radiation propagation path, wherein a dielectric element is provided between the radiating element and the reflector in the radiation propagation path, the dielectric element being configured by shape and/or thickness to modify the phase of the wave profile differentially between an azimuth plane and an elevation plane in the direction of the radiated wave along the propagation path.

The reflector may be a parabolic reflector. The reflector may be a truncated parabolic reflector.

The dielectric element may be substantially flat in configuration across the propagation path. The dielectric element may be arranged to be substantially orthogonal to the propagation path. The dielectric element may be of uniform thickness. Alternatively, the dielectric element may have a variable thickness across the propagation path. For example, the dielectric element may have a variable thickness in sections across the propagation path. The dielectric element may be of a consistent material type across the propagation path. The antenna may include a plurality of dielectric elements. The plurality of dielectric elements may be separated with a gap between them. The dielectric elements may be mounted upon means to allow movement to vary their relative location to each other and/or vary the gap between them. The plurality of dielectric elements may be stacked in a stack one upon the other in the propagation path. The stack of the plurality of dielectric elements may be in direct contact, surface to surface, with each other or have a gap between them. The stack may include means to allow separation of the plurality of dielectric elements in the stack to vary the gap separation between them.

The dielectric element may have a thickness relative to the expected wavelength of the electromagnetic wave in the order of λ/2 or a multiple of that. The dielectric element may have a dielectric constant in the range 2.3 to 3.0.

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

Figure 1 is a representation of a known antenna measurement coordinate system;

Figure 2a is a perspective view of a known pyramidal horn of a sector antenna;

Figure 2b is a perspective view of a known parabolic horn of a sector antenna;

Figure 3a is a graphical representation of the azimuth plane pattern of a typical sector antenna;

Figure 3b is a graphical representation of the azimuth plane pattern of the antenna of Figure 3a with a dielectric element in accordance with the present invention;

Figure 4a is a side view of an antenna according to a first embodiment of the present invention;

Figure 4b is a front view of the antenna of Figure 4a;

Figure 5 is a plot of a radiation pattern in the azimuth plane of the antenna of Figures 4a and 4b; and

Figure 6 is a plot of a radiation pattern in the elevation plane of the antenna of Figures 4a and 4b.

Whilst the teachings of this application are applicable to both omnidirectional and directional antennas, only directional antennas, and sector antennas in particular, will be hereinafter described.

As described above in relation to Figure 1, an antenna is defined by the radiation wave front pattern or profile created in operation in terms of the azimuth plane and the elevation plane. An antenna can be designed and specified for installation but that specification or local conditions or requirements may change. It has previously been difficult to adapt the antenna to new conditions or package an antenna in a desired small compact space envelope for the equipment to be used with the antenna. It is known to provide dielectric loaded antennas (see Dielectric Loaded Antennas - L Shafai, University of Manitoba - Encyclopaedia of RF and Microwave Engineering - Wiley Online Library) but these have a lens format to adapt the emergent wave front profile so provision of the antenna can be difficult and costly requiring a specifically formed dielectric lens to provide the active adaptation of the profile particularly with differences in the azimuth and elevation planes.

With a sector or directional antenna, a parabolic reflector is active in terms of adaption of the first emitted electromagnetic waves to provide a desired wave front profile so that in accordance with the present invention a dielectric element of a desired configuration, geometry and depth/thickness can be used to further adapt the profile or radiation pattern.

Figures 3a and 3b illustrate a parabolic reflector 302 as a reflector in accordance with the present invention with Figures 3a and 3b showing ray paths in the horizontal (azimuthal) plane without and with, respectively, a dielectric element 314 in the propagation path. A radiating element in the form of a horn is located at or very near a focus 300 so that as can be seen in Figure 3a electromagnetic waves or rays will be incident upon the parabolic reflector 302 such that they are reflected as a wave front or profile 303 in accordance with conventional parabolic antenna responses. Thus, the profile 303 is determined principally by the shape of the parabolic reflector 302 and the position of the horn relative to the focus 300 of the reflector.

In Figure 3b again the horn can be positioned about the focus 300 but by the action of the dielectric element 314 the apparent focal length of the parabolic reflector 302 is shortened to a ‘virtual’ ray path 301a (shown by broken lines). This shift leads to reflection by the reflector 302 to create an altered wave front profile or pattern 304 which has a phase gradient that substantially lags the conventional wave front profile or pattern 303 as depicted in Figure 3a. The shift is due to refraction at 299 through the dielectric element 314 within the limits of Snell’s Law such that dependent upon angle of incidence by the incident ray 301 then refraction within limits occurs with the refracted ray 305 then reflected by the reflector 302. It will be understood that there will be some scatter and/or radiation absorption losses so overall configuration of the arrangement will be such that these are acceptable in the operational context. The principal reflected rays 304a will then be substantially orthogonal to the element 314 and pass through without further refraction.

As stated above there will be some scatter and losses as will be described below but by appropriate configuration, choice of materials for the element 314 and thickness/depth of element 314 it will be understood that an acceptable adaptation of the emitted pattern or profile can be achieved. In some respects the wave profile 304 will be that of a horn positioned at the focal point of the virtual ray path 301a rather than the true position in combination with the parabolic reflector 302.

It will be understood that the element 314 can be arranged so that the distances 306, 307 as spacings from the reflector 302 and the focal point 300 of the reflector can be changed; the element 314 can be specified with a thickness relative to the expected wavelength; the element 314 can be flat or tilted up or down relative to the reflector or even slightly curved; the element can have a constant thickness or a variable thickness in sections and be made with different materials and so dielectric properties in different parts. In any event the emergent wave front pattern or profile 304 is adapted and adjusted as compared to a wave profile or pattern for the reflector without the element 314.

It will be appreciated that the above adaptation of the antenna provides a limited need for re-construction of the antenna using a simple flat dielectric element 314 in the form of a plate in combination with the reflective response of the reflector 302. The reflector 302 may be a ‘perfect’ reflector but as indicated above normally with a sector antenna the reflector 302 will itself provide a difference in the wave front profile or pattern emerging from the antenna. The reflector can be circular, elliptical, square, rectangular or whatever necessary to give the reflective response required. In accordance with the present invention the reflector 302 and the dielectric element 314 will act collectively to provide the wave front profile desired whether that be an OEM specification for a compact accommodation space or a post installation change needed to an existing antenna installation due to altered operational conditions.

Figure 4a shows a side view of an antenna 10 in accordance with the present invention, whilst Figure 4b shows a front view of the antenna 10.

The antenna 10 comprises a radiating element in the form of a feed horn 14 and a reflector in the form of a parabolic reflector 12. Such antenna configurations are well known in the art, whereby a radiating element (feed horn 14) produces an electromagnetic wave front which is reflected by the surface of a parabolic reflector 12 having a shape corresponding to that obtained by revolving a parabola about its principal axis. The parabolic reflector 12 may be formed of a sheet metal e.g. aluminium, copper metal screen, or wire grill construction and modified to the desired size and shape as required so that a desired electromagnetic wave pattern/distribution or profile is achieved for the operational conditions and/or requirements. In accordance with the present invention the reflector 12 and the horn 14 are arranged with dielectric elements in the form of first and second plates 20 and 21 between them. The first and second plates 20 and 21 alter the ray paths by refraction so by shaping and configuration of the plates 20 and 21 changes in the radiated pattern can be achieved.

The feed horn 14 is located, exactly or substantially, at the focal point (not shown) of the surface of the parabolic reflector 12. Microwave signals for transmission, for example, 1GHz to 30GHz, and preferably 24.5GHz to 29.5GHz, are fed to the feed horn 14 from a feed line 18 coupled to the feed horn 14. These signals are then emitted and reflected by the reflector 12 as desired. Alternatively, the feed horn 14 may be formed as part of the same structure as the feed line 18.

The feed horn 14 is designed to ‘illuminate’ the reflector 12 with the microwave signals radiating from a conductive splash plate (not shown) extending from the feed horn 14. In alternative embodiments the feed horn 14 may be any suitable configuration such as, for example, rectangular, pyramidal or a conical horn antenna. As will be appreciated by a person skilled in the art, the choice of horn will depend on factors such as frequency and polarisation. The combination of the feed horn 14 and the reflector 12 along a propagation path define the electromagnetic wave profile or pattern emitted by the antenna.

In accordance with the present invention, the two dielectric plates 20, 21 (Figure 4b) are co-located along the same vertical plane, orthogonal to the axis of symmetry of the parabolic reflector 12 and located between the parabolic reflector 12 and the radiating horn 14 as shown in Figures 4a and 4b in the propagation path. In the present illustrative embodiment, a space or gap 23 is also provided between the plates 20, 21 so the plates 20, 21 may be separate or separable or a single plate provided with an electromagnetically ‘clear’ or neutral material between distinctly separable sections or zones of dielectric material as required to define the plates 20, 21 as seen by the radiated or propagating electromagnetic waves.

It will be understood that the plates 20, 21 may be formed of the same dielectric material, have the same thickness, have the same shape and the same orientation/geometry relative to the horn 14 and more importantly the reflector 12 or these factors could be different for each plate 20, 21 giving a different wave front pattern or profile with symmetry or asymmetry with less dependence upon an active specific lens shape to create the wave front profile.

In accordance with aspects of the present embodiment, the depth or thickness of the plates 20, 21 (Dp) is approximately half the wavelength of the radiated electromagnetic radiation in the dielectric

Dp = Xo/(2*Vsr) where λο is the free space wavelength and εΓ is the relative permittivity (or dielectric constant) of the material of the plates.

The positioning of the plates 20, 21, between the horn 14 and reflector 12, causes the radiation propagating through the plates 20, 21 to be variously refracted and scattered, and, therefore, the wave front incurs a phase delay. A further delay is incurred when the rays are reflected by the surface of the parabolic reflector 12 again through the plates 20, 21. The waves propagating through the space 23 are not further delayed across the wave profile. The shaping and configuration of the plates 20, 21 determine the proportion of the antenna aperture with a modified phase profile in comparison to an antenna where the plates 20, 21 are absent i.e. the gap 23. The spacing between the plates 20, 21 may be a fixed width or variable width in a stepped or tapering change from one side to the other so again altering the radiation pattern provided by the antenna. The spacing may be permanent or variable in use on an appropriate mechanism to change the configuration (flat or slightly tilted) and spacing between the plates 20, 21.

In the present embodiment, the plates 20, 21 are fabricated of low loss materials with dielectric constants εΓ = 2.3 - 3.0. However, it will be appreciated, upon reading this specification, that various other materials with higher dielectric constants can also be used.

Careful choice of the sizes and/or shapes of and/or gaps between the plates 20, 21 combined with the amplitude profile across the reflector 12 produces a radiation pattern that exhibits a broad, flat top pattern across the axis of the plates (30 in Figure 5) and only a marginal increase in beam width in the orthogonal axis (40 in Figure 6).

Advantageously, the principle of the design is independent of the sense of polarization being transmitted or received.

As is well known, the ratio f/D (focal length/diameter) of the parabolic reflector 12 is the fundamental factor governing the design of the feed for a parabolic dish reflector. In the present embodiment, the minimum f/D ratio of the parabolic reflector 12 is determined by the half angle subtended by the reflector 12 to the feed that must not exceed the Brewster angle for the plate 20, 21 materials.

Further shaping of the horizontal beam width can be achieved by overlaying further dielectric plates on the plates 20, 21 in a stack. These further plates may be of a much smaller profile to cover an area closer to the perimeter of the reflector 12.

With the above in mind, in conventional systems, (when the dielectric plates 20 and 21 are absent), radiation emerging from a feed tube is reflected by a splash plate and directed onto the reflector surface 12 to produce an outgoing wave front of constant phase.

In the present embodiment, the dielectric plates 20, 21 modify the phase characteristics of the radiation across the reflector 12 aperture resulting in beam widths or profiles wider than if the plates 20, 21 were absent. The greatest change in beam width for the antenna 10 is in the plane where its central axis passes through the plates 20, 21.

The plates 20, 21 generally will be fixed to provide a consistent wave front profile or pattern emerging from the antenna 10. However, if some degree of tuning is desired then the plates could be mounted upon suitable means such as ratchet or screw thread for manual or motor driven relative movement of the plates and so altering the wave front profile. The movement will be to change the gap 23 between plates. The plates 20, 21 are arranged substantially in the same plane so the gap 23 will also be adjusted in that plane.

Dielectric plates in accordance with aspects of the present invention can be stacked one upon the other. Thus, localised sections or parts of the antenna will have a different refractive and phase shift response along with an altered loss profile. The plates in the stack may lie flat one upon the other or have a gap between them which may be adjusted by appropriate means to allow adaption of the wave front profile.

Figure 5 is a plot of the antenna’s radiation in the azimuth plane, whereby traces 30 and 32 relate to the radiation profile with and without plates 20, 21 respectively, whilst Figure 6 is a plot of the antenna’s radiation profile in the elevation plane, whereby traces 40 and 42 relate to the radiation profile with and without plates 20, 21 respectively.

From the plot of Figure 5, traces 30 and 32 show that the maximum gain in the azimuth plane is approximately 6dB lower for the antenna 10 having the dielectric plate 20 in comparison to a similar antenna without the plates 20, 21 whilst the beam width is wider for the antenna 10 having the dielectric plates 20, 21 in comparison to a similar antenna without the plates 20, 21. This lower gain can be attributed to the dielectric plates 20, 21 but will typically be acceptable in comparison with the additional adaptation of the profile for operational performance.

Furthermore, from the plot of Figure 6, traces 40 and 42 show that the maximum gain is approximately 6dB lower for the antenna 10 having the dielectric plates 20, 21 in comparison to a similar antenna without the plates 20, 21 for the elevation plane whilst the beam widths or profiles are similar for both antennas with and without plates 20, 21. Thus, again there is reduction in gain but this will generally be acceptable for the added adaptation of the radiation profile emerging from the antenna without major or replacement of an installed antenna or a need to design and manufacture a bespoke antenna for a desired profile.

It will be seen, therefore, that dielectric plates 20, 21 advantageously provide a wider beam width or profile in the azimuth plane for antennas in comparison with antennas without such plates.

It is intended that the invention encompass all such modifications and variations as fall within the scope of the appended claims.

Whilst the antenna hereinbefore described is a centre fed shaped parabolic reflector, the principle of locating a dielectric plate in the propagation path of the radiated radiation is equally applicable to other reflector geometries, directional or omnidirectional, including, but not limited to, the micro strip antenna, truncated parabolic (dish) antenna, offset fed parabolic reflector, Cassegrain or Gregorian antenna.

For example, the shape of the parabolic reflector itself may be modified so as to provide specific functionality in addition to the dielectric plate. For example the reflector may be parabolic in the vertical plane whilst truncated horizontally, which would provide a sector beam having a wide profile in the azimuth plane, but a narrow profile in the elevation plane. It will be appreciated that using a dielectric plate with a truncated reflector would provide an extended beam profile.

Due to reciprocity, the antenna works whether transmitting or receiving radiation. Therefore, it will be appreciated that an antenna 10 having dielectric plates 20, 21 is operable to receive radiation from a wider horizontal range than would otherwise be achievable without the plates 20, 21.

Furthermore, if such a shaped reflector is rotated 90°, then the beam would spread out in the elevation plane, whilst being narrow in the azimuth plane. Alternative configurations of reflector may also provide a sector beam, e.g. “orange peel” reflectors, or a cylindrical parabolic fed, for example, by a linear array of dipoles.

In accordance with the present invention an antenna is configured generally to create a fan beam wave profile or pattern using a conventional parabolic antenna configuration but fitted with two dielectric plates of specified geometry to combine reflection and refraction. The dielectric plates can be made from any low loss dielectric including Polyethylene, ABS, PTFE and ceramics. The thickness of plates is approximately a half wavelength in the dielectric. At frequencies in the 25 - 29 GFIz band that means that the plates with a dielectric constant sr of 2.5 would be 3.5mm thick.

As indicated above generally the antenna is based on a conventional parabolic design. The f/D ratio of the parabolic dish is typically 0.35 - 0.45. The beamwidth in the plane containing the fan shape profile is typically 25 - 35 degrees. The diameter of the dish is between 9 to 14 wavelengths so with electromagnetic waves at 27 GFIz the dish diameter is between 100mm and 150mm.

Claims

1. An antenna comprising a radiating element operable to radiate an electromagnetic wave, and a reflector operable to direct at least a portion of the electromagnetic wave in a wave profile in a direction along a radiation propagation path; wherein a dielectric element is provided between the radiating element and the reflector in the radiation propagation path, and the dielectric element being configured by shape and/or thickness to modify the phase of the wave profile differentially between an azimuth plane and an elevation plane in the direction of the radiated wave along the propagation path.

2. An antenna as claimed in claim 1, wherein the reflector is a parabolic reflector.

3. An antenna as claimed in claim 2, wherein the reflector is a truncated parabolic reflector.

4. An antenna as claimed in any preceding claim, wherein the dielectric element has a substantially flat configuration across the propagation path.

5. An antenna as claimed in any preceding claim, wherein the dielectric element is substantially orthogonal to the propagation path.

6. An antenna as claimed in any of claims 1 to 5, wherein the dielectric element has a uniform thickness.

7. An antenna as claimed in any of claims 1 to 5, wherein the dielectric element has a variable thickness in sections across the propagation path.

8. An antenna as claimed in any preceding claim, wherein the dielectric element has a consistent material type across the propagation path.

9. An antenna as claimed in any preceding claim, wherein the antenna includes a plurality of dielectric elements.

10. An antenna as claimed in claim 9, wherein the plurality of dielectric elements are separated with a gap between them.

11. An antenna as claimed in claim 10, wherein the plurality of dielectric elements are laterally separated in a plane with a gap between them.

12. An antenna as claimed in any of claims 9 to 11, wherein the dielectric elements are mounted upon means to allow movement to vary their relative location to each other and/or vary the gap between them.

13. An antenna as claimed in claim 9 or any claim dependent therefrom, wherein the plurality of dielectric elements are in a stack one upon the other in the propagation path.

14. An antenna as claimed in claim 13, wherein the stack of dielectric elements are in direct contact, surface to surface, with each other or have a gap between them.

15. An antenna as claimed in claim 13 or claim 14, wherein the stack includes means to allow separation of the plurality of dielectric elements in the stack to vary the gap separation between them.

16. An antenna as claimed in any preceding claim, wherein the dielectric element has a thickness relative to the expected wavelength of the electromagnetic wave in the order of λ/2 of the expected wavelength of the electromagnetic wave or a multiple of that.

17. An antenna as claimed in any preceding claim, wherein the dielectric element has a dielectric constant in the range 2.3 to 3.0.

18. An antenna substantially as hereinbefore described and with reference to Figures 4a and 4b of the accompanying drawings.