This invention relates to antennas and in particular, but not exclusively, to a lens for a deployable antenna. The present invention finds particular application in large aperture, wideband, deployable antenna structures suitable for space applications operating in the UHF to EHF frequency bands, generally defined as being in the frequency range from 300 MHz to 300 GHz.
Large aperture antennas, particularly those intended for use in space communications applications, are often based upon a large reflector element which may be packed into a relatively compact package and unfurled, once in position, by a suitable deployment mechanism. The deployment mechanism may comprise a mechanical tensioning device or an inflatable support structure, for example. However, reflectors tend, to be less tolerant of path length errors, caused for example by deformation of the reflector surface, causing greater phase errors than certain other types of antenna, in particular those based upon radio frequency (RF) lenses.
Large RF dielectric lenses are generally considered unsuitable for space or similar applications as they are difficult to fabricate and to deploy. Certain types of “artificial” lens are known, such as Fresnel lenses, which have been used in certain applications. In particular an artificial lens comprising printed parallel plate waveguides is known to have been used in a large aperture antenna structure.
Another known type of RF lens—a planar lens—typically comprises three layers:—a layer of antenna feed-facing planar elements; a layer of signal paths of varying length; and a layer of non-feed-facing planar elements. The layers of planar elements are interconnected via the signal path layer. The planar array elements in this type of planar lens receive and radiate in a broadside direction. However, in this type of lens, the following constraints generally apply:
From a first aspect, the present invention resides in a deployable lens for an antenna, comprising an array of metallic lens elements formed on a plurality of interlinked sections of an at least partially flexible dielectric substrate, wherein each lens element comprises a first end-fire element directed towards a feed side of the lens, a second end-fire element directed towards a non-feed side of the lens and a section of transmission line for coupling signals between the first and second end-fire elements, wherein the section of transmission line is arranged to apply a delay to said signals according to the position of the lens element within the aperture of the lens as deployed.
For the purposes of the present patent specification, and as is generally recognised in the art, the term “feed” is intended to refer to any antenna element or group of elements positioned to receive signals or to transmit signals via the lens. A “feed” may also comprise a waveguide positioned and oriented so as to receive or transmit signals via the lens. Thus, the term “feed side” of the lens refers to that side of the lens that faces towards the antenna—towards the antenna feed element(s) or waveguide. The, “non-feed side” of the lens is that side facing away from the antenna, towards the Earth in a space application for example.
Preferably, the structure of the lens, when deployed, comprises a cellular structure of open-ended cells wherein one of the lens elements is formed on at least one wall of each cell. The cells are preferably defined by intersections of the plurality of planar sections and the cells are preferably square-sectioned.
The use of end-fire elements in particular enables a lens to be made in the form of an open cellular structure which is not only particularly simple to make and to deploy, but when in the deployed configuration the open-ended cells offer much reduced air resistance in at least one direction in comparison with known planar lens structures; the latter advantage being of interest to applications involving low earth orbits in particular.
The preferred use of square-sectioned cells, in particular, provides for a particularly simple structure that may be collapsed for stowage and may be deployed by means of a simple mechanical extender or an inflatable support structure.
Where only linear polarisation is required, the array of lens elements comprises lens elements formed only on a first plurality of parallel cell walls. For dual polarisation, lens elements may additionally be formed on a second plurality of parallel cell walls which, when the lens is deployed, lie substantially perpendicular to the first plurality of walls.
Preferably, each of the first and second end-fire elements is a Vivaldi end-fire element. Such an element design makes for a particularly simple lens element, especially when the coupling signal path of each lens element is provided by a section of slot-line transmission line integrated with the first and second end-fire elements, as in a preferred embodiment of the present invention. In this case, the entire lens element lies in a single plane and so may be cut into a metal layer applied to only one side of the dielectric substrate.
According to preferred embodiments of the present invention, the section of transmission line of each lens element may be provided by a section of micro-strip transmission line or by a section of strip-line transmission line. These options preserve the substantially planar nature of the lens element. However, the micro-strip option requires a metal layer to be applied to both sides of the dielectric substrate so that the end-fire elements may be cut into the metal on one side of the dielectric substrate and the micro-strip coupling section may be cut from the metal layer on the reverse side of the substrate. The strip-line option requires a slightly more complex structure of two layers of dielectric substrate, each with a metal layer on their outward facing surfaces to accommodate the end-fire elements on one such surface and a ground plane layer on the other such surface. Sandwiched between the dielectric substrate layers, and hence between the end-fire element layer and the ground plane layer, is a section of strip-line conductor which, in conjunction with the outer ground plane layers forms a section of transmission line to couple signals between the end-fire elements, with a appropriate level of delay. However, despite requiring two dielectric substrate layers, optionally a third dielectric layer to maintain precise spacing between the different layers, and three metal layers, the total thickness of the cell walls in the strip-line option need be no more than approximately one fiftieth of a wavelength of the signals that the lens is required to operate with.
According to the requirements for the aperture and focal length of the lens in preferred embodiments of the present invention, the signal path length for each of the lens elements is set so as to provide an appropriate profile of delay across the aperture of the lens to signals passing through the lens. In general, the lens will be required to focus incoming RF signals onto an antenna feed element or into a waveguide or to form a substantially parallel beam of signals from those emitted by an antenna feed element or waveguide by way of output from the antenna.
By increasing the range of signal delays that are applied by lens elements from the edge to the centre of the lens aperture, the focal length of the lens may be reduced. If the required difference in delay makes it difficult to provide a sufficiently long signal path within the preferred length of a lens element, then a so-called “zoned” lens may be provided in which integer multiples of the intended operational wavelength for signals may be removed from the signal path length in lens elements across the lens, as will be discussed in more detail in the description of preferred embodiments below. Alternatively, those lens elements towards the centre of the lens may be made longer and hence the lens may be made locally thicker, enabling the respective lens elements to accommodate a longer signal path length.
In a preferred embodiment of the present invention, one or more passive components are integrated within the signal path of the lens elements. For example, a simple filter may be incorporated. In other applications, it may be desirable to integrate one or more active components within the signal path of the lens elements, for example to provide amplification of other forms of manipulation to, signals passing therethrough. Advantageously, the open cell structure of the lens provides a natural route for escaping heat from any such active components.
From a second aspect, the present invention resides in a deployable antenna that uses a deployable lens according to the first aspect of the present invention summarised above and as discussed in more detail below.
From a third aspect, the present invention resides in a method for manufacturing a deployable lens for an antenna, wherein the lens as deployed comprises a cellular structure of open-ended cells, the method comprising the steps of:
(i) from a flexible sheet of dielectric material having a metal layer applied thereto, forming, in the metal layer, one or more rows of lens elements required to form the lens, each lens element comprising a back-to-back pair of end-fire antenna elements;
(ii) selecting a first row and a second row of lens elements from those formed at step (i) comprising those lens elements that will form a given group of cells of lens elements in the lens as deployed and placing those rows in parallel association such that, for a given cell in the lens as deployed, a lens element in the first row that will be adjacent within the cell to a lens element in the second row are aligned;
(iii) bonding the first and second rows together along a bonding line or bonding region associated with each aligned pair of lens elements; and
(iv) repeating steps (ii) and (iii) until all rows of lens elements required to form the lens have been selected and bonded together.
Preferably, fold lines are formed in the dielectric material and/or the metal layer between adjacent lens elements such that there is flexing along those fold lines, when the lens is deployed, to enable the respective cells to open.
Preferred embodiments of the present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings of which:
According to a first embodiment of the present invention, a constrained planar end-fire lens is provided for use in a deployable antenna. In the context of the present invention, the term “constrained planar” is intended to mean that the signal paths are constrained to transmission lines (not free space signal paths) and that the end-fire elements terminate in a substantially planar surface across the aperture of the lens on both the feed side and the non-feed side of the lens. Alternatively, in a preferred embodiment of the present invention, the end-fire elements may terminate in a stepped planar surface, on either or both of the feed side and the non-feed side surface of the lens, and this type of surface will be considered for the purposes of the present patent application to fall within the scope of “constrained planar”.
The structure of the lens in this first embodiment, being based upon end-fire elements, has been found to be particularly suited to compact stowage and subsequent deployment. Moreover, the method of fabrication of the lens, as will be described in more detail below, is particularly simple in comparison with that for known planar lens structures. In particular, where a large aperture lens is required and the structure comprises a large number of lens elements, the lens elements in a preferred embodiment of the present invention may be simply fabricated on a flexible dielectric substrate as large sheets of elements which may be cut into strips, folded and bonded together along weld lines.
To begin, the structure and the key principles in operation of a lens according to this first embodiment of the present invention will now be described with reference to
Referring to
Thus a signal received by a first Vivaldi element 115 on a receiving side of the lens is coupled by means of the slot-line transmission line 125 to a second Vivaldi element 120 on the transmitting side of the lens where the signal is transmitted. The length of the slot-line transmission line 125 for that particular lens element 110 determines the time delay that is applied to the signal between receipt and transmission by the lens element 110.
Across the aperture of the lens, the length of the slot-line 125 in each of the lens elements 110 is set according to the requirements of the lens to focus incident radiation. In the small section of lens shown in
As will be described below, the cell-like lens structure is formed and held together by welded or otherwise bonded sections 130 of the metal-clad dielectric substrate, for example by means of weld lines. The width of these welded sections 130 shown in
As mentioned above, a required delay profile across the aperture of the lens is implemented by providing a different length of slot-line 125 in each lens element 110 according to its distance from the centre of the lens when deployed. Lens elements 110 that lie at substantially the same distance from the centre of the deployed lens have slot-lines of the same length. However, a more coarsely profiled lens may be provided in which lens elements 110 lying in annular regions several lens elements wide are provided with slot-lines of the same length so that the delay profile is more coarsely stepped across the aperture of the lens. Such a delay profile may be tolerable in certain applications and provides for a simpler construction in that a smaller number of different lens elements need to be fabricated.
If the lens is required to have a relatively short focal length such that the range of delays required between the centre and the outer regions of the lens is relatively large, it may not be possible to accommodate a slot-line of the length required for the central lens elements within a lens element of a certain preferred length (corresponding to the thickness of the lens). One possible technique, mentioned above, is to make the lens elements in the central region of the lens longer than those towards the edge of the lens, so that at least one surface, e.g. the feed-side surface of the lens, is stepped to some degree. An alternative technique that may be applied where a large variation in delay is required is to apply zoning to the lens. Zoning is a known technique for containing the thickness of a radio-frequency lens by removing a whole number of wavelengths of the intended operating wavelength from the delays that are applied across different regions of the lens. Thus, the delay profile of lens elements may range from the same minimum delay to the same maximum delay repeatedly, between the outer regions of the lens and the centre of the lens, according to the radial distance from the centre, so that lens elements in no one region of the lens needs to apply a delay outside this range and so exceed a certain predetermined thickness. However, the zoning technique is only suited to relatively narrow bandwidth applications as the zoning is designed with respect to a certain operational wavelength of signals. The performance of the lens can, be expected to degrade at operating wavelengths significantly different to the design wavelength.
Two examples of lens elements 110 with different slot-line lengths are shown in and will now be described with reference to
Referring to
A perspective representation of a deployed portion of a lens according to this first embodiment is shown in
A preferred method for manufacturing a lens according to this first embodiment will now be described with reference to
Referring to
In the example of
Referring to
Preferably, the strips 500-510 are placed with their metal faces 520 similarly oriented, and hence their dielectric faces 525 similarly oriented so that all welded regions 130 comprise dielectric-to-metal bonds. Alternatively, the strips 500-510 may be oriented in pairs, metal face 520 to metal face 520, dielectric face 525 to dielectric face 525, such that bonding of materials is like-with-like, i.e. metal to metal and dielectric to dielectric. However, in the event that the dielectric substrate is provided with a metal layer on both sides, all weld regions 130 will have metal surfaces and all welds will thus be metal-to-metal. If metal is removed between lens elements 110 to leave gaps between lens elements 110, then all weld regions 130 will be bare dielectric and all welds will be dielectric-to-dielectric.
It is not necessary that the metal-faced portion 520 of the lens element 110 is flexible to the same degree as that portion in the vicinity of the weld region 130. In a lens structure as described in this first embodiment of the present invention, stowage and deployment of a lens requires only that the dielectric material is able to bend in the region of the fold lines 135.
In this first embodiment of the present invention, the Vivaldi end-fire elements 115, 120 are coupled using a slot-line transmission line 125. There are particular advantages associated with remaining in slot-line for the entire lens element, in particular that of enabling the lens element 110 to be fabricated in a single planar layer of metal. However, the Vivaldi end-fire elements 115, 120 may alternatively be coupled by sections of micro-strip or strip-line transmission line in further preferred embodiments of the present invention, if necessary incorporating a separate delay line. To illustrate the differences between the three transmission line embodiments, the structure of corresponding cells of lens elements, for example in pre-assembly arrangements corresponding to that described above with reference to and as shown in
Referring firstly to
The structure of a lens element, and hence of the lens, comprising end-fire Vivaldi elements coupled by means of a section of micro-strip transmission line will now be described with reference to
Referring initially to
b shows the second part 705 of the lens element, comprising the micro-strip conductor 750 cut from the metal layer on that opposed face of the dielectric substrate. The micro-strip conductor 750 corresponds in its function to the section of slot-line 125 in the lens element 110 of the first embodiment described above. That is, it provides an appropriate delay to signals passing between the Vivaldi elements 710, 715, the length of delay being determined by the length of the conductor 750. The length of conductor 750 is varied according to the distance of the lens element from the centre of the lens, as discussed above. Signals received by one of the Vivaldi elements 710, 715 are coupled from the respective Vivaldi element 710, 715 to the micro-strip conductor 750 by means an angled portion 755 formed at the respective end of the micro-strip conductor 750. Having passed along the transmission line section provided by the micro-strip conductor 750 and the corresponding section of ground plane 730, the signal is similarly coupled to the other Vivaldi element 715, 710 for transmission by the lens element.
Referring to
The structure of a lens element, and hence of the lens, comprising end-fire Vivaldi elements coupled by means of a section of strip-line transmission line will now be described with reference to
Referring initially to
Referring to
Referring to
Referring to
The lens element of this third embodiment is assembled by bringing the first and second dielectric sheets together so that the strip-line conductor 920 is sandwiched between them. The strip-line conductor 920 is thus separated by dielectric layers from the metal sections 930 and 915 of the first and third parts 900, 910 which act as ground plane layers in a section of strip-line transmission line, coupling the Vivaldi elements 935, 940. Preferably, in order to maintain a precise spacing between the two dielectric layers, a third layer of dielectric material, of a different type of dielectric material to that of the first and second layers, may be used as a thin spacer to fill what would otherwise be air gaps in the strip-line conductor layer between the first and second dielectric sheets. Preferably, all three layers of dielectric material are bonded together.
Referring additionally to
In the example shown in
Whereas one preferred technique has been described for making a lens according to preferred embodiments of the present invention, it will be clear to a person of ordinary skill in this field that other techniques may be used to make a deployable lens of the structure preferred in the present invention, substantially as shown in its deployed state in
Number | Date | Country | Kind |
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07270043.8 | Aug 2007 | EP | regional |
0716356.1 | Aug 2007 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2008/050678 | 8/8/2008 | WO | 00 | 11/4/2008 |