LEACH, Thomas Robin (36 North Court, Northfield Road, Sheffield S10 1QR, GB)
ORTON, Richard Stanley (BAE SYSTEMS Advanced Technology Centre, West Hanningfield RoadGreat Baddow, Chelmsford Essex CM2 8HN, GB)
LEACH, Thomas Robin (36 North Court, Northfield Road, Sheffield S10 1QR, GB)
CLAIMS
1. An electromagnetic band-gap structure, comprising: an electrically conducting ground plane; and a periodic planar arrangement of electrically conducting surface elements mounted parallel to and at a predetermined distance from the ground plane, wherein each of the surface elements is supported in said planar arrangement by at least one electrically conducting support element extending from an edge of the surface element to the ground plane and wherein for no two adjacent surface elements are their respective support elements disposed in a parallel back-to-back arrangement.
2. An electromagnetic band-gap structure according to Claim 1 , wherein the surface elements are all of the same close-packing shape.
3. An electromagnetic band-gap structure according to any one of the preceding claims, wherein the surface elements and the respective support elements are formed by folding flat metal sheet templates.
4. An electromagnetic band-gap structure according to Claim 3, wherein each of the flat metal sheet templates forms a unit cell comprising one or more surface elements and respective support elements.
5. An electromagnetic band-gap structure according to Claim 4, wherein a unit cell comprises four adjacent square surface elements and respective support elements, and a base element.
6. An electromagnetic band-gap structure according to Claim 4, wherein a unit cell comprises one hexagonal surface element and at least one respective support element.
7. An electromagnetic band-gap structure according to Claim 4, wherein a unit cell comprises a plurality of triangular surface elements and respective support elements.
8. An electromagnetic band-gap structure according to Claim 5, wherein the support elements of any two adjacent surface elements are disposed at right angles to one another.
9. An electromagnetic band-gap structure according to any one of the preceding claims, arranged to operate as a high impedance surface with electromagnetic signals at a frequency in the range 100MHz to 1GHz.
10. An antenna, comprising an antenna element mounted on an electromagnetic band-gap structure according to any one of claims 1 to 9, wherein the electromagnetic band-gap structure is arranged to operate as a high impedance surface at an operating frequency of the antenna and hence as a ground plane for the antenna.
11. An antenna according to Claim 10, wherein the antenna element is a dipole antenna.
12. A low-profile antenna, comprising an antenna according to Claim 10 or Claim 11 , further comprising an antenna element mounted parallel to and at a level substantially coincident with the plane of surface elements in the electromagnetic band-gap structure. |
ELECTROMAGNETIC BAND-GAP STRUCTURE
This invention relates to electromagnetic band-gap structures and in particular, but not exclusively, to an improved structure operable as a high impedance surface for use in low-profile antenna applications operating with electromagnetic radiation in the frequency range 100MHz to 1 GHz.
In a known arrangement, described for example by D. Sievenpiper, L. Zhang, R. F. J. Broas, N. G. Alexopolous and E. Yablonovitch in "High- Impedance Electromagnetic Surfaces with a Forbidden Frequency Band," IEEE Trans. On Microwave Theory and Technology, Vol. 47, No. 1 1 , November 1999, a high impedance surface has been created using a structure in the form if a close-packed periodic array of square-topped "thumb tack' or mushroom-shaped metal elements connected a ground plane surface by means of vias. A representation of the structure of Sievenpiper et al. is shown in Figure 1 . This structure is intended for use with signals of microwave frequency, usually defined to lie in the region of the electromagnetic spectrum between infra-red and radio waves and so of frequency in the range of 1 to 300GHz typically. The structure shown in Figure 1 is therefore typically of a scale that allows for fabrication using printed circuit techniques. If the structure described by Sievenpiper et al. were to be scaled up in size to be suitable for use with signals in the range of 100MHz to 1 GHz, the result would be a bulky and heavy structure.
From a first aspect, the present invention resides in a electromagnetic band-gap structure, comprising: an electrically conducting ground plane; and a periodic planar arrangement of electrically conducting surface elements mounted parallel to and at a predetermined distance from the ground plane, wherein each of the surface elements is supported in said planar arrangement by at least one electrically conducting support element extending from an edge of the surface element to the ground plane and wherein for no two
adjacent surface elements are their respective support elements disposed in a parallel back-to-back arrangement.
Electromagnetic band-gap structures according to this first aspect of the present invention comprise a periodic array of unit cells, each unit cell comprising at least one electrically conducting surface element of a close- packing shape supported at its edge and electrically connected to an electrically conducting ground plane by at least one electrically conducting support element. The support elements are placed so that for no two adjacent surface elements are their support elements arranged in close proximity to one another in a parallel back-to-back arrangement. This has the advantage that undesirable or unpredictable effects affecting the performance of the structure as a high impedance surface at a desired frequency may be avoided. Support elements of adjacent surface elements may be arranged parallel to one another so long as they are placed apart, for example at non-adjacent edges of the adjacent surface elements.
Advantageously, this particularly simple form of structure enables a high impedance surface to be constructed much more easily and with less expense than known high impedance surfaces designed for use in the preferred frequency range of 100MHz to 1 GHz. Preferably, the surface elements are all of the same close-packing shape. Close packing shapes that may be used where all the surface elements are of the same shape include triangles, squares and hexagons. However, in an alternative embodiment, a mixture of different shapes may be used to achieve a close-packed arrangement of surface elements. In a preferred example that makes use of two different shapes for the surface elements, a periodic arrangement of octagons and squares may be used. Of course any periodic combination of shapes may in theory be used that results in a close-packed arrangement of surface elements. However, there may be a corresponding reduction in the ease of manufacture of structures using more complex arrangements of shapes.
In the prior art arrangement of Sievenpiper et al. for example, as shown in Figure 1 , a high impedance surface 100 comprises an arrangement of surface elements 105 connected to a ground plane 110 by means of vias 115.
However, in the present invention, the surface elements are connected to the ground plane and supported at one or more of their edges by flat metal support elements arranged preferably at approximately 90° to the ground plane and to the plane of surface elements. Preferably the surface elements and their support elements may be folded from flat metal templates, greatly simplifying their manufacture in comparison with a structure made according to the prior art design referenced above.
Preferably, where the support elements are square, the support elements of any two adjacent surface elements are disposed at right angles to one another. This provides for a more uniform structure.
Electromagnetic band-gap structures according to the first aspect of the present invention are designed for use, preferably, with electromagnetic signals in the frequency range 100MHz to 1 GHz.
From a second aspect, the present invention resides in an antenna, comprising an antenna element mounted on an electromagnetic band-gap structure defined according to the first aspect above, wherein the electromagnetic band-gap structure is arranged to operate as a high impedance surface at an operating frequency of the antenna and hence as a ground plane for the antenna.
Advantageously, structures according to preferred embodiments of the present invention are structurally and electromagnetically similar in more than one direction, for example in the x direction and the y direction, parallel to the edges of the surface elements in the case of square surface elements. Thus a dipole antenna would be substantially unaffected by its mounting orientation on the surface of the structure, enabling crossed dipole antennae to be mounted for example. Preferably, structures according to preferred embodiments of the present invention may be filled with a light-weight dielectric foam material in order to
- A - increase their robustness and rigidity without adding significantly to their weight. This is of particular advantage in those embodiments in which surface elements are supported by only one edge.
From a third aspect, the present invention resides in a low-profile antenna, comprising an antenna as defined according to the second aspect above, further comprising an antenna element mounted parallel to and at a level substantially coincident with the plane of surface elements in the electromagnetic band-gap structure.
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:
Figure 1 shows a prior art electromagnetic band-gap structure;
Figure 2 shows an electromagnetic band-gap structure according to a first preferred embodiment of the present invention; Figure 3 shows a portion of the structure of Figure 1 for the purpose of explaining its behaviour as a high impedance surface;
Figure 4 shows a plan view of a low-profile antenna comprising a structure according to the first preferred embodiment of the present invention;
Figure 5 shows a flat metal template for use in constructing unit cells that make up the structure in the first preferred embodiment of the present invention;
Figure 6 shows a flat metal template for use in constructing unit cells that make up the structure according to a second preferred embodiment of the present invention;
Figure 7 shows an electromagnetic band-gap structure according to the second preferred embodiment of the present invention;
Figure 8 shows a plan view of an electromagnetic band-gap structure according to a third preferred embodiment of the present invention; and
Figure 9 shows a flat metal template for use in constructing unit cells that make up the structure according to the third preferred embodiment of the present invention.
Electromagnetic band-gap (EBG) structures according to preferred embodiments of the present invention have been designed to provide a high- impedance surface to electromagnetic radiation at selected frequencies in the range 100MHz to 1 GHz in particular. These preferred EBG structures are particularly suited for application to low-profile antennae in which they are used to provide a ground plane. At frequencies in the range 100MHz to 1 GHz, known high-impedance surfaces require large and heavy structures. However, preferred embodiments of the present invention aim to provide a light-weight structure and one that is simple and inexpensive to make.
An EBG structure according to a first preferred embodiment of the present invention will now be described with reference to Figure 2. Referring to Figure 2, a perspective view of a portion of an EBG structure
200 is shown comprising a periodic arrangement of square surface elements 205, each of width w and each one separated from its adjacent plates 205 by a distance g. Each surface element 205 is connected to a ground plane 210 and supported by one edge at a height h above and parallel to the ground plane 210 by a support element 215. Each support element 215 is preferably oriented at substantially 90° to the ground plane 210 and to the surface element 205 that it supports. Preferably, the surface elements 205 are manufactured as groups of four adjacent elements, each group of four elements forming what will be referred to as a unit cell. One of these unit cells is represented in Figure 2 by the surface elements 205a, 205b, 205c, 205d and their support elements 215. By adjusting the dimensions w, g and h, the structure 200 may be arranged to behave as a high-impedance surface over a required frequency range. The techniques for determining appropriate values for w, g and h are well known in the art and will not be described here. Further information on the theory and practice of high impedance surfaces applicable to that of the present invention may be found in the prior art document of Sievenpiper et al. referenced above.
The EBG structure 200 operates on the basis of a parallel resonant LC circuit in which resonance occurs when ωo 2 = 1/LC. The basis of operation of the EBG structure 200 as a high-impedance surface can be understood in more detail with reference to Figure 3 in which a perspective view of a small portion of the structure 200 is shown.
Referring to Figure 3, capacitance C is shown to exist between adjacent edges 300 of the surface elements 205, and also between adjacent edges 305 of the support elements 215. Inductance L arises in the structure 200 when magnetic fields are created by current flowing through the support elements 215 and the ground plane 210. These capacitance and inductance properties are not confined to one unit cell or another, but also arise between unit cells across the structure 200.
In a preferred application of the EBG structure 200, the structure 200 is required to behave as a high impedance surface at the frequency of operation of an antenna and so provide a suitable ground plane to enable a low-profile antenna to be constructed, as will now be described with reference to Figure 4.
Referring to Figure 4, a plan view is provided of a low profile antenna structure 400, from a direction perpendicular to the plane of the structure 400, comprising an EBG structure similar to that of Figure 2. The structure 400 comprises a 5x5 array of unit cells of the type (205a-d) shown in Figure 2, providing a periodic structure of square surface elements 405 supportably connected to a ground plane 410. An antenna 420 comprising antenna elements with a total length that is half the intended operating wavelength of the antenna 420 is installed in the centre of the structure 400. Preferably, the antenna 420 is mounted at substantially the same height above the ground plane 410 as that of the surface elements 405 above the ground plane 410, preferably leaving a small gap so as to avoid actual contact with the underlying surface elements 405.
By way of example, if a low-profile antenna structure 400 is required for operation with signals of frequency 432 MHz, then the EBG structure is required to have a band-gap at 432MHz in which the phase of its reflection coefficient is
0°, so imitating the behaviour of a perfect magnetic conductor (PMC) at that frequency. To achieve this with a 5x5 unit cell EBG structure as shown in Figure 4, the dimensions referred to above may be determined using standard techniques to be: w = 128mm, g = 8mm, and h = 80mm. The half-wavelength antenna 420 is approximately 700mm long and is mounted at a height of 80mm above and parallel with the ground plane 410. Preferably the unit cell components are constructed from aluminium sheet 1.5mm thick and the ground plane 410 is constructed from aluminium sheet 3mm thick.
Conventionally, a radiating dipole antenna would need to be mounted approximately 174 mm above a perfect electric conductor (PEC) ground plane for operation at 432MHz. Hence, mounting the antenna 420 above an EBG structure according to this first preferred embodiment of the present invention reduces the overall height of the antenna by approximately 94mm in this particular example. Conveniently, each of the unit cells of the EBG structure 200 of Figure 2 or that (400) of Figure 4 may be constructed using flat metal sheet templates. Each flat metal template is designed to be folded along predetermined fold lines to form the 3D structure of a unit cell. Once folded into shape, each unit cell may be bolted or otherwise fixed to a metal ground plane at a required spacing. Examples of templates and the corresponding unit cells will now be described according to preferred embodiments of the present invention, beginning, as shown in Figure 5, with a template for making unit cells used in the EBG structures 200 and 400 described above.
Referring to Figure 5, and further with reference to Figure 2, a flat metal sheet template 500 is shown from which a unit cell comprising the surface elements 205a-d may be constructed. The template 500 is shown with dimensions marked by way of example for making a unit cell according to the particular 432MHz low-profile antenna embodiment described above with reference to Figure 4. The surface elements 205a-d are formed by folding along fold lines 505a-d, respectively, each through substantially 90°. The support elements 215 are formed by folding along the fold lines 515, also through substantially 90°, and in the same folding direction as for the respective
adjacent fold line 505a-d, to complete the unit cell. A base 520 of the unit cell may be drilled to form fixing holes 525 to enable the unit cell to be bolted or otherwise attached to the ground plane 210.
Referring to Figure 6, a further template 600 is shown for making a unit cell based upon square surface elements that is similar to that used in the structure 200 of Figure 2, but with a different arrangement of support elements 215. In the template 600, folding along fold lines 605a-d through substantially 90° results in surface elements 605a-d similarly disposed to those plates 205a-d of Figure 2. However, after folding along lines 610 through substantially 90° in the same folding direction as for the respective adjacent fold line 605a-d, the arrangement of support elements 615 around the base 620 of the unit cell is different to that resulting with the template 500 of Figure 5. Fixing holes 625 may be drilled through the base 620 to enable fixing of the unit cell to the ground plane 210, as above, ensuring that no two support elements 615 of adjacent unit cells are in a back-to back parallel arrangement. A preferred arrangement of unit cells constructed from the template 600 of Figure 6, after fixing to a ground plane 210, is shown in Figure 7.
Referring to Figure 7, a perspective view of an EBG structure 700 is shown, constructed from unit cells folded from the template 600 of Figure 6. As can be seen, the arrangement of surface elements 605 above the ground plane 710 is similar to that in the structure 200 of Figure 2, but the arrangement of supporting upright plates 615 is different to that involving the support elements 215 of Figure 2. Nevertheless, the dimensions required to achieve the characteristics of a high impedance surface at a desired frequency are similar to those for the EBG structure 200 of Figure 2 as described above given that the structure 700 possesses similar characteristics of capacitance and inductance as the structure 200.
Whereas the EBG structures 200, 400, 700 in preferred embodiments of the present invention use square surface elements 205, 405, 605, close- packing shapes other than squares may also be used for the surface elements, as would be apparent to a person of ordinary skill in this field. In particular, periodic arrangements of triangular or hexagonal surface elements may be
used with various arrangements of support elements to connect them to a ground plane. Alternatively, close-packed arrangements may be realised with combinations of surface elements of different shapes, for example octagons and squares. In a further preferred embodiment of the present invention, an arrangement based upon the use of hexagonal surface elements will now be described with reference to Figure 8.
Referring to Figure 8, a representational view is provided of an EBG structure 800, viewed from a direction perpendicular to a surface comprising a periodic arrangement of hexagonal surface elements 805, rather than square surface elements 205 as in Figure 2 or 405 as in Figure 4. Each of the hexagonal surface elements 805 is supported preferably by three support elements 815. However, only one or two support elements 815 may be used, alternatively, though with some loss of rigidity in the structure 800. A preferred arrangement of the support elements 815 is shown in Figure 8, consistent with the rule that no pair of support elements 815 of adjacent unit cells - a unit cell in this embodiment comprising a single hexagonal surface element 805 and its support elements 815— shall lie in a back-to-back parallel arrangement.
In the EBG structure 800 of Figure 8, each unit cell may be folded from a flat metal template as for the unit cells based upon a square surface element 205, 405, 605 in preferred embodiments described above. A preferred template for the unit cell in Figure 8 is shown in Figure 9.
Referring to Figure 9, a flat metal template 900 is shown for use in constructing unit cells based upon a single hexagonal surface element 805 with three support elements 815. Each of the support elements is formed by folding along a fold line 905 through substantially 90°. For convenience, mounting flanges 910 may be formed by making a further fold through substantially 90° along a respective adjacent fold line 915, preferably in the same folding direction as for the adjacent fold line 905. Fixing holes 920 may be drilled in the mounting flanges 910 to enable each unit cell to be securely attached to a ground plane to form the EBG structure 800 shown in Figure 8.
As will be apparent to a person of ordinary skill in the relevant field, templates of other designs may be made to create alternative arrangements of surface elements and their respective support elements, according to the shape or shapes of surface elements required. Furthermore, unit cells comprising different numbers of surface elements to those unit cells defined above may be chosen and the corresponding templates designed for their manufacture.
A preferred antenna element 405 for use in a low profile antenna as described above with reference to Figure 4 will now be described with reference to Figure 10. Referring to Figure 10, a perspective view of an antenna element 405 is provided, in which the antenna element is a dipole antenna having two arms 1005 of length equal to one quarter of the operational wavelength of the antenna, supported and fed in anti-phase (180° phase difference) substantially at the level of the surface elements of a electromagnetic band-gap structure (not shown in Figure 10) by semi-rigid sections 1010 of co-axial cable. The coaxial cables 1010 pass through and are insulated from the ground plane 410. Preferably, the outer lines of the co-axial cable sections 1010 are soldered together at a point 1015 where they terminate.
Other types of antenna may also be used in conjunction with electromagnetic band-gap structures according to preferred embodiments of the present invention. For example, in the antenna shown in Figure 10, a simple antenna element may be formed with a single arm of the required length by bending the inner core of a single section 1010 of coaxial cable through 90° at an appropriate height above the ground plane 410. In another example, a crossed dipole arrangement may be mounted above the ground plane 410.
Further choices of antenna design would be apparent to a person of ordinary skill in this field and are intended to fall within the scope of the present invention.
While electromagnetic band-gap structures according to preferred embodiments of the present invention are particularly suited for use in low-
profile antennae, antennae that do make use of these structures are not limited to being of the low-profile type.
References
The following additional publications are listed by way of background reading:
[1] Y. Rahmat-Samii, 'The Marvels of Electromagnetic Band-gap (EBG) Structures," A CES Journal, Vol. 18, No. 4, November 2003.
[2] S. Ramo, J. R. Whinnery, T. Van Douzer, "Fields And Waves In Communication Electronics," John Wiley & Sons, Second Edition, pp. 285-286, 1984.
[3] R. E. CoIMn, "Antennas And Radiowave Propagation," McGraw-Hill Book Co., International Student Edition, pp. 16-17, 1987.
[4] E. V. D. Glazier, H. R. L. Lamont, The Services' Textbook of Radio, Volume 5, Transmission and Propagation," Her Majesty's Stationery Office, pp. 54-61, 1958.
[5] S. Ramo, J. R. Whinnery, T. Van Douzer, "Fields And Waves In Communication Electronics," John Wiley & Sons, Second Edition, pp. 596-598, 1984. [6] F. Yang, Y. Rahmat-Samii, "Microstrip Antennas Integrated With Electromagnetic Band-Gap (EBG) Structures: A Low Mutual Coupling Design For Array Applications," IEEE Trans. On Antennas And Propagation, Vol. 51, No. 10, October 2003.
[7] F. R. Yang, K. P. Ma, Y. Qian, T. Itoh, "A Uniplanar Compact Photonic- Bandgap (UC-PBG) Structure And Its Applications For Microwave Circuits," IEEE Trans. On Microwave Theory And Techniques, Vol. 47, No. 8, August 1999.
[8] F. R. Yang, R. Coccioli, Y. Qian, T. Itoh, "PBG-Assisted Gain Enhancement of Patch Antennas on High-Dielectric Constant Substrate,"
Antennas And Propagation Society, IEEE International Symposium, Vol. 3, July 1999.
[9] F. R. Connor, "Introduction Topics In Electronics And Telecommunications-Antennas," Edward Arnold Ltd., pp. 33-34, 1981.