AU2011202962B2 - Low-tilt collinear array antenna - Google Patents

Abstract

A collinear antenna (100) is formed as a planar circuit on a dielectric substrate (102). The antenna comprises a central planar conductive transmission line (108) disposed along a central axis for conducting radio frequency (RF) signals to or from the antenna. A planar conductive ground element (110) is formed about the central axis. A collinear array of planar conductive transmission line segments (112) is disposed in opposed pairs adjacent to the ground element about the central axis. Each transmission line segment is conductively coupled to corresponding outer radiating dipole elements (114), and arranged longitudinally adjacent to at least one other such segment. As a result, RF signals are electromagnetically coupled between adjacent segments. A feed point (116) comprises at least one slot formed in the ground element, whereby RF signals are electromagnetically coupled between the central transmission line and a transversely adjacent one of the transmission line segments. 122 120 118 112 114 110 Figure 1 KL~7j27~104 Figure 2 7T Figure 3

Description

1 LOW-TILT COLLINEAR ARRAY ANTENNA FIELD OF THE INVENTION The present invention relates to antenna devices, and more particularly to collinear array antennas. 5 BACKGROUND OF THE INVENTION Wireless communications systems, including cellular telephony, indoor and outdoor wireless local area networks (LANs), and so forth, are becoming increasingly ubiquitous. Such systems generally require antennas having wide-angle radiation 10 patterns (often omnidirectional antennas are desirable), and commonly with vertical polarisation state. Collinear array antennas are well-known, and are able to satisfy these requirements. However, the simplest arrangements of collinear array antennas are end-fed designs, which have limited bandwidth. In particular, it is known in 15 the art that as the physical length of an electrical element (such as an antenna) is increased the bandwidth of the element, as measured relative to a fixed centre frequency, will decrease. In the case of collinear array antennas, this bandwidth reduction arises due to phase mismatch between radiating elements close to the feed point, and distant from the feed point, at frequencies deviating from the fixed 20 centre frequency. This phase mismatch also results in a frequency-dependent tilt, ie a deviation of the elevation-plane radiation pattern from that at the centre frequency. Known solutions to the bandwidth and tilt problems with collinear array antennas include centre-fed or corporate-fed designs, both of which aim to 25 reduce the difference in electrical length between a common signal input point and the various radiating elements in the array and/or to produce a symmetrical radiation pattern to suppress tilt. However, these types of array antenna are generally quite complex and expensive to manufacture. The antennas typically take the form of coaxial cylindrical dipole radiating elements, with elaborate feed 30 networks, which are prone to failure due to mechanical stress. For centre-fed designs, in order to get an isolated feed point, a triaxial-type transmission line network is required (ie a coaxial cable within a coaxial cable). Corporate-fed 2 designs require even more elaborate feed networks, generally requiring multiple power dividers. Aside from the cost of manufacture, and the susceptibility to mechanical failure, elaborate collinear array antenna designs generally involve large numbers 5 of electrical and/or mechanical joints between individual components, which are a known cause of passive intermodulation distortion (PIM). In practice, PIM can result in crosstalk between signals on different RF carriers within the antenna bandwidth, and it is therefore essential to minimise this type of distortion. The use of printed circuit board (PCB) design and manufacturing methods 10 can produce antennas having significantly reduced numbers of distinct mechanical and electrical components, thus addressing the aforementioned disadvantages of conventional cylindrical dipole antenna arrays. For example, collinear array antennas based on coplanar waveguide (CPW) arrangements are disclosed by Jakal and McEwan in "Collinear Antennas Based on Coplanar 15 Waveguide", 10th International Conference on Antennas and Propagation, 14-17 April 1997, No. 436, and by Nesic and Nesic in "Omnidirectional Uniplanar Electromagnetically Coupled Antenna Array", Microwave and Optical Technology Letters, Volume 40, No. 6, 20 March 2004. However, these designs are inherently end-fed, and therefore inevitably suffer from the reduced bandwidth 20 and tilt problems discussed above. The utility of such antennas is therefore limited to single frequency and narrowband applications. To date, no collinear array antenna has been developed that has been able to combine the bandwidth and low-tilt advantages of centre-fed cylindrical dipole array antennas with the reduced manufacturing costs and improved 25 mechanical and electrical performance achievable using planar design techniques. The present invention seeks to provide such an antenna. SUMMARY OF THE INVENTION In order to address the aforementioned problems and needs in the art, the present invention generally provides a collinear antenna having a central 30 longitudinal axis which is formed as a planar circuit on a dielectric substrate, and which comprises: a central planar conductive transmission line disposed along said central axis for conducting radio frequency (RF) signals to or from the antenna; 3 a planar conductive ground element formed about said central axis; a collinear array of planar conductive transmission line segments disposed in opposed pairs adjacent to the ground element about said central axis, each transmission line segment being conductively coupled to corresponding outer 5 radiating dipole elements, and arranged longitudinally adjacent to at least one other such segment, whereby said RF signals are electromagnetically coupled therebetween; and a feed point comprising at least one slot formed in the ground element whereby said RF signals are electromagnetically coupled between the central 10 transmission line and a transversely adjacent one of said transmission line segments. Embodiments of the invention therefore uniquely provide a "two-layer" transmission line structure. In particular, signals may be fed in (or received) via the central transmission line, wherein they are coupled at the feed point to the 15 outer array of transmission line segments. The signals propagate bi-directionally along this second layer transmission line system, comprising the electromagnetically coupled array of transmission line segments. The antenna radiates (or receives) signals via the outer radiating dipole elements that are conductively coupled to each transmission line segment. 20 Advantageously, the feed point may be located any desired distance along the collinear array. In particular, the feed point may be located centrally along the array, thereby providing a centre-fed collinear array antenna design exhibiting low frequency-dependent tilt. In alternative embodiments the feed point may be displaced from the central location, in order to achieve a desired beam tilt. For 25 example, if the antenna is to be placed at an elevated location, such as on a building rooftop, it may be desirable to have a downwardly tilting beam pattern. Alternatively, desired beam tilt may be introduced by appropriate modification of the dielectric substrate, eg by applying a dielectric loading to either the upper or lower half of the antenna, thereby increasing its electrical length, while 30 maintaining the feed point at a central location. In a preferred embodiment, the antenna may be formed as a coplanar waveguide (CPW) structure on the dielectric substrate. Such an arrangement is presently preferred, due to its relatively simpler design and fabrication, ie by 4 forming the required conductive elements on a single side of the dielectric substrate. However, alternative constructions are also possible. For example, in an embodiment based upon a microstrip design, the central planar conductive transmission line is disposed on a first side of the dielectric substrate, while the 5 ground element and array of transmission line segments are disposed on the opposing side of the substrate. Such a design is, however, presently considered less desirable, due to the requirements of forming conductive elements on both sides of the dielectric substrate, as well as the additional processing steps required to form an electrical connection between the transmission line and the 10 ground element on the opposing side of the substrate, eg using a via. Advantageously, impedance matching (eg to 50 ohms) may be achieved by appropriate design of the central transmission line. For example, in a preferred embodiment a stepped series impedance-matching transformer is implemented, in the form of a short section of the central transmission line that is 15 slightly narrowed so as to exhibit an increased series impedance. In the preferred embodiment, the antenna is terminated at each end by further radiating elements that are electromagnetically coupled to adjacent ones of the array of transmission line segments, and conductively coupled to the ground element. Advantageously, a short transmission line segment may be 20 used to provide conductive coupling between the radiating elements and the ground element, the length of which may be determined so as to provide appropriate phase compensation. The slot formed in the ground element at the feed point may be a simple gap in the ground element. However, in optimised antenna designs the slot will 25 generally have a more complex form designed to optimise the electromagnetic coupling between the central transmission line and the transversely adjacent second layer transmission line segments. Similarly, the outer radiating dipole elements may be generally rectangular in shape, and simply coupled to the corresponding transmission line segments by 30 a short length of transmission line. However, in an optimised design for a particular application the radiating elements may include beam pattern augmentation features, such as slots and/or extending "fingers".

5 As will be appreciated by those skilled in the art, the detailed design of high-frequency RF circuits, including antennas, is often an iterative process including computer simulation and prototyping steps, having the objective of tailoring the circuit configuration in order to achieve a desired level of 5 performance, according to predetermined design criteria, for a specific application. Accordingly, while further advantages and features of the invention will be apparent from the following description of preferred embodiments, these should be understood as exemplary, and not limiting of the scope of the invention as defined in any of the preceding statements, or in the claims appended hereto. 10 Note: the words 'comprises/comprising', and grammatical variations, are used in this specification to specify the presence of stated features, integers, steps or components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 15 BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which like reference numerals represent like features, and wherein: Figure 1 is a schematic diagram illustrating a four-element CPW antenna 20 design according to an embodiment of the invention; Figure 2 is a computed H-field pattern for the four-element CPW antenna of Figure 1; Figure 3 is a chart showing computed elevation and azimuth patterns for the four-element CPW antenna of Figure 1; 25 Figure 4(a) shows the layout of a more detailed antenna design embodying the invention; Figure 4(b) is a schematic diagram illustrating the cross-section of the array antenna of Figure 4(a); Figure 5 is a detailed view of a central section of the antenna shown in 30 Figure 4(a), showing radiating elements, shunt feed and feed point; Figure 6(a) is a detailed view of an impedance-matching transformer in the antenna shown in Figure 4(a); 6 Figure 6(b) shows an equivalent circuit of the impedance-matching transformer shown in Figure 6(a); Figure 7 shows a detailed view of the phase compensation arrangement of the antenna shown in Figure 4(a); 5 Figure 8(a) is a schematic diagram illustrating a four-element microstrip antenna design according to an embodiment of the invention; Figure 8(b) is a cross-section view of the antenna shown in Figure 8(a); and Figure 9 illustrates applications of antennas embodying the present 10 invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A coplanar waveguide (CPW) antenna 100 embodying the invention is illustrated schematically in Figure 1. The antenna 100 comprises a number of planar conductive elements disposed on a dielectric substrate 102. As a practical 15 matter, the antenna 100 may be manufactured using conventional printed circuit board (PCB) fabrication techniques. As will be appreciated, the ability to use well-established PCB design and manufacturing methods not only simplifies design and fabrication, but also provides a very high degree of precision and repeatability in the formation of the conducting elements, at a relatively low cost. 20 At one end 104 of the antenna structure, signals may be input to, or output from, the antenna 100 depending on whether it is used for transmission, reception, or both. In practice, a suitable RF connector will typically be provided, in order to facilitate connection to a transmitter and/or receiver, for example via a coaxial cable. Depending upon the operating frequency of the antenna 100, and 25 other design or commercial considerations, suitable RF connectors may include BNC, TNC, N, SMA, or any of a variety of other conventional types of connector that are well-known in the art. The antenna further comprises a plurality of pairs of radiating dipoles 106, formed from the conductive elements disposed on the substrate 102. For 30 convenience, it will be assumed throughout the remainder of this specification that the antennas described herein are used for transmission, ie that the dipole elements 106 are radiating RF energy corresponding with a signal input at the 7 end 104 of the antenna structure. However, as those skilled in the art will appreciate, the antennas may equally be used for reception of signals. A planar conductive transmission line 108 is disposed along a central axis 101 of the antenna 100. The transmission line 108 conducts signals from the 5 input end 104 into the antenna structure. Surrounding the central conductive transmission line 108 are planar conductive ground elements 110, also formed about the central axis 101 of the antenna 100. Moving further out from the central axis, opposing pairs of conductive transmission line segments, eg 112, are arranged in collinear arrays on either 10 side of the ground elements 110. Each transmission line segment 112 is conductively coupled to corresponding outer radiating dipole elements 114, adjacent pairs of which comprise the array of antenna dipoles 106. Each one of the planar conductive transmission line segments 112 is arranged longitudinally adjacent to at least one other such neighbouring segment, in close proximity such 15 that RF signals are electromagnetically coupled between such adjacent pairs of transmission line segments 112. The antenna 100 also comprises a substantially centrally located feed point 116, comprising slots 118 formed in the ground elements 110, creating a gap in the surrounding ground elements through which RF energy may be 20 coupled from the central transmission line 104 to transversely adjacent transmission line segments 112. The antenna 100 therefore operates (for radiation of RF energy) in the following manner. An RF signal input at the end 104 of the central transmission line 108 propagates towards the central feed point 116. At this point, RF energy 25 is able to radiate through the slots 118 in the ground elements, and is coupled to the outer transmission line segments 112 immediately adjacent to the slots 118. The signals then propagate in both directions along these transmission line segments, and are successively coupled to adjacent segments in the collinear array. As they do so, energy is conductively coupled to the outer radiating dipole 30 elements 114, whereby RF energy radiates from the antenna 100. Two other features are notable in the schematic CPW antenna design 100 shown in Figure 1. Firstly, the central transmission line 108 extends beyond the central feed point 116 and terminates at a far end point 120. At this point, the 8 transmission line 108 may be unconnected (ie open circuit, capacitive) or may be connected to the adjacent ground elements 110 (ie short-circuit, inductive). Secondly, at each end of the antenna 100 there are disposed pairs of dipole elements 122 that are conductively coupled to the ground elements 110. These 5 elements provide terminations for the propagating signals, and additional design features (not shown in Figure 1, but discussed below with reference to Figure 7) may be incorporated for phase compensation. As will be appreciated, the various dimensions of elements of the antenna 100, the number of dipole radiating elements 106, and the detailed structure of the various elements, will determine 10 the various parameters and performance criteria of the resulting antenna. These parameters and criteria include, for example, the antenna central frequency, bandwidth, gain and radiation patterns. Figure 2 shows a computed cross-sectional H-field pattern 200 for the four-element CPW antenna 100 shown in Figure 1. As can be seen, the current 15 density (ie radiation pattern) of the four dipoles is substantially symmetrical about the central feed point 116. It is therefore anticipated that the antenna will exhibit low tilt. While the bandwidth, defined, for example, in terms of gain or voltage standing wave ratio (VSWR) as a function of frequency, will be finite, the symmetrical distribution of current density will be substantially maintained over a 20 wide range of frequencies, thereby avoiding frequency-dependent tilt, as is expected for a centre-fed antenna. Alternatively, if it is desired to design an antenna having a predetermined, non-zero, beam tilt, this may readily be achieved either by displacing the feed point 116 away from the centre of the antenna 100, or by applying a dielectric 25 loading over half of the antenna length, on one side of the feed point 116, in order to increase its effective electrical length. Figure 3 is a chart 300 showing a computed elevation pattern 302, and a corresponding computed azimuth pattern 304, for the four-element CPW antenna 100 shown in Figure 1. It can be seen from the chart 300 that the antenna 100 30 exhibits zero tilt in combination with almost uniform (within 1.6 dB peak-to-peak) omnidirectional radiation. The slight oval deviation in the azimuth pattern 304 is due to the planar geometry of the antenna 100. Maximum radiation occurs at 90 degrees to the plane of the substrate 102.

9 Figure 4(a) illustrates the layout of a more-detailed practical antenna design 400 embodying the invention. Figure 4(b) is a schematic diagram illustrating a cross-section, eg through the plane A-A' of the antenna 400. The antenna 400 comprises the following features, many of which are described in 5 greater detail below with reference to Figures 5 to 7: 0 pattern augmentation elements (1); e phase compensation structures (2); e a central feed point (3); e dielectric substrate (4); 10 e an upper terminal element (5); e a series impedance-matching transformed (6); e central coplanar waveguide structure (7) comprising ground elements disposed about a central planar conductive transmission line (9); e planar radiating elements (8); 15 e outer electromagnetically coupled transmission line segments (10); e shunt (ie short circuit) feed (11); e coplanar strip transmission line (12); e outer protective radome (13), eg made from fibreglass; e copper cladding (14), from which the conductive elements are 20 etched; e adhesive dielectric tape (15); e rigid support (16), eg made from polycarbonate; and e high-density spacers (17), eg made from ether foam. Many of these features are more readily visible in the detailed views of 25 sections 402, 404 and 406 of the antenna 400, shown respectively in Figures 5, 6(a) and 7. Figure 5 shows a more-detailed view of the central section 402 of the antenna 400, in which the structures of the radiating elements 8, the shunt feed 11 and feed point 3 are more clearly visible. The shunt feed 11 comprises 30 conductive coupling (ie a short circuit) between the central transmission line 9 and the adjacent ground elements. The structure of the slots in the ground elements at the feed point 3 is relatively complex, and has been designed to 10 substantially enhance the coupling of the RF energy from the central transmission line 9 to the adjacent conductive transmission line segments 10. The basic form of the radiating elements 8 remains substantially rectangular, within the basic antenna design 100 shown in Figure 1, however additional slots and projecting 5 "fingers", eg pattern augmentation element 1, have been included in order to improve the beam pattern of the antenna 400. Figure 6(a) shows a detailed view of the section 404 of antenna 400, including the impedance-matching transformer 6. The impedance-matching transformer 6 comprises a stepped series transformer, implemented by a 10 narrowing of the central transmission line 9 so as to increase the characteristic impedance. For example, in an exemplary embodiment the feed point impedance is around Zfeed=( 2 8 +17j) ohm. As shown in the equivalent circuit diagram 600 in Figure 6(b), a short 50 ohm transmission line segment 602 is positioned between the feed point 3 and the narrowed segment 6, which removes the reactants 15 transforming to a non-reactive impedance of 103 ohm. The narrow segment 6 has a characteristic impedance of 70 ohm, which transforms the impedance to 50 ohm, matching with the input section 604 of the central transmission line 9. Figure 7 shows in greater detail the end section 406 of the antenna 400, comprising the upper terminal element 5. The terminal element 5 is a ground 20 element, to which the corresponding dipole elements 8 are conductively coupled. In particular, this coupling is via a short section of transmission line, electromagnetically coupled to the transmission line segment 10 adjacent thereto. The length of this transmission line, ie the location of the end point of slot 2, provides phase compensation. Correct phase compensation ensures that a 25 standing wave is established in the antenna 400 at the design centre frequency. While the CPW structure described above with reference to Figures 1 to 7 is presently preferred, due to its ease and relatively low cost of manufacture, the principles of the invention can be applied to other planar waveguide structures. In particular, Figure 8(a) is a schematic diagram illustrating a four-element antenna 30 800 using an alternative microstrip design, and embodying the present invention. Figure 8(b) shows a corresponding cross-sectional view taken through the plane

B-B'.

11 In the microstrip design the central planar conductive transmission line 804 is disposed on one surface (eg an upper surface) of a dielectric substrate 802. The ground elements 806, outer conductive transmission line segments and associated radiating elements (collectively 808) are disposed on the opposed, 5 eg bottom, side of the substrate 802. A central feed point comprises a slot 810 formed across the ground element 806, and running under the central transmission line 804. Formation of the shunt feed 812, ie conductive coupling between the end of the transmission line 804 and the adjacent ground plane, may be achieved using a via through the substrate 802. 10 The microstrip design 800 is directly analogous to the CPW design 100. However, the microstrip design 800 is presently less preferred, due to the additional costs and processing steps associated with formation of circuit elements on both sides of the substrate 802, and in establishing the shunt feed 812 between the central transmission line 804 and the ground element 806. 15 Figure 9 illustrates a number of potential applications of the antenna 400. These include antenna arrays 902, for use in MIMO systems, single vertically mounted antennas for use in mesh 904 and trunking 906 applications, and slimline wall-mounted units 908, for mini-cell applications. As will be apparent from the foregoing description, many variations are 20 possible falling within the scope of the present invention, depending upon antenna design requirements for particular applications. Accordingly, the invention is not to be limited to the particular embodiments described herein, but rather the scope of the invention is defined by the claims appended hereto. 25

Claims (9)

1. A collinear antenna having a central longitudinal axis which is formed as a planar circuit on a dielectric substrate, and which comprises: a central planar conductive transmission line disposed along said central 5 axis for conducting radio frequency (RF) signals to or from the antenna; a planar conductive ground element formed about said central axis; a collinear array of planar conductive transmission line segments disposed in opposed pairs adjacent to the ground element about said central axis, each transmission line segment being conductively coupled to corresponding outer 10 radiating dipole elements, and arranged longitudinally adjacent to at least one other such segment, whereby said RF signals are electromagnetically coupled therebetween; and a feed point comprising at least one slot formed in the ground element whereby said RF signals are electromagnetically coupled between the central 15 transmission line and a transversely adjacent one of said transmission line segments.

2. The antenna of claim 1 wherein the feed point is located centrally along the array, thereby providing a centre-fed collinear array antenna

3. The antenna of claim 1 wherein the feed point is displaced from a central 20 location so as to provide predetermined beam tilt in an elevation plane.

4. The antenna of claim 2 wherein a dielectric loading is applied to either the upper or lower half of the antenna so as to provide a predetermined beam tilt in an elevation plane.

5. The antenna of claim 1 which is formed as a coplanar waveguide (CPW) 25 structure on the dielectric substrate.

6. The antenna of claim 1 which is formed as a microstrip structure on the dielectric substrate, wherein the central planar conductive transmission line is disposed on a first side of the dielectric substrate, and the ground element and 13 array of transmission line segments are disposed on the opposing side of the substrate.

7. The antenna of claim 1 further comprising a stepped series impedance-matching transformer disposed along the central transmission line. 5

8. The antenna of claim 1 which is terminated at opposing ends by further radiating elements that are electromagnetically coupled to adjacent ones of the array of transmission line segments, and conductively coupled to the ground element.

9. The antenna of claim 8 further comprising a transmission line segment to 10 provide conductive coupling between the radiating elements and the ground element, and configured to provide a predetermined phase compensation. RF INDUSTRIES PTY LTD 15 WATERMARK PATENT & TRADE MARK ATTORNEYS P33264AU00