GB2486675A

GB2486675A - An antenna for generating omnidirectional circularly polarized radiation

Info

Publication number: GB2486675A
Application number: GB201021679A
Authority: GB
Inventors: Max Ammann; Xiulong Bao; Adam Narbudowicz
Original assignee: Dublin Institute of Technology
Current assignee: Dublin Institute of Technology
Priority date: 2010-12-22
Filing date: 2010-12-22
Publication date: 2012-06-27
Also published as: GB201021679D0; GB2486675A8

Abstract

An antenna, suitable for generating omnidirectional circularly polarised radiation, comprises a planar ground electrode 110 sandwiched between two planar radiating electrodes 115 and where a dielectric medium 120 is interposed between each of the said electrodes. Each of the radiating electrodes 115 includes a perturbation 130 and a feed 140 which is arranged to couple, in-phase signals, to each radiating element 115. The three electrodes 110, 115 are in a spatial arrangement such that, when the radiation electrodes are excited, their respective electric fields electrically couple together to generate an omnidirectional radiation pattern. The footprint of the ground electrode 110 may be 10% to 80% larger than that of the resonant electrodes 115. The electrodes may, for example, be square, rectangular and/or circular shaped. Circular radiating electrodes may be concentric. The perturbation associated with the resonant electrodes may be stubs or other phase perturbation means such as truncated corners or slot voids or any suitable geometric formation. The feed may include a tapered electrode located in a slot. The antenna may be used in global positioning systems, RFID, vehicle communications, tolling and wireless communication systems.

Description

An antenna for generating omnidirectional circularly polarized radiation

Field of the Invention

The present application relates to an antenna for generating omnidirectional circularly polarized radiation, and in particular to an antenna which includes a pair of spaced apart radiation electrodes that provide electric fields that are proximity coupled together for facilitating the generation of an omnidirectional radiation pattern.

Background

Antennas known heretofore for Global Positioning Systems (GPS) may be categorized into two broad categories, namely, Directional Circularly Polarised (CP) antennas and omnidirectional linearly polarised antennas.

Directional Circularly Polarised (CP) antennas are typically mounted so that their main beam points upwards towards satellites orbiting the earth. Such antennas are typically used in fixed applications or mounted on the roof of vehicles.

Omnidirectional linearly polarised antennas are more vulnerable to distortion due to reflected signals and polarisation mismatch losses as a result of antenna orientation misalignment than CR antennas but may be mounted in any orientation. Such antennas are particularly useful for applications where the antenna orientation may change, for example on mobile phones.

There is therefore a need for a circularly polarised antenna that can be used in applications where the orientation of the antenna may change or where it can be mounted in different orientations.

Summary

These and other problems are addressed by an antenna for generating omnidirectional circularly polarized radiation pattern, the antenna comprises a pair of spaced apart radiation electrodes that provide electric fields that are proximity coupled together for facilitating the generation of an omnidirectional radiation pattern.

Accordingly, a first embodiment provides an antenna as detailed in claim 1.The present teaching also relates to an electronic device as detailed in claim 39. Advantageous embodiments are provided in the dependent claims.

These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the application.

Brief Description Of The Drawings

The present teaching will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of an antenna for generating omnidirectional circularly polarized radiation.

Figure 2 is a plan view of a detail of the antenna of Figure 1.

Figure 3 is a plan view of the antenna of Figure 1.

Figure 4 is a bottom plan view of another antenna for generating omnidirectional circularly polarized radiation.

Figure 5 is a side cross sectional view of the antenna of Figure 4.

Figure 6 is a top plan view of the antenna of Figure 4.

Figure 7 is graph of return loss (Si 1) versus frequency for the antenna of Figurei.

Figure 8 is a graph of axial ratio versus the angle of the antenna of Figure 1.

Figure 9 is a graph of the gain versus the angle of the antenna of Figure 1.

Figure 10 is graph of return loss (Si 1) versus frequency for the antenna of Figure 4.

Figure ii is a graph of axial ratio versus the angle of the antenna of Figure 4.

Figure 12 is a graph of the gain versus the angle of the antenna of Figure 4.

Detailed Description Of The Drawings

The present teaching will now be described with reference to some exemplary antennas which are provided to assist in an understanding of the present teaching.

Referring initially to Figures 1 to 3, there is illustrated an exemplary antenna 100 for generating omnidirectional circularly polarized radiation. The antenna 100 is a type of patch antenna which comprises a stack of three axially spaced apart electrodes each defining respective planar surfaces 1 05a, 1 OSb and 1 05c. The electrodes may be of copper or any other suitable conducting material. The stack has a ground electrode 110 located between two spaced apart radiation electrodes 115. The respective radiation 115 electrodes are an equal distance from the ground electrode 110. A dielectric medium 120 is interposed between the ground electrodes 110 and the respective radiation electrodes 115 which provide mechanical stability to the electrodes. In this example, the dielectric medium is Taconic RF-35, with a relative permittivity Er of 3.5 and a tangent delta loss 3k of 0.0018. Taconic RF-35 is given by way of example only and it will be appreciated by those skilled in the art that other dielectric materials may be employed.

The radiation electrodes 115 are operably coupled together by a coupling element 125 provided by an elongated conductive strip. The two radiation electrodes 115 are connected together by the coupling element 125 adjacent a feed point so that the radiation electrodes 115 are excited in phase. The present inventors have realised that the radiation electrodes 115 are excited in phase when the coupling element 125 is located close to the feed point. In this example, the coupling element 125 is located 7mm from the feed point.

However, it will be appreciated by those skilled in the art that the exact distance is subject to optimisation which is dependent on the electrical field and the current distribution. In this exemplary arrangement, the surfaces 105a and 105c are rectangular while the surface 105b is square. The three electrodes are arranged in a parallel spaced relationship. The feed point includes an excitation formation 140 provided on the ground electrode 110 which is used to excite the radiation electrodes 115 with an electric field. While the radiation electrodes 115 are spaced apart, in operation their electric fields are proximity coupled together. Proximity coupling refers to the arrangement whereby the radiation electrodes 115 are not directly touching each other but their electric fields are electrically coupled together.

Phase perturbation is generated as a result of the geometric shape of the radiation electrodes 115. Each radiation electrode 115 has a pair of spaced apart ends 128 with a pair of spaced apart sides 129 extending there between.

The lengths of the sides 129 are marginally longer than the lengths of the ends 128 such that the radiation electrodes 115 are nearly square. The surfaces I USa and 1 05c of the two electrodes 115 are arranged orthogonally to each other in order to achieve right-handed circular polarisation in the respective electrodes 115. A perturbation formation, in this case, a rectangular stub 130 extends outwardly from one of the ends 128 of each electrode 115. The stubs are arranged orthogonally to each other in order to achieve right-handed circular polarisation in the respective electrodes 115. Phase perturbation is caused by the stubs 130 and by providing the electrodes 115 with dimensions that almost resemble a square. It will be appreciated by those skilled in the art that other means of achieving phase perturbation may be employed for example by providing truncated corner or voids (slots) in the radiation electrodes 115.

The footprint of the surface I 05b of the ground electrode 110 is marginally larger than the footprint of either one of the surfaces I 05a or I 05c of the respective radiation electrodes 115. It will be appreciated by those skilled in the art that typically for patch antennas to operate the ground electrode needs to have a substantially larger footprint than the radiating electrodes. The present inventors have realised that by keeping the footprint of the ground electrode 110 only marginally larger than the footprint of the electrodes 115 allows the electrical fields of the radiating electrodes 115 to proximity couple together across the outer perimeter of the ground electrode 110. The proximity coupling of the electrical fields of the radiating electrodes 115 results in omnidirectional circular polarisation.

While the perturbation formation is described as being a rectangular stub in Figure 1, it will be appreciated by those skilled in the art that other polygonal shapes may be used to provide the perturbation formation. For example, the perturbation formation may be provided as a square, triangle, ellipse, arc or void. In the exemplary embodiment, the radiation electrodes 115 have of length of 51 mm, a width of 49 mm and a depth of approximately 0.02mm. The ground electrode 110 has a length of 56 mm, a width of 56 mm and a depth of approximately 0.02 mm. The coupling element 125 has a length of 3.3 mm, a width of 2.5 mm and a depth of approximately 0.05 mm. The area of the surface I 05b is approximately 25% larger than the area of the respective surfaces 1 05a and 1 05c. Ideally, the footprint of the ground electrode 110 is larger than the footprint of the respective radiation electrodes 115 by a percentage falling within one of the following ranges 10% to 80%, 15% to 75%, 20% to 70%, 25% to 65%, 30% to 60%, 35% to 50%, and 40% to 45%. It will be appreciated by those skilled in the art that the dimensions are given by way of example only and it is not intended to limit the present teaching to these exemplary dimensions.

The excitation formation 140 is provided by a coplanar waveguide (CPW) feed which consists of a rectangular slot 142 formed on the ground electrode 115 with a central electrode 145 located within the slot 142. The electrode 145 tapers in a V-shape from a 50 ohm line width to a point near the end of the slot 142. The 50 Ohm line width is given by way of example only and it will be appreciated that other values may be employed. The length of the taper and the dimensions of the slot 142 are selected to achieve the desired performance.

The purpose of the CPW feed is to feed the radiating electrodes 115 with an electric field. It will be appreciated by those skilled in the art that the CPW feed is given by way of example only, and it is not intended to limit the present teaching to this exemplary feed as other alternative feed arrangements may be employed.

Referring now to Figures 4 to 6, there is illustrated another exemplary antenna 200 for generating omnidirectional circularly polarized radiation. The antenna 200 is substantially similar to the antenna 100 and like components are indicated by similar reference numerals. The main differences between the antenna 200 and the antenna 100 is that the radiating electrodes are circular rather than rectangular and the perturbation formation is provided by a pair of concentric circular voids 220 formed on the radiation electrodes 115. Otherwise the antennas 100 and 200 operate similarly. The voids 220 provide the same function as the stubs 130 by providing phase perturbation. The radiuses of the voids 220 are substantially equal. The radiation electrodes 115 are arranged in a spaced concentric relationship. The ground electrode 110 is rectangular with a footprint than is marginally larger than the footprint of the respective radiating electrodes 115 as best illustrated in Figures 4 and 6.

In the exemplary arrangement the radius of the voids 220 is 11 mm, and the radius of the radiating electrodes 215 is 25 mm. The ground electrode has a length of 63 mm, a width of 50 mm and a depth of approximately 0.02 mm. The concentric voids 220 define a first central axis 230 of origin which is spaced apart from a second central axis 240 of origin defined by the concentric arrangement of the radiation electrodes 215 such that the first and second axes 230, 240 are not co-axial. Thus the centres of the voids 220 are located at a distance offset from the centres of the radiation electrodes 215 which is required to generate circular polarisation. The coupling element 125 which electrically couples the radiation electrodes 215 together is provided by a conductive pin that extends through an aperture 250 formed on the ground electrode 210. The conductive pin passes through the ground electrode 210 via the aperture 250 but does not contact the ground electrode 210 so that the radiation electrodes 215 are electrically isolated from the ground electrode 210.

The electrical device (not shown) on which the antenna 200 is mounted is electrically coupled to the antenna 200 via an electrical connector 270 that is operably coupled to upper radiation electrode 115. In this exemplary embodiment, the dielectric medium defines a first layer 255 located between the upper radiation electrode 215 and the ground electrode 210, and a second layer 260 located between the lower radiation electrode 215 and the ground electrode 210.

Referring now to the graphs of Figures 7 to 9, that illustrates computer simulated results for the antenna 100 in operation. The graph of Figure 7 of frequency versus Si 1 (return loss) demonstrates that the configuration of Figures 1 to 3 operates as an antenna which is well matched to 50 C) for frequencies where Si 1<-i 0 dB. By having a low absolute reflection coefficient (Si 1) indicates a good match which results in more radiation being generated.

The graph of Figure 8 of axial ratio versus the angle for antenna 100 in the YZ plane is a measure of the quality of circular polarisation. It is accepted by international standards, that a signal is circularly polarised if its axial ratio is below 3 dB. As the highest axial ratio in Figure 8 is less than 2.9 dB it demonstrates that the antenna 100 radiates circular polarisation with an omnidirectional pattern. The graph of Figure 9 of gain versus the angle for the antenna 100 in the YZ plane shows the gain for right handed (RHCP) and left handed circular polarisation (LHCP). Left-handed circular polarisation is in this case an unwanted signal and is kept as low as possible. Left-handed circular polarisation (LHCP) is a polarisation exactly orthogonal to the right-handed circular polarisation (RHCP). Since GPS uses RHCP, the LHCP signals are considered as interference and therefore should be rejected by the antenna.

The antenna can be made LHCP antenna and in this case the RHCP radiation level should be kept low.

Referring now to the graphs of Figures 10 to 12, that illustrates computer simulated results for the antenna 200 in operation. The graph of Figure 10 of Sil (return loss) versus frequency demonstrates that the configuration of Figures 4 to 6 operates as an antenna which is well matched to 50 C) for frequencies where Sil <-10 dB. The graph of Figure ii of axial ratio versus the angle for antenna 200 in the YZ plane is a measure of the quality of circular polarisation. As the highest axial ratio in Figure 8 is less than 1.5 dB demonstrates that the antenna 200 radiates circular polarisation with an omnidirectional pattern. The graph of Figure 12 of gain versus the angle for the antenna 200 in the YZ plane shows the gain for right handed (RHCP) and left handed circular polarisation (LHCP).

It will be understood that what has been described herein are exemplary embodiments of antennas for generating omnidirectional circularly polarized radiation. While the exemplary embodiments have been described with reference to some exemplary arrangements, it will be understood that it is not intended to limit the present teaching to such arrangements as modifications can be made without departing from the spirit and scope of the present teaching. In particular, it will be appreciated by a person skilled in the art that the three stacked electrodes can be any arbitrary shape other than rectangular or circular. Similarly, it will be understood that the perturbation formation may be provided by any suitable geometric formation. While the radiation electrodes are described as being an equal distance from the ground electrode, it will be appreciated that other arrangements are possible.

The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Furthermore, it will be understood that Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems ("sat nay") that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line-of-sight by radio from satellites.

Receivers calculate the precise time as well as position, which can be used as a reference for scientific experiments. Whilst there are several systems in existence or proposed, the primary system is currently the United States NAVSTAR Global Positioning System (GPS). Thus in the present application whilst reference has been made to GPS it will be understood that this applies to all satellite navigation systems. Moreover, the antenna of the present application is not limited to use in satellite navigation systems and may be employed in a variety of different applications. In particular, the antenna is suitable for use in systems which employ circular polarization where orientation of the device is arbitrary and the direction of signal is unknown. Examples of such systems may include global positioning systems, RFID systems, vehicle communications, tolling and wireless communications systems.

Thus the present application is not intended to be limited to the specific examples provided herein but instead is only intended to be limited to the scope of the claims which follow.

Claims

Claims 1. An antenna for generating omnidirectional circularly polarized radiation, the antenna comprising: a stack of three electrodes defining respective planar surfaces with a ground electrode located between two spaced apart radiation electrodes, an dielectric medium interposed between the ground electrode and the respective radiation electrodes, an excitation feed for exciting the radiation electrodes, a coupling element coupling the radiation electrodes together for facilitating exciting the radiation electrodes in phase, an perturbation formation being provided on the respective radiation electrode for implementing phase perturbation; wherein the three electrodes are in a spatial arrangement such that when the radiation electrodes are excited their respective electric fields electrically couple together for facilitating the generation of an omnidirectional radiation pattern.
2. An antenna as claimed in claim 1, wherein the footprint of the ground electrode is dimensioned to be larger than the footprint of the respective radiation electrodes.
3. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 10% to 80% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
4. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 15% to 75% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
5. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 20% to 70% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
6. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 25% to 65% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
7. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 30% to 60%, larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
8. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 35% to 50% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
9. An antenna as claimed in claim 2, wherein the footprint of the ground electrode is 40% to 45% larger than the footprint of the respective radiation electrodes for facilitating the electric fields of the radiation electrodes proximity coupling together about the outer perimeter of the ground electrode.
10. An antenna as claimed in any preceding claim, wherein the excitation feed includes an excitation formation formed on the ground electrode.
11. An antenna as claimed in any preceding claim, wherein the perturbation formation defines a geometric formation on the respective radiation electrodes.
12. An antenna as claimed in claim 11, wherein the perturbation formation defines a polygonal.
13. An antenna as claimed in claim 11, wherein the perturbation formation defines a circle.
14. An antenna as claimed in claim 11, wherein the perturbation formation defines at least one of a rectangle, square, ellipse, arc, or void.
15. An antenna as claimed in claim 11, wherein the perturbation formation comprises a first formation on one of the radiation electrodes, and a second formation on the other one of the radiation electrodes.
16. An antenna as claimed in claim 15, wherein the first formation and the second formation are co-axial with respect to an axis of origin.
17. An antenna as claimed in claim 15, wherein the perturbation formation is defined by a void.
18. An antenna as claimed in claim 15, wherein the first formation and the second formation are arranged orthogonally.
19. An antenna as claimed in claim 15, wherein the perturbation formation is defined by stubs.
20. An antenna as claimed in claim 15, wherein the first formation defines a circular void.
21. An antenna as claimed in claim 20, wherein the second formation defines acircularvoid.
22. An antenna as claimed in claim 21, wherein the first and second formations are arranged concentrically such that they have a common axis of origin.
23. An antenna as claimed in claim 22, wherein the radiation electrodes are circular.
24. An antenna as claimed in claim 23, wherein the radiation electrodes are arranged concentrically.
25. An antenna as claimed in claim 24, wherein the concentric arrangement of the first and second formations defines a first axis of origin which is spaced apart from a second axis of origin defined by the concentric arrangement of the radiation electrodes such that the first and second axes of origin are not co-axial.
26. An antenna as claimed in any of claims 1 to 15, wherein the three electrodes have a polygonal shape when viewed in plan view.
27. An antenna as claimed in claim 26, where the radiating electrodes are rectangular, and the ground electrode is square when viewed in plan view.
28. An antenna as claimed in any of claims 1 to 15, wherein the two radiating electrodes are circular, and the ground electrode is polygonal when viewed in plan view.
29. An antenna as claimed in any preceding claim, wherein the excitation feed is defined by a co-planar waveguide.
30. An antenna as claimed in any preceding claim, wherein the excitation feed comprises a slot.
31. An antenna as claimed in claim 30, wherein the excitation feed further comprises a tapered electrode located in the slot.
32. An antenna as claimed in any preceding claim, wherein the excitation feed is located adjacent to the coupling element for facilitating exciting the radiating electrodes in phase.
33. An antenna as claimed in any one of the preceding claims, wherein the dielectric medium comprises a first layer located between the ground electrode and one of the radiation electrodes, and a second layer located between the ground electrode and the other one of the radiation electrodes.
34. An antenna as claimed in any preceding claim, wherein the three electrodes are in a parallel spaced relationship.
35. An antenna as claimed in any one of claims 1 to 18, wherein the two radiation electrodes are in a concentric spaced relationship.
36. An antenna as claimed in any one of claims 1 to 12, wherein the perturbation formation is provided by a rectangular void.
37. An antenna as claimed in claim 18, wherein the radiation electrodes are arranged orthogonally to each other.
38. An antenna as claimed in any preceding claim, wherein the ground electrode is located an equal distance from each of the radiation electrodes.
39. An electronic device being operably coupled to the antenna as claimed in any of claims 1 to 38.