Dual band antenna with a dome-shaped radiator

Abstract

A dual band antenna (AN) configured for being position on a surface of a pit lid and capable of wireless signal transmission at two frequency bands in response to an electrical signal applied via a feed wire. A convex conductive surface, e.g. 5 dome-shaped, is placed above a conductive ground plane element, wherein at least a part of an edge, e.g. 20-40% of the edge, of the dome-shaped radiator element is in electrical contact with the conductive ground plane element. Further, the convex conductive surface is connected via the feed wire. This antenna design allows first and second resonance frequencies within a factor of such as 1.8-2.2, 10 which allows the antenna to be designed e.g. for both of the frequency bands 450-470 MHz and 902-928 MHz which are relevant for meter reading data and with smaller dimension than what could be expected from conventional antennas. A housing with a convex top surface forms an enclosure around the antenna parts conductive ground plane element and the dome-shaped radiator element, the 15 housing has a bottom surface arranged to face the surface of the pit lid, and where the feed wire exits the housing.

Claims

1. A dual band pit lid antenna configured for being positioned on a surface of a pit lid and being arranged to transmit a wireless signal at first and second wireless transmission frequencies in response to an electrical signal being applied via a feed wire, the antenna comprising a planar conductive ground plane element, a conductive dome shaped radiator element positioned above the conductive ground plane element with a convex upper surface facing away from the conductive ground plane element, wherein a part of an circumferential edge of the dome shaped radiator element is in electrical contact with the conductive ground plane element, wherein the dome shaped radiator element is electrically connected to the feed wire, and wherein the conductive ground plane element and the dome shaped radiator element are configured to provide first and second resonance frequencies to match the first and second wireless transmission frequencies, and a housing arranged to form an enclosure around the conductive ground plane element and the dome shaped radiator element, where the housing is provided with a convex top surface and a plane bottom surface arranged to face a surface of a pit lid.

2. The dual band pit lid antenna according to claim 1, wherein one or more areas of the dome shaped radiator are cut away, thereby reducing its surface area.

3. The dual band pit lid antenna according to claim 1, wherein the dome shaped radiator element is in electrical connection with the conductive ground plane element along 25-50% of the length of its circumferential edge.

4. The dual band pit lid antenna according to claim 1, wherein the dome shaped radiator element is formed by a metal plate.

5. The dual band pit lid antenna according to claim 1, wherein the electrical connection between the circumferential edge of the dome shaped radiator element and the conductive ground plane element is provided by a conductive plate element arranged perpendicular to the conductive ground plane element along the circumferential edge of the dome shaped radiator element.

6. The dual band pit lid antenna according to claim 1, wherein the dome shaped radiator element has a length of 70-200 mm along its major axis of extension.

7. The dual band pit lid antenna according to claim 1, wherein a diameter or major axis length of the dome shaped radiator element is 10-15 times a height of the dome shaped radiator element.

8. The dual band pit lid antenna according to claim 1, wherein the feed wire is in electrical contact with the dome shaped radiator element at a feeding point located a distance away from the circumferential edge of the dome shape radiator element.

9. The dual band pit lid antenna according to claim 1, comprising an additional electrical connection between the dome shaped radiator element and the conductive ground plane element, the additional electrical connection being arranged away from the circumferential edge of the dome shaped radiator element.

10. The dual band pit lid antenna according to claim 9, wherein at least a portion of the dome shaped radiator element is arranged in parallel with the convex top surface of the housing.

11. The dual band pit lid antenna according to claim 9, wherein the dome shaped radiator element and the conductive ground plane element are designed to provide a transmission gain directivity pattern at the first and second transmission frequencies having a maximum gain within an angle interval of 15°-75° measured from the conductive ground plane element.

12. The dual band pit lid antenna according to claim 9, wherein the second resonance frequency is 1.8-2.2 times the first resonance frequency.

13. The dual band pit lid antenna according to claim 9, wherein the first resonance frequency is within 200-600 MHz.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will now be described in more detail with regard to the accompanying figures of which:

(2) FIG. 1a shows a side view of the conductive parts of an antenna with a dome-shaped radiator element arranged above a plane conductive ground plane element,

(3) FIG. 1b shows a side view of the conductive parts of an antenna, where the circumferential edge of the dome shaped radiator is extended to enable a direct electrical contact to the conductive ground plane,

(4) FIG. 2 shows a front view of the antenna without polymeric casing,

(5) FIG. 3 shows a top view of the antenna with the conductive ground plane element marked,

(6) FIG. 4 shows a cross-section of the antenna of FIG. 3 along line AA,

(7) FIG. 5 shows in perspective a complete pit lid antenna including an encapsulating housing,

(8) FIG. 6 shows a bottom view of the antenna with the feed wire exiting through the bottom of the housing, and

(9) FIG. 7 shows the antenna seen from the top, with the radiator element marked.

(10) The figures illustrate specific ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1a shows the basic antenna parts of a pit lid antenna embodiment. A plane circular metal plate forms a conductive ground plane element GP, a dome-shaped conductive radiator element RE formed by a metal plate is positioned above the conductive ground plane element GP, and a vertical conductive part VC arranged 20 along a circumferential edge of the radiator element electrically connecting the ground plane element and the radiator element. The radiator element is double-curved and arranged with a convex upper surface facing away from the conductive ground plane element GP, i.e. with a concave lower surface facing towards the conductive ground plane element GP.

(12) The shown radiator element has an overall rotational symmetric dome shape. However, other variations of an overall dome shape may be applied to obtain specific antenna properties. For example, the shape of the radiator element may comply with the one of a super-ellipsoid or a super-spheroid to provide a super-ellipsoidal or super-spheroidal dome, respectively. Alternatively or additionally the length, width and height dimensions of the dome may be shortened or protracted to change the shape of the dome.

(13) A part of a circumferential edge of the dome-shaped radiator element RE is in electrical contact with the conductive ground plane element GP by means of a vertical conductive part VC, also formed by a metal plate. This vertical conductive part VS is preferably soldered to the circumferential edge of the dome-shaped radiator element RE and to the conductive ground plane element. Preferably, the vertical conductive part VC extends along 20%-40% of the edge of the dome-shaped radiator element RE. In some embodiments the vertical conductive part VC may even extend along 1-50% of the edge of the dome-shaped radiator element RE. Further one or more single point conductive connections between the conductive ground plane element GP and the dome-shaped radiator element RE can be added at a distance away from the vertical conductive part VC.

(14) As an alternative to the vertical connective part, the dome shaped radiator element may have a part of the circumferential edge extended to enable a direct electrical contact between the dome shaped radiator and the conductive ground plane, this is illustrated in FIG. 1b.

(15) Further, the dome-shaped radiator element RE is electrically connected to an inner conductor IC of a feed wire FW at a position away from its circumferential edge and the vertical conductive part VC, while the conductive ground plane element GP is connected to the outer conductor, which is the voltage reference, of the feed wire FW.

(16) In the shown embodiment, the dome-shaped radiation element has a full dome shape. It is to be understood that at least some of the advantageous effect can also be obtained by a dome shape with parts cut away.

(17) With such antenna design, the resulting first resonance frequency of the antenna will be remarkably low for its size. The antenna design provides antennas having a size relative to wavelength in the range from λ/10 to λ/2. With the actual antenna size DG being defined as the diameter or major axis length of the conductive ground plane element, the relationship between antenna size DG and wavelength for the proposed antenna design is thus given by the equation:

(18) $\mathrm{DG}=\frac{\lambda }{n}$
where n is an integer between 2 and 10. Compared to know antenna concepts, n=10 is a high number, allowing a relatively small antennas to operate at high wavelength, i.e. low frequencies, while at the same time having a good radiation performance in form of a relatively high gain and radiation efficiency.

(19) For an antenna of a given size DG, the resonance frequency or wavelength may be tuned by changing the length of the extension of the electrical connection between the ground plane element and the circumferential edge of the radiator element, e.g. by changing the length of the extension of the conducting plate electrically connecting the ground plane element and the radiator element.

(20) In an exemplary embodiment, an antenna according to the invention may be designed with a conductive ground plane element having a diameter of 140 mm, i.e. DG=140 mm. The maximum wavelength at which the antenna is designed to resonated may thus be found using the above equation:

(21) $\mathrm{DG}=\frac{\lambda }{n}\iff \lambda =\mathrm{DG}\times n=140\times 10=1400\mathrm{mm}$
which corresponds to a minimum resonance frequency of 214 MHz in free space.

(22) Similarly, the minimum resonance wavelength can be found by:
λ=DG×n=140×2=280 mm
which corresponds to a maximum resonance frequency of 1071 MHz.

(23) Further, it has been found that the design allows first and second resonance frequencies differing by a factor of about 2.0, hereby giving a good match to first and second wireless transmission frequencies in respective bands of e.g. 450-470 MHz and 902-928 MHz, which are relevant bands for meter reading purposes.

(24) From FIG. 2 it is seen that the feed wire FE is connected to a feeding point FP located at a sloping part of the radiator element RE. The feeding point is thus arranged offset from a centre of the dome-shaped radiator element. Alternatively, the feeding point may be located at the centre of the dome shaped radiator element. Further, the feed wire FW penetrates through the conductive ground plane element GP and connects to the conductive ground plane element GP with its outer conductor, while the inner conductor is connected at the feeding point FP.

(25) Additionally, it is seen from FIG. 1a and FIG. 2 that on opposite sides of the dome-shaped radiator element, sections of the radiator are cut away thereby changing the radiator geometry. However, even though sections of the dome are cut away, the radiator element is still considered to have an overall dome shape and various radiator geometries defining an overall dome shape are considered to be within the scope of the invention.

(26) FIG. 3 shows a top view of the antenna with the dashed line indication the outer periphery of the conductive ground plane element GP. As seen the conductive ground plane element GP has a circular shape. In other embodiments the conductive ground plane element may have the shape of an ellipse or super-ellipse. In the shown embodiment, the conductive ground plane element is substantially planar. However, in other embodiments the ground plane element may be curved or double curved. Further, the dome-shaped radiator element RE is arranged above the conductive ground plane element with its centre arranged concentrically with the centre of the ground plane element. Further, as seen from FIG. 3 and FIG. 7, the diameter or major axis length DG of the conductive ground plane element GP exceed the diameter or major axis length DR of the radiator element RE.

(27) Referring to FIG. 4, it is to be understood that the conductive antenna elements RE, GP, VC are arranged to be enclosed by a housing HS, preferably provided by a polymer. The housing has a bottom surface BS arranged to face the surface of the pit lid, and the feed wire exits the housing at the bottom surface. The housing has a convex top surface TS arranged to withstand passage of a vehicle. Further, the housing has a circular circumference, and thus matches the shape of the conductive antenna parts. To utilize the full size of the housing, the conductive ground plane element GP is preferably arranged parallel with the bottom surface of the housing, while the dome-shaped convex upper surface of the dome-shaped radiator element RE is arranged parallel with the convex top surface of the housing.

(28) The dome-shaped radiator element RE has a limited height, with its length along its major axis of extension or diameter DR being preferably 10-15 times its height. In one embodiment, the conductive ground plane element and the radiator element has an aggregated height of 21 mm and the total height of the housing HH is 25 mm. Combined with the conductive ground plane element GP having a diameter dose to the diameter of the housing DH, the conductive elements of the antenna effectively utilize the available space to maximize antenna performance under the given constraints.

(29) Further, the material constituting the housing is arranged to support the conductive ground plane element GP and the dome-shaped radiator element RE relative to each other, thereby providing a solid antenna construction. The housing material may be provided in the form of a resins, foam or other material known to the skilled person and cast around the conductive antenna elements. In other embodiments (not shown) pockets of air or other material may be arranged inside the antenna construction, while still provided a rigid and durable antenna construction.

(30) FIG. 5 and FIG. 6 show the complete antenna with an off-centre positioned feed wire FW extending from the bottom surface. The housing is provided with mounting holes MH for fastening the antenna to a pit lid or other structure using appropriated fastening means.

(31) To sum up: the invention provides a dual band antenna (AN) configured for being positioned on a surface of a pit lid and capable of wireless signal transmission at two frequencies in response to an electrical signal applied via a feed wire. A convex radiator element providing a conductive surface, e.g. dome-shaped, is placed above a conductive ground plane element, wherein at least a part of an edge, e.g. 20-50% of the edge, of the dome-shaped radiator element is in electrical contact with the conductive ground plane element. Further, the radiator element is connected via the feed wire. The antenna design provides first and 35 second resonance frequencies within a factor of such as 1.8-2.2. A housing with a convex top surface forms an enclosure around the conductive ground plane element—and radiator elements and provides a bottom surface arranged to face the surface of the pit lid.

(32) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms “including” or “includes” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.