GB2495119A - Spacer arrangement for mounting an antenna on a convex conductive surface - Google Patents

Abstract

An item of street furniture, a spacer or a method for spacing, comprising: a spacer 44 located between an antenna 34 and a conductive surface 18 such that they are separated by a distance of a quarter wavelength. The conductive surface 18 may be a substantially horizontal convex surface of an item of street furniture such as a telephone kiosk roof or the roof of a bus shelter 14 or the curved surface of an advertising display. A low profile antenna 34, a convex conductive surface 18 and spacer 44 assembly may be used in a wireless base station arrangement. The antenna arrangement may provide an improved performance from small cells installed to improve coverage by wireless networks by filling-in areas in the network where reception is unacceptably bad.

Description

Antenna Spacer The invention relates to Spacers for antennas for transmitting and/or receiving electromagnetic radiation.

Many mobile phone and Wi-Fi service providers have areas within their local wireless networks where reception is unacceptably bad. There is a need to improve reception for their customers in order to increase customer satisfaction and reduce the incentive for customers to switch to another provider. Services providers are seeking to improve coverage and increase capacity by deploying additional, small cells to fill in so-called "not-spots" -i.e. areas in the network where reception is unacceptably bad. By installing more base stations and antennas, reception can be improved by ensuring that the user is always close enough to an antenna to get good reception. There are obstacles in the way of erecting ever-increasing numbers of mobile phone masts, especially in conservation or other sensitive areas, Apart for the cost of erecting the masts, permission may have to be obtained from local planning authorities before a new mast can be erected. Local pubic opinion may be hostile to applications for new masts and this may contribute towards planning apphcations being rejected.

One way of reducing the impact to the local environment would be to attach antennas to existing structures such as street furniture, for example bus shelters, telephone kiosks or advertising displays. Telephone kiosks are particularly suitable for this as most are provided with a supply of electric power suitable for powering mobile telephony or Wi-Fi transmitting and receiving equipment and are connected to the terrestrial telephone network, allowing the exchange of voice and data traffic with remote locations.

Statement of Invention

The present inventors found that mounting antennas to certain items of street furniture resulted in impaired reception due to distortion of the field generated by the antenna. The present inventors found that, whereas mounting an antenna to the flat roof of a modern, K100-style telephone kiosk resulted in good reception at or near ground level, mounting the same antenna directly to the curved roof of a 1<2-or 1<6-style telephone kiosk resulted in a distorted radio frequency radiation pattern with impaired reception at or near ground level (i.e. the region where most mobile phone users will be located).

The present invention addresses the above issues by using a specially dimensioned spacer to space an antenna relative to a curved conductive surface so as to improve reception.

The present invention provides an item of street furniture comprising a convex conductive surface. The item of street furniture also comprises a spacer and an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength. The antenna is spaced by the spacer from the convex conductive surface by a spacing equivalent to a quarter of the wavelength of the radiation. The spacing introduced by the spacer improves the field pattern of the antenna, to give better coupling (e.g. better reception) at or around street level.

According to a further aspect, the present invention provides a spacer for spacing, from a convex conductive surface, an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength. The spacer is arranged to provide a spacing between the antenna and the surface of a quarter of the wavelength of the radiation.

According to a further aspect, the present invention provides an antenna assembly comprising the above spacer and an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength. The antenna and spacer are arranged for mounting to a convex conductive surface with a spacing between the antenna and the surface of a quarter of the wavelength of the radiation.

According to a further aspect, the present invention provides a method for spacing an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength, comprising: positioning a spacer between the antenna and a convex conductive surface so that the antenna is spaced at a spacing to the convex conductive surface of a quarter of the wavelength of the radiation.

Brief Description of the Drawings

In order to aid understanding, embodiments of the invention will now be described by way of example with reference to the drawings in which: Figures la and lb show, in line-drawing form, conventional street furniture; Figures 2a and 2b show schematics of conventional radio antenna; Figures 3a and 3b shows radio antennas mounted to the Street furniture of Figure 1; Figure 4, Ga and 6b show in line drawing form a spacer according to aspects of the present invention; Figures 5a and 5b show in line-drawing form an antenna mounted to Street furniture according to an aspect of the present invention; Detailed Description of Embodiments of the Invention A spacer and a method for spacing an antenna from a convex conductive surface is described. The spacer compensates for distortion introduced by the convex conductive surface in the electromagnetic field generated by the antenna. The convex conductive surface may form part of an item of street furniture. In a preferred embodiment, the invention provides for improved performance from small cells installed to improve coverage in wireless networks by filling-in areas in the network where reception is unacceptably bad.

To minimise site deployment and operational costs, mobile and Wi-Fi operators are seeking to deploy small cells (base stations with antennas) on existing sites, including various forms of street furniture. Figures 1 and 2 show, respectively, an example of a telephone kiosk and bus shelter design. Planning objections can greatly increase the time taken to deploy a new small cell. For deployments of new cells, this can cause major delays in delivery and incur large additional costs for the service provider. Owing to their closeness to the ground, highly visible antennas located on street furniture may also be subject to vandalism leading to damage and malfunctioning.

A deployment involving small and unobtrusive antennas is desirabe so that minimal additional planning approval is required. However, the use of antennas at such sites can incur a performance compromise in terms of base station coverage and capacity. We now describe embodiments of the invention.

Figure 2a shows a schematic (not to scale) of a half-wave dipole antenna 20. Dipole antenna 20 comprises two conductors 22, 24 normally of rod or wire arranged in line with each other with a small space in between. The two arms are driven at central feed terminals 26 at which point a radio frequency voltage is applied to the antenna. The dipole is basically symmetrical; about a centre line (indicated in the Figure by dashed line 28).

The dipole may be thought of as having a main axis extending along the two arms. A folded dipole, as shown at 21 in Figure 2b (not to scale) has a similar, omnidirectional field pattern to the half-wave dipole, described above. As with the half-wave dipole, the folded dipole is basicafly symmetrical; about a centre line, indicated in the Figure by dashed line 28 passing through the feed terminals 26. A loop antenna (not shown) where the loop circumference is in the range of a wavelength has, whether fundamentally circular or square in configuration, a similar omnidirectional field pattern.

The radiation pattern of an antenna is a plot of the relative electro-magnetic field strength of the radio waves emitted by the antenna at different angles. A dipole antenna, for example, emits substantially equal power in all directions in the plane perpendicular to the main axis of the antenna, with the power dropping off as the angle to the plane increases.; this is known as an "omnidirectional pattern'. Many items of Street furniture have curved upper sections which can adversely modify the field pattern of an antenna mounted to them. The telephone kiosk 10 of Figure 1 is a classic design with convex curved roof section 12. The example bus shelter design 14 of Figure 2 also has a convex-(i.e. outwardly-) curved roof section 18. Use of the spacer described below and claimed in the appended claims allows mobile phone, WiFi or similar antennas to be fitted to common items of Street furniture whose upper sections are not flat but convex-(i.e. outwardly-) curved. The improvement in field pattern brought about by the invention widens the range of street furniture to which unobtrusive antennas can be fitted to provide local cells to improve on the reception for users in a local area. A spacer for a mobile phone or WiFi antenna is described which counter-acts the effects on the radiation pattern of an antenna caused by fixing the antenna to a curved, rather than a flat surface.

An obvious choice for providing good reception from a kiosk-top antenna would be an omnidirectional, rod antenna such as W5013 from Pulse Electronics (Figure 3a). Figure 3a shows the upper part of telephone kiosk 10 of Figure 1, with a rod antenna 32 mounted to the convex-curved roof section 12. This antenna takes the form of a 400 mm rod, which, tests have shown, gives good reception at street-level when fitted to the convex curved roof of a K6 telephone kiosk. Such a rod antenna is very visible when mounted on the kiosk, giving rise to aesthetic concerns which may make the obtaining of planning approval problematic. Due to its high visibility, the antenna may be subject to vandalism, leading to damage which may render the antenna unusable. Use of a low-profile "hockey puck" antenna, such as the SmartDiscTM from Smarteq Wireless AB, Enebyberg, Sweden, addresses both problems due to its compact, low-profile shape. A low-profile antenna 34 is shown mounted to a K6 kiosk-top in Figure 3b (what is actually shown in the Figures is the antenna housing, rather than the antenna itself which is contained within the housing).

Figure 3b shows the upper part of telephone kiosk 10 of Figure 1, with a low-profile antenna 34 mounted to the convex-curved roof section 12. The manufacturer's fitting instructions for the SmartDisclM require fitting the antenna directly to the mounting surface, as shown in Figure 3b. Unfortunately, tests carried out by the present inventors have shown, that such low-profile antennas provide an unacceptably low performance when mounted directly to the kiosk roof.

The invention relates to a mount for a low-profile radio antenna suitable for fitting the antenna to the curved roof a kiosk or other street furniture or to similar convex conductive surfaces and which provides improved wireless transmission characteristics of the antenna resulting in improved wireless reception, while maintaining a very low profile. The invention also relates to a corresponding method for spacing a low-profile radio antenna for fitting to the roof of a kiosk or to similar convex conductive surfaces.

Figure 4 shows fitting of low-profile antenna 34 of Figure 3b to convex surface 42 with spacer 44 fitted immediately between the antenna and the surface. The spacer separates antenna 34 from convex-convex surface 42 by a distance of one quarter of the wavelength of the transmitted radio signal. Where the radio signal occupies a range of wavelengths, this will be a nominal or mean value. The whole spacer-antenna assembly (34, 44) may be fixed to surface 42 by means of a fixing stub 46 fitted through a hole (not shown) in mounting surface 42. Alternatively, fixing may be achieved by means of a suitable adhesive or a mounting bracket independently of the spacer. The spacer is carefully dimensioned so as to introduce between the antenna within the housing and the mounting surface, a spacing of a quarter of the nominal wavelength of the radio signal being transmitted (or received) by the antenna. Use of the spacer changes the radio frequency radiation pattern of the antenna with the result that signal strength increases close to the ground and mobile phone users in the street experience improved reception.

The improved electromagnetic linkage between mobile and antenna also results in improved reception at the antenna of signals generated at the mobile device, since the coupling between mobile and antenna is reciprocal.

For a dipole antenna, such as those illustrated in Figure 2, when oriented with the main axis perpendicular to the convex surface, the spacing is measured from the centre line 26 of the antenna. When oriented with the main axis parallel to the convex surface, the spacing is measured from the main axis of the antenna, i.e. the line extending along arms 22, 24.

By positioning by means of a quarter-wavelength spacer a low profile antenna in relation to the conductive convex surface of a supporting structure, the invention exploits the behaviour of the conductive convex surface as a passive antenna element to improve antenna performance. The modified arrangement of antenna and surface results in the downwards biasing of the radiation pattern, which improves linkage between the antenna and mobile devices at Street level. Other spacings, such as half-wavelength spacers, were found not to provide the same benefit. The spacer was designed so as to distance the antenna from the surface by a quarter-wavelength, so as to provide the optimal field pattern.

Figures 5a and 5b show the antenna-spacer arrangement of Figure 4 in the context of fitting an antenna to the kiosk of Figure la and the bus shelter of Figure lb. Figure 5a shows antenna 34 mounted via spacer 44 to convex-curved surface 12 of the roof section of kiosk 10. Figure 5b shows antenna 34 mounted via spacer 44 to convex-curved surface 18 of the roof section of bus shelter 14. From Figure 5a and Sb it is evident that the low-profile of the antenna is preserved when mounted according to the present invention.

The kiosk and bus shelter roofs comprise a convex-curved, conductive sheet. This curved sheet is not to be confused with the concave-curved sheet, such as is found in satellite dishes and which tends to focus radio waves directed to or from an antenna. In contrast to such concave reflectors, the convex kiosk roof tends to disperse the radio waves generated by the antenna. When fined to such a convex-curved structure, the field generated by the low-profile antenna is shifted away from the plane of the structure. In a typical installation, this will result in the field being shifted upwards -diverting the energy in the transmitted field way from receivers (e.g. mobile phones) at street level. The shift in the radiation pattern of the antenna reduces electromagnetic linkage between mobile devices at street level and the antenna which also results in worse reception at the antenna of signals generated at mobile devices at street level.

Figures 6a and 6b show a detailed engineering drawing of an embodiment of spacer 44 of the present invention. Figure 6a shows spacer 44 in cross-section and Figure 6b shows spacer 44 in plan view. Double-headed arrows and lead lines are used to indicate, as is customary key dimensions (in millimetres). These dimensions are given by way of example only. Spacer 44 comprises body 70 of substantially cylindrical overall appearance provided with a central aperture 72 for accommodating, according to a preferred embodiment, antenna mounting stub 46. As can be seen from Figure 6a, main body 70 is partially hollow with a stepped internal cavity 74 which reduces the amount of material used.

We now describe some typical spacings for commonly-used radio frequencies. Common frequency ranges used for mobile telephony and Wi-Fi include: 800, 900, 1800, 2100 and 2400 MHz. We can calculate wavelength from frequency by using the formula: A=v/f; where A is wavelength in metres; is frequency in Hertz; and v is the speed of light in metres per second (taking a value of 3x108m/s in air).

Using the above formula, we obtain the following values for quarter-wavelength spacings: At 800MHz: f= 8x1 08 A=3/8x1 00 = 0.38 metres; so that W4 = 0.09m or 90mm At 900MHz: f= 9x108 A=3/9x10° = 0.33 metres; so that 1J4 = 0.08m or 80mm; at 2100MHz f=21x108 A= 3/21x108=0.14m metres, so that W4 = 0.035m or 35mm; at 2400MHz f= 24x1 0 A=3/24x1 00 = 0.13 metres; so that V4 = 0.03m or 30mm.

Spacers may be constructed for either conductive or insulating material and may be either substantially hollow or solid. Figures 6a and 6b show detailed and dimensioned engineering drawings of a suitable spacer construction for an antenna operating at 2400MHz. Spacer 44 comprises substantially cylindrical body 70 with central aperture 72 in an upper surface opening out into central cavity 74. The aperture and central cavity may be convenient features in some embodiments, depending on the mounting arrangement for the antenna housing but may be omitted in other embodiments. It will be noted that the overall height of the spacer is not the 30mm, determined as the correct spacing for an antenna operating at 2400MHz, but 23mm. This is because part of the spacing is made up by the positioning of the antenna within the antenna housing, which, in this case, adds 7mm to the overall spacing from the surface. It will be understood that, given an antenna which extends across the curved surface, the spacing between the two will vary slightly along the extent of the antenna. The spacing described above and referred to in the claims will be understood to indicate the smallest value of the spacing between the two.

The spacer may, according to a preferred embodiment, act to fix or mount the antenna to the convex surface using one of the methods described above, however, its main role is to establish the desired spacing between the antenna and the curved conductive surface including in cases where the mounting of the antenna is achieved by means not involving the spacer.

The technique and spacer described above allows the deployment of small wireless base stations with a compact, low-profile antenna on a variety of existing or planned structures where the antenna is small enough to avoid issues relating to planning approval and to avoid becoming a target for vandalism. Advantageously, the technique exploits the metalwork (or other conductive elements) within the structure to provide a passive antenna element that improves electromagnetic linkage between mobile devices located at street level and the antenna.

The technique and spacer described above provide a method of deploying low-profile antennas suitable for establishing base stations located on existing structures (including telephone kiosks) where the curved profile of the structure is detrimental to the wireless transmission characteristics of the antenna. This enables new outdoor small cells (base stations with antennas) to be deployed at street sites such as at phone kiosks, or other street furniture that offer base station performance in terms of coverage and capacity equivalent to that obtained when using a larger antenna. The invention is not limited to telephone kiosks or other street furniture. This technique can be applied to a variety of other convex-curved, conductive structures, such as vehicle bodies which can be harnessed to improve the performance of a suitably designed and low-profile antenna mounted in accordance with the present invention.

The above embodiments are to be understood as illustrative examples of the invention.

Further embodiments of the invention are envisaged and will be evident to the skilled reader. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of another of the embodiments, or any combination of the embodiments. Furthermore, equivalents and modifications not described above will be evident to the skirled reader and may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims (1)

1. <claim-text>Claims 1. An item of street furniture comprising a convex conductive surface, in which the item of street furniture further comprises a spacer and an antenna for transmithng and/or receiving electromagnetic radiation at a wavelength; in which the antenna is spaced by the spacer from the convex conductive surface by a spacing equivalent to a quarter of the wavelength.</claim-text> <claim-text>2. The item of Street furniture of claim 1, in which the item of Street furniture is a telephone kiosk.</claim-text> <claim-text>3. The item of Street furniture of claim 1 or claim 2, in which the convex conductive surface forms part of a roof of the item Street furniture.</claim-text> <claim-text>4. A spacer for spacing, from a convex conductive surface, an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength; in which the spacer is arranged to provide a spacing between the antenna and the surface of a quarter of the wavelength.</claim-text> <claim-text>5. The spacer of claim 4, in which the spacer is arranged to mount the antenna to the surface.</claim-text> <claim-text>6. The spacer of claim 4, in which convex conductive surface forms a part of Street furniture.</claim-text> <claim-text>7. The spacer of claim 4, in which convex conductive Surface forms part of a roof of a telephone kiosk.</claim-text> <claim-text>8. The spacer of claim 4, in which convex conductive surface is substantially horizontal in use.</claim-text> <claim-text>9. A method for spacing an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength; in which the method comprises: positioning a spacer between the antenna and a convex conductive surface so that the antenna is spaced at a spacing to the convex conductive surface of a quarter of the wavelength.</claim-text> <claim-text>10. The method of claim 9 comprising mounting the antenna to the convex conductive surface by means of the spacer.il. An antenna assembly, in which the antenna assembly comprises the spacer of any one of claims 4 to 8 and an antenna for transmitting and/or receiving electromagnetic radiation at a wavelength; in which the antenna and spacer are arranged for mounting to a convex conductive surface with the antenna spaced by the spacer from the surface of a quarter of the wavelength.12. A wireless base station comprising a wireless transmitter and/or receiver and the antenna assembly of claim 11.13. A wireless base station comprising a wireless transmitter and/or receiver and the item of Street furniture of claim 1.</claim-text>