GB2494922A - External and flexible groundplane extensions for antennas - Google Patents

Abstract

A radio device comprises a housing containing a radio transmitter and/or receiver, an antenna and a conductive ground plane. The conductive ground plane is electrically connected to at least one elongate flexible conductive member 13 that extends outside the housing 11, 14. The said conductive member 13 may act as a groundplane extension to provide an extended bandwidth for a small device 11, 14 with a longest dimension substantially less than half a wavelength at an operational frequency. The said conductive member 13 may be a coiled, convoluted or meandered structure formed within a wrist-strap 12, neck-cord 15, belt, clothing, hat glove, collar, saddle, wearable by, or attached to, a person or animal or item for identification or tracking.

Description

GROUNDPLANE EXTENSIONS FOR ANTENNAS

(0001] This invention relates to techniques for improving antenna performance in small radio devices by way of groundplane extensions.

BACKGROUND

(0002] The growth of the use of personal services delivered by radio has led to the development of small compact mobile terminals such as smartphones, and personal navigation and tracking devices. Many of these devices rely on radio connectivity to an increasing variety of network services such as cellular radio, wireless local area networks and satellite navigation services. Progress in the design and manufacture of integrated circuits has allowed increasingly complex functionality to be incorporated into small hand-held or body-worn devices.

(0003] The antennas needed to support radio connectivity are subject to well-known physical constraints which relate their efficiency and bandwidth to their dimensions measured in wavelengths at the frequencies at which they operate. In order to minimise antenna dimensions it has become usual to employ unbalanced antennas. These comprise a radiating element or configuration of radiating elements having an excitation terminal. The radio signal to be transmitted is connected between this radiating structure and the ground conductor or groundplane of the radio circuits associated with the antenna.

Such a configuration is known as an unbalanced antenna, by contrast with a balanced antenna such as a dipole.

(0004] It is well known that the performance of an unbalanced antenna, in particular its bandwidth, depends strongly on the dimensions of the ground system, commonly known as the groundplane, against which it is driven. A curve showing the typical relationship between bandwidth and groundplane dimensions is shown in Handset Antennas, Chapter 2 of Antennas for Portable Devices, Zhi Ning Chen (Editor), John Wiley, 2007, ISBN: 978-0-470-03073-8, and in further technical papers referenced therein. Given suitable antenna placement, typically near one end of the groundplane, the optimum dimension for the groundplane is approximately a half wavelength at the centre of the operating frequency band.

(0005] The use of groundplane extensions to extend operating bandwidth is well known, for example from US 5,561,436 (Ground extension of flip cover of a handset) and US 5,337,061 (Ground extension of flip cover of a handset). However, these groundplane extensions are specific to particular handset constructions, and are internal to the handset cover. Another type of groundplane extension is known from EP2343772, but this is a non-flexible groundplane extension internal to a handset.

[0006] According to the principle of reciprocity, the main characteristics of a passive antenna device remain the same whether the antenna is used for the transmission or reception or radio signals. For the sake of simplicity the descriptions herein will refer to the antenna as operating in transmitting mode.

BRIEF SUMMARY OF THE DISCLOSURE

[0007] According to a first aspect, there is provided a radio device comprising a housing containing a radio transmitter/receiver, an antenna and a conductive groundplane, and further provided with at least one elongate flexible conductive member electrically connected to the conductive groundplane and extending outside the housing.

[0008] Embodiments of the present invention seek to facilitate extended bandwidth from a small device having a longest dimension substantially less than a half wavelength at the frequency of operation. Embodiments of the invention are particularly suitable for use in connection with small devices intended to be worn by a user, such as alarms and trackers.

[0009] Embodiments of the present invention further seek to provide a flexible groundplane extension that can be incorporated in a functional or decorative feature attached to the small radio device.

[0010] Advantageously, the operating frequency bandwidth of the device is extended by means of the at least one attached elongate flexible conductive member electrically connected to the groundplane. The at least one elongate flexible member may be directly or galvanically connected to the groundplane, or may be connected by means of a capacitor or an arrangement providing a capacitive reactance.

[0011] In order to reduce the area or volume occupied by the elongate flexible conductive member, the member may be coiled, convoluted or meandered.

[0012] The elongate flexible conductive member connected to the groundplane may be contained within or form a strap or cord by which the device is attached to a person, animal or supporting structure.

[0013] For example, the member may be contained within or form a wrist-strap of a device designed to be worn on the wrist like a wristwatch. Alternatively, the member may be contained within or form a neck-cord of a pendant-like device designed to be worn around the neck. Alternatively, the member may be contained within or form a belt to be secured around a user's waist or around some other body part or support structure. In some embodiments, the member may be contained within or form an identification tag or tracking device. In some embodiments the member may be contained within or form a strap or cord forming part of a hat, glove or other piece of clothing, or may be contained within or form part of a collar, saddle, tag or other device wearable by or attached to an animal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: FIGURE 1 shows a balanced antenna; FIGURE 2 shows an unbalanced antenna directly driven from an unbalanced source; FIGURE 3 shows a typical relationship between frequency bandwidth and the long dimension of the groundplane; FIGURE 4 shows a typical unbalanced PlEA antenna mounted at one end of a groundplane FIGURE 5 shows a conductive groundplane extension; FIGURE 6 shows a watch having a flexible groundplane incorporated in a non-conducting strap; and FIGURE 7 shows two groundplane extensions forming part of the neck-strap of a wearable radio device.

DETAILED DESCRIPTION

[0015] The balanced antenna shown in Figure 1 comprises two substantially symmetrical conductive members 1 and 2. Such an antenna may be excited by a balanced (two- terminal) source 3. It may also be fed from an unbalance source through a balanced-to-unbalanced circuit arrangement (a balun).

[0016] The unbalanced antenna in Figure 2 comprises a single conductive member 4, driven by a source 5 having one terminal connected to a conductive groundplane 6 and the other to the conductive member 4. The groundplane 6 may comprise the system of ground conductors integral with a printed circuit board that carries the electronic components comprising the device! together with other components electrically connected thereto, such as a keyboard, display screen, loudspeaker or other associated components.

To provide the maximum operational frequency bandwidth, the long dimension of this groundplane 6 should be of the order of one half wavelength at the centre of the operating frequency band, as shown for example in Figure 3. In fact, given that the wavelength of a 900MHz signal in tree space is around 333mm, Figure 3 shows that the maximum bandwidth is obtained at a chassis length of around 125mm, which is somewhat less than one half of one wavelength. Accordingly, in some arrangements a long dimension of the groundplane 6 in the region of from one quarter to one half of a wavelength may be appropriate for the best bandwidth performance. For devices operating in more than one frequency band, for example a mobile phone operating in two bands having centre frequencies in the region of 900MHz and 1800MHz, the longest dimension should approach a half wavelength in the lower frequency operating band.

[0017] Many methods are known whereby the dimensions of the driven conductor 4, commonly called the antenna or radiating element, can be reduced in overall dimensions by coiling, meandering or bending, applied singly or in combination, together with the use of geometries such as an inverted-L, planar inverted-F (PIFA) and inverted-T. By way of example! Figure 4 shows a PIFA antenna 7 in an optimum position at one end of the groundplane 6. The antenna 7 may be directly driven from the transmitter as shown in Figures 2 and 4, or may be fed through a coupling dielectric or conductive shunt-feed arrangement. Such arrangements can reduce the dimensions of the radiating structure, but despite strategies for the reduction of the dimensions of the antenna 7, the optimum dimension for the groundplane 6 remains as approximately a half wavelength.

[0018] Figure 5 shows an arrangement using an electrically small groundplane 6 in which the effective electrical length has been increased by the addition of an additional conductor 8 at the end remote from the antenna 7. Such arrangements have commonly been incorporated into the rigid case of a mobile handset or have been created using printed circuit techniques into the lid of a folding clamshell" handset.

[0019] In certain embodiments of the present invention, the groundplane 6 is extended by means of a conductive member embedded in a strap, neck-cord or decorative feature attached to the radio device. Preferably such a conductor is flexible such that its presence does not detract from the aesthetic or practical features of the device. The means of connection of the groundplane extension conductor to the internal groundplane of the radio device may be conductive or by means of a capacitance provided between the groundplane and the strap end. Where convenient, more than one conductive extension may be used.

[0020] An exemplary embodiment is shown in Figure 6 where a device in the form of a wristwatch 11 is provided with non-conducting straps 12, 12', within one of which is a flexible groundplane extension 13. The groundplane extension 13 is embedded in the strap 12 and is galvanically connected to the watch body 11, preferably at a position approximately 180 degrees from the location of the antenna feed. For example, if the antenna feed is at 12 o'clock on the dial, the conductor is preferably placed in the strap attached at 6 o'clock. By way of example, the groundplane extension conductive member 13 may be made from a conductive braid within a leather or plastic strap, the end proximate to the radio device being connected thereto using a conductive case and strap attachment bar. The length of the conductive extension 13 may be chosen to provide the optimum radio frequency performance in the frequency bands of interest.

[0021] A further exemplary implementation is shown in Figure 7, wherein a radio device 14 is attached to a neck-cord 15, those parts 13, 13' of the cord proximate to the device 14 being made of a flexible conductive material whose length may be chosen to provide optimum radio performance. The conductive material may have a decorative finish or may be concealed in a decorative non-conductive sleeve. Preferably the groundplane extension is connected to the top of the device 14, while the antenna is positioned at the bottom.

[0022] The flexible conductive groundplane extension may be meandered, for example by etching it on a flexible printed circuit substrate material; it may also be coiled or formed into other shapes for decorative or aesthetic purposes. The electrical characteristics of the groundplane extension may also be modified by the incorporation of capacitive or inductive structures or components.

[0023] For some applications multiple groundplane extensions having different physical lengths or shapes may be provided in order to provide resonant behaviour in more than one operating frequency band.

[0024] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0025] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0026] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims (1)

1. <claim-text>CLAIMS: 1. A radio device comprising a housing containing a radio transmitter/receiver, an antenna and a conductive groundplane, and further provided with at least one elongate flexible conductive member electrically connected to the conductive groundplane and extending outside the housing.</claim-text> <claim-text>2. A device as claimed in claim 1, wherein the device has a longest dimension less than a half wavelength at a frequency of operation.</claim-text> <claim-text>3 A device as claimed in claim 1 or 2, wherein the at least one elongate flexible conductive member extends an operating frequency bandwidth of the device.</claim-text> <claim-text>4. A device as claimed in any preceding claim, wherein the at least one elongate flexible member is connected to the groundplane by way of a capacitor or an arrangement providing a capacitive reactance.</claim-text> <claim-text>5. A device as claimed in any preceding claim, wherein the at least one elongate flexible member is coiled, convoluted or meandered.</claim-text> <claim-text>6. A device as claimed in any preceding claim, wherein the at least one elongate flexible is contained within or forms a strap or cord by which the device may be attached to a person, animal or supporting structure.</claim-text> <claim-text>7. A device as claimed in any preceding claim, wherein the at least one elongate flexible conductive member forms or is contained within a wrist-strap.</claim-text> <claim-text>8. A device as claimed in any one of claims 1 to 6, wherein the at least one elongate flexible conductive member forms or is contained within a neck-cord.</claim-text> <claim-text>9. A device as claimed in any one of claims 1 to 6, wherein the at least one elongate flexible conductive member forms or is contained within a belt.</claim-text> <claim-text>10. A device as claimed in any preceding claim, wherein the at least one elongate flexible conductive member forms or is contained within an identification tag or tracking device.</claim-text> <claim-text>11. A device as claimed in any preceding claim, wherein the at least one elongate flexible conductive member is contained within or forms a strap or cord forming part of a hat, glove or other piece of clothing.</claim-text> <claim-text>12. A device as claimed in any one ot claims 1 to 10, wherein the at least one elongate flexible conductive member is contained within or forms a collar, saddle, tag or other device wearable by or attached to an animal.</claim-text> <claim-text>13. A radio device substantially as hereinbefore described with reference to or as shown in the accompanying drawings.</claim-text>