GB2406217A

GB2406217A - Tuneable antenna

Info

Publication number: GB2406217A
Application number: GB0321222A
Authority: GB
Inventors: Gert Frolund Pedersen
Original assignee: ITT Manufacturing Enterprises LLC
Current assignee: ITT Manufacturing Enterprises LLC
Priority date: 2003-09-10
Filing date: 2003-09-10
Publication date: 2005-03-23
Also published as: GB0321222D0

Abstract

A tuneable antenna suitable for mobile telecommunications or small terminal applications comprises an electrically conductive antenna 8 having antenna elements 40, 48 tuned to different frequencies, capacitance plates 30, 32 adjacent to respective antenna elements 40,48, and a switch (27) for connecting one of the plates 30, 32 to ground to tune the tuneable antenna to corresponding different resonant frequencies. In a first switch position one of the plates 30,32 is connected to ground whilst the other is not connected and has a floating potential. In a second switch position the ground connected plate and the floating potential plate are changed over. This allows the frequency of one antenna element portion to increase whilst decreasing that of the other using a single switch. The switch may be electrically actuated or may be MEMS or PIN diode switches. One or more capacitive plates may be arranged adjacent an antenna element portion which may be conneted to ground by one or more switches.

Description

24062 1 7

TUNEABLE ANTENNA

This invention relates to a tuneable antenna. More particularly, but not exclusively, this invention relates to a tuneable antenna for use with mobile telecommunication equipment operating on at least two frequency bands.

Mobile telecommunication equipment or small terminal equipment such as a mobile, or cellular, telephone will typically have an antenna that is used for the wireless transmission and receipt of electromagnetic signals. In a mobile telephone, this antenna is used to communicate with the base transceiver station for the cell in which the telephone is operating.

The frequency band in which the mobile telephone and base transceiver station communicate depends on the communications standard to which the mobile network, of which the base transceiver station is a part, conforms. For example, the GSM (Global System for Mobile communications) standard used throughout Europe and Asia defines two alternative frequency bands, 900MHz and 1800MHz, more exactly EGSM from 880MHz to 960MHz, and GSM1800 from 1710MHz to 1880MHz. In North America several systems are used, including PCS (Personal Communications Service), which operate in frequency bands around 850MHz and 1900MHz respectively, for example GSM850 from 824MHz to 894MHz, and PCS from 1850MHz to 1990MHz.

Further frequency bands have been defined for the UMTS (Universal Mobile Telephone Standard) "third generation" standard, and for other radio communications applications such as GPS.

Antennas for mobile telephones are well known. Early mobile telephones were designed to operate in single, fixed frequency bands, and were therefore only compatible with mobile networks operating in that frequency band. Antenna designs for these mobile telephones were resonant around a single, fixed frequency. Capacity constraints and different bandwidth allocations in different countries have resulted in the use of a variety of frequency bands. To deal with these multiple bands, mobile telephones that are capable of operating in two or three frequency bands were developed. Antenna designs for these mobile telephones must therefore be capable of operating in at least two defined frequency bands.

A number of antenna designs that are capable of operating in at least two defined frequency bands have been proposed. One approach for providing a multiple band antenna is to use separate antennas, each corresponding to a different frequency band. For example, EP 1109251 and WO 01/03238 disclose antenna arrangements in which two antennas for emitting two independent frequencies are used.

It is more usual however to use a single antenna capable of resonance in a plurality of frequency bands.

US 6452551 discloses a capacitor loaded planar antenna that is capable of operating in two defined frequency bands. A high band antenna stub is attached to a low band antenna.

US 6346919 discloses an antenna capable of operating on two or more unrelated frequencies. The antenna has a number of notches in a patch element so as to provide more than one resonant frequency.

EP 1168495 discloses an antenna that is capable of operating in two defined frequency bands. An antenna element is parallel to, and connected at one end to, an earth plate. An electrical switch that is capable of making a low impedance connection between the opposite end of the antenna and the earth plate allows the antennae to operate at two alternative resonant frequencies.

EP1109251 describes an arrangement having two antenna elements on a ceramic substrate which may be capacitatively coupled together by a switch.

However, there remains a need for improved multiple band antennas.

According to the invention there is provided a multi-band tuneable antenna comprising: an electrically conductive antenna element having a plurality of antenna element portions for radiating at respective first communications frequency bands, the plurality of respective first communications frequency bands including a plurality of different frequency bands; a capacitance element adjacent to and coupled to a respective antenna element portion; and a switch switchably coupling the capacitance element to ground to tune the communications frequency of the respective antenna element portion to a respective second communications frequency different to the first communications frequency.

Note that any do voltage acts as ground to RF frequencies. The chosen ground may conveniently be the ground plane of the mobile device in which the antenna is installed.

By arranging a capacitance element adjacent to an individual antenna element portion, to couple predominantly to that portion, or a plurality of capacitance elements adjacent to respective individual elements, the change in frequency of the individual element or elements can be individually controlled. Thus, by adjusting the size and location of the capacitance elements it is straightforward to design an antenna in which each antenna element portion can be switched between precisely determined frequencies.

The inventors are aware of a document in which a capacitance element is used in a multiple antenna element portion system, namely EP 1109251. This document is concerned not with multiple frequency bands but with enlarging a single frequency band. Moreover, this document teaches using a control electrode coupled to two element portions to controllably couple the ends of the element portions together. Thus this prior art arrangement offers much less design flexibility. In particular, the frequencies of both element portions are shifted together. This is suitable in the application envisaged in EP1109251, for broadening a single frequency band, but unsuitable for switching between different frequency bands at well-defined frequencies, as is readily achieved using the present invention.

Normally, the communications frequency or frequencies of an antenna element portion will be given by the resonant frequency or frequencies of the antenna element, especially at the frequencies used for mobile telephony. However, the skilled person will appreciate that it is possible, for example using suitable impedance matching techniques, to transmit and receive signals away from resonance. In this case, the communications frequency of the antenna element will be given by the antenna element in combination with the impedance matching components. Another way to obtain resonance of an antenna which is electrically too small is to employ capacitative loading at the end of the element whereby the element electrical becomes longer.

The invention may be applied to different types of antenna. In particular, the invention may be applied to a planar antenna with or without a ground plane.

Thus, the antenna element may extend laterally. The antenna element portions may be planar. The antenna element portions corresponding to each frequency may be defined by a number of longitudinal and lateral notches.

In embodiments, the tuneable antenna also comprises an electrically conductive ground element extending parallel to the antenna and spaced from it. Preferably, the capacitance element or elements is or are arranged closer to the antenna element portions to which they are adjacent than the distance of the ground plane to the antenna element portion.

The antenna may in particular include first and second antenna element portions and respective first and second capacitance elements adjacent to the first and second antenna elements respectively, and a common switch arranged to connect the first capacitance element leaving the second capacitance element floating in a first position and the second capacitance element portion in a second position leaving the first capacitance element floating. In this way the frequency of a switchable one of the first and second elements is adjusted by its adjacent capacitance element.

In preferred arrangements, the or each respective communications frequency varies by no more than 30% as the capacitance element is switched. By requiring only relatively small variation of frequencies in a multiband antenna it becomes practical to realise the invention without requiring the capacitance element or elements to be excessively large.

In a particularly useful embodiment a first antenna element portion having a respective first capacitance element switchably connected to ground, the first antenna element portion having a resonant frequency in the EGSM frequency band of 880MHz to 960MHz when a the first capacitance element is not connected to ground and a lower frequency in the GSM 850 frequency band of 824MHz to 894MHz when the switch is connected to ground; and a second antenna element portion having a respective second capacitance element switchably connected to ground, the second antenna element portion having a resonant frequency in the PCS1900 frequency band of 1850 to 1990 MHz when the second capacitance element is not connected to ground and a lower frequency in the GSM1800 frequency band of 1710MHz to 1880MHz when the second capacitance element is connected to ground.

In this way the antenna can operate at the frequencies presently most common in Europe and North America.

A single switch may be used to switch one or the other of the capacitance plates to ground. Thus, in a first state the antenna operates at the frequencies particularly used in North America, whereas in a second state the antenna can operate at the GSM frequencies used in Europe and some other countries.

Note that this change in frequencies of bands does not occur in the arrangement of EP1109251 which describes a method of broadening a single band. Moreover, in the arrangement of EP1109251 the frequency of each element portion is switched in the same direction using a single switch, rather than increasing one frequency and decreasing another.

In a particularly preferred embodiment the or each antenna element is a planar element and each respective capacitance element is a planar element spaced perpendicularly from and extending substantially parallel to the respective antenna element.

l0 In alternative embodiments the capacitance elements may be coplanar with the antenna elements. Alternative arrangements are also considered possible.

A control circuit may provide control signals to the electromagnetic device so that the position of the switch or switches may be controlled. The control circuit may, for example, provide the control signals automatically in response to the electromagnetic signals that it detects, under the control of a computer program.

In a further arrangement, the antenna element further comprises a fixed antenna element portion for a constant communications frequency. This fixed antenna element portion may be used for a communications service with a fixed or worldwide frequency. One example of such a frequency is that of the Universal Mobile Telephone Standard (UMTS), for use with UMTS networks.

Other fixed antenna element portions may be used for other fixed frequency services, such as the global positioning system (GPS).

The invention also provides mobile communications apparatus comprising the tuneable antenna described above, such as a mobile telephone, or other mobile communication equipment or small terminal equipment.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a sectional view of an embodiment of the tuneable antenna including ground plane, spacer frame and antenna; Figure 2 shows a perspective view of the spacer frame and associated components of the embodiment of Figure 1; Figure 3 shows a detail top view of the components associated with the spacer frame; Figure 4 shows a top view of the antenna elements of the embodiment of Figure 1; and Figure 5 shows a perspective view of the arrangement of Figure 1.

It should be noted that the figures are diagrammatic and not drawn to scale.

Relative dimensions and proportions of parts of these figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings.

Referring to Figure 1, a mobile telephone according to the invention includes a circuit board 2 defining a ground plane 4, an insulating spacer frame 6 and an antenna 8. The spacer frame 6 is associated with a number of further components, represented in Figure 1 schematically as a capacitance plate element 10 extending substantially adjacent to and parallel to part of the antenna and a switch 12 arranged to switchably connect the capacitance plate element 10 to ground.

Figure 1 also shows schematically a signal source 14 feeding into a feed point 16 on the antenna 8 as well as a direct connection of a ground point 17 on the antenna 8 to ground. Note that the ground connection shown in Figure 1 is optional and in some alternative embodiments may be omitted. A housing 18 is provided, shown schematically by a dotted line.

Figure 2 shows the spacer frame 6 which is formed of insulating plastics in the specific embodiment. The spacer frame 6 comprises a number of ribs 20 for supporting the antenna 8 away from the ground plane 4. In the specific embodiment shown the ribs are 5mm-9mm in the direction perpendicular to the ground plane, though this may vary depending on the available space within the mobile telephone housing.

Figure 2 also shows a number of components associated with the spacer frame, and a top view of these components is provided in Figure 3. An electrically controlled actuator 24 is held in a hole 26 defined in one of the ribs 20. The actuator is connected to a flexible switch foil 27 of metal connected in turn to a stud 28 for connection to the ground plane 4.

A pair of capacitance plate elements 10, comprising first capacitance plate element 30 and second capacitance plate element 32, are supported on insulating capacitance plate support rib 36. The actuator is connected to the switch foil 27 by rod 34 which passes under the second capacitance plate element 30 and which is arranged to move the switch foil 27 from a first position (shown dotted in Figure 3) in contact with and electrically connecting the first capacitance plate element 30 to a second position (shown in a full line) in contact with and electrically connecting the second capacitance plate element 32. In this way, either of the first and second capacitance plate elements 30,32 can be connected to ground.

As shown in Figure 4, the antenna 8 includes a feed point 16. A first antenna element portion 40 made up of first arm 42 and second arm 44 separated by notch 46 extends in one direction from feed point 16. A second antenna element portion 48 in the form of an "L" extends in another direction from feed point 16. The ground connection 17 is made at the location indicated by reference numeral 17, opposite the feed point 16. Note that although the feed point 16 and ground 17 are indicated to be between the two antenna elements, it is also possible for the skilled person to arrange for different feeding and grounding points.

The mobile telephone is assembled within housing 18 with antenna 8 spaced from ground plane 2 as shown in Figures 1 and 5. Note that the first and second capacitance plate elements 30, 32 are in fact arranged parallel to and closely spaced from the ends of the first and second antenna elements 40,48 to couple to individual elements. By arranging the plates adjacent to the ends significant frequency shifts are most easily achieved. Stud 28 is connected to ground plane 4.

The use of parallel closely spaced capacitance plates SO, 32 allows significant changes in frequency without using excessively large plates. In the specific embodiment, the plates may be of linear dimension 2mm by 4mm to 6mm by 6mm, depending on the available space and frequency shift required. As the skilled person will appreciate, the capacitance of a plate of area A spaced by a distance I from the antenna element is indicated by C=ò A/l. Thus, a closer spacing between the capacitance plates and the antenna elements may allow smaller linear dimensions to be used, and the converse is also true. Larger or smaller plates may also be used if necessary to achieve the required frequency shifts.

In use, actuator 24 switches one of the capacitor plate elements 30,32 to connect it to ground, leaving the other floating. When the capacitance element is grounded, the capacitance is effectively on and this capacitative loading on the antenna element causes a drop in frequency.

In the described embodiment, both plates 30,32 are controlled by the action of a single switch which grounds the first plate 30 in a first position and the second plate 32 in its second position.

In the preferred embodiment, the first antenna element 40 has a resonant frequency in the EGSM frequency band of 880MHz to 960MHz when the switch is in a second position so that the first capacitance plate 30 is not grounded and at a lower frequency in the GSM 850 frequency band of 824MHz to 894MHz when the switch is in a first position so that the first capacitance plate is grounded.

The second antenna element 48 has a resonant frequency in the PCS1900 frequency band of 1850 to 1990 MHz when the switch is in the first position so that the second capacitance plate 32 is not grounded and a lower frequency in the GSM1800 frequency band of 1710MHz to 1880MHz when the switch is in the second position so that the second capacitance plate 32 is grounded.

In this way the antenna operates at the frequencies suitable for North America with the switch in the first position and at frequencies suitable for Europe with the switch in the second position.

The invention is not limited to the embodiment described above and the skilled person will be aware of alternative possibilities for implementing many of the features.

The number of frequency bands is not restricted to two and three or more frequency bands may be provided if required.

In alternative embodiments, each of the capacitance plates 3O,32 is connected by separate switches 12 to ground.

Instead of the switch described, any alternative switching arrangement may be used.

For example, MEMS switches may be used, i.e. a small electronic switch defined on a piece of silicon. It is presently quite difficult to make a MEMS switch reliable over a large number of duty cycles, making them unsuitable for some applications. However, the number of times that the switch in the present invention is actuated is rather small, making the MEMS switch highly suitable for this application.

Alternatively, PIN diode switches may be used. These are slightly nonlinear leading to some harmonic distortion, but this can be reduced by correct design as will be appreciated.

The antenna 8 may include an arm of fixed frequency, for example for a fixed band.

In less preferred embodiments the capacitance elements may be arranged elsewhere than at the ends of the antenna element arms.

The above described embodiment uses an essentially planar antenna but in embodiments the antenna elements may be, for example, at different levels or not planar. The elements may be shaped to conform to the inside of the housing in which they reside.

Although the embodiment described above uses a spacer to separate the antenna element from the ground plane, a spacer is not necessary, and the antenna element may simply be fixed to the inside of the housing, or otherwise.

In the present application, the term mobile telephone or mobile phone is used to cover all mobile communications devices, including for example, PDAs, cordless telephones, and other wireless communications devices.

Claims

CLAIMS: 1. A multi-band tuneable antenna comprising: an electrically

conductive antenna element having a plurality of antenna element portions for radiating at respective first communications frequency bands, the plurality of respective first communications frequency bands including a plurality of different frequency bands; a capacitance element adjacent to and coupled to a respective antenna element portion; and a switch switchably coupling the capacitance element to a supply voltage to tune the communications frequency of the respective antenna element portion to a respective second communications frequency different to the first communications frequency.
2. A tuneable antenna according to claim 1 having a plurality of capacitance elements adjacent to and coupled to respective antenna element portions, the capacitance elements being switchably coupled to the ground by at least one switch.
3. A tuneable antenna according to claim 1 or 2 wherein each antenna element portion is a planar element and the or each capacitance element is a planar element spaced perpendicularly from and extending substantially parallel to the respective antenna element portion.
4. A tuneable antenna according to any one of claims 1 to 3 wherein the antenna element portions corresponding to each frequency are defined by a number of longitudinal and lateral notches.
5. A tuneable antenna according to any preceding claim further comprising an electrically conductive ground plane spaced away from and extending parallel to the antenna element.
6. A tuneable antenna according to claim 5 wherein each capacitance element is arranged closer to the antenna element than the ground plane.
7. A tuneable antenna according to any preceding claim wherein the or each first communications frequency of each respective antenna element portion differs by no more than 30% from the respective second communications frequency of the same antenna element portion.
8. A tuneable antenna comprising first and second antenna element portions and respective first and second capacitance elements adjacent to the first and second antenna element portions respectively, wherein the switch is arranged to connect the first capacitance element portion leaving the second capacitance element floating in a first switch position and the second capacitance element portion in a second switch position leaving the first capacitance element floating.
9. A tuneable antenna according to claim 7 or 8 comprising: a first antenna element portion having a respective first capacitance element switchably connected to ground, the first antenna element portion having a resonant frequency in the EGSM frequency band of 880MHz to 960MHz when a the first capacitance element is not connected to ground and a lower frequency in the GSM 850 frequency band of 824MHz to 894MHz when the switch is connected to ground; and a second antenna element portion having a respective second capacitance element switchably connected to ground, the second antenna element portion having a resonant frequency in the PCS1900 frequency band of 1850 to 1990 MHz when the second capacitance element is not connected to ground and a lower frequency in the GSM1800 frequency band of 1710MHz to 1880MHz when the second capacitance element is connected to ground.
10. A mobile communications apparatus comprising a tuneable antenna according to any preceding claim.