WO2011095207A1

WO2011095207A1 - Antenna device and portable electronic device comprising such an antenna device

Info

Publication number: WO2011095207A1
Application number: PCT/EP2010/051289
Authority: WO
Inventors: Stefan Irmscher; Peter Lindberg
Original assignee: Laird Technologies AB
Current assignee: Laird Technologies AB
Priority date: 2010-02-03
Filing date: 2010-02-03
Publication date: 2011-08-11
Anticipated expiration: 2012-08-03
Also published as: US20120268330A1; DE112010005220T5

Abstract

An antenna device comprises a loop element (27) having a length (L1) providing loop resonance at a first wavelength, where a resonance frequency of this wavelength is used in a desired frequency band. A capacitance (28) is provided between a first position (P1) on the element and ground, thereby dividing the element into a first and a second section (S1, S2 ). The second section has an inductance that depends on the length (L2) and forms a resonance circuit with the capacitance which causes the element to function as a monopole element at the resonance frequency of the resonance circuit. The first position (P1) and capacitance (28) are so selected that the resonance circuit resonance frequency lies in the desired frequency band. The first position (P1) is also selected such that the length (L3) of the first section (S1) provides a monopole resonance at a second wavelength having one resonance frequency at the resonance circuit resonance frequency.

Description

ANTENNA DEVICE AND PORTABLE ELECTRONIC

DEVICE COMPRISING SUCH AN ANTENNA DEVICE FIELD OF INVENTION

The present invention relates generally to antenna devices and more particularly to an antenna device for use in a portable radio communication device, such as a mobile phone. The present invention also relates to a portable radio communication device having such an antenna device.

BACKGROUND

Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages connected with using internal antennas, of which can be mentioned that they are small and light, making them suitable for applications wherein size and weight are of importance, such as in mobile phones. One type of frequently used antenna in this regard is the Planar Inverted F Antenna (PIFA), which

essentially uses the whole device as radiator. This antenna functions well and provides good multi-band functionality . However, there may be a problem when a portable radio communication device or terminal having this type of antenna is used by a person having hearing aid

equipment. There might be interference in this hearing aid equipment caused by such an antenna. Therefore there exists a so-called Hearing Aid Compatibility

(HAC) requirement in some countries. This complicates the use of the PIFA antenna. In order to fulfill the requirement research has been made into alternative antennas .

One antenna that is promising is the loop antenna. One reason for this is that this antenna, at some

frequencies, does not use the whole terminal as radiator and therefore it is possible to place the antenna far from the end of the terminal intended to face a hearing aid and thereby obtain interference reduction.

However, there is a problem with this type of antenna and that is the bandwidth covered. Today's antennas for use in cellular communication, like Long Term Evolution (LTE), are to cover a number of wide

frequency bands, where a first band is around 900 MHz and a second band is between 1710 and 2170 MHz. The loop antenna has problems in being able to cover the very wide second band.

There is therefore a need for providing a loop antenna that has a better wide band capacity, for instance when covering a first lower band of medium width together with a second higher band of higher width.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an internal antenna device for use in a portable radio communication device, which provides a loop antenna having good wideband properties.

The invention is based on the realization that a desired wide band can be covered by a loop antenna element through merely connecting a capacitance to the ground end of the loop antenna element. With a good choice of the connection point on the loop antenna element and the capacitance it is possible to make the loop antenna element function as a monopole element in a frequency range inside the desired band and thereby obtain a further resonance in order to cover the desired band.

According to the present invention there is provided an antenna device for operating in at least one desired operational frequency band, the antenna device comprising : a loop element having a feeding end for connection to a radio communication circuit and a grounding end for connection to ground, wherein the length of the loop element between the feeding and grounding ends is selected to provide loop resonance at a first

wavelength, where one resonance frequency associated with this first wavelength is a base resonance

frequency for providing coverage of said desired frequency band, and a first capacitance provided between a first position on the loop element and ground, thereby dividing the loop element into a first section stretching between the feeding end and the first position and a second section stretching between the first position and the grounding end, wherein the second section has an inductance depending on the length, said inductance forming a resonance circuit together with the first capacitance, which resonance circuit has a resonance frequency causing the loop element to function as a

monopole element in a frequency range, where the first position and the first capacitance are selected for the frequency range to lie in said desired frequency band, and - the first position is selected for giving the

first section a length providing the loop element with a monopole resonance at a second wavelength, which second wavelength is selected so that one resonance frequency associated with this second wavelength lies within the frequency range in order to provide a first assisting resonance frequency, which assist in the coverage of the desired frequency band.

The invention is also directed towards a portable radio communication device comprising in its interior such an antenna device, a ground plane and a radio communication circuit connected to the antenna device.

The antenna device according to the invention provides operation with good performance throughout a wide frequency band. This is furthermore done with a minimum of components and elements, making the antenna device economical and easy to produce. The size can furthermore also be small.

Further preferred embodiments are defined in the dependent claims. The term "base resonance frequency" used here is intended to mean a resonance frequency which is used as a basic building block when providing coverage of a desired frequency band. The term "assisting resonance frequency" is intended to mean a resonance frequency used as a further building block that is added to a base resonance frequency for providing coverage of the desired frequency band. BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a front view of one exemplifying portable radio communication device according to the present invention;

Fig. 2 shows a sectional view of the portable

electronic device according to the present invention;

Fig. 3 schematically shows a general antenna device according to a first variation of the invention together with a ground plane and a radio communication circuit ,

Fig. 4 schematically shows a first embodiment of the antenna device according to the first variation together with a ground plane and a radio communication circuit;

Fig. 5 shows a resonance circuit formed by the antenna device, Fig. 6 schematically shows a return loss diagram for the antenna device according to the first variation;

Fig. 7 schematically shows a second embodiment of the antenna device according to the first variation together with a ground plane and a radio communication circuit ;

Fig. 8 schematically shows a third embodiment of the antenna device according to the first variation together with a ground plane and a radio communication circuit, and

Fig. 9 schematically shows a general antenna device according to a second variation of the invention together with a ground plane and a radio communication circuit . DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of a

preferred embodiment of an antenna device and portable radio communication device according to the invention will be given. In the description, for purposes of explanation and not limitation, specific details are set forth, in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be utilized in other embodiments that depart from these specific details. In other instances, detailed

descriptions of well-known units, entities and circuits are omitted so as not to obscure the description of the present invention with unnecessary details. The invention is generally directed towards an antenna device and a portable radio communication device including an antenna device, where the antenna device may be supposed to receive radio signals in a first and a second operating frequency band.

Fig. 1 shows a front view of a portable radio

communication device 10, such as a mobile phone. The portable radio communication device 10 can however be another type of device, such as a lap top computer, a palm top computer or an electronic organizer such as a personal digital assistant (PDA). The device 10 is, as an example, provided with a speaker 12 placed close to an upper end of the device, a keypad 14 placed close to a lower end of the device and a display 16 in- between the speaker 12 and the keypad 14. These are here provided on the casing of the device 10. It should however be realized that the device may just as well be provided without a display, speaker and/or keypad. The device 10 is also provided with at least one antenna. However, all antennas are provided inside the device, i.e. in the interior of the device.

Fig. 2 shows a schematic side view of the device 10, which is a cross section through the casing 18. On the casing the speaker 12 is indicated. In order to clarify the description of the present invention only elements that are relevant for understanding the present invention are included. Thus a number of units in the device have here been omitted, like for

instance the display and the keypad shown in fig. 1. The device 10 here includes a circuit board 20 on which an antenna device 22 according to the present invention is mounted. On the board 20 there is

furthermore a radio communication circuit 24, here the a cellular radio communication circuit, which may for instance be used to communicate in at least two separate frequency bands. The circuit board 20, which may be a multi-layer PCB (printed circuit board), furthermore includes a ground plane (not shown), which is used together with the antenna device 22 of the present invention. In fig. 2 the antenna device is shown as being placed on the circuit board. It should however be realized that it may include elements, like a radiating and/or radiation receiving element that extends beyond this circuit board or is even provided outside of this circuit board. It may for instance be folded around an edge of the circuit board or be placed outside of the board.

As can be seen the antenna device 22 is placed as far as possible from the speaker 12. The reason for this is that then interference caused by it on hearing aid is minimal, which helps in fulfilling HAC requirements being placed on the portable radio communication device .

There exist two general variations of the present invention. According to the first variation the antenna device comprises a loop element together with two capacitances. According to the second variation the antenna device comprises a loop element together with only one capacitance, a capacitance near a grounding end of the loop element. Now a number of embodiments of the invention will be disclosed all based on the first variation of the invention

Fig. 3 schematically shows a general antenna device according to the first variation of the invention together with a ground plane 26 on the circuit board 20. On the board there is furthermore provided the radio communication circuit 24 as well as a radiating and/or radiation receiving element 27. This element here has a first end, a feeding end, connected to the radio communication circuit 24 using the feeding conductor and a second end, a grounding end, connected to the ground plane 26. On the circuit board the radio communication circuit 24 is placed above the ground plane. The feeding conductor is not in contact with this ground plane, which in the figure shown as a channel provided on the board through the ground plane where the connection is provided. Normally this connection can be provided through the use of a coaxial cable or a microstrip line. The ground plane will normally not include such a channel. The ground plane and connection is only shown in this way in order to clearly show that the feeding end is not connected to ground. The radiating and/or radiation receiving element 27 is according to the present invention a loop element. The antenna device 22 according to the first variation of the invention includes this loop element 27 together with a first CI and a second capacitance C2 both provided between the loop element 27 and ground. The loop element 27 thus has a first end, a feeding end, connected to the radio communication circuit 24 and then runs in a loop to a grounding end, which is connected to the ground plane 26. This loop is here disclosed as elliptical. It should however be realized that the loop may have any suitable shape as long as the feeding end is connected to the radio

communication circuit 24 and the grounding end is grounded. The feeding end is furthermore only

connected to the radio communication circuit for receiving radio signals. There is no grounding at this end as for a Planar Inverted F Antenna (PIFA) element.

The capacitances CI and C2 can be provided in

different ways and therefore they are in fig. 3 indicated as boxes including the symbol of a

capacitor. The first capacitance CI is here provided between the loop element 27 and ground 26 close to the ground end, while the second capacitance C2 is

provided between the loop element and ground 26 close to the feed end. Fig. 4 schematically shows the antenna device

according to a first embodiment of the invention. Here the loop element 27 has the same shape as in fig. 3. The radio communication circuit 24 is shown as an AC voltage source. The first and second capacitances CI and C2 are here provided as discrete capacitor

components and are therefore disclosed as ordinary capacitor symbols. There is here a first component 28 providing the first capacitance CI and a second component 30 providing the second capacitance C2. The figure furthermore shows that the loop element 27 has a length LI from the feeding to the grounding end and that the first capacitor 28 is connected between a first position PI on the loop element 27 and ground. This means that the first capacitance CI is provided between the position PI on the loop element and ground. Through this first position PI the loop element is furthermore divided into two sections a first section SI and a second section S2, where the first section SI stretches between the feeding end and the first position PI and the second section S2 stretches between the first position PI and the grounding end. The first section furthermore has a length L3, while the second section has a length L2.

The functioning of the antenna device according to the first variation of the invention will now be described with reference being made also to fig. 5, which shows a resonance circuit formed by the antenna device and to fig. 6, which schematically shows a return loss diagram for the antenna device according to the first variation . The length LI of the loop element 27, which is also the circumference of the loop, is here selected to provide loop resonance at a first fundamental

frequency f₁₀ in a first frequency band Bl and at a first harmonics frequency f in a second frequency band B2. The first frequency band may here be the 900 MHz frequency band, while the second frequency band may be the 1710 - 2170 MHz band. The first band is here a lower band of medium width, while the second band B2 a higher band of higher width. The loop element also has a second harmonics frequency f₁₂.The second harmonics frequency is here the second order harmonics frequency. This latter frequency is

furthermore provided outside and also above the second frequency band. It may for instance lie at about 2400 MHz For a first wavelength λ₁ at which this loop resonance occurs, the first fundamental frequency f₁₀ is provided at λ_χ/2 , the first order harmonics frequency f_n provided at λ₁ and the second order harmonics frequency f₁₂ at 3λ₁/2. These frequencies apply when the loop element operates in a loop antenna mode. LI is then selected to be λ₁/2. All these frequencies are thus frequencies associated with this first wavelength.

From this it can be understood that the length LI of the loop element is selected to provide loop resonance at this first wavelength. One of these resonance frequencies is furthermore selected to be a base resonance frequency. A base resonance frequency is a frequency, which is to be used as a basic building block in order to cover a desired frequency band. To this base resonance frequency is then added one or more assisting resonance frequencies for providing the frequency band coverage. In the example given here the second frequency band B2 is this desired frequency band and the first order harmonics of the first wavelength is the base resonance frequency. In the first variation of the invention there are two

assisting resonance frequencies, while in the second variation of the invention there is only one assisting resonance frequency. In this first variation of the invention, the second order harmonics associated with the first wavelength is used for providing one such assisting resonance frequency.

The fundamental frequency described above provides sufficient coverage of the first band Bl. However, the first order harmonics frequency is not able to cover the second band B2 by itself. Something has to be done .

Through placing the first capacitance CI at the first position PI, the loop element is divided into the first and the second sections SI and S2, where the second section has a length L2 and the first section SI has a length L3. The loop element 27 is thus divided into a first section SI stretching between the feeding end and the first position PI and a second section S2 stretching between the first position PI and the grounding end. The second section S2 has an inductance L_L2 that is dependent on the length L2. This furthermore means that as the first capacitance CI and the grounding end of the loop are connected to ground there is created a resonance circuit made up of the first capacitance CI and the inductance L_L2 of the second section S2 of the loop element 27.

This resonance circuit has a resonance frequency f_rc determined through ( 27t*SQR(L_L1C₁ ) ) ^_1. The resonance circuit therefore provides resonance in a frequency range covering the resonance frequency f_rc. In this range the functioning of the loop element 27 is changed. This means that in this range the loop element 27 no longer acts as if the second end is grounded. Instead it is acting as an open-ended monopole element. It here operates in a monopole mode where it may act as a long monopole element. More particularly, it is in fact the first section SI of the element 27 that acts as a monopole element. According to the invention the first position PI and the first capacitance CI are selected for the

resonance frequency f_rc of the resonance circuit to lie in said desired frequency band, i.e. to here lie in the second frequency band B2. According to the invention the length L3 of this first section is then selected for providing a further resonance. The first position PI is thus selected for giving the first section SI a length L3 for providing the loop element with a monopole resonance at a further frequency and here this further frequency is in the second frequency band B2.

For a second wavelength λ₂ associated with this

monopole resonance, the first fundamental frequency f₂₀ is provided at λ₂/4, the first order harmonics

frequency f₂₁ provided at λ₂/2 and the second order harmonics frequency f₂₂ at 3λ₂/4. The length L3 is in this second embodiment selected to correspond to λ₂/4. If then one of these frequencies f₂₀, f₂₁ or f₂₂ lies in the frequency range of the resonance circuit, the loop element will have a further resonance, a monopole resonance, at this frequency. In this embodiment of the invention the frequency that is selected to lie within the frequency range is the second order

harmonics frequency f₂₂. It can thereby be seen that the first position PI is selected for giving the first section SI a length L3 providing the loop element with a monopole resonance at a second wavelength λ₂, which second wavelength λ₂ is selected so that one resonance frequency, here the second harmonics frequency f₂₂, associated with this second wavelength lies within the frequency range of the resonance circuit. In this way a first assisting resonance frequency is provided, which first assisting frequency in this example is the second harmonics frequency f₂₂. This first assisting resonance frequency thereby assists the base resonance frequency f_n in the coverage of the desired frequency band. Put slightly differently, the position of the first point PI on the loop element 27 and thereby also the length L3 is selected for the second order monopole harmonics resonance frequency f₂₂ of the first section SI to lie in the frequency range where the resonance circuit resonates. In this embodiment this first assisting resonance frequency f₂₂ is furthermore set to be equal to the resonance frequency f_rc of the resonance

circuit .

This first assisting frequency is here furthermore selected so that it lies in the second band B2

adjacent a loop resonance frequency, i.e. a resonance frequency associated with the first wavelength and thus the coverage of the second band B2 is improved. The length of the first section is thus chosen for providing the first assisting resonance frequency close to a loop resonance frequency associated with the first wavelength in the desired frequency band. In this way improved wideband coverage in the second band B2 is obtained.

According to the first embodiment of the present invention this coverage is improved even further through the use of the second capacitance C2. As mentioned above, the second order harmonics frequency f₁₂ of the basic loop element lies outside of the second band B2. According to the first variation of the invention this frequency is shifted through the use of the second capacitor C2 such that it will appear in the second band B2. This means that in the present example, the second capacitance C2 is selected to have a value that causes the second order harmonics frequency to be shifted into to the second band B2. The value of the second capacitance C2 is thus

selected to provide a shift of the second order harmonics frequency f₁₂ into the second frequency band. In this way this frequency is made into a second assisting resonance frequency assisting in the

coverage of the desired frequency band.

The second capacitance is here furthermore selected so that the second assisting resonance frequency is shifted to lie adjacent at least one other resonance frequency contributing to the coverage of the desired frequency band, in this example adjacent either the base resonance frequency or the first assisting resonance frequency.

In the first variation of the invention it is shifted to lie adjacent both, through being shifted to lie in- between the first harmonics frequency of the loop resonance and the first harmonics frequency of the monopole resonance. Thus in this example, the first position PI and first and second capacitances CI and C2 are selected such that the second assisting

resonance frequency is placed between the base

resonance frequency and the first assisting resonance frequency. This can be seen in fig. 6.

In this way it is possible to provide a very wide second band with a limited number of components. This also means that the antenna device is easy to produce and the production costs are low. The antenna device can also be kept small.

Here it should be mentioned that it is possible to vary which resonance is to lie beside which. It is for instance possible that the first assisting resonance frequency is provided in the middle of the band as well as to provide the base resonance frequency in the middle. It is furthermore possible to select other frequencies associated with the first and second wavelengths to be used as assisting resonance

frequencies, for instance harmonics resonance

frequencies of other orders. The base resonance frequency may therefore also be another harmonics frequency than of the first order. It may also be a fundamental frequency as may the first assisting resonance frequency. From this it can be understood that the present invention does not require the coverage of the above-mentioned first frequency band. There may therefore be only be one band covered. The invention is furthermore not limited to the two specific bands described above, but can be applied on any frequency bands.

As mentioned above the first and second capacitances can be provided in different ways. In a second

embodiment of the invention shown in fig. 7, they are provided as plates 32 and 34 attached to the loop element 27 at the positions PI and P2, which plates stretch towards the ground plane 26. The capacitances CI and C2 are then determined through the widths of these plates and the distance between the plates and the ground plane.

According to a third embodiment of the invention they can also be provided through a bending of the loop structure as can be seen in fig. 8. A part 36, 38 of the loop element 27 at each end is thus placed at a distance from a ground plane, where the width of the part and the distance to the ground plane determines the capacitance.

In the drawings the first and the second capacitances are shown as provided symmetrically on two sides of the middle of the loop element. It should be realized that this is in no way any requirement. It should furthermore be understood that the realization of a first capacitance according to any of the above mentioned embodiments can be combined with the realization of the second capacitance according to any of the other embodiments .

It should also be known that according to a second variation of the invention the present invention can be realized without shifting the second harmonics frequency of the loop resonance. Then of course the second capacitance is omitted. This is schematically shown in fig. 9, which in all other respects is similar to fig. 3. A number of different embodiments and variations of the invention have been disclosed. It should here be realised that these are but just a few ways in which the present invention may be varied. Therefore the invention is only to be limited by the following claims.

Claims

1. An antenna device (22) for operation in at least one desired operational frequency band (B2), said antenna device (22) comprising: a loop element (27) having a feeding end for

connection to a radio communication circuit (24) and a grounding end for connection to ground (26), wherein the length (LI) of the loop element between the feeding and grounding ends is selected to provide loop resonance at a first wavelength, where one resonance frequency (f_n) associated with this first wavelength is a base resonance frequency for providing coverage of said desired frequency band, and a first capacitance (CI) provided between a first position (PI) on the loop element and ground, thereby dividing the loop element into a first section (SI) stretching between the feeding end and the first position and a second section (S2) stretching between the first position and the grounding end, wherein - the second section has an inductance (L_L2)

depending on the length (L2), said inductance forming a resonance circuit together with the first capacitance (CI), which resonance circuit has a resonance frequency causing the loop element to function as a monopole element in a frequency range, where the first position (PI) and the first capacitance (CI) are selected for the frequency range to lie in said desired frequency band, and - the first position (PI) is selected for giving the first section (SI) a length (L3) providing the loop element with a monopole resonance at a second wavelength, which second wavelength is selected so that one resonance frequency (f₂₂) associated with this second wavelength lies within said frequency range in order to provide a first assisting resonance frequency, which assist in the coverage of said desired frequency band.

2. The antenna device according to claim 1, wherein the resonance frequency of the resonance circuit and said first assisting resonance frequency are the same.

3. The antenna device according to claim 1 or 2, wherein the length of the first section is chosen for providing said first assisting resonance frequency close to a loop resonance frequency associated with the first wavelength in the desired frequency band.

4. The antenna device according to any previous claim, wherein the first assisting resonance frequency is a harmonics frequency associated with the second wavelength.

5. The antenna device according to any previous claim, wherein the base resonance frequency is a harmonics frequency associated with the first

wavelength.

6. The antenna device according to any previous claim, wherein a second assisting resonance frequency (fi₂) associated with the first wavelength lies outside said desired frequency band and further comprising a second capacitance (C2) provided at a second position (P2) of the loop element between the feeding end and the first position (PI) and selected to shift this second assisting resonance frequency into to the desired frequency band for assisting in the coverage.

7. The antenna device according to claim 6,

wherein the second capacitance is selected so that the second assisting resonance frequency is shifted to lie adjacent at least one other resonance frequency contributing to the coverage of the desired frequency band.

8. The antenna device according to claim 7,

wherein the first position and first and second capacitances are selected such that the second

assisting resonance frequency is placed between said base resonance frequency and said first assisting resonance frequency.

9. The antenna device according to any previous claim, wherein at least one capacitance is realized through a capacitor component (28, 30).

10. The antenna device according to any previous claim, wherein at least one capacitance is realized through placing a part (36, 38) of the loop element at a distance from a ground plane (26) causing the capacitance to occur.

11. The antenna device according to claim 10, wherein said capacitance is determined through the width of the part and the distance to ground

12. The antenna device according to any previous claim, wherein at least one capacitance is provided through a plate (32, 34) being attached to the loop element and stretching towards the ground plane (26).

13. The antenna device according to claim 12, wherein said capacitance is determined through the width of the plate and its distance to ground.

14. The antenna device according to any previous claim, wherein the loop element is operable in a first (Bl) and a second (B2) frequency band and said desired frequency band is the second frequency band, the length (LI) of the loop element between the feeding and grounding ends is selected to provide loop

resonance at a fundamental frequency (f₁₀) associated with the first wavelength in the first frequency band and at a first harmonics frequency (f_n) in the second frequency band, where the base resonance frequency is said first harmonics frequency.

15. A portable radio communication device (10) comprising in its interior, an antenna device (22) according to any of claims 1 - 14, a ground plane (26) and a radio communication circuit (24) connected to the antenna device.