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. The demand for various types of communication needed in a portable radio communication device is

increasing. Today it is often necessary to cover several cellular frequency communication bands, near field communication like Bluetooth, positioning like GPS and radio like FM. Examples on other types of possible communications is television like DBM.

However, the portable radio communication devices are getting increasingly smaller and thus the space available for different frequency bands is getting more and more limited. There is therefore a need for combining the radiator elements of an antenna device for operation in

different frequency bands and for different types of radio communication technologies. This combination should also allow simultaneous use of the radiator elements, which is not so easy to do, since they will often interfere with each other. These devices do in many cases not allow provision of resonance in the same band. There is therefore a need for providing an antenna device which allows the simultaneous use of the same radiator element for simultaneous operation in

different frequency bands and different communication technologies . 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 combines simultaneous use in two different frequency bands with a small size. The invention is based on the realization that a radiator structure being connected to the first ends of a set of capacitors, the second ends of which are provided at ground potential at least for a first frequency band, and where the sum of the values of the capacitors in the set is below 15 pF, provides

adequate performance in the two bands together with a small size.

According to the present invention there is provided an antenna device for operation in at least a first lower and a second higher frequency band, the antenna device comprising: a radiator structure, the radiator structure comprising at least one first radiator element and having a first and a second end, a first feeding connection coupling the radiator structure to a first radio circuit for operation in the first frequency band, a second feeding connection for coupling the radiator structure to a second radio circuit for operation in the second frequency band, and a set of capacitors connected to the radiator structure and including at least one capacitor, where a first end of each capacitor in the set is connected to the radiator structure and a second end is provided at ground potential, at least for the first frequency band, and the sum of the values of the capacitors in the set is below 15 PF.

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

The antenna device according to the invention provides operation with fair performance in both a first lower and a second higher frequency band. This is

furthermore done with a small amount of electrical components and radiator 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.

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 radio communication device according to the present

invention; Fig. 3 schematically shows the antenna device

according to a first embodiment of the invention together with two radio circuits,

Fig. 4 schematically shows some parts of the antenna device according to the first embodiment of the invention together with the radio circuits on the circuit board as well as a ground plane of the circuit board;

Fig. 5 schematically shows the antenna device

according to a second embodiment of the invention together with two radio circuits, and

Fig. 6 schematically shows some parts of the antenna device according to the second embodiment of the invention together with the radio circuits on the circuit board and ground plane.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments 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 simultaneously receive and/or transmit 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 with which the present invention is concerned 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. 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, the keypad and the speaker 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 first radio circuit 24, here an FM radio circuit and a second radio circuit 26, here a cellular radio circuit. The circuit board 20, which may be a multi-layer PCB

(printed circuit board), furthermore includes a ground plane (not shown).

Fig. 3 schematically shows the antenna device

according to a first embodiment of the invention, in the form of a dashed box 22, including a radiator elements and a number of electric components in the form of filters, capacitors and matching units. The connection of the antenna device to the first and the second radio communication circuit 24 and 26 is also shown .

Fig.4 schematically shows the radiator elements of the antenna device according to the first embodiment being connected to the radio circuits 24 and 26. However, in fig. 4 the above mentioned electric components have been omitted in order to provide a better

understanding of the physical placing of the radiator elements on the board 20. In the figure the placing of the ground plane is also shown.

Now the first embodiment of the invention will be described.

The antenna device 22 of the present invention is provided for operation in at least a first lower and a second higher frequency band, where the lower

frequency band in this embodiment is the FM frequency band, which is 88 - 108 MHz in Europe and 76 - 110 MHz in the USA, and the higher band is a cellular

frequency band for instance the GSM 850 or 900 band. It should however be realized that it is possible to operate the antenna device in other bands such as GSM 1800 and 1900 MHz in for instance GSM, WCDMA or LTE, as well as in Bluetooth and GPS bands. One of the bands could also be a DVB band in about 400 — 800 MHz. The antenna device according to the first embodiment is provided for coupling of a radiator structure to two separate radio circuits, and therefore it includes a first feeding connection FC1 between the radiator structure and the first radio circuit 24 and a second feeding connection FC2 between the radiator structure and the second radio circuit 26. The radiator

structure is in this first embodiment solely made up of a first radiator element RE1 and has a first and a second end, where the first end is to receive radio signals. The first feeding connection FC1 here

includes a first low pass filter LP1 connected between the radiating structure and the first radio circuit 24. This low pass filter LP1 is in this first

embodiment provided as an inductor, for instance of 100 nH, connected in series between the first end of the radiator structure and the first radio circuit 24. This filter has the function to block signals in and above the second frequency band and to allow signals in and below the first frequency band to pass. There is here furthermore also a matching unit MU, here in the form of an inductance, for instance having a value of above 200 nH. This is provided for matching the radiating element of the antenna device to the

frequencies used in the first frequency band. The matching unit is connected between the first end of the radiator structure and ground.

The second feeding connection FC2 includes a series- connected first capacitor CI between the first end of the radiator structure and the second radio circuit 26. The first capacitor CI thus has a first end coupled to the radiator structure and a second end coupled to the second radio circuit 26. The first capacitor CI here allows signals in the second

frequency band to pass through the second feeding connection FC2 while assisting in stopping signals in the first frequency band from reaching the second radio communication circuit 26. The second feeding connection also includes a second low pass filter LP2 connected between the second feeding connection and ground. More particularly, it is connected between the second end of the first capacitor CI and ground. This second low pass filter LP2 is here also provided in the form of an inductor, typically of about 10 nH, in order to provide ground for the first frequency band and also in order to provide electrostatic Discharge (ESD) protection of the antenna device. Thus the provision of the second low pass filter LP2 ensures that the second end of the first capacitor CI is provided at ground potential for the first frequency band. However in the second frequency band and at higher frequencies it is not. Through the provision of the second low pass filter LP2 the first capacitor CI becomes a shunt capacitor for the first frequency band but a series capacitor for the second frequency band. The first capacitor CI may furthermore act as a pure conductor for these higher frequencies, i.e. act as a short-circuit.

The first radiator element RE1 of the radiator

structure is here shaped for providing resonance. It may as an example be shaped as a planar rectangular element, which provides resonance with the help of the matching unit MU for the first frequency band. The first end of radiator structure, which is also the first end of the first radiator element RE1 is here coupled to the two feeding connections FCl and FC2 via a common feeding connection CC, here in the form of a thin conductor, and at the second end, which is also the second end of the first radiator element RE1, optionally coupled to ground via a second capacitor C2. This second capacitor C2 is designed to ground the second end of the structure for signals in the second and higher frequency bands. The common feeding

connection CC is provided at right angles to the longitudinal extension of the radiator structure.

As can be seen in fig. 4, the radiator structure is here essentially provided along the whole length of a first, short side of the circuit board 20, and the area where it is provided does optionally not have any ground plane, while the common feeding connection is provided at right angles to this first side and the radiator structure and stretches along a second, long side of the circuit board. In the first embodiment there is, as was mentioned above, only one radiator element in the structure and it stretches along the full length of the first short side. This means that the radiating element operating in the first frequency band stretches along the whole first short side, is being fed along the second long side and is thus provided above a part of the circuit board optionally lacking ground plane. This improves the performance if operating in an FM band.

In the first embodiment the whole radiator structure is used in both bands simultaneously. The second optional capacitor C2 is here a shunt capacitor for the radiator structure and also makes the radiator structure electrically floating in the first frequency band. It is thus not connected to any potential at this point. This means that the radiator structure in this embodiment functions as a monopole element in the first frequency band, which is matched to an operating frequency by the matching unit MU, i.e. it is matched for obtaining resonance in this range. The first low pass filter LPl furthermore makes the first capacitor CI a shunt capacitance in relation to the first frequency band and also provides ESD protection and stops radio signals in the first frequency band from reaching the second radio circuit. In this way radio signals can be received and transmitted to and from the first radio circuit via the radiator structure in the first frequency band, where the first low pass filter LPl ensures that signals in the second and higher frequency bands do not reach the first radio circuit 24.

The first radio circuit 24 may here include an

amplifier, for instance a low noise amplifier for amplifying the radio signals in the first frequency band. Because of this the radiator structure with components and this amplifier may be considered to be a so-called active antenna. The radiator structure, shunt capacitors at the first and second ends and the inductor of the matching unit may here be selected to provide a resonance circuit having an impedance close to the optimal noise impedance S _opt of the low noise amplifier at the first frequency band. The impedance of the radiator structure and the impedance of the amplifier may therefore be matched to each other at an impedance considerably higher than the impedance of 50 Ω normally provided for electrical circuits.

The shunt capacitances give a slight degradation of the performance in the first frequency band compared to operation without the capacitors. However, this is acceptable considering the fact that at the same time it is possible to combine the antenna device with use also for the second frequency band. If the sum of the shunt capacitances connected to the radiator structure is 15 pF a 5 db eae (effective antenna efficiency) performance degradation is obtained, while if the sum is 10 pF a 3dB performance degradation is obtained. However, these values are acceptable in most

applications . It can therefore be seen that the shunt capacitors connected to the radiator structure should together have a capacitance below 15 pF. This means that the sum of the capacitances of these capacitors have to be below this value. When the second capacitor C2 is present the radiator structure of the first embodiment is in the example shown in fig. 3 provided as a loop or rather a half loop antenna when operating in the second frequency band. This is because in this embodiment the second end of the radiator structure is grounded. If the second capacitor has another placing along the

structure, the structure may instead act as a PIFA or IFA antenna in the second frequency band, while if the second capacitor is missing the structure will

function as a monopole antenna also in the second band.

In the second frequency band the radiator structure is in this example in fig. 3 thus made to operate as a magnetic dipole for radio signals transmitted to and from the second radio circuit 26. In a similar way the combination of first capacitor and second low pass filter here ensures that signals in the first

frequency band do not reach the second radio circuit.

There are number variations that are possible to make of the first embodiment. It is possible that the matching unit is omitted. In this case it is possible that the first radio circuit may need to include an amplifier. It is here also possible that the second capacitor is omitted. The above-described amplifier may of course also be omitted from the first radio circuit. Also the second low pass filter and the second capacitor may be omitted.

Fig. 5 and 6 show a second embodiment of the antenna device according to the invention that provided in the same way as fig. 3 and 4.

The difference from the first embodiment is that the radiator structure comprises two radiator elements RE1 and RE2 provided along the short edge, i.e. the first side, of the circuit board. These radiator elements RE1 and RE2 are interconnected via a third low pass filter LP3, which can be realized in the form of an inductor, typically having a value of 30 nH, which low pass filter is arranged to let signals in the first frequency band to pass and to block signals in the second frequency band. In this second embodiment there is furthermore a parasitic element PE, placed along the common feeding connection CC along the second long side of the circuit board, it is thus provided at essentially right angles to the first and second radiator elements RE1 and RE2. This parasitic element PE is here connected to ground via a third capacitor C3. The capacitor C3 is set for grounding the

parasitic element PE in the second and higher

frequency bands but not in the first frequency band. This parasitic element has the function of providing high band resonance.

In the second embodiment the first and second radiator elements RE1 and RE2 are together provided all along the short side of the circuit board. They are thus connected in series after each other along this short side for providing operation in the first frequency band, where the third low pass filter LP3 ensures that the first and second radiator elements are considered as one single element. They can therefore together be considered as one element for providing resonance in the first frequency band. However the filter LP3 also ensures that the second radiator element RE2 does not contribute to the operation in the second frequency band. Therefore only the first radiator element RE1 contributes to resonance in this band. It can thus be seen that in this second embodiment the second end of the first radiator element RE1 is floating or not connected to any potential in relation to the second frequency band. Thus in this embodiment the radiator structure functions as a monopole element in both the first and the second frequency band.

Also here the radiator elements stretch along the whole of the short side and are fed from the long side as in the first embodiment. Also here the sum of the shunt capacitors connected to the radiator structure is below 15 pF. However, in this embodiment there is only one such capacitor, the first capacitor CI, and therefore it is enough that this capacitor has a value below 15 pF.

There are a number of variations that can be made also of this second embodiment. Also here the matching inductor can be removed and the first radio circuits include an amplifier or be provided without. It is here also possible to add the second capacitor, provide or omit an amplifier in or from the first radio circuit as well as to omit the second low pass filter. It is of course also possible to remove the parasitic element.

Here it is also possible that the radiator structure is used as a PIFA antenna for the cellular bands. In this case there might be needed a further ground connection at the first end of the radiator structure, which should also be provided via a capacitor. In this case also this additional capacitor has to be

considered in the comparison with the limit of 15 pF. In case the radiator structure is not DC grounded, it is possible to provide such DC grounding through a large resistor, typically of about 560 1.Ω, between the radiator structure and ground. This resistor may be connected to the first end of the radiator structure. 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. It should for instance be realized that the radiator structure need not be stretching along a short side. It may just as well be stretching along a long side of the circuit board. The second feeding connection need not be made coupled to the radiator structure at the first end. It can be coupled anywhere along the length of the radiator structure between the first and the second ends. It is furthermore possible to connect more radio circuits to the radiator structure. In this case they should be connected with a feeding

connection including a series-capacitor and preferably with means for setting the second end of this

capacitor connection to ground in the first frequency band, for instance using a low pass filter. Then this capacitance would have to be considered when

performing the above-mentioned summing.

Therefore the invention is only to be limited by the following claims.