FIELD OF INVENTION

The present invention relates generally to antenna devices and more particularly to an antenna device for a portable radio communication device, such as a mobile phone or similar 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 generally 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 HAC requirement research has been made into alternative antennas.

One antenna type that is promising in this regard 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 generally for use in cellular communication, like Long Term Evolution (LTE), are to cover a number of wide frequency bands, where a first operational frequency band is around 700 MHz and a second frequency 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 multi-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 antenna device for a portable radio communication device, which provides good frequency band coverage based on a loop element. A way of realizing multiple frequency band coverage of an antenna device is to use a series-switched loop radiator, as illustrated in Fig. 1. Such an antenna device comprises a loop element 1 having a grounding end and a feeding end.

The grounding end is connected to ground. The feeding end is connected to radio communication circuitry 4 through a series inductor 2, typically called a bypass inductor. The feeding end also comprises a shunt inductor 10 as well as ESD protection 9 arranged parallel with the shunt inductor. Further, switching means 3 is arranged parallel with the series inductor 2, to provide multiple matching of the loop element 1. The switching means 3 is in a first state configured to connect the feeding end to the radio communication circuitry 4 through a series inductor 5, and in a second state configured to connect the feeding end to an open end. The open end of the switching means will intrinsically have a parasitic capacitance 8 coupling the feed point to ground. Both the input and outputs of the switching means are preferably provided with DC blocking capacitors 6 and 7.

This solution however has some drawbacks. The parasitic capacitance 8 short-circuits the antenna device for high frequencies, typically from about 2 GHz. To reduce this influence the shunt inductor 10 is provided, making the bypass inductor 2 large and creating a large voltage swing over the switching means at low frequencies, for e.g. a desired 700 MHz . The large voltage swing over the switching means will generate harmonics due to non-linearity in the switching means and will also in turn require ESD protection at the feeding end.

The present invention is based on the realization that by moving the switching means from the feed end of the loop element to the ground end thereof all drawback mentioned above are mitigated and 1-2 components can be removed.

According to the present invention there is provided an antenna device for operation in at least at least two operational frequency bands, the antenna device comprising: a loop element having a feeding end for connection to radio communication circuitry and a grounding end for connection to ground, first filtering means connecting the grounding end to ground, and switching means provided parallel with the first filtering means, wherein the switching means is configured to in a first state connect the grounding end to ground parallel with the first filtering means to match the loop element to a first operational frequency band of the at least two operational frequency bands, and in a second state connect the grounding end to an open end parallel with the first filtering means to match the loop element to a second operational frequency band of the at least two operational frequency bands, which removes the need of a component for the antenna device, i.e. the shunt inductor, and generally another component for the antenna device, i.e. ESD protection. For additional matching of the loop element the antenna device preferably comprises second filtering means provided between the ground and the first state of the switching means. The first and second filtering means preferably each comprises a series inductor, for simple matching.

For further alternative matching of the loop element the antenna device preferably comprises fourth filtering means connecting a third state of the switching means to ground, to provide matching to a third frequency band.

The invention is also directed towards a portable radio communication device. Further preferred embodiments are defined in the dependent claims. BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from the detailed description of embodiments given below and the accompanying figures, which are given by way of illustration only, and thus, are not limitative of the present invention, wherein:

Fig. 1 illustrates an antenna device having a series-switched loop radiator.

Fig. 2 illustrates an antenna device according to a first embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

In the following description, for purpose of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent for a person skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed description of well- known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details.

An antenna device for operation in at least at least two operational frequency bands in a portable radio communication device according to a first embodiment of the present invention will now be described with reference to Fig. 2.

The antenna device comprises a loop element 1 having a feeding end and a grounding end. The feeding end is connected to radio communication circuitry 4. The grounding end is connected to ground through first filtering means 2, acting as a bypass filter for switching means 3. The switching means 3 is provided parallel with the first filtering means 2, wherein the switching means 3 is configured to in a first state connect the grounding end of the loop element 1 to ground parallel with the first filtering means 2 to match the loop element 1 to a first operational frequency band of the at least two operational frequency bands. In a second state the switching means 3 is configured to connect the grounding end of the loop element 1 to an open end parallel with the first filtering means 2 to match the loop element 1 to a second operational frequency band of the at least two operational frequency bands.

The antenna device intrinsically comprises a parasitic capacitance 8 parasitically connecting second open state of the switching means 3 to ground. A typical parasitic capacitance is in the order of 0.5-2 pF.

The antenna device preferably comprises second filtering means 5 provided between ground and the first state of the switching means 3.

Further, the antenna device preferably comprises DC blocking means 6 and 7 arranged on the input and outputs of the switching means 3, preferably realized as series capacitors of about 100 pF. For configuration of an antenna device to provide operation in the LTE operational frequency band 700 and the cellular operational frequency bands 850, 1800, 1900 and 2100 the following component values are e.g. used. The loop element 1 has an electrical length corresponding to λ for 1850 MHz The first filtering means comprises a series inductor of about 13 nH. The second filtering means comprises a series inductor of about 0 nH. The switching means 3 is a SP4T switch with one input and four outputs, one output for each of four states of the switch. In the first state of the switching means frequency band coverage of cellular operational frequency bands 850, 1800, 1900 and 2100 are thus provided, and in the second state of the switching means frequency band coverage of the LTE 700 is thus provided.

For improved antenna efficiency further switching states of the switching means 3 is preferably provided. The antenna device, in such a case, comprises fourth filtering means provided between ground and a third state of the switching means, for matching of the loop element 1 to a third frequency band. In the third state the switching means is configured to connect the grounding end of the loop element 1 to ground parallel with the first filtering means 2 to match the loop element 1 to a third operational frequency band. The fourth filtering means preferably comprises a series capacitor of about 2.7 pF. In the third state of the switching means frequency band coverage of cellular operational frequency bands 900, 1800, 1900 and 2100 are thus provided.

Although series inductors have been described as realization means for matching of the loop element, series (or grounded parallel) capacitors could alternatively be used.

An important advantage of series-switching the grounding end of the loop element, compared to series-switching the feeding end of the loop element, is that the impact of the parasitic capacitance 8 is reduced thereby mitigating the impact on high frequency bands, which in turn removes the need for a shunt inductor at the loop element, in turn reducing the bypass inductor value. This also entails a significantly improved ESD protection, generally removing the need of additional ESD protection at the loop element.

It will be obvious that the present invention may be varied in a plurality of ways. Such variations are not to be regarded as departure from the scope of the present invention as defined by the appended claims. All such variations as would be obvious for a person skilled in the art are intended to be included within the scope of the present invention as defined by the appended claims.