EP2270925A1

EP2270925A1 - Antenna device and portable radio communication device comprising such an antenna device

Info

Publication number: EP2270925A1
Application number: EP09008751A
Authority: EP
Inventors: Stefan Irmscher; Christian Braun; Johan Bäckman
Original assignee: Laird Technologies AB
Current assignee: Laird Technologies AB
Priority date: 2009-07-03
Filing date: 2009-07-03
Publication date: 2011-01-05

Abstract

The present invention relates to an antenna device for a portable radio communication device operable in at least a first high-frequency band and a second low-frequency band. The antenna device comprises: an on-ground radiating element (2) having at least a first port (3) for feeding of the first high-frequency band and a second port (4) for feeding of the second low-frequency band; first selecting means (7) connected to the first port, and second selecting means (8) connected to the second port; the first selecting means is configured to open-end (P1) the first port and the second selecting means is configured to connect the second port to a second matching network (P6, P7, P8) through which the second low-frequency band is fed (10), when the antenna device operates in the second low-frequency band; and the second selecting means is configured to ground (P5) the second port and the first selecting means is configured to connect the first port to a first matching network (P2, P3, P4) through which the first high-frequency band is fed (9), when the antenna device operates in the first high-frequency band.

Description

FIELD OF INVENTION

The present invention relates generally to antenna devices and more particularly to an antenna device for a portable radio communication device operable in at least a first high-frequency band and a second low-frequency band. The invention also relates to a portable radio communication device comprising such an antenna device.

BACKGROUND

Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages associated with the use of 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.
In such portable radio communication devices, a desire exists to add support for more and more different radio communication standards. These then typically require different frequency bands. One standard may here also use more than one frequency band.
It is then customary to use a radiating element in the antenna device that resonates in a desired frequency band.
It is then often desired that one antenna device is to be used for communication in many such different frequency bands. The radiating element is then to resonate in more than one frequency band.
This is hard to accomplish in a small portable radio communication device.
A problem in prior art antenna devices is thus to provide a small sized multi-band antenna covering at least a high-frequency band and a low-frequency band while retaining good performance.

SUMMARY OF THE INVENTION

In order for an antenna device to provide a broad bandwidth it should preferably comprise a radiating element without a ground plane, i.e. a so called off- ground antenna. However, due to limited space in a portable radio communication device the antenna device generally is required to have a radiating element positioned over, or at least partly over, a ground plane means, i.e. a so called on-ground antenna which limits the bandwidth it can cover. A further problem with on-ground antennas, particularly for mobile phones, is that many times a so called bar phone has a length of about 90-100 mm, which is inappropriate for antenna operation around 900 MHz.
An object of the present invention is to provide an antenna device that covers at least a first high-frequency band and a low-frequency band while still keeping the overall size of the antenna device small and retaining good performance.
The invention is based on the realization that muliband coverage ability can be provided in a small sized antenna device operable in high and low frequency bands through modifying this antenna device by providing selection means for different feeding of low-frequency band operation and high-frequency band operation, respectively, wherein the antenna device is configured to operate as different antennas for high-frequency bands and low-frequency bands, respectively.
According to a first aspect of the present invention there is provided an antenna device as defined in claim 1.
According to a second aspect of the present invention there is provided portable radio communication device as defined in claim 10.
The present invention provides an antenna device and a portable radio communication device, respectively, wherein the problem of providing an antenna device that covers at least a first high-frequency band and a low-frequency band while still keeping the overall size of the antenna device small and retaining good performance is solved through an antenna device for a portable radio communication device operable in at least a first high-frequency band and a second low-frequency band, the antenna device comprising: an on-ground radiating element having at least a first port for feeding of the first high-frequency band and a second port for feeding of the second low-frequency band; first selecting means connected to the first port, and second selecting means connected to the second port; the first selecting means is configured to open-end the first port and the second selecting means is configured to connect the second port to a second matching network through which the second low-frequency band is fed, when the antenna device operates in the second low-frequency band; and the second selecting means is configured to ground the second port and the first selecting means is configured to connect the first port to a first matching network through which the first high-frequency band is fed, when the antenna device operates in the first high-frequency band.
In order to provide the antenna device with improved bandwidth for multiple high-frequency bands the first selecting means comprises a first open-ended connection point and at least a second and a third connection point providing different matching networks for feeding of the multiple high-frequency bands.
To provide the antenna device with improved bandwidth for multiple low-frequency bands the second selecting means comprises a first grounded connection point and at least a second and a third connection point providing different matching networks for feeding of the multiple low-frequency bands.
The radiating element preferably has a rectangular general outline, to improve the bandwidth by utilizing maximum volume for the antenna device.
The first and second selection means are preferably realized as:

switching means, providing switching between an state that is short-circuited or open-circuited, respectively, and a state that passes the signal to RF circuitry, optionally going through a matching circuit with components, wherein the provided switching further can be extended to include several states of the matching circuit;
filtering means, producing a state that short-circuited or open-circuited, respectively, in one frequency band and in the other frequency band passes the signal to the RF circuitry, optionally going through a matching circuit; or
filtering means, combined with switching means switching between different matching circuits, wherein the filtering means is arranged between the switching means and the radiating element in order to improve for example the open-circuited state.

The first and second selecting means are preferably provided as switches, to improve the bandwidth of the antenna device.
For a robust solution, the antenna device preferably comprises a filter between the second port and ground, configured to pass the first high-frequency band and to block the second low-frequency band.
The first port is preferably connected at or near a top corner of the rectangular general outline of the radiating element to improve the bandwidth of the antenna device.
Advantageously, the first and second ports are connected to positions at the radiating element based on optimization of the Q0 factor for the first high-frequency band and the second low-frequency band.
Further features and advantages of the present invention will be evident from the following description.

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 schematically illustrates an antenna device for a portable radio communication device.
Fig. 2 shows an antenna device for a portable radio communication device according to a first embodiment of the present invention.
Fig. 3 illustrates the antenna device shown in Fig. 2 operable for the portable radio communication device in a high-frequency mode.
Fig. 4 illustrates the antenna device shown in Fig. 2 operable for the portable radio communication device in a low-frequency mode.
Fig. 5 shows Q0 curves for a plurality of low-frequency and high-frequency combinations according to the present invention.
Fig. 6 shows an antenna device for a portable radio communication device according to a second embodiment of the present invention, having low-frequency selecting means configured as switching means with reduced switching loss.
Fig. 7 shows an antenna device for a portable radio communication device according to a third embodiment of the present invention, having selecting means configured as filtering means.
Fig. 8 shows an antenna device, which is a combination of the second and third embodiments of the present invention, illustrated in Figs. 6 and 7.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of an antenna device according to the invention will be given. In the description, for purposes of explanation and not limitation, specific details are set forth, such as particular hardware, applications, techniques etc. 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 methods, apparatuses, and circuits are omitted so as not to obscure the description of the present invention with unnecessary details.
An antenna device according to a first embodiment of the present invention will now be described with reference to Figs. 1-5.
Fig. 1 shows the outlines of an on-ground antenna device in a portable radio communication device, such as a mobile phone or similar device. A radiating element 2 is arranged at the top of the portable radio communication device, over a ground plane means 1, such as the printed circuit board (PCB) thereof. Between the radiating element 2 and the PCB 1 there are two electrical connections through a first port 3 and a second port 4. The electrical connections to the radiating element 2 is in this embodiment on the top of the radiating element 2, but could less preferably be on the sides of the radiating element 2, on the bottom of the radiating element 2 or inside the outline of the radiating element 2. The connection placement can be optimized for maximum bandwidth and can vary depending on size and shape of the radiating element. On the PCB 1 there are provided RF circuitry and grounding devices (not shown) that are connectable to the antenna device of the invention. The length extension of the portable radio communication device is illustrated by arrow 5 and the width extension of the portable radio communication device is illustrated by arrow 6.
Although the radiating element 2 is described as an on-ground radiating element it is not necessary for the radiating element 2 to be completely over the ground plane means 1. As long as at least a major portion of the radiating element is on-ground the negative effects mentioned above is evident.
In Fig. 2, there is shown a general outline of the ground plane means 1 shown in Fig. 1, on which the on-ground radiating element 2 is arranged in the top portion of the mobile phone. Fig. 2 also illustrates first selecting means 7, a switching element, connected to the first port 3 and second selecting means 8, a switching element, connected to the second port 4, both selecting means with related matching components. The first and second selecting means 7 and 8, together with their respective related matching components, are arranged on the PCB 1, but for clarity illustrated flattened out above the general outline of the mobile phone. It is also possible to provide the first and second selecting means on the antenna device, such as on a dielectric carrier supporting the radiating element 2. An advantage with providing selecting means on the antenna device is that a complete antenna module connectable to a PCB of a mobile phone is achieved.
The antenna device is operable in at least a first high-frequency band, e.g. covering GSM1800, GSM1900 and WCDMA, and a second low-frequency band, e.g. covering GSM850 and GSM900. To cover these operating bands the low-frequency band covers 824-960 MHz and the high-frequency band covers 1710-2170 MHz. In order to provide such operation, the antenna device comprises a single radiating element 2 having a first port 3 for feeding of the high-frequency band and a second port 4 for feeding the low-frequency band.
The antenna device utilizes available space efficiently by operating the radiating element 2 as a PIFA antenna, see Fig. 3, for the high-frequency band and operating the radiating element 2 as a patch antenna, see Fig. 4, for the low-frequency band. During operation of the antenna device in the high-frequency band the first port 3 is connected 9 to the RF circuitry through connection point P2, P3 or P4, and the second port 4 is connected to ground through connection point P5. During operation of the antenna device in the low-frequency band the first port 3 is open-ended through connection point P1 and the second port 4 is connected 10, through a connection separate from the high-frequency band feeding, to the RF circuitry through connection point P6, P7 or P8.
The three different connection points to RF circuitry for the first and second selecting means, respectively, represent different tuning and matching elements/networks for the antenna device for operation of high-frequency bands and low-frequency bands, respectively. The different tuning elements/networks tune the radiating element to different centre frequencies for operation at different operating bands. The antenna device is e.g. configured for multiple frequency band operation by having connection point P2 configured to tune the radiating element 2 as a PIFA antenna for Gsy1800 operation, connection point P3 configured to tune the radiating element 2 as a PIFA antenna for GSM1900 operation, connection point P4 configured to tune the radiating element 2 as a PIFA antenna for WCDMA operation, connection point P6 configured to tune the radiating element 2 as a patch antenna for GSM850 operation, connection point P7 configured to tune the radiating element 2 as a patch antenna for GSM900 operation, and connection point P8 configured to tune the radiating element 2 as a patch antenna for LTE operation.
The described selecting means are illustrated as switching elements having four switch positions. The switching element is preferably a single pole, four- throw switching element (SP4T), for instance a GaAs FET device that is reflective in the off state.
The different connection points P2, P3, P4 and P6, P7, P8, respectively, defines, apart from dedicated tuning for different operating bands, different matching networks between the radiating element and the RF circuitry of the portable radio communication device, such that the antenna device can be optimized for a plurality of high-frequency bands and a plurality of low-frequency bands, all utilizing a PIFA structure for the high-frequency bands and a patch structure for the low-frequency bands. Although three different tuning and matching networks have been' described for each selecting means 7 and 8, fewer or more running and matching networks might be used for operation of fewer or more frequency bands.
Further, the matching elements preferably match the connections to the RF circuitry to 50 Ω. Also, in this embodiment the matching and reactive elements are preferably provided as components. These can be lumped components attached to the antenna structure of the antenna device, for instance through soldering to the corresponding contact pads.
The, distance between the first port 3 and the second port 4, as well as their respective position on the radiating element 2 is optimized for the high-frequency band and low-frequency band operation by means of the total best bandwidth, which in turn is calculated by means of Q0 calculations. This calculation also considers the dimensions of the radiating element, such that they provide fundamental resonance in the first high-frequency band and in the second low-frequency band. Q0 represents the quality factor of the antenna at resonance (i.e., the antenna is tuned to zero reactance). The quality factor of an antenna relates the power stored in the reactive field to the radiated power. The bandwidth defined for a certain return loss or voltage-standing-wave-ratio (VSWR) is inversely proportional to Q.
The broadband properties of the antenna device according to this embodiment can be seen in Fig. 5, which shows the Q0 factor for the antenna device in dependence of frequency in GHz. In the figure there are four curves. A first curve LB1 shows the Q0 factor for low-frequency operation for an antenna device having a radiating element primarily sized for high-frequency band operation. A second curve LB2 shows the Q0 factor for low-frequency operation for an antenna device having a radiating element primarily sized for low-frequency band operation. A third curve HB1 shows the Q0 factor for high-frequency operation for an antenna device having a radiating element primarily sized for high-frequency band operation. Finally, a fourth curve HB2 shows the Q0 factor for high-frequency operation for an antenna device having a radiating element primarily sized for low-frequency band operation.
The optimal bandwidth for low-frequency band operation is provided in the dashed region of LB, and the optimal bandwidth for high-frequency band operation is provided in the dashed region of HB. By having the radiating element selectively working in two different antenna modes, for low-frequency bands and high-frequency bands, respectively, the antenna device can work both in the LB and HB region. The best bandwidth for the antenna device is achieved with optimization of the distance between the first port 3 and the second port 4 and the position of the first port 3 and the second port 4 on the radiating element. The first port 3, for high-frequency band operation is preferably positioned on the top edge of the radiating element 2, close to the side of the radiating element 2. This optimization is based on numerical analysis for thousands of variations.
The bandwidth of the antenna device increases for a wider radiating element, wherein a mobile phone typically having a width of about 40 mm, thereby creating a maximum width of the radiating element. The bandwidth of the antenna device is optimized for a radiating element having a length of about 15-20 mm, wherein a shorter radiating element decreases the bandwidth and a longer radiating element also decreases the bandwidth. These calculations are based on that the radiating element is arranged at a height of about 4 mm above the ground plane means.
The radiating element have been shown as having a rectangular-shaped general outline to provide broad bandwidth for both the first high-frequency band and the second low-frequency band, but different shapes of the radiating elements as well as added slots and similar modification is foreseen to fine tune the antenna device or to add further frequency bands. By utilizing as large area as possible, of the available area, maximum bandwidth for the available volume is achieved.
Thus, by providing different behaviour for the antenna device for low-frequency band behaviour and for high-frequency band behaviour, broad band coverage in the desired operating bands is provided for a basic single band structure provided by the basic resonances thereof.
In operation the first selecting means is operated for varying matching of the radiating element in the first high-frequency bands while the second selecting means is grounded. When operating in the low-frequency bands the first selecting means is open-ended while the second selecting means varies matching of the radiating element in the second low-frequency bands.
Although the radiating element has been described as an on-ground radiating element the inventive principle can be applied also for off-ground radiating elements, wherein the radiating element would operate as a monopole antenna for low-frequency bands and as an IFA for high-frequency bands.
An antenna device according to a second embodiment of the present invention will now be described with reference to Fig. 6. The second embodiment is identical to the first embodiment described above, apart from the following.
The first and second selecting means 7 and 8 are configured as SP3T switches'. The first selecting means 7 is configured for open-end in P1, GSM1800 and GSM1900 in P2 and WCDMA in P3. The second selecting means 8 is configured for grounding in P5, GSM850 in P6 and GSM900 in P7. Further, in order to reduce switch losses, an inductor is arranged in parallel with the switch 8.
With an inductor in parallel with the switch the currents are divided between the switch and the parallel inductor, thereby reducing the loss caused by the switch for most configurations. By appropriately setting the values of the components at P6, P7 and the parallel inductor, full tuning range (switching range) can still be kept.
An antenna device according to a third embodiment of the present invention will now be described with reference to Fig. 7. The third embodiment is identical to the first embodiment described above, apart from the following.
The first and second selecting means 7 and 8 are configured as filtering means. The open-ended function for port 3, during low-frequency band operation, can e.g. be provided by a two-pole filter combined with a high-frequency band matching net connected to ground. The grounding function for port 4, during high-frequency band operation, can e.g. be provided by a two-pole filter combined with a low-frequency band matching net connected to ground.
An antenna device according to the third embodiment of the present invention can be combined with an antenna device according to the second embodiment of the present invention, which is illustrated in Fig. 8.
Filtering means 13 is connected between first switching means 7 and the radiating element 2, and is configure to block low-frequency bands and to pass high-frequency bands. In this way also the first switching means can be bypassed by an inductor, reducing switching loss, at the same time providing an open-ended circuit for the low-frequency band.
Filtering means 13 is e.g. realized by a 5 pF capacitor in parallel with a 6 nH inductor. Several similar stages can be cascaded for better performance of the filtering means 13.
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.

Claims

An antenna device for a portable radio communication device operable in at least a first high-frequency band and a second low-frequency band, said antenna device comprising:
a radiating element (2) having at least a first port (3) for feeding of said first high-frequency band and a second port (4) for feeding of said second low-frequency band;

first selecting means (7) connected to said first port, and second selecting means (8) connected to said second port;

said first selecting means is configured to provide an open-ended circuit (P1) at said first port for said second low-frequency band, and said second selecting means is configured to provide a second tuning and

matching network (P6, P7, P8) through which said second low-frequency band is fed (10) at said second port, when said antenna device operates in said second low-frequency band; and

said second selecting means is configured provide grounding (P5) at said second port for said first high-frequency band, and said first selecting means is configured to provide a first tuning and matching network (P2, P3, P4) through which said first high-frequency band is fed (9) at said first port, when said antenna device operates in said first high-frequency band.
The antenna device according to claim 1, wherein said first selecting means is provided as switching means comprising a first open-ended connection point (P1) and at least a second and a third connection point (P2, P3) providing different tuning and matching networks for feeding of said high-frequency band at different centre frequencies.
The antenna device according to claim 1 or 2, wherein said second selecting means is provided as switching means comprising a first grounded connection point (P5) and at least a second and a third connection point (P6, P7) providing different tuning and matching networks for feeding of said low-frequency band at different centre frequencies.
The antenna device according to claim 3 or 4, wherein said switching means comprises a switch having a parallel inductive element.
The antenna device according to claim 4, comprising filtering means (13) between said first port and said first selecting means, configured to block said second low-frequency bands and to pass said first high-frequency bands.
The antenna device according to any previous claim, wherein said radiating element has a rectangular general outline.
The antenna device according to any previous claim, wherein said second selecting means comprises filtering means between said second port and ground, configured to ground said first high-frequency band and to pass said second low-frequency band.
The antenna device according to any previous claim, wherein said first port is connected at a corner of said rectangular general outline of said radiating element.
The antenna device according to any previous claim, wherein said first and second ports are connected to positions at said radiating element based on optimization of the Q0 factor for said first high-frequency band and said second low-frequency band.
A portable radio communication device comprising an antenna device according to any previous claim.