Multiband antenna system

The invention relates to an internal antenna system of a radio device with separate operating bands. The system is intended for use especially in small-sized mobile stations.

In small-sized, mobile radio devices the antenna is preferably placed inside the casing of the device for convenience. This makes the design of the antenna a more demanding task compared to an external antenna. Extra difficulties in the design are caused when the radio device has to function in a plurality of frequency ranges, the more the wider these ranges or one of them are.

Internal antennas most often have a planar structure, in which case they have a radiating plane and a parallel ground plane at a certain distance from it. The radiating plane is provided with a short-circuit and feed point of the antenna. The short-circuit conductor belonging to the structure extends from the short-circuit point to the ground plane, and the feed conductor of the antenna extends from the feed point to the antenna port of the device. For increasing the number of operating bands of the antenna, the radiating plane can be divided into two or more branches of different length as seen from the short-circuit point. The number of bands can also be increased by a parasitic auxiliary element. As an alternative, a parasitic element can be used for widening an operating band by arranging the resonance frequency corresponding to it relatively close to the resonance frequency corresponding to a branch of the radiating plane.

In this description and the claims, the terms "radiating plane", "radiating element" and "radiator" mean an antenna element, which can function as a part transmitting radio-frequency electromagnetic waves, as a part receiving them or as a part which both transmits and receives them. Correspondingly, "feed conductor" means a conductor which can also function as a receiving conductor.

The antennas of the kind described above have the drawback that their characteristics are insufficient when the number of radio systems in accordance with which the radio device must function increases. The insufficiency appears from that e.g. the matching of the antenna is poor in the band used by one of the radio systems or in a part of at least one of such bands. This drawback can be diminished by providing the antenna structure with a switch by which the operating band of a relatively narrow-band antenna can be displaced from the transmitting band of the radio system to the receiving band and vice versa or to a sub-band within the

transmitting or receiving band. However, the switch causes additional losses and thus reduces the efficiency of the antenna. The efficiency of the antenna can thus remain unsatisfactory because of poor matching or switch losses, for example. Said drawbacks are emphasized when the antenna size has to be compromised because of the lack of space. The size is reduced by shortening the distance between the radiating plane and the ground plane or by using dielectric material between them, for example. In addition, these antennas have the drawback that it is difficult to make sufficient isolation between the antenna parts corresponding to different bands.

It is also possible to arrange two radiators in the antenna structure so that they both have a feed conductor of their own. This can be done when the radio device has a separate transmitter and receiver for some radio system. Fig. 1 shows an example of such an antenna structure known from the publication WO 02/078123. It comprises a ground plane 101 , a radiating plane 1 10, a parasitic element 113 of the radiating plane and a segregated radiator 107. The radiating plane has a feed conductor 102 and a short-circuit conductor, and thus it forms a PIFA (Planar Inverted F-Antenna) together with the ground plane. The PIFA has two bands, because the radiating plane is divided into a first 1 11 and a second 1 12 branch as seen from the short-circuit and feed point. The first branch functions as a radiator in the frequency range of the GSM900 (Global System for Mobile communications) system and the second branch in the range of the DCS (Digital Cellular Standard) system. The parasitic element 1 13 is connected to the ground plane and it functions as a radiator in the range of the PCS (Personal Communication Service) system. The segregated radiator 107 has its own feed conductor 103 and short-circuit conductor. Together with the ground plane it forms an IFA, which functions as a Bluetooth antenna. The segregated radiator is located near the radiating plane and its parasitic element so that the short-circuit and feed conductors of the radiating plane, the short-circuit conductor of the parasitic element and the short-circuit and feed conductors of the segregated radiator are in a row in a relatively small area compared to the dimensions of the antenna structure. The support structure of the antenna elements is not visible in the drawing.

The segregated radiator mentioned above, provided with its own feed, is thus for the Bluetooth system. Such a radiator can similarly be e.g. for the WCDMA (Wideband Code Division Multiple Access) system. In general, the use of a segregated radiator provided with its own feed reduces the drawbacks mentioned above to

such an extent that the matching can be made good at least in the frequency range of the radio system for which the segregated radiator is provided.

The use of dielectric material for reducing the physical size of the antenna was mentioned above. Fig. 2 shows an example of such a known antenna. This com- prises a dielectric substrate 21 1 , a radiator 212 and its feed element 213. The radiator and the feed element are conductor strips on the surface of the substrate. All three together form an antenna component, which is mounted on the circuit board PCB of a radio device.

The object of the invention is to reduce the above-mentioned drawbacks of the prior art. The arrangement according to the invention is characterized in what is set forth in the independent claim 1. Some preferred embodiments of the invention are set forth in the other claims.

The basic idea of the invention is the following: The antenna system of a multiband radio device is implemented as internal and decentralized in a way that the device has a plurality of separate antennas. Each antenna is based on (a) radiating elements) on the surface of a dielectric substrate. The substrate can be, for example, a piece of ceramics or a part of the outer casing of the device. The antennas are located at suitable places in the device. The operating band of an individual antenna covers the frequency range used by one radio system, the frequency ranges close to each other and used by two different radio systems or only the transmitting or receiving band of the frequency range used by a radio system. If the device has a shared transmitter and a shared receiver for the radio systems using frequency ranges close to each other, there can anyway be a separate antenna for each system or the antenna can also be shared.

The invention has the advantage that the size of the antennas can be made small. This is due to that when there is a plurality of antennas, a relatively small bandwidth is sufficient for an individual antenna. When the bandwidth is small, a material with higher permittivity can be chosen for the antenna than for an antenna having a wider band, in which case the antenna dimensions can be made correspond- ingly smaller. In addition, the invention has the advantage that a good matching is achieved on the whole width of the band of each radio system. This is due to that the matching of a separate antenna having a relatively narrow band is easier to arrange than the matching of a combined multiband antenna. The invention further has the advantage that the antenna system has a good efficiency in different bands. This is partly due to the quality of the matching and partly to that no

switches for choosing a sub-band are needed in the structure. Both the matching and the efficiency are also improved by the fact that in a decentralized system the antennas can each be located in a place which is advantageous with regard to its function. The invention further has the advantage that the isolation between the antennas is good. This is due to the sensible decentralization of the antennas and the fact that a substrate with a relatively high permittivity collapses the near field of the antenna.

In the following, the invention will be described in detail. Reference will be made to the accompanying drawings, in which

Fig. 1 shows an example of a known multiband antenna,

Fig. 2 shows an example of a known antenna component using a dielectric substrate,

Fig. 3 shows an example of the placement of the antennas in an antenna system according to the invention,

Fig. 4 shows another example of the placement of an antenna belonging to the antenna system according to the invention,

Figs. 5a-d show examples of the composition of an antenna system according to the invention,

Fig. 6 shows an example of the matching of a pair of antennas in an antenna system according to Fig. 3, and

Fig. 7 shows an example of the efficiency of a pair of antennas in an antenna system according to Fig. 3.

Figs. 1 and 2 were already described in connection with the prior art.

Fig. 3 shows an example of an antenna system according to the invention as a layout drawing. There is a radio device 300 with a circuit board PCB, plastic frame FRM and casing CAS in the drawing. A large part of the surface of the circuit board on the side visible in the drawing consists of a conductive ground plane GND. In this example the antenna system includes six antennas. Each one of these comprises an elongated antenna component with a ceramic substrate and two radiating elements. The ground plane around the antenna component is also considered to be a part of the antenna here. In this example, the radiating elements of each antenna component are of the same size so that they resonate in

the same, relatively narrow frequency range. The feed conductor of an antenna is connected to one element, and the other element is parasitic.

The first 310, the second 320, the third 330, the fourth 340 and the fifth 350 antenna component are mounted on the same side of the circuit board PCB, visible in the drawing. The first antenna component 310 is located in the middle of the first end of the circuit board, parallel with the end. The second antenna component 320 is located in a corner defined by the second end and the first long side of the circuit board, parallel with the end. The third antenna component 330 is located near the corner defined by the second end and the second long side of the circuit board, parallel with the long side. The fourth antenna component 340 is located beside the first long side of the circuit board parallel with it, slightly closer to the first than the second end. The fifth antenna component 350 is located beside the second long side of the circuit board parallel with it, opposite to the fourth antenna component. The sixth antenna component 360 is mounted on the side surface of the frame FRM, which surface is perpendicular to the plane of the circuit board. The antenna components are located at places which are advantageous with regard to the other RF parts and so that they do not much interfere with each other.

Fig. 3 also shows an example of the ground arrangement of the antennas. The ground plane of the surface of the circuit board has been removed from below and beside the first antenna component 310 to a certain distance. However, a narrow part of the ground plane extends to one or more points of the radiators. Such an arrangement increases the electric size of the antenna compared to that the ground plane would continue as wide to the area under the component. In that case e.g. the height of an antenna component operating in a certain frequency range can be correspondingly reduced. The other antennas can have a similar ground arrangement. In theory, the whole ground plane is naturally shared between all the antennas. In practice, the system has mainly antenna-dedicated ground planes, because of the decentralization of the antenna components. This becomes evident from the fact that the distance along the ground plane between two radiators belonging to different antennas is at least the combined length of these radiators.

The antennas according to Fig. 3 can be designed e.g. as follows: - the antenna based on the component 310 is an antenna for the GSM850 system; - the antenna based on the component 320 is an antenna for the GSM900 system;

- the antenna based on the component 330 is an antenna for the GSM1800 system;

- the antenna based on the component 340 is a transmitting antenna for the WCDMA system; - the antenna based on the component 350 is a receiving antenna for the WCDMA system;

- the antenna based on the component 360 is an antenna for the GSM1900 system.

Fig. 4 shows another example of the placement of an antenna belonging to the antenna system according to the invention. The rear portion CAS of the outer casing of a radio device and a radiator 412 on its inner surface are seen in the drawing. In this example the radiator is a dense meander pattern by shape, and it has been implemented by growing conductor material on the surface of the casing. Thus the part of the casing under the radiator functions as the substrate belonging to the antenna.

Figs. 5a-5d show examples of the composition of the antenna system according to the invention as schematic diagrams. In Fig. 5a there are three antennas. One of them is shared between the GSM850 and GSM900 systems, the second is shared between the GSM1800 and GSM1900 systems, and the third is for the WCDMA system. In Fig. 5b, there are six antennas for the same bands as above in the example mentioned in the description of Fig. 3. So, one of them is for the GSM850 system, the second for the GSM900, the third for the GSM1800, the fourth for the GSM1900, the fifth for the transmitting side of the WCDMA system, and the sixth for the receiving side of the WCDMA system, listed in the order of Fig. 5b. In Fig. 5c there are twelve antennas. One of them is for the transmitting side of the GSM850 system, and the second and the third for the receiving side of the GSM850 system. The latter two are used to implement the space diversity in the receiving. There is a corresponding group of three antennas for the GSM900, GSM1800 and GSM1900 system as well. Fig. 5d presents a modification of the composition according to Fig. 5a. Now the all four GSM systems have their own antenna. However, the GSM850 and GSM900 antennas, the operating bands of which are close to each other, are connected to the same feed line. After the separation of the transfer directions, the antennas then become connected to the shared transmitter and the shared receiver of these systems. In the same way the GSM1800 and GSM1900 antennas, the operating bands of which are close to

each other, are connected to a shared feed line. The WCDMA antenna can also be connected to this line.

Fig. 6 presents an example of the matching of the antenna system according to

Fig. 3 for the antennas corresponding to the fourth 340 and the fifth 350 antenna component, when these are designed to function as the transmitting and receiving antennas of the WCDMA system. The substrate of the antenna components is of a ceramics, and its dimensions are 10-3-2 mm ³ (length, width, height). The figure shows the curve of the reflection coefficient S1 1 as a function of frequency. It is seen from the curve that the reflection coefficient is -10 dB or better in the range of both the transmitting and the receiving band. The matching of the antenna pair is then good.

Fig. 7 shows a curve of the efficiency of the same antenna pair to which Fig. 6 applies as a function of frequency. It is seen that the efficiency is approx. 0.76 on the average in the transmitting band and approx. 0.72 in the receiving band. The effi- ciency of the antenna pair is thus excellent considering the small size of the antenna components. The maximum gain of the transmitting antenna is approx. 1.3 dB and the maximum gain of the receiving antenna approx. 2.3 dB on an average as measured in free space

A decentralized antenna system according to the invention has been described above. As appears from the examples described, the number and the location of the antennas can vary greatly. The invention does not limit the method of manufacture of individual antenna components. The manufacture can take place for example by coating a piece of ceramics partly with conductive material or by growing a metal layer on the surface of e.g. silicon and removing a part of it by the tech- nique used in the manufacture of semiconductor components. The inventive idea can be applied in different ways within the scope defined by the independent claim 1.