The invention relates to an antenna designed for portable radio devices, which antenna includes a radiating slot in the ground plane of the radio device. The invention also relates to a radio device with an antenna according to the invention.

The space available is an important factor in the design of antennas for portable radio devices. Without any limit on size, it is relatively easy to make a high-quality antenna. However, in portable devices the antenna is preferably placed within the cover of the device, and along with the reduction of the size of the devices, the space available for the antenna has become smaller and smaller. This means that design has become ever more demanding. The fact that the antenna often has to operate in two or more frequency bands contributes to the same.

In practice, an antenna with satisfactory characteristics that fits into a small-sized device is most easily obtained by a planar structure: The antenna includes a radiating plane and a ground plane parallel with it. These planes are usually con- nected to each other at a suitable point by a short-circuit conductor, in which case a structure of the PIFA (planar inverted F antenna) type is created. The size of the ground plane naturally has an effect on the characteristics of the antenna. Like in the case of a monopole whip, the ground plane of an ideal planar antenna is also very wide. When the ground plane becomes smaller, the performance of the an- tenna deteriorates because the antenna resonances are weakened and because of the resonances of the ground plane itself. In addition, resonances are then more easily generated in other conductive parts of the device, too. The deterioration of performance appears as a reduction of the bandwidth and the antenna gain. In dualband antennas, this drawback is naturally more pronounced in the lower band, which is most often located in the range of 0.9 GHz. Another drawback of the PIFA is that it is not suitable for flat radio devices, because bringing the radiating plane and the ground plane closer to each other deteriorates its characteristics.

In the invention to be described here, the antenna is based on a radiating slot in the ground plane. A slot like that is known as such. For example, the application publication Fl 20021668 discloses an antenna structure with a radiating slot in its ground plane. This antenna structure is seen in Fig. 1. It shows the circuit board 105 of a radio device, where most of the upper surface of the circuit board is conductive. This conductive surface functions as the ground plane 110 of the antenna. At the one end of the circuit board there is the radiating plane 120 of the antenna, to which plane the antenna feed conductor 131 and the short-circuit conductor 132

are connected. The radiating plane of the example is divided, as viewed from its short circuit point, into two branches B1 , B2 of different lengths for forming two separate operating bands. In the ground plane 110 there are two parallel slots starting from its edge. The first slot 115 runs beside the short circuit point S of the antenna, and the second slot 116 in a way that the antenna feed point F on the surface of the circuit board 105 remains between the first and the second slot. The second slot can be dimensioned so that oscillation is excited in it in the antenna's upper operating band, in which the slot then improves the matching.

The slot radiator according to Fig. 1 is only an auxiliary radiator of the PIFA in the upper operating band of the antenna. The lower operating band is based on the lower resonance of the PIFA. Thus the radiating plane of the PIFA is necessary, in which case the above-mentioned drawbacks of the PIFA also exist. A proper operating band based on a slot resonance in the range of 0.9 GHz is not obtained.

The object of the invention is to reduce the above mentioned drawbacks of the prior art. The slot antenna according to the invention is characterized in what is set forth in the independent claim 1. The radio device according to the invention is characterized in what is set forth in the independent claim 7. Some preferred embodiments of the invention are set forth in the other claims.

The basic idea of the invention is the following: The functional main part of the an- tenna of a small-sized radio device is a slot in the ground plane of the device, open at its one end. The slot is located in a way that a relatively narrow strip of the ground plane remains between it and an edge of the ground plane. At the end of this ground strip, beside the open end of the slot, there is connected an elongated auxiliary element, which elevates slightly from the ground plane and is then di- rected towards the closed end of the slot. The auxiliary element increases the electric length of the slot so that the basic resonance frequency of the slot occurs in the range of the lower operating band of the antenna intended as a dual-band antenna. The slot thus functions as a radiator in the lower operating band. The antenna is fed from a point that is advantageous with regard to the matching, near the closed end of the slot. In addition, the auxiliary element is dimensioned so that when the antenna is fed by the frequencies of the upper operating band, vibration of its own is excited in the auxiliary element, in which case it functions as a radiator in the upper operating band.

The invention has the advantage that the lower operating band of a dualband an- tenna can also be implemented by a structure that takes a relatively small amount

of space. This is due to the fact that a slot radiator in the ground plane is used, and the required auxiliary element has a significantly smaller area and is closer to the ground plane than the radiating plane of a PIFA, for example. In addition, the invention has the advantage that in the antenna according to it, the space between the auxiliary element and the ground plane can be utilized by placing other components of the radio device in that space. Furthermore, the invention has the advantage that the antenna according to it can be situated more advantageously with regard to SAR (specific absorption rate) than the prior art antennas. Yet another advantage of the invention is that the matching of the antenna according to it does not require separate matching components.

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

Figure 1 presents an example of a ground plane slot radiator according to the prior art, Figure 2 presents an example of an antenna according to the invention,

Figure 3 presents an example of the feed and ground arrangement of an antenna according to the invention,

Figure 4 presents examples of the tuning of an antenna according to the invention, Figure 5 presents an example of the location of an antenna according to the invention in the radio device,

Figure 6 presents another example of the location of an antenna according to the invention in the radio device,

Figure 7 presents an example of the band characteristics of an antenna accord- ing to the invention,

Figure 8 presents an example of the efficiency of an antenna according to the invention, and

Figure 9 presents an example of the influence of the user's head on the efficiency of antennas.

Figure 1 was dealt with above in connection with the description of the prior art.

Fig. 2 shows an example of an antenna according to the invention. A circuit board

205 of a radio device, most of the upper surface of which board is conductive, is seen in the figure. This conductive surface functions as the signal ground, or ground plane 210, of the radio device. At one end of the circuit board there is a non-conductive slot 220 in the ground plane. This slot starts from an edge of the ground plane, i.e. it opens to the edge of the ground plane. The slot 220 runs in the direction of the end of the ground plane, and its other end, or the closed end, extends relatively close to the opposite long side of the ground plane, as seen from the open end of the slot. A relatively narrow area 212 of the ground plane, which is called a ground strip here, remains between the slot and the end of the ground plane. In this description and the claims, the qualifier "relatively" in connection with a distance refers to such a distance, which is smaller by at least an order than the wavelength corresponding to the highest operating frequency of the antenna.

An oscillation can be excited in the slot 220 by feeding electromagnetic energy to it at the natural frequency of the slot. Then the slot functions as a radiator, sending energy to its environment. In Fig. 2 the feed point FP of the slot radiator is marked by a dashed line, which point is near the closed end of the slot in this example. However, the properties of such a slot radiator, like efficiency, are not yet satisfactory. In addition, it is difficult to arrange its natural frequency sufficiently low in a pocket-sized device, when an operating band below one gigahertz is required. The fact that the slot opens to the edge of the ground plane makes it a quarter-wave resonator. This helps to achieve lower natural frequencies as compared to half- wave resonators, but not enough.

For the above-mentioned reasons, the antenna structure according to the inven- tion also includes a conductive auxiliary element 230. This is a piece made of relatively rigid sheet metal, for example, with an elongated first portion and a second portion essentially perpendicular to it. At its end on the side of the second portion, the auxiliary element is galvanically connected to the end of the ground strip 212 beside the open end of the slot 220 so that the first portion of the auxiliary element is above the ground strip 212. It can also extend above the slot 220 in its transverse direction. In its longitudinal direction, the first portion extends roughly to the closed end of the slot. Its distance from the ground plane, or the height of the antenna corresponding to the second portion of the auxiliary element, is relatively small, e.g. 5 mm.

The auxiliary element 230 naturally has an effect on the characteristics of the antenna. The equivalent circuit of the slot at its natural frequency is a parallel reso-

nance circuit. The auxiliary element increases both the inductance and the capacitance in the equivalent circuit, in which case the natural frequency of the structure becomes lower. The same thing can also be expressed by saying that the auxiliary element increases the electric length of the slot. The auxiliary element can be di- mensioned so that the natural frequency of the antenna falls in the frequency range of the GSM850 or the GSM900 (Global System for Mobile communications), for example, even if the physical length of the radiating slot were under 4 cm. An additional feature in the operation of the antenna is that an oscillation can be excited in the auxiliary element at its own natural frequency. The natural frequency of the auxiliary element can be arranged in the frequency range of the GSM1800 or the GSM 1900 system, for example, in which case the antenna has two usable operating bands. The lower operating band is based on the slot of the ground plane lengthened by the auxiliary element, and the upper operating band is based on the auxiliary element.

Fig. 3 presents an example of the feed and ground arrangement of the antenna according to the invention. In the part drawing (a) there is the circuit board 305 of a radio device as seen from above and in the part drawing (b) as seen from below. On the upper surface of the circuit board there is the radiating slot 320 of the antenna in the ground plane 310, like in Fig. 2. The auxiliary element 330 as cut is also seen in the part drawing (a). The antenna is fed electromagnetically by a feed conductor 340. The feed conductor is a conductor strip on the lower surface of the circuit board 305. The strip extends from the antenna port essentially perpendicularly over the line of the radiating slot at the feed point FP near the closed end of the slot. The signal ground GND is on the lower surface of the circuit board at a certain distance from the slot 320, so there is no ground at the slot or quite near it.

The impedance of the antenna structure as measured from the antenna port is influenced by, among other things, the location of the feed point FP and the length of the free portion of the feed conductor 340. When these are chosen suitably, the antenna can be matched without separate matching components. For the match- ing, the feed conductor can also be continued through the circuit board to its upper surface and be connected there to the edge of the radiating slot on the side of the ground strip 312.

Fig. 4 shows two examples of the tuning of the antenna according to the invention.

One way is to make the radiating slot winding. In the example of the figure, the slot 420 forms a meander pattern. Another way is to shape the auxiliary element. In the example of the figure, a part CUT has been cut off from the first portion of the

auxiliary element 430 above the ground strip 412. The third way is to change the height h of the auxiliary element, or the distance of the first portion from the ground. The marking Δh in the figure means such a change.

Fig. 5 shows an example of the location of an antenna according to the invention in the radio device. The radio device RD1 of the example is a foldable mobile station having a first turning part TP1 and a second turning part TP2, both connected to a hinge HG. The radio device is shown as a simplified longitudinal cross- section. The first turning part includes, among other things, the keyboard and microphone of the device, and the second turning part includes, among other things, the display and the earphone EP of the device. The second turning part is thus the part of the device held on the user's ear. In this example, the antenna 500 is located at the opposite end of the first turning part as viewed from the hinge HG. The radiating slot and the auxiliary element are on the outer side of the circuit board 505 of the first turning part in the normal using position as seen from the user. Such a location means a relatively small SAR value when the earphone EP is adjacent to the user's ear.

Fig. 6 shows another example of the location of an antenna according to the invention in the radio device. The radio device RD2 of this example is also mechanically two-part, the parts being now slidable in relation to each other. So the device has a first sliding part SP1 and a second sliding part SP2. The radio device is shown as a simplified longitudinal cross-section. The first sliding part includes, among other things, the keyboard and microphone of the device, and the second sliding part includes, among other things, the display and the earphone EP of the device. The second sliding part is thus the part of the device held on the user's ear. In this example, the antenna 600 is located at the end of the first sliding part that is farther from the second sliding part when the radio device is in its extended position. The radiating slot and the auxiliary element are on the outer side of the circuit board 605 of the first turning part in the normal using position as seen from the user. Such a location means a relatively small SAR value when the earphone EP is adjacent to the user's ear.

Fig. 7 shows an example of the band characteristics of an antenna according to the invention. The curve 71 shows the changing of the return loss RL as a function of frequency when the antenna is in the free state. The antenna has two clear operating bands. The lower operating band is in the range of 850 MHz, and its width is approx. 90 MHz when the value -5 dB of the return loss is used as the criterion of the cut-off frequency. This bandwidth is considerably large. The upper operating

band is in the range of 1.9 GHz, and its width is approx. 70 MHz when the value -5 dB of the return loss is used as the criterion of the cut-off frequency. In the centre of the operating bands the return loss is about 15 dB, which indicates good matching.

Fig. 8 shows an example of the efficiency of an antenna according to the invention. The curve 81 shows the change of the efficiency of the dualband antenna corresponding to the return loss curve of Fig. 7 as a function of frequency when the antenna is in the free state. In the lower operating band the efficiency varies on both sides of the value 0.6. In the centre of the upper operating band the efficiency is approx. 0.85 and decreases from it to the value of approx. 0.5 when reaching the edges of the band. For comparison, Fig. 8 also shows an efficiency curve 82 of a PIFA having the same operating bands. In the lower operating band, the efficiency of the PIFA is almost the same as that of the antenna according to the invention. In the upper operating band, the PIFA is better than the antenna accord- ing to the invention, its efficiency being over 0.8 on the average. However, the difference in the height of the antennas must be taken into account here. The tested antenna according to the invention is 5 mm high, and the PIFA in comparison is 9 mm high. The circuit board with its ground plane is of equal size in both antennas.

Fig. 9 shows an example of the influence of the user's head on the efficiency of the antennas. The curve 91 concerns the same antenna according to the invention as the curve 81 , and the curve 92 concerns the same PIFA as the curve 82. To the antennas are in this case in the normal position for use beside the user's head. It is seen that the efficiencies fall very clearly, but less for the antenna according to the invention than for the PIFA. In the lower operating band, the efficiency of the antenna according to the invention is on the average 0.13, but that of the PIFA only 0.08. In the upper operating band, the efficiency of the antenna according to the invention is on the average 0.11 , but that of the PIFA only 0.075.

In this description and the claims, the qualifiers "lower" and "upper" refer to a position of the antenna structure, in which the circuit board of the radio device is hori- zontal and the radiating slot of the antenna is on its upper surface. Naturally, the use position of the device can be whichever.

Some antenna structures according to the invention have been described above. The shapes of the parts of the antenna can naturally differ from those shown in the figures. For example, the auxiliary element may also have a slot for increasing the

bandwidth, and there may be more than one auxiliary element. The radiating slot and the ground strip separated by it can also run in the same direction as the long side of the circuit board of the radio device. The circuit board of the radio device can be a multilayer board, in which case the antenna feed conductor can be located in one of its intermediate layers. The inventive idea can be applied in different ways within the scope defined by the independent claim 1.