WO2007054616A1

WO2007054616A1 - Internal monopole antenna

Info

Publication number: WO2007054616A1
Application number: PCT/FI2006/050439
Authority: WO
Inventors: Heikki Korva; Petteri Annamaa
Original assignee: Pulse Finland Oy
Priority date: 2005-11-11
Filing date: 2006-10-12
Publication date: 2007-05-18
Also published as: FI20051146A0; FI20051146A

Abstract

An internal monopole antenna especially intended for flat radio devices. The radiator (320) of the antenna is for the most part in one geometrical plane, forming thus a radiation plane. A planar and conductive ground element (330) is added to the antenna below the radiating plane and parallel with it. This is galvanically connected to the ground plane (310) of the radio device through an inductive element (335), which has relatively high impedance at the operating frequencies of the antenna. The radiator has preferably two branches for forming separate operating bands. The antenna is pretuned upwards so that the effect of both the user's tissues and the ground element, which lowers the resonance frequencies, is taken into account. The antenna is installed in the radio device so that in the operating position of the device it is as far as possible from the user's head with the ground element toward the user. The efficiency of the antenna is relatively good when the device is beside the user's head, the SAR value being small at the same time. In addition, the space between the radiator and the ground element can be utilized by placing other components of the radio device in it.

Description

Internal monopole antenna

The invention relates to an internal monopole antenna especially intended for flat radio devices.

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 casing 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 the design becomes more demanding. It also contributes to the matter that often the antenna has to operate in two or more frequency bands.

In practice, an antenna with satisfactory characteristics that fits into a small-sized device is most easily made with a planar structure. The antenna includes a radiating plane and a ground plane parallel with it. These planes are generally connected to each other by a short-circuit conductor for matching, in which case a structure of the PIFA (planar inverted F-antenna) type is created. However, when the distance between the radiating plane and the ground plane is reduced from the optimal value, the characteristics of the PIFA, such as the bandwidth, are deteriorated. This means a problem in the flat radio devices, which are common today. Especially the parts of a two-part device, which are movable in relation to each other, are generally relatively flat. In addition, the reduction of the size of the devices has led to that their ground plane has also become smaller. In this case the capability of the PIFA deteriorates because the antenna resonances become weaker, and because of the ground plane's own resonances occurring on useless frequencies. In dual-band mobile antennas, the drawbacks are more pronounced in the lower operating band, which is located in the range of 0.9 GHz.

The problem caused by the flatness of the radio device can be reduced by making the antenna with a monopole structure. Fig. 1 shows an example of such a mono- pole antenna internal to the device. The circuit board 105 of the device and the radiating element 120 are seen in the drawing. On the circuit board there are natu- rally components of the device, and its surface mostly consists of the conductive ground plane GND. The radiating element, or the radiator, is located at the end of the circuit board, outside it as viewed from above, and so the ground plane is located aside from the radiator. As seen from the feed point FP, the radiator of the example is divided into two branches of different length for forming two separate operating bands. The radiator 120 with its branches is for the most part in a plane which is parallel with the circuit board 105. For lengthening the longer branch 121 and for shaping the radiation pattern, its outer end has a portion directed toward the geometrical plane of the circuit board, and as a result, the antenna has a certain height. However, this height remains smaller than the height required by the other parts of the device together.

Monopole antennas of the type described above have a better capability than a PIFA made in the same space, but the capability leaves room for improvement in a situation when the device is used as a telephone and held on the user's ear. The reason may be that the resonance frequencies of the antenna shift downward from the effect of the conductive tissues of the user, in which case the efficiency of the antenna deteriorates. In addition, the radiation is further weakened by the fact that part of the energy of the antenna's near field converted to heat in the tissues of the user. The abbreviation SAR (Specific Absorption Rate) is used for this phenomenon. The antenna can naturally be pretuned in such a way that its bands are ap- proximately in place in a normal situation of use. This improves efficiency, but on the other hand, the SAR increases, which is not regarded as desirable.

Fig. 2 shows an example of a monopole antenna internal to the device, known from the publication EP 03396111.1 The main parts of the antenna are a mono- pole-type base element 220 and a parasitic element 230. The base element is a rigid conductor wire resembling a rectangular circle on one end of the circuit board, and it is connected to the antenna port of the radio device from the feed point FP by a feed conductor 225. The upper surface of the circuit board is for the most part part of the ground plane 210, which does not, however, extend to the antenna. The radiator formed by the base element has two bands. The upper op- erating band is based on the third harmonic of the basic resonance frequency. In order to set the proportion of the harmonic frequency to the basic resonance frequency as correct, the base element is divided into two branches as seen from the feed point FP, between the outer ends of which there is a significant electromagnetic coupling. This decreases said proportion of the harmonic frequency to the basic resonance frequency so that those frequencies fall on the frequency ranges used by the GSM900 and the GSM 1800 systems (Global System for Mobile communications), for example. The parasitic element 230 is also a rigid frame-like conductor wire and is located below the base element, approximately in the plane of the circuit board 205. The parasitic element is connected at a point G to the ground plane 210 through an inductive element 235. This is a conductor wire making approximately one turn. Point G divides the parasitic element into two parts. One part together with the inductive element resonates in the lower operating band of the antenna, and the other part together with the inductive element in the upper operating band of the antenna. The parasitic element thus functions as an auxiliary radiator. The parts of the parasitic element and the inductive element 235 are dimensioned so that the matching of the antenna is optimized in both bands.

The structure according to Fig. 2 provides a better efficiency than a mere base element. The parasitic element can also be used for widening at least one operating band by arranging its resonance frequency suitably aside from the corresponding resonance frequency of the base element. However, the efficiency of the struo ture remains quite low during an actual call. It does not provide a possibility for reducing the SAR, either.

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

The basic idea of the invention is the following: The radiator of an internal antenna of a radio device is a monopole element, which is mostly in one geometrical plane and thus forms a radiation plane. A planar and conductive ground element parallel with the radiation plane is added to the antenna below a radiation plane. This is galvanically connected to the ground plane of the radio device through an inductive element, which has a relatively high impedance on the operating frequencies of the antenna. At low frequencies, the inductive element represents a short circuit in practice. The radiator has preferably two branches for forming separate operating bands. The antenna is pretuned upwards so that the effect of both the user's tissues and the ground element, which lowers the resonance frequencies, is taken into account. The antenna is installed in the radio device so that in the operating position of the device it is as far as possible from the user's head with the ground element toward the user.

The invention has the advantage that a relatively good efficiency is achieved for the antenna when the device is beside the user's head and the SAR value is low at the same time. In addition, the invention has the advantage that the space between the radiator and the ground element can be utilized by placing other components of the radio device in it. This is due to that these components can be grounded against electro static discharge (ESD) simply through the ground ele- merit and the inductive element according to the invention without deteriorating the function of the antenna significantly.

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

Fig. 1 presents an example of a known internal monopole antenna,

Fig. 2 presents another example of a known internal monopole antenna,

Figs. 3a, b presents an example of an internal monopole antenna according to the invention,

Fig. 4 presents examples of the coupling between the ground element according to the invention and the ground plane of the radio device,

Fig. 5 presents an example of the location of an antenna according to the invention in the radio device,

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

Fig. 7 presents an example of the efficiency of an antenna according to the invention,

Fig. 8 presents an example of the effect of the user's head on the efficiency of the antennas. Fig. 9 presents another example of an internal monopole antenna according to the invention,

Fig. 10 presents a third example of an internal monopole antenna according to the invention, and

Fig. 11 presents a fourth example of an internal monopole antenna ao cording to the invention.

Figures 1 and 2 were already explained in connection with the description of the prior art.

Figs. 3a and 3b present an example of an internal monopole antenna according to the invention. Fig. 3a is a perspective drawing, and in Fig. 3b the antenna struc- ture is seen from the side and enlarged in relation to Fig. 3a. A circuit board 305 having a mostly conductive upper surface is seen in both drawings. This conduc- tive surface functions as the signal ground GND, or the ground plane 310, of the radio device. The radiator 320 of the antenna is a monopole element having a similar shape as the one in Fig. 1 ; it has two branches 321 , 322 as seen from the feed point FP, for forming two separate operating bands, and in the outer end of the longer branch 321 there is a fold directed toward the geometrical plane of the circuit board 305. Apart from the fold, the radiator is planar. This main plane of the radiator is called the "radiation plane". In this example, the radiation plane is slightly above the level of the upper surface of the circuit board, because in any case there are components that increase the thickness of the device on the circuit board and above it. The battery BT seen in Fig. 3b is one such component. The radiator is connected from its feed point to the antenna port of the radio device on the circuit board by a feed conductor 325.

The difference compared to the structure shown in Fig. 1 is that the antenna now comprises a planar conductor element 330, which is located under the radiation plane, parallel with it and called the "ground element" here. In this example, the ground element is the conductive coating of a small, dielectric plate. The ground element 330 covers the area on which the branches of the radiator are, as seen in the direction of the normal of the ground element and the radiation plane. It is thus located outside the end of the circuit board 305. In this example, the ground ele- ment is coupled to the ground plane by a coil 335, which has a relatively high impedance at the operating frequencies of the antenna. The distance between the ground element and the radiation plane is relatively small, e.g. less than 5 mm, in order to fit the antenna also in a flat radio device.

Fig. 3b also shows a dielectric frame 370, which supports the antenna for the part of both the radiator and the ground element.

Fig. 4 shows two examples of the coupling between the ground element 430 and the ground plane of the radio device. In partial figure (a), the coupling is similar to that in Figs. 3a, b: The ground element is coupled to the ground plane from the centre of its longitudinal side by a coil L. The inductance of the coil is for example 30 nH. In partial figure (b), the ground element is coupled to the ground plane from the ends of its longitudinal side by two coils L1 and L2. Their inductances are for example 60 nH. The absolute value of the impedance of coil L in the range of 900 MHz is approximately 170 Ω, and that of coils L1 and L2 approximately 350 Ω. These values are so high that with regard to the antenna operation, the ground element is practically isolated from the ground plane. At low frequencies the impedance of the coils is close to zero. This means that the ground element is then practically a continuation of the ground plane, which is in fact the intention of the whole coupling. Namely, the space between the radiator and the ground element can then be utilized by placing other components of the radio device in it. There is a reason to ground many of the components against ESD, and by the ground ele- ment the grounding is simple. Naturally, there can also be more coils, or in more general terms, inductive elements, implementing a galvanic coupling.

According to the above description, the coupling of the ground element to the ground plane is in all cases weak at the operating frequencies of the antenna. A "weak" coupling means here and in the claims the fact mentioned above that in view of the antenna operation, the ground element is isolated from the ground plane.

Fig. 5 shows an example of the location of the antenna according to the invention in a 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 tied with the hinge HG. The radio device is presented as a simplified, longitudinal cross-section. The first turning part includes the keyboard and microphone of the device, among other things, and the second turning part includes the display and speaker EP of the device, among other things. The antenna 500 is located at the opposite end of the first turning part, as seen from the hinge HG, in which case the ground plane, es- sential with regard to the antenna operation, is included in the circuit board 505 of the first turning part. The location of the antenna means a relatively low SAR value when the speaker EP is beside the user's ear. In the operating position of the device, the radiator 520 of the antenna is on its outer side as seen from the user, while the ground element 530 is correspondingly on its inner side.

Fig. 6 shows another example of the location of the antenna according to the invention in the radio device. The radio device RD2 of this example, too, has mechanically two parts, while the parts are now slidable in relation to each other. Thus 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 the keyboard and microphone of the device, among other things, and the second sliding part includes the display and speaker EP of the device, among other things. The antenna 600 is located at the end of the first sliding part which is farther from the second sliding part when the radio device is in its extended state, in which case the ground plane, essential for the antenna operation, is included in the circuit board 605 of the first sliding part. The location of the antenna means a relatively low SAR value when the speaker EP is beside the user's ear. In the op- erating position of the device, the radiator 620 of the antenna is on its outer side as seen from the user, while the ground element 630 is correspondingly on its inner side.

Fig. 7 shows an example of the efficiency of a dual-band antenna according to the invention. The lower operating band is designed for the GSN850 system, which has the frequency range of 824-894 MHz. The upper operating band is designed to cover the frequency range of 1850-1990 MHz used by the GSM 1900 system, for example. These ranges are marked by arrows in the drawing. Curve 71 shows the changing of the efficiency as a function of frequency when the antenna is in a free space. In the lower operating band, the efficiency varies between 0.44 and 0.62, the average value being 0.54. In the upper operating band, the efficiency varies between the values 0.70 and 0.88, the average being 0.82. For comparison, the efficiency curve 72 of a corresponding antenna, which does not have a ground element according to the invention, is also in Fig. 7. In the lower operating band, its efficiency varies between the values 0.61 and 0.69, being 0.65 on the average, and in the upper operating band between 0.66 and 0.70, the average being 0.68. It is seen that the ground element according to the invention deteriorates the efficiency by 0.11 on the average in the lower operating band, but improves it in the upper operating band by 0.14 on the average.

Fig. 8 shows an example of the effect of the user's head on the efficiency of the antennas. Curve 81 relates to the same antenna according to the invention as curve 71 , and curve 82 relates to the same prior art monopole antenna as curve 72. The radio devices then are in this case in the normal operating position beside the user's head. The efficiencies are naturally lower than when measured in a free space. In the lower operating band, the efficiency of the antenna according to the invention is 0.30 on the average, and the efficiency of the prior art reference antenna is 0.24. In the upper operating band, the efficiency of the antenna according to the invention is 0.51 on the average, and the efficiency of the prior art reference antenna is 0.42. It is seen that in a normal situation of use, the ground element ac- cording to the invention improves the efficiency in both operating bands.

The SAR values of the antenna represented by Fig. 8 are approximately the same as with such a corresponding antenna, which does not have a ground element and which has been tuned upwards at the cost of efficiency so that the bands are slightly aside even in the use situation for reducing the SAR. When a ground ele- ment is added to such an antenna, the operating bands are made to shift downwards approximately to the spectrums of the radio signals. Then the radiation of the antenna naturally becomes more powerful and its efficiency improves. On the other hand, the ground element placed between the radiator and the user attenuates the field being directed to the user's head, for which reason the strengthening of the radiation does not cause an increase in the SAR value.

Fig. 9 shows another example of an internal monopole antenna as seen from above. A circuit board 905 of a radio device, the upper surface of which is for the most part a conductive ground plane 910, is seen in the drawing. In principle, the main radiator of the antenna is similar to the one shown in Fig. 3a, and it thus has two branches 921 and 922, as seen from the feed point FP, for forming two sepa- rate operating bands. Under the radiator there is a ground element 930 according to the invention, which in this example is connected to the ground plane through a planar coil 935 on the circuit board. In addition, the antenna now includes a radiating parasitic element 940, which is known per se. This is located beside the end of the main radiator on the side of the feed point FP, in the same plane as the main radiator. The parasitic element is connected to the ground plane from the grounding point GP close to the feed point. Its resonance frequency can be arranged in the upper operating band of the antenna for widening this. The upper operating band is primarily based on the resonance of the shorter branch 922 of the main radiator. In addition, when the harmonic of the basic resonance frequency of the longer branch 921 is arranged in the upper operating band, this can be made to cover the frequency range of 1.7 to 2.0 GHz, for example.

Fig. 10 shows a third example of an internal monopole antenna according to the invention as seen from above. In principle, the structure is similar to that shown in Fig. 3a, but the difference is that the radiator A20 does not branch into two as seen from the feed point FP. Thus the antenna has only one operating band. There is the ground element A30 according to the invention under the radiator.

Fig. 11 shows a fourth example of an internal monopole antenna according to the invention as seen from the side. When the structure is compared to that shown in Fig. 3b, the difference is that the radiator B20 is now fastened directly to the upper surface of the circuit board B05. The radiation plane is thus the geometrical plane of the upper surface of the circuit board. In order to keep a sufficient distance between the ground element B30 and the radiator, the ground element is now beneath the lower surface of the circuit board. Between the circuit board and the ground element there is a coil B35 with a vertical axis. The coil couples the ground element as a continuation of the ground plane B10 at low frequencies, and supports the ground element mechanically at the same time. In the example of Fig. 11 , the ground plane is on the lower surface of the circuit board, although it could naturally also be on the upper surface in this case, too. A part of the dielectric support structure B70 is also seen on the farther end of the antenna. The ground element B30 is a relatively rigid metal sheet in this example.

In this description and the claims, the qualifiers "lower", "upper", "below", "above" and "vertical" refer to a position of the antenna structure, in which the circuit board of the radio device is horizontal and the radiator is located higher than the ground element. The device can naturally be in any position when used.

Some antenna structures according to the invention have been described above. The shapes and location of the parts of the antenna can naturally differ from those presented in the drawings. The invention does not limit the method of manufacture of the antenna, either. The inventive idea can be applied in different ways within the limits set by the independent claim 1.

Claims

1. An internal antenna of a radio device, the radio device comprising a circuit board (305; 505), at least one surface of which is for the most part a conductive ground plane (310; 910), a main radiator (320; 520) of the antenna being a mono- pole element, which is for the most part located in a radiation plane parallel with the circuit board and outside the circuit board as seen from above, and coupled to an antenna port from its feed point (FP), characterized in that it further comprises a planar ground element (330; 530; 930) parallel with the radiating plane and being located below it, which ground element is galvanically coupled to the ground plane by at least one inductive element (335; 935) for an ESD protection of components placed between the radiator and the ground element, the coupling between the ground element and the ground plane being weak at the operating frequencies of the antenna.

2. An antenna according to Claim 1 , characterized in that said main radiator is divided into two branches (321 , 322; 921 , 922) of different length as seen from the feed point (FP), to form two separate operating bands for the antenna.

3. An antenna according to Claim 1 , characterized in that said inductive element is a discrete coil (335; L; B35) connected to central area of a longer side of the ground element.

4. An antenna according to Claim 3, characterized in that the impedance of said inductive component is at least 100 Ω at the operating frequencies of the antenna to implement said weak coupling.

5. An antenna according to Claim 1 , characterized in that there are two of said inductive elements, one of which (L1 ) is connected to a first end of the ground element (430) and the other one (L2) to a second end of the ground element.

6. An antenna according to Claim 1 , characterized in that it further comprises a radiating parasitic element (940) being located in said radiation plane, which parasitic element is connected to the ground plane from a grounding point (GP) being located close to the feed point (FP).

7. An antenna according to Claim 1 , characterized in that the distance between the ground element and the radiation plane is at the most 5 mm.

8. A radio device comprising a circuit board, at least one surface of which is for the most part a conductive ground plane, and an antenna having a monopole radiator, which is for the most part located in a radiation plane parallel with the circuit board and outside the circuit board as seen from above and connected to an an- tenna port of the radio device from its feed point, characterized in that it further comprises a planar ground element parallel with the radiating plane and below it, having a weak coupling to the ground plane at the operating frequencies of the antenna.

9. A radio device (RD1 ; RD2) according to Claim 8, comprising a first (TP1 ; SP1 ) and a second (TP2; SP2) part movable in relation to each other, which second part is the part held on a user's ear in a normal use situation, characterized in that said circuit board means a circuit board (505; 605) of the first part (TP1 ; SP1 ) and that said antenna (500; 600) is located at the end of the first part which is farther from the second part when the radio device is in its extended state, and in the operating position of the radio device the radiator of the antenna is on its outer side as seen from the user, while the ground element is correspondingly on its inner side.