BACKGROUND OF THE INVENTION Modern mobile phones are getting smaller and smaller and thus the interaction between antenna, phone body and user will become more important than earlier. It is well known that the size of an antenna is critical for its performance, see Johnsson, Antenna Engineering Handbook, McGrawHill 1993, chapter 6. There is also normally a requirement today that two or more frequency bands are supported.

Mobile telephones generally exchange radio signals with a radio base station. Some signal exchange occurs during standby when no call is going on and the phone is located for instance in the hand, in a pocket, or at the waist of the user. Signal exchange of course occurs when a call is going on and the phone is then typically located between the ear and mouth of the user, or still in a pocket or at the waist of the user with an earpiece and a microphone connected.

A fundamental and efficient antenna type for mobile telephone is the monopole consisting of an antenna whip having a length generally a fraction of a wavelength and a phone circuit board acting as a corresponding ground conductor. Among them a length of half a wavelength was used in many older phones and gives a very low feeding current (corresponding to high impedance) with low currents on the telephone body or circuit board. This type of antenna provides very low electromagnetic

fields on the phone itself and thus little interaction with head, hands etc close to the phone. The size is however much bigger than complying with modern telephone design so generally much smaller antennas are required for the sake of easy handling.

However, since the small antenna has to radiate the same power as a big one (due to the requirements of the phone system) the currents or voltages (depending on the type of antenna) on the small antenna will be bigger. This is especially true when the structure is small as compared to a wavelength. Thus the possible interaction with various objects close to the antenna will inherently be bigger and so will the currents along the phone body or circuit board. This applies to all typical screeners in telephone surroundings which means that the electromagnetic fields of the antenna will interact significantly with the user's body during call mode. The interaction would generally occur during standby as well as if the phone is close to the user's body.

When dealing with interaction between the telephone antenna and its immediate surroundings, the electromagnetic near field of the antenna is more important than the far field. In this interaction, there are at least two different quantities to consider.

One is the power loss in the surroundings consisting of losses in for instance a table, a bag, a hand, a head and other human tissue. Such losses could be referred to as Power Obstruction near Phone (POP), quantified in watts. Another quantity is Specific Absorption Rate (SAR) which is relevant in countries where there is legislation and regulation defining SAR upper limits as the power loss per a certain unit of body tissue, generally quantified as an average in watts per a certain amount of body tissue. For instance, the FCC (Federal Communications Commission) in the USA requires that SAR be

less than 1.6 mW in average per gram of body tissue. The phone systems require a certain power level (such as 2W peak and 0.25 W in average for GSM). Different antennas and phones exhibit different SAR for the same radiated power. According to standards (FCC, CENELEC and others), SAR is measured inside a dummy head, or may be calculated.

The present invention has the aim to provide an antenna that generally attains low POP for saving power. Naturally, there is also an aim of the present invention is to fulfil SAR regulations and any other applicable legislation as may be required in various countries. However, the term POP used herein has a technical and non-legal meaning. Thus it should not be confused with SAR which is a quantity of different nature.

Due to SAR's nature of power density, a smaller antenna structure carrying the same power as a bigger structure is more likely to show a high POP. This is the case for most phones using small antennas. The general development of the phones thus calls for SAR-optimized solutions. Larger antenna structures will generally cause lower SAR-values, but modern telephone design requirements do not support increasing size.

Antenna efficiency is an important characteristic and efficiency and SAR are somewhat correlated as high SAR obviously means high POP.

It can be expected that antenna structures isolated from the phone body (by distance or symmetry) would have more favourable SAR performance as many phones show maximum SAR somewhere at the phone body, due to the currents along the same.

The electromagnetic fields around any antenna structure customary are described as"near fields"and"far fields" (see for instance Krauss: Antennas, McGraw Hill 1988) with the

following boundaries (r is the distance and X is the wavelength).

Reactive near field: r < Transition zone: k < r < 2D2/k Far field: r > 2D2/S D is the largest measure of the radiating structure ("diameter") and for a phone (with D ranging from k/3 to , the limits means that the"transition zone"generally vanishes. For this invention it is important to note that the near field and the far field generally are very different with a transition zone in between. Regions of high SAR values (i. e. strong fields close to the phone) may occur in different directions compared to direction of strong radiation (in the far field).

There have been a large variety of attempts in order to reduce the near field towards a user. Some of these are directed towards arrangements and methods for increasing the distance between the active antenna and the user or for achieving directivity, and yet others are employing different kinds of shielding means. For instance, CA 2,088,724, DE 19518125, and DE 19600041 disclose all shielding devices comprising a two- dimensional plate, which shields the radiation towards the user. Such shielding means may be mounted externally with respect to the antenna, at the antenna itself or at the casing of the mobile telephone, or they may be formed integrally with the antenna or with the telephone casing.

US 5,335,366 discloses a shielding means comprising an absorber, reflector or blocker integrally disposed on the antenna, e. g. as a coating, along the longitudinal axis thereof.

US 5,666,125 comprises an antenna having at one side thereof an elongated shielding means disposed along the longitudinal axis of the antenna. An elongated element is arranged in parallel with the antenna at opposite side thereof, said element assisting in redirection of the radiation.

WO 94/28595 describes an antenna arrangement comprising a dielectric cylinder at which four vertically extending rods are arranged, and of which rods one is an antenna element and the others are passive interconnected elements that may be grounded or left in an open circuit. The elements may operate so as to direct and/or reflect the radiation. It is mentioned that a higher degree of directionality is achieved by using grounded passive elements. Furthermore, said document discloses a further two element arrangement with one active element and one passive element connected to ground.

US 5,826,201 discloses a vertical rod antenna and a passive rod antenna disposed parallel with each other. It is, however, not clear whether the passive element should be connected to ground or to the rod antenna, or if it should be left in open circuit.

Problems of these prior art arrangements include occupation of large volumes, large weight, insufficient POP reduction, severe deterioration of far field radiation characteristics, complex design, difficult or expensive implementation, or poor mechanical duration. Many of the so-called"SAR reduction devices"are thus far from practical in a modern small antenna device for a mobile telephone of lightweight. Furthermore, none of the patent documents quoted above seems to fully utilize the large difference between far fields and near fields, but presumably they rather attempt to create far fields which are low in certain directions which does not necessarily imply low near fields.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an antenna device comprising a dielectric supporting means mountable on a radio communications device usable to a human user, and a radiating first element operable in a first frequency band, said supporting means and said radiating first element having a common central longitudinal axis and said radiating first element being supported by said supporting means, having first and second ends being a first feed point and an open end, respectively, and being disposed at least partly around said central longitudinal axis, which device avoids at least some of the problems that is related with prior art.

In this respect, it is a particular object of the invention to provide such an antenna device, which exhibits improved near field radiation characteristics, i. e. reduced POP values, and/or provides for a more compact design as compared with prior art. The main tool for this is to create a near field substantially different from the far field.

It is a further object of the invention to be integratable in existing antenna device designs, still maintaining sufficient performances of the same.

It is yet a further object to provide such an antenna device that is simple, of lightweight, and inexpensive.

It is still a further object to provide said antenna device being efficient, easy to install and reliable, particularly mechanically durable, even after long use.

These objects among others are, according to the invention, attained by an antenna device as claimed in Claim 1.

An advantage of the present invention is a substantial reduction in POP, while still maintaining an antenna device geometry that provides for small sizes.

Further characteristics of the invention, and advantages thereof, will be evident from the following detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description of embodiments of the present invention given hereinbelow and the accompanying Figs. 1-2 which are given by way of illustration only, and thus are not limitative of the invention.

Fig. la displays schematically a front view of an antenna device according to a first embodiment of the present invention as mounted on a mobile telephone of which only a portion is shown, Figs. lb and lc display schematically same mounted antenna device in a lateral view and from above, respectively, the former also comprising portion of the head of a user of said telephone, and Figs. ld, le and lf show an electrically conductive or semi-conductive means and two alternative radiation elements, respectively, of said antenna device.

Fig. 2a displays schematically a front view of an antenna device according to a second embodiment of the present invention as mounted on a mobile telephone of which only a portion is shown, Figs. lb and lc display schematically same mounted antenna device in a lateral view and from above, respectively, the former also comprising portion of the head of a user of said telephone, and Figs. ld and le show an electrically conductive or semi-conductive means and two radiation elements, respectively, of said antenna device.

DETAILED DESCRIPTION OF EMBODIMENTS In the following description, for purposes of explanation and not limitation, specific details are set fourth, 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 practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, protocols, apparatuses, and circuits are omitted so as not to obscure the description of the present invention with unnecessary details.

With reference to Figs. la-c, a radiating first element, here in the form of a meander conductor radiating element 1, is supported by a dielectric cylindrical hollow frame 2 such that radiating element and the frame have a common central longitudinal axis as indicated by dash-dotted line 3. Frame 2 is mounted extending outwards on a chassis 4 of a hand portable mobile telephone 5. The position of the meander element 1 on the chassis 4 is selected such that radiation of the meander conductor 1, at least within a first frequency band, is transmitted and received effectively in different positions chosen by an operator during standby or during a telephone call. In Figs. la-c the frame 2 is located at one side of a top portion of the chassis 4 projecting upwards. The meander element 1 has first and second ends being a first feed point 6 and an open end 7, respectively. Feed point 6 is connected to feeding/receiving circuitry (not shown) of mobile telephone 5 through an electrical connector 15 mounted inside of frame 2 as indicated in Fig. lc. Hence, there is an aperture or similar in frame 2 enabling electrical connection between radiating element 1 and conductor 15 to be made.

According to the invention, an elongated electrically conductive or semi-conductive means, here in the form of pin

8, is supported by said frame 2. Pin 8 extends substantially parallel with said longitudinal axis 3 and has first and second ends being a ground point 9 arranged to be connected to ground of mobile telephone 5, and an open end 10, respectively. This is preferably achieved by connecting point 9 to a circuit board (not shown) of mobile telephone 5 or, alternatively, to other connection point considered to be ground.

Frame 2, radiation element 1, and pin 8 are further covered by a dielectric cover 11 (indicated by dashed line 11 in Figs. la-c), which provides electrical isolation and mechanical protection against various environments which mobile telephone 5 may be exposed to.

Frame 2 has in the illustrated case a circular cross section.

However, the present invention covers other designs such as e. g. elliptical, rectangular and quadratic cross sections. The frame may further be designed in a curved configuration in such manner that the central longitudinal axis through the frame is curved.

Electrically conductive or semi-conductive pin 8 extends substantially along the entire length of meander conductor element 1 in the direction of longitudinal axis 3 and is supported by frame 2 substantially at the radially outermost portion of frame 2, i. e. at the outer surface thereof, which is best seen in Fig. lc. Optionally, there may be provided a groove or the like (not shown) in the outer surface of frame 2 wherein pin 8 may be disposed. In the present embodiment, pin 8 is further supported by frame 2 radially outwards in the direction of a user's head 12 (see Fig. lb) during use of mobile telephone 5. That is, pin 8 is positioned at the front side 13 of frame 2.

Besides, further pins or conductive elements may be disposed at the antenna device. Particularly, a second electrically conductive or semi-conductive pin (not shown) may extend along longitudinal axis 3 and being connected to ground or being left in open circuit.

Fig. ld shows pin 8 removed from frame 2, which preferably is an electrically conductive or semi-conductive pin made of, e. g. copper, stainless steel or a polymer. Typical dimensions of pin 8 are a length of 20-80 mm, and a cross section of 0.1- 1 x 0.1-1 mm2. However, for each mobile telephone design, and particularly for each frame design and radiating element design, pin 8 may be designed and sized in order to achieve optimal performance, e. g. as regards gain and near field radiation. Electrically conductive or semi-conductive pin 8 may be replaced with an electrically conductive or semi- conductive wire, band, pattern, or the like.

Typical for the invention is that conductive pin 8 is designed to carry a current comparable to that in the meander and thus giving near fields of comparable amplitude but with a phase which is at least partly opposite. By a suitable choice of parameters it is possible to reduce the near field considerable. The pin 8 has poor radiating performance which gives an influence on the far field (radiating) which is small compared to the influence on the near field.

Fig. le shows a possible shape of the meander radiating element 1 being an etched or printed conductor pattern on a dielectric flexible film 14 in a flat configuration. The radiating element extends from the feed point 6 at one edge of film 14, which has an essentially rectangular shape, in an alternating curve including parallel sections and turns to the free end 7 at an opposite edge of film 14. The single meander radiating element is to be formed from the flat configuration into a configuration wherein film 14 is tubular or, at least

forms part of a cylinder as indicated in Figs. la-c. This is achieved by arching meander element 1 about frame 2, so that the angle relative the longitudinal axis 3 increases and decreases alternately. Such a configuration is compact, provides high durability and can be used in a wide variety of antenna applications. Alternatively, meander element may be arched and mounted on the interior surface of hollow frame 2 (not shown).

Alternatively, instead of using flexible film 14 as carrier, the meander radiating element may be mounted on other carrier or directly on frame 2.

Meander radiating element 1 may have other geometries such as a meander pattern diagonally on film 14 or rotated 90° in the plane of film 14 as compared to the illustrated case.

The term meander is used to describe the radiating element because it is generally a preferred shape. As anyone skilled in the art will recognize an infinite number of other shapes will yield equivalent result by creating an elongation of the conductor making it typically a quarter of a wavelength, such as jointed line pattern, e. g., the pattern 18 as shown in Fig. lf, having a feed point 19 and a free end 20, respectively.

Particularly, an antenna pattern for operation in a high frequency band has only 1-2 bends.

Furthermore, in other configurations pin 8 (or other electrically conductive or semi-conductive means) may be integrally disposed on film 14 (not shown in the Figs.), whereby also pin 8 may be formed through etching or printing.

Preferably, radiating element 1 extends tangentially in angular ranges measuring less than 360° around longitudinal axis 3, i. e. extending less than a full turn as is best seen in Fig. lc. Electrically conductive or semi-conductive pin 8

extends preferably in angular ranges where radiating element 1 is absent.

Furthermore, said frame may further support an impedance matching circuitry (not shown) connecting feed point 6 of radiating element 1 to circuitry of mobile telephone 5.

Considering next Fig. 2a-e, a second embodiment of the present invention will be described, which embodiment is identical with said first embodiment except of the designs and positions of the electrically conductive or semi-conductive pin 8'and the meander radiating element 1', 1", respectively. All other parts in Figs. 2a-e have same reference numerals as in Figs. la-e.

Pin 8'is in this embodiment supported by frame 2 at a side which is located opposite to the side facing a user's head 12 (see Fig. 2b) during use of mobile telephone 5. That is, pin 8'is positioned at the back side 16 of frame 2.

Furthermore, pin 8'has a lateral extension 16 in the vicinity of its first end (9'). In Fig. 2d this is clearly visible, said lateral extension existing in a tangential direction with respect to said central longitudinal axis 3 from the back side 16 to the front side 13 of frame 2, i. e. arched around frame 2, to be connected to ground of the mobile telephone 5.

Alternatively, pin 8'may extend in a radial direction, i. e. straight through frame 2, which in this case is provided with suitable apertures.

In other respects, e. g. regarding size and material, pin 8' may comprise the various features as depicted in previous embodiment.

With reference now to fig. 2e, dual meander elements 1', 1'' on a common film 14'are shown, which are tunable to two different frequencies allowing operation of the antenna device

in two separated frequency bands. These elements are fed at feed points 6', 6'', which in the illustrated case coincide.

Said feed points are to be coupled to circuitry of mobile telephone 5, possibly via an impedance matching means (not shown).

It would also be possible to arrange more than two meander elements together in order to achieve operability in more than two frequency bands or still wider band (s) than could be achieved by two elements. Although depicted in a well- functioning flat configuration in fig. 2e, the flexible film 14'of the dual meander elements 1', 1''is preferably intended to be formed in a cylindrical configuration as described with reference to previous embodiment of the invention. Alternative designs of radiating elements may be employed such as those depicted in previous embodiment.

A third embodiment of the present invention (not shown in the Figs.) comprises an electrically conductive or semi-conductive pin as in previous embodiments, but supported by said frame radially outwards and towards a direction which forms a predetermined angle together with the direction towards an intended location of a user's head during use of the radio communications device with the frame mounted. The predetermined angle may range from 0 to 360°, but ranges more preferably from 2 to 10°, and most preferably from 5 to 10°.

The reason for the large reduction, which can be achieved by employing the invention, may be understood if the fields around the phone are seen as a sum of elementary spherical wave functions. As described in J. E. Hansen: Spherical Near- Field Antenna measurements, Peter Peregrinus 1988, the fields outside of any radiating structure (like a telephone in free space) can with full accuracy be described as a sum of spherical wave functions."Outside of"mean outside of any sphere enclosing the structure and the wave functions can

(without details) be written Snm (r, O,) where r is the radial distance, A the angle from zenith and (p the azimuth angle, all according to spherical co-ordinates. n and m are indices and the order of this two-dimensional set and m is never larger than n. Sn, m (r, O, (p) forms a kind of basis for a two- dimensional Fourier series if r is fixed and thus can be used to describe any wave pattern if sufficiently many terms are included.

Two important conclusions from the theory of the spherical wave functions are (see the earlier mentioned reference by Hansen): If the sphere has a radius of a and the wave number is k (=27t/, being the wavelength), then Sn, m having il > 1.5 ka gives very little contribution in the far field. A limited number of Sn, m can thus be used to describe the radiation field far away with good accuracy. For a small phone at < 1 GHz, n=2 may already be too large, which means a dipole like field (n=1 corresponds to dipoles).

Sn, m (r, O,) will in case of large n decrease sharply for increasing r until approximately kr > n. That means that even if there is a complicated current pattern at small r (i. e."on the phone") the field at large distances (kr » n) will not change.

Together this means that it is possible to find a current distribution on a radiating body, which at least in some directions will give a fairly weak field at small distances but not affect the radiation field at big distances. The extra element on the antenna has been found to give this desired modification of the near field without changing the far field too much.

Practical tests on modern mobile phones with fixed exterior meander antennas have given a robust SAR reduction of 40% or more and some tests indicate that more than 90% reduction is a realistic goal.

As a conclusion, POP is heavily reduced by employing the present invention, and lowest values are obtained by the prototype in accordance with the first embodiment of the present invention.

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