An antenna arrangement for small-size radio communication devices

Antenna arrangement for small radio communication equipment

The present invention relates to an antenna arrangement of a radio communication device, such as a radiotelephone, for establishing a two-way connection. The purpose of the antenna arrangement is to reduce the radiation of the antenna in the direction of the body and on the other hand to reduce the effect of the body on the properties of the antenna.

In the known antenna structures for personal radio communication devices, the aim has been to achieve the most symmetrical radiation pattern possible so that the position of the communication device does not affect the quality of the radio connection. However, the symmetry of the radiation pattern is realized only in free space. Various obstacles in the vicinity of the radio communication device affect the directional pattern of the antenna. Personal radio communication devices are now so small that, for example, in radiotelephones where the headset is fixedly connected to the telephone, the antenna in the operating position of the telephone comes so close to the body that the body affects the directional pattern of the antenna. The symmetry of the directional pattern is not realized in typical operating positions of the radiotelephone if the radiotelephone is small in size and the antenna comes very close to the head and hand of the user of the communication device.

Efforts have been made to reduce the effect of the body on the directional pattern of the antenna and on the other hand the radiation towards the body by solutions in which various shields placed between the antenna and the body attenuate the radiation or change the shape of the radiation pattern. Various solutions for directing the radiation pattern of the antenna away from the user's end are known, for example, from EP 588 365, EP 588 271 and US 5,451,965. The disadvantage of these is that in part the user's hand can still interfere with the antenna signal. Solutions are also known in which the radiating antenna element is sought to be moved as far as possible from the user of the telephone. An example of this is given in US 5,513,383. However, these solutions are quite difficult to implement if the radiotelephone is very small.

It is noteworthy that international standardization organizations are defining measurement methods and limit values ​​for radiation emitted by mobile phone transmitters into the body, so this must be taken into account in antenna design in the future. Radiation is measured in typical operating positions of the phone, the so-called SAR measurements. (Specific Absorption Rate). This needs to be taken into account in particular for cellular handsets, which operate at relatively high transmission power in relation to the size of the phone, such as AMPS, GSM, DCS and in the future

UMTS. The radius of the cell, i.e. the maximum distance of the telephone to the nearest base station, in these systems is usually several kilometers, even tens of kilometers, and the power of the handset transmitter is up to 2W.

The size of personal mobile stations is constantly decreasing, which makes it more difficult to use antenna solutions according to the prior art. The problems of the solutions according to the prior art are particularly emphasized if the radiotelephone is fixed to the body, for example with a wristband to the user's wrist. In this case, it should be possible to minimize both the effect of the hand on the antenna and, in the operating position, the effect of the head on the antenna.

The prior art discloses antenna solutions for pager devices for attachment to the wrist. However, the pager does not include a transmitter, but only a receiver, so the radiation of the antenna towards the body does not become a problem with the pagers. In addition, the operating position of the pager is different. The pager is not brought close to the head in the operating position like a wristband phone. Some wristband antennas have been designed to reduce the effect of the hand on the antenna, but not the head. In addition, the pagers have sought an antenna directional pattern that is as comprehensive as possible in each direction.

U.S. Pat. No. 5,564,082 discloses a wrist-mounted radiotelephone in which one antenna is arranged on the wristband and the other antenna on a speaker holder detached from the wristband. Both antennas remain in the operating position of the phone when the loudspeaker is in the ear, in the blind spot between the hand and the head.

The requirements of the cellular network for the antenna used in the telephone should also be noted. The connection between the mobile telephone and the base station is two-way, in which case the base station of the mobile telephone network must be able to receive the signal transmitted by the mobile telephone. When the mobile phone is placed on the wristband, or else you want to make the phone as small as possible, the transmitter cannot be oversized to ensure a connection. Using a higher power transmitter would result in the need for a larger power source (battery), making the phone large and heavy and making it very difficult to place on the wrist. The requirements for the radiotelephone antenna are therefore considerably stricter than for the antennas of devices with only a receiver. In broadcast receiver and paging systems, the transmitters are fixed, so that they can be large, and the power supplies are connected to a fixed electrical network, so that the transmitters can be oversized and the quality of the antennas of the receivers can be compromised, respectively.

It is an object of the present invention to avoid the disadvantages of the prior art and to implement an antenna arrangement for two-way radio communication devices which is suitable for use in personal mobile stations. The directional pattern of the antenna is arranged so that the effect of the body on the directional pattern and on the other hand the radiation applied by the antenna to the body are as small as possible. The directional pattern of the antenna is oriented in the opposite direction to the radiating direction of the speaker, and the antenna is or can be arranged in a position where it is substantially in the body of the device outside the speaker. The asymmetric radiation pattern of the antenna is a disadvantage in many applications, but this feature is exploited in the present invention.

The radio communication devices according to the invention and their features are apparent from the appended independent claims 1 and 8 and the preferred embodiments from the dependent claims.

Thanks to the invention, the body does not substantially affect the radiation pattern of the antenna, and the radiation of the antenna is not directed at the body in typical operating positions of the mobile station. The antenna element according to the invention can be implemented as a thin planar structure, whereby it is very suitable for very small personal mobile stations. It is particularly well suited, for example, for a wrist-mounted mobile phone which is intended to be used without detaching the phone from the wrist during a call. At the ground level, which is part of the antenna structure, the application of radiation to the hand is attenuated and, on the other hand, the effect of the hand on the shape of the antenna radiation pattern is reduced. The antenna arrangements according to the invention, on the other hand, solve the fact that the radiation pattern of the antenna is directed away from the user's end in the position in which the telephone is used during the call.

In some embodiments of the invention, antenna solutions are provided in which the invention can be utilized in connection with conventional handsets.

In one embodiment of the invention, an antenna solution for a mobile station which can be folded into a smaller space is further presented.

The invention and its other features and advantages will be described in more detail below by way of example and with reference to the accompanying drawings, in which Figure 1 shows a prior art handset in its typical operating position, Figure 2 shows the operating position of a wrist-mounted mobile phone during a call, Figures 3a, 3b and 3c show prior art a microstrip antenna viewed from different directions in Figures 4a, 4b and 4c show a prior art PIFA antenna seen from different directions. Fig. 5 shows a symmetrical planar antenna structure according to the invention with two radiators; Fig. 6 shows a radiation pattern of a planar antenna when a wide ground plane is used; is close to the size of the radiator, Figures 8a and 8b show an arrangement of an antenna element according to the invention, Fig. 9 shows an arrangement of an antenna element according to the invention, Fig. 10 shows an arrangement of an antenna element according to the invention, Fig. 11 shows a hand-held radio communication device and an arrangement of an antenna element according to the invention when the radio message is in standby mode, Fig. 12 shows radio communication according to Fig. 11 , Fig. 13 shows an arrangement of an antenna element according to the invention, Fig. 14 is a block diagram of a radiator selection circuit according to the invention when the antenna element comprises two radiators.

In the following description, call time refers to a situation in which a call, i.e. a two-way radio connection, is established between a telephone and a base station.

In this case, both the transmitter and the receiver are in use on the phone. In the description, standby means the operating mode of the telephone in which it is ready to receive a call, in which case the receiver is active, the transmitter is not active.

In the description, a wristband telephone is a radio communication intended to establish a two-way radio connection, intended to be attached to the user's wrist and intended to be used attached to the wrist in both standby and call mode. In the description, a handset means a radio communication intended to establish a two-way connection and which is of a size and shape suitable for hand-holding.

Figure 1 shows a prior art handset 10 in a typical operating position. In such handsets of sufficient size, the antenna 11 can be placed at the top of the telephone so that the antenna is placed in a free space from which there is a sufficient distance for both the user's hand and the user's head for the operation of the antenna. In this type of telephone, it is generally also possible to implement an antenna structure in which the antenna can be pushed into the telephone. This protects the antenna from being broken when the phone is in standby mode and is held, for example, in your pocket or belt.

Figure 2 shows a typical operating position of a radiotelephone 20 for attachment to the wrist. Prior art handset antenna solutions are not suitable for such a telephone. The length of the rod antenna according to Figure 1 must be proportional to the wavelength of the reception and transmission frequency used by the telephone. This results in a well-functioning rod antenna being so long that it is very inconvenient to use in a wristband phone. The length of the rod antenna in a handset operating in the 900 MHz frequency range is typically about 10 cm. The phone used in the wristband is so small that the rod antenna cannot be pushed into the body of the phone in standby mode. Another antenna commonly used in handsets is a cylindrical helical antenna, which is shorter but rather large in diameter and thus also difficult to place in a compact telephone.

The rod antennas and helix antennas used in handsets are omnidirectional and aim for a symmetrical radiation pattern, so when using such antennas in a wristband phone, a sufficient distance should be provided to the antenna both in the user's head and hand, at least in a call situation.

Antennas for wrist-mounted devices are used, for example, in pagers. Paging devices only have a receiver, not a transmitter. Wristband phones also have a transmitter and phone standards that limit the effect of the electromagnetic field generated by the antenna on the user's body (SAR) during transmission. The effect of the electromagnetic field is measured in typical operating positions of the phone. Wrist-mounted pagers have sought antenna structures in which the antenna would receive a signal as well as possible from all directions. In addition, the operating position of the pager is different from that of the wrist unit, so the antennas of the pager are not suitable for use in wristphones.

Figures 3a, 3b and 3c show a microstrip patch antenna 30. It consists of a conductive film 31 (patch) formed on a plate-like base material (substrate) 32 having certain electrical properties (permittivity). The conductive membrane 31 acts as a radiator part of the antenna structure and on the opposite side of the plate there is a uniform ground plane 33 (formed, for example, from a conductive membrane). The radiator supply 34 takes place, for example, coaxially through the ground plane or on the transmission line 35 from the edge of the plate.

A square radiator is in resonance when its side length corresponds approximately to half the wavelength of the base material used. By using a base material with a high permittivity, the size of the radiator and thus also of the entire antenna can be reduced. However, this leads to a deterioration in the efficiency of the antenna, so that the dimensions of the antenna cannot be made very small. The radiation pattern and bandwidth of the antenna are strongly influenced by the shape of the radiator. The dimensioning of the microstrip patch antenna has been explained in more detail e.g. In: '' Microstrip antennas, IJ Bahl and P.Bhartia published in 1980 by Artech House Inc. of Delman, MD, USA. When using a microstrip antenna in the present invention, the microstrip antenna generates a radiation pattern asymmetrical with respect to the radiator 31.62.72 so that the gain maximum 74 of the directional pattern 61.71 is directed to the radiator side and the radiation to the ground plane 33.63.73 is small. The shape of the radiation pattern is roughly shown in Figures 6 and 7.

Figure 4 shows another planar antenna solution 40 PIFA (Planar Inverted F-Antenna). It is radiated by a metal plate 41 attached at one end to the ground plane 42. The supply can be made, for example, coaxially to the radiator 43. The electrical properties of the antenna structure in question can be influenced e.g. the shape of the radiator, the location of the feed point in the radiator and the distance of the radiator from the ground plane. The dimensioning of PIFA antennas has been explained in more detail, e.g. in: Analysis of a Probe-Fed Short Circuited Micro Strip Antenna, IEEE Transactions on Vehicular technology, Vol 45 No.3, August 1996. The PIFA antenna is also used in the present invention so that the radiation pattern formed by it is asymmetric so that the gain maximum is directed at the radiator. 41 side and radiation to the ground side 42 side is negligible.

The shapes of the radiation patterns shown in Figures 6 and 7 thus also apply to a PIFA-type antenna.

Many modifications can be made to the above antenna structures in order to make the antenna suitable for mass production and mechanically robust. For example, in a PIFA-type antenna, a solid having a permittivity approximately equal to the permittivity without air permittivity can be used in the space between the radiator and the ground plane, in which case the antenna structure is made more stable. Likewise, the supply to the radiator can be arranged in many different ways, for example by making the antenna a Surface Mounted Device (SMD), in which the supply point of the antenna is connected from the radiator to its solder pin at the edge of the ground plane. The antenna element may also comprise a plurality of radiators parallel to the ground plane on the same side of the ground plane. The shape of the radiator is also not limited to a planar structure, but the radiator may be, for example, a wire bent parallel to the ground plane, as is the case with an antenna of the IFA (Inverted F-Antenna) type.

Common to all these antenna structures that can be utilized in the invention is a large ground plane and a substantially ground plane radiator or radiators, or that the planar antenna is intended to be mounted on a wide ground plane to form a similar structure characterized by an asymmetrical radiation pattern with maximum orientation away from the ground plane.

Regarding the magnitude of the surface area required for the antennas, it can be mentioned that planar antennas implemented with current technology suitable for current cellular radiotelephone systems require a minimum surface area of ​​several square centimeters.

Fig. 6 shows roughly the direction pattern 61 of the antenna when a very large ground plane 63 is used relative to the size of the radiator 62. However, in small telephones the size of the ground plane 63.73 has to be limited to close to the radiator 62.72, in which case the antenna also starts to radiate somewhat. side. Figure 7 roughly shows the shape of the radiation pattern 71 when a lower ground plane 73 is used. The circumferential distance 74 of the radiator from the radiator describes the relative strength of the electromagnetic field generated by the antenna in that direction when the antenna is connected to the transmitter.

By placing such a planar antenna on the wristband or the phone attached to the wristband so that the ground plane is between the wrist and the radiator, the maximum radiation pattern of the antenna is directed away from the wrist.

The minimum of the antenna radiation is directed towards the hand and, on the other hand, the effect of the hand on the directional pattern of the antenna is reduced.

In a wristband phone, at least a portion of the phone, such as a microphone 84 and a headset 85 (Figure 8b), is placed on the palm side of the hand to allow use of the phone without removing it from the wrist. In some solutions, all the functions of the phone can be placed on the palm side of the wristband. Sometimes it may be justified to place all the functions of the phone on the back of your hand.

However, if the phone as a whole is placed on the palm side of the hand, the antenna 15 cannot be placed on the same side, because in the wristband operating position of Figure 2, when the antenna ground plane is towards the wrist, the maximum radiation pattern would be directed towards the user's head. In addition, the antenna would come completely blind in the space between the hand and the head.

Figures 8a and 8b show the location of the antenna, which can avoid the antenna problem of this type of telephone. The planar antenna element 81 is positioned on the back of the hand so that the ground plane of the antenna faces the hand and the ground plane is between the radiator 86 and the wrist. The supply of the antenna from the radio part 87 to the antenna can be arranged inside the wristband 82, for example with a flexible coaxial cable 83. The advantage is that the antenna thus has a rather large surface area and thus a good efficiency. The radiator of the antenna is made large, whereby the efficiency of the antenna is good and the ground plane is large compared to the radiator, whereby the radiation in the direction of the hand is small. In addition, the directional pattern of the antenna remains the same and parallel regardless of the diameter of the wristband, which can vary quite a bit depending on the user.

If the phone is placed almost entirely on the back of the hand, the problem will be the rather large area required by the phone's user interface (e.g. keyboard and display) and antenna. Current digital cellular telephones 35 include, for example, short message services (SMS) that send text messages. To make writing and reading text messages as easy as possible, the display 91 (see Fig. 9) should be as large as possible and also the keys 92 should be as large as possible. As previously mentioned, the surface area of ​​the antenna element 93 should be as large as possible to achieve the correct directional pattern and good efficiency. The antenna should be positioned so that the radiator side 94 of the antenna is not covered by components or material that attenuate the signal transmitted or received by the antenna or substantially change the direction pattern of the antenna. The radiator element can be coated with a suitable protective material, for example low-loss thin plastic, but placing the display or keyboard on top of the antenna would present a variety of problems.

Also, the antenna cannot be placed on the palm side of the hand, which would now have space, as the parts of the phone except the microphone and headset are on the back of the hand. The antenna on the palm side of the hand would be in the shadow between the hand and the head and the maximum of the directional pattern would be directed towards the head in the operating position of the phone.

Placing an antenna with a sufficiently large surface area on the side of the wristband would cause problems, as the antenna should not be substantially wider than the wristband and the antenna should adapt to different sized wrists to make it comfortable for the wearer. In addition, the requirement for the shape of the radiation pattern must be taken into account. The radiation pattern should not be directed towards the user's head in typical operating positions. If, for example, the antenna were placed on a wristband on the thumb side, it would be quite easy to accidentally use the telephone momentarily, for example with the antenna's radiation directed directly towards the user's eye, even with the antenna in direct contact with the eye.

Figure 9 shows a solution that allows the use of a planar antenna with a sufficiently large surface area in a wrist phone. There, the ground plane 95 of the antenna faces the user's hand, with the directional pattern of the radiator 94 facing away from the hand. The antenna is placed in the back of the hand so that its direction pattern is directed sector of the side back of the hand between the finger and the little. In this case, it is very difficult to accidentally use the telephone so that the maximum of the directional pattern is directed towards the user's head. The antenna can be hinged 98 to both the body 96 of the telephone and the wristband 97 to accommodate different sized wrists. The antenna and the antenna connection to the telephone can be made symmetrical so that the antenna can be connected to the telephone for either a right- or left-handed telephone user. The ground plane of the antenna and the ground of the telephone can be connected via a joint. The antenna can be fed, for example, by a separate flexible coaxial joint or via a joint (not shown). If the telephone body 96 can be made small enough, the antenna element can also be divided into several parts or shaped to surround the keypad 92 and the display 91.

Because the use of a large-area phone on the wristband is cumbersome, one solution is to make the phone more multi-part so that the parts are folded on top of each other when the phone is in standby mode. The phone is only opened in a call situation so that all parts of the user interface are visible and accessible.

Fig. 10 shows a solution in which the antenna 100 is placed on a protective cover 101 which can be turned in the standby mode to protect the keyboard and / or the display. With this solution, an antenna with a fairly large surface area and thus also a good efficiency can be realized. Since the protective cover pivots about 150180 degrees when opened, the directional pattern of the antenna also pivots accordingly towards the wrist of the phone user. This problem can be solved by making the cover with a symmetrical antenna structure according to Fig. 5, with a ground plane 51 in the middle of the cover and radiators 52,53 on both sides of the cover. The cover hinge 54 may be provided with a switch or the hinge may control a switch that detects the position of the cover. Depending on the position of the protective cover, the one whose radiation pattern is directed substantially away from the hand in that position is selected as the radiator. The radiators can be replaced, for example, in a position where the cover is at an angle of about 90 degrees to the phone.

In addition, it is possible to use an antenna in the wristband phone so that both radiators are utilized when the protective cover is opened at an angle of about 90 degrees. The antenna can be used either so that both radiators are connected in parallel when the cover is opened, or the receiver can, for example, at certain intervals switch both radiators on alternately and compare which one gives a better signal in terms of quality or strength. Radiotelephones have measuring circuits that monitor the strength and quality of the received signal. The selection of the radiator is made on the basis of these measurement results. The radiator selection circuit is illustrated in Figure 14 and the corresponding explanation. Even in this position of the protective cover, the maximum of the directional pattern of the antenna is directed away from the end. The distance from the antenna to the hand increases, and the maximum of the directional pattern does not point directly at the hand.

If one radiator is used in standby mode and two radiators in parallel in call mode, it must be taken into account that the impedance of the antenna is different in standby mode and in call mode. In this case, the telephone must be equipped with two separate antenna supply circuits or the impedance of the antenna supply circuit is made adjustable so that the impedance of the supply circuit is always substantially the same as the impedance of the antenna in use. The impedances of the antennas can be dimensioned using the previously mentioned publications on planar antenna structures and the publications mentioned therein. The dimensioning of the matching circuits is known to those skilled in the art. publication: Antenna Impedance Matching, Wilfred N. Caron published in 1989 by American Radio Relay League, Newington, USA.

In a wristband phone, the protective cover can also be made slidable and equipped with a planar antenna, in which case two radiators are not required. In the standby mode, the cover is pushed onto the keypad and / or display and opened during a call. The ground plane comes to the bottom surface of the cover and the radiator to the top surface. The grounding of the antenna and the connection of the radiator to the antenna take place, for example, by sliding contacts or flexible cables.

The invention can also be applied to handsets which are not intended to be attached to the user's wrist. In the call mode, the handset is brought close to the user's head so that the telephone's handset 116 comes close to the user's ear and the sound 114 produced by the handset is directed toward the ear. Correspondingly, in the operating position, the microphone 117 is sought to be brought as close as possible to the user's mouth so that the speech 115 is directed as closely as possible to the microphone 117. The handset is brought into such an operating position by grasping the body of the handset, typically the portion 118,138 below the handset 116. To cover the sound and modify the frequency response, covers 113, 119, 134, 136 covering the headset and the microphone are used, which are provided with openings of suitable size and shape.

The antenna is shielded by the user's hand if the planar antenna is placed in the handset body in the body portion 118 between the headset 116 and the microphone 117, or at least substantially in said area, or the antenna is in the keypad cover 110, which is opened downward in the user's mouth. Accordingly, in Figure 13, placing the antenna substantially in the area 138 of the body that the telephone user grips with his hand would cause a shading problem. All of the above antenna arrangements have the disadvantage that in a typical operating position the antenna remains in the space between the user's hand and head. In these cases, the characteristics of the antenna may change significantly due to hand and head. The aim of the placement of the antenna element according to the invention is to avoid the above-mentioned disadvantages by placing the antenna substantially outside the area which the user typically grips with his hand.

The present invention can also be applied to a conventional handset as shown in Figs. In the call position, the antenna located on the cover 111 is pivoted upward away from the blindfold between the hand and the head, whereby the effect of the hand and the head on the antenna is minimized and further the antenna directional pattern 112 is directed away from the end 120. If the protective cover is on the same side as the telephone handset according to Fig. 11, the structure of the two radiators according to Fig. 5 is not required, because in the standby position the direction pattern is directed away from the telephone and in the call position away from the end.

The present invention can also be applied using, for example, the symmetrical fixed antenna structure 130 of the two radiators 131,132 of Fig. 13. There is a ground plane 133 between the radiators. The selection of the radiator can be performed, for example, as follows. In a call situation, the radiator (132) on the opposite side to the headset 134 is selected for use, whereby the radiation pattern is always directed away from the end in the call situation. The telephone controller section 148 always knows when an incoming call has been answered or an outgoing call is initiated, i.e. when the transmitter is on. This same information can be used to select the antenna radiator in call mode.

The radiator 131 can be switched on in standby mode. For example, if the telephone is placed on an electrically conductive surface so that the radiator 132 is between the ground plane of the antenna and the electrically conductive surface, the operation of the antenna may be disturbed. In standby mode, the transmitter is not on, so there is no radiation to the body and the selection of the antenna radiator can be made on the basis of which radiator works better.

Fig. 14 is a block diagram of a radiator selection circuit 140 when the antenna element has two radiators 141, 142 and the selection of the radiator is performed based on the received signal strength or quality. The received high frequency signal is amplified in amplifier 144 and mixed in mixer 145 into an intermediate frequency or baseband signal and further amplified in amplifier 146. Mobile telephones have a measuring circuit which monitors the strength of the received signal in a so-called RSSI circuit (Received Signal Strength Indicator and Signal Quality (BER.Bit Error Rate)). The signal is passed to circuit 147 where these signal strength and quality measurements are performed. The measurement results are passed to controller section 148 which interprets the measurement results and signal strength and / or quality degradation. directs the antenna select switch 143 to another position.

The signal measurement is performed again in this position of the switch. After the measurements, the antenna selector switch is left in a position that produces a better signal.

The measurement can be repeated at certain intervals so that the telephone performs the comparison while performing RSSI measurements, for example on the basis of instructions given by the telephone network.

The comparison can also be performed only if necessary, i.e. when it is noticed that the received signal strength has decreased, the radiator is switched and it is tested whether another radiator produces a stronger signal, if it produces it, then it is connected to an antenna.

The preferred applications of the invention are small cellular radiotelephones, the scope of application of the invention is of course not limited to these primary applications, but the invention can also be used in other radio stations of different constructions, in which the use of properties. Examples are lower power cordless telephones and personal radio communication devices divided into several parts. A radio communication device divided into several parts means, for example, a structure in which the user interface of a communication device is a separate unit that communicates with the radio part of the communication device using a low-power radio link.