WO2005086277A2

WO2005086277A2 - Antenna array with a first and second antenna for use in mobile applications

Info

Publication number: WO2005086277A2
Application number: PCT/IB2005/050632
Authority: WO
Inventors: Heiko Pelzer
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2004-02-25
Filing date: 2005-02-22
Publication date: 2005-09-15
Also published as: WO2005086277A3

Abstract

A compact antenna array comprising a first antenna and a second antenna particularly for use in mobile applications, which are both designed for operation in a common frequency range and comprising a control device, which selects an operating mode depending on the signals assigned to the antennae. The operating mode determines whether both antennae operate or only one of them operates, whether the antennae are operated with a phase offset and which antenna is influenced by what power.

Description

Antenna array

An integrated antenna array having a multiplicity of STUB antennae is known from US 2003/0052828 Al. The antennae of the antenna array are divided into sub-arrays. The sub-arrays make it possible to receive different channels from a provider or several providers simultaneously. Even personal settings can be provided by using a personalized card with sub-arrays being allocated to each card.

It is an object of the invention to improve the reception and transmission characteristics of mobile applications. Furthermore, it is an object of the invention to reduce the radiation burden of a user of such applications. The object of the invention is achieved by the features mentioned in the patent claim 1. As a result of the measure of feeding a control device with signals from at least two antennae of an antenna array, it is possible to select an operating mode in dependence on the signals fed to the control device. The antennae of the antenna array are then equally suitable for reception and transmission of identical frequencies. As there are at least two antennae, it is possible in case reception by one antenna is poor, to use the other antennae for reception and also for transmission. Poor reception can be the result of an absorber in the immediate surroundings of the antenna, such as the head of a user, for example. Reception can thus be improved by proper selection. Similarly, the antenna that has a better connection to the nearest base station can be used for transmission. By selecting an operating mode in which several antennae of the antenna array can be operated, it is possible to align the radiation field to the antenna array in a purposeful manner. The radiation burden of the user can be reduced by selecting a radiation field directed away from a user. Absorption by the user's head can thus also be reduced. If the absorption is reduced, transmission can be effected with less power, its quality remaining the same. The lower use of transmission power leads to a reduction in the power needed and thus to an increase in the operating lifetime. Detectors could be assigned to the antennae through which the distance from the antennae to the head can be detected. If these signals are fed to the control device, an operating mode can be selected in which the radiation field is directed away from the user's head. Transmitter power control, part of for example Radio Subsystem Link Control in the GSM system, can be used for controlling the operating mode. By operating antennae of the antenna array with a phase offset, the transmission and reception frequencies can be fine-tuned. Furthermore, a phase offset will also allow a spatial alignment of the radiation field The use of ceramic antennae allows the antenna array to be particularly compact. This helps enhance the insulation between the individual antennae and this also makes it possible to use such antenna arrays in smaller mobile devices such as mobile telephones. The power can be divided over the operated antennae as desired by providing a power divider. The effect of this is that it is possible to make a purpose- oriented modification and alignment of the radiation field. A memory in which a multiplicity of operating modes is stored can be provided in the control device. Several parameters can be taken into consideration when the operating mode is selected. Thus, the signals received by a base station can be taken into consideration for setting the power level. Furthermore, there may also be noticed whether the signal received by one antenna becomes stronger and what orientation the signal has. Accordingly, an operating mode can be selected by which the transmitter power is aligned in the direction of this base station. When selecting an operating mode, in which only one of the antennae is operated, it can be arranged such that the other or further antennae in the antenna array can be terminated by defined impedance to increase the efficiency of the operated antenna. In the examples of embodiment discussed above, a short circuit to ground metalization of the application is generated, an open 50-ohm line is used or a 50-ohm resistor is used as the defined impedance. If the antennae are arranged angular to one another, the radiation fields of the antennae are aligned at an angle. The antennae are preferably arranged at right angles to one another. This has the effect that the antenna array has better sensitivity to radiation with different polarities. These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter. In the drawings are shown in:

Fig. 1 user with antenna array, Fig. 2 antenna array in top view, Fig. 3 antenna array shown in Fig. 2 in three-dimensional view, Fig. 4 S parameters of the antenna arrays shown in Figs. 2 and 3, Fig. 5 S parameters of the top antenna, if the side antenna is terminated by various resistors, Fig. 6 S parameters of the side antenna, if the top antenna is terminated by various resistors, Fig. 7 S parameters of the top antenna and of the side antenna plotted against the position with respect to the head, if the inactive antenna is terminated with 50 ohms, Fig. 8 shift of the frequency position plotted against in relation to a phase shift, Fig. 9 a control device, Fig. 10 switch configuration 1 with antennae, and Fig. 11 switch configuration 2 with antennae

Fig. 1 shows schematically a user with an application of an antenna array 1. The application in this case is a mobile telephone. The antennae 2, 3 are shown schematically as bold areas. The connecting line stands for a PCB 4. Figs. 2 and 3 give a detailed view of an antenna array. It can be seen that a first antenna 2, designated hereafter as top antenna and a second antenna 3, designated hereafter as side antenna, are arranged on the PCB 4. Top stands for the arrangement of the antenna on one of the shorter lateral edges of the PCB 4. Side stands for the arrangement of the antenna on one of the longer lateral edges of the PCB 4. Fig. 1 has left out illustrating a housing surrounding the PCB 4 and the antennae 2, 3 of the mobile telephone, so that the position of the antennae 2, 3 relative to the head 24 of the user becomes clear. In a typical situation of use, the user holds the telephone to his ear. In such a situation of use, the user's head represents an absorber 12 in the near field of the antenna array 1. Various relative positions of the PCB and, thus also of the antennae, with respect to a head or to a dummy head are shown. The various positions have been designated as Position 1, Position 2 and Position 3. In position 1 the top antenna is arranged at the shortest distance to the head and in position 3 at the longest distance. The line symbolically represents the short top edge of the PCB 4, on which the top antenna 2 is arranged. Before discussing the effect of the position of the antennae, the structure of a possible antenna array will first be further described with the help of Figs. 2 and 3. Ceramic antennae have been used as antennae 2, 3. The design of the antennae shown in Fig. 2 and Fig. 3 is designed for the UMTS frequency range. Antennae with a different design can also be used, which are tuned to another frequency range. For example, antennae tuned to GSM, DCS, PCS and TD-SCDMA can be provided. Both antennae are always of the same type. Such ceramic antennae have a substrate 32 of a material with a dielectric constant greater than 1, preferably greater than 19. This kind of antennae 2, 3 are as a rule very compact and can be arranged in any arbitrary orientation to the PCB 4. The antennae 2, 3 are connected to a power source through a high-frequency feeding line 34. The high-frequency feeding line 34 is connected to an excitation line 36, which is arranged on the substrate 32. Furthermore, a metalization designated below as ground metalization 38 is arranged on the substrate 32. The ground metalization 38 is connected to a metalization 5 through a ground terminal 40 of the PCB. The PCB 4 has an edge zone 6, which is not provided with metalization. This edge zone 6 is needed so that the coupling of the antenna to the metalization of the PCB 4 is not too strong. There may also be provided to mount the antennae separately from the PCB. Apart from such an antenna array, antennae of another type, e.g. PIFA, can also be used. Besides the depicted use in a mobile telephone, such antenna arrays can also be provided in any other applications, especially in mobile devices, such as a mobile computer or in an electronic appointments book. The effect of using such antenna arrays with ceramic antennae is that assembly space can be saved or with identical assembly space, the reception and transmission power can be improved. For example, if two antennae are provided instead of only one, and the two can be operated in the same frequency range and if both antennae are arranged at a distance and preferably at an angle to each other, then both antennae can be used for receiving and transmitting signals. Then, if one of the antennae has poor reception or none at all, transmitting and receiving will still be possible with the other antenna. Fig. 9 shows a control device 8, which is designed as an example for the operation of two antennae 2, 3. This control device 8 has a power divider 11. The power divider, in its simplest form, is a switch. The power divider divides the power provided by a power source 18 in two lines. A phase shifter 10 is arranged in one of these lines. This phase shifter 10 can shift the phase with which the antenna 2 is operated in comparison with the antenna 3. A signal received from a base station 13 is fed back to the control device 8 by the first antenna 2 as well as the second antenna 3 to set the power level. The distance to the absorbers present in the near field, such as the head in the immediate surroundings, can be concluded from this signal. The distance to the absorbers 12 is determined in the one position detector 16 in a controller 9. The distance to an absorber present in the near field of the antenna can also be determined by means of a separately provided sensor e.g. an IR sensor. In this example of embodiment, the signal received by the next base station 13 is forwarded to the control device 8 for power control of the application in the form of a mobile station 7. The method in the mobile telephone is given as an example here. Participants of the radio interface are a base station 13 and a mobile station 7 with the above-mentioned antenna array. The base station. 13 determines the quality of the radio interface. For example, the quality of the radio interface is determined every 480 ms in the GSM system. The base station conveys to the mobile station 7, whether any adaptation is needed in the transmitter power of the mobile station 7, or even whether the connection should be transferred to another base station 13, called Handover. At the same time, the control signal of the base station 13 can be used to change the operating mode of the antenna array 1. A poorer connection may for example result from a change in the position of the mobile station 7 relative to the user's head. The head absorbs in this example a larger part of the radiated energy, so that an adaptation must be made. This adaptation can consist in raising the power level of the mobile station 7, which on the one hand further raises the radiation exposure of the user and, on the other, consumes the energy reserves of the mobile station 7 faster. The described antenna array 1 makes it alternatively possible to select another operating mode which, for example, modifies the radiation field of the antenna array such that the antenna array radiates less energy in the user's direction. The user's head absorbs less energy. This in turn further reduces the user's exposure to radiation. As a result of this, there is more energy available for the radio interface. Therefore, the power level of the mobile station 7 need not be raised, which simultaneously saves the energy reserves of the mobile station 7. Depending on this control signal, the control device 8 thus selects an operating mode stored in a memory 17. The operating mode defines which antenna 2, 3 is charged with what component of the power made available and with what phase offset the antennae 2, 3 are operated. It may also happen that just one of the antennae is controlled. Accordingly, the power divider 11 and the phase shifter 10 are controlled by the control device 8. This can be realized by means of the control signals generated by the control device 8, which are fed to the phase shifter and the power divider. There may also be provided that the power source 18 is selected by means of a signal generated by the control device 8 for making a pre-defined power available, which depends on the operating mode or the control signal for setting the power level. The different operating modes can be stored in a memory 17 in the control device. There may also be provided that the operating mode is generated by computer routines in dependence on the signals fed to the control device 8. Furthermore, a switch 30 can be provided in each of the lines. With this switch it is possible to terminate the respective antenna 2, 3 by different resistors. The switch 30 is arranged in the 50-ohm high-frequency feeding line and has three switching states. In a switching state designated as open, the switch is open and the high-frequency feeding line 34 of the antenna 2, 3 is not connected to the PCB 4 or to the metalization 5 of the PCB. In a second switching state of the switch, the high- frequency feeding line 34 is connected to the metalization 5 of the PCB 4 through a 50- ohm resistor. The antenna is not controlled, however. In a third switching state, the high-frequency feeding line 34 is connected directly to the metalization 5 of the PCB 4. The ground terminal 40 of the antenna 2, 3 is also connected to this metalization 5. The antenna is thus short-circuited through the metalization of the PCB 4. This switching state is designated as short. The termination of an antenna through different resistors can also be pre-defined by the selected operating mode. When selecting the operating mode, it can also be considered whether the signal received by the antenna 2, 3 originates from the same base station or from various base stations. If the received signal originates from various base stations, by selecting the operating mode the radiation field can be aligned to the direction of the base station from which the stronger signal was received by an antenna 2, 3. The radiation field may also be provided to be aligned in the direction of the base station from which the signal received by an antenna becomes stronger in the course of time. Then it may be provided that the strength of the signal received by this base station must exceed a predefined threshold value or must not be for example, 10 or 20 % weaker than that of the signal received by the other base station. In this manner, it is possible to ensure improved readiness for reception and transmission. If one of the antennae becomes defective, then the receiving and transmitting operation will automatically be continued by the remaining operable antenna. Fig. 4 is a representation of the S parameters of the antenna array 1 shown in Figs. 3 and 2. It can be seen from this representation how large the interaction is between the antennae 2, 3. The maximum total efficiency of the side antennae determined in an antenna measuring chamber amounts to 75 %, if the top antenna is terminated with 50 ohms, and that of the top antenna, due to the better adaptation, to 80%, if the side antenna is terminated with 50 ohms. The maximum radiation efficiency, i.e. the total efficiency corrected by the adaptation is between 85 and 90 %. The relative accuracy of the efficiency measurements is better than 5 % while during comparison with measurements carried out in other measuring chambers, deviations up to 10 % are possible. The radiation efficiency is influenced by an absorber in the near field of the antenna array to the extent that the absorber itself represents an additional loss mechanism to the Ohmic and dielectric losses of the antenna i.e. a part of the energy, which is absorbed and then radiated by the antenna(e), is converted to heat in the absorber e.g. induction of loss-prone eddy currents or consequent to dielectric losses, and is thus not available for radiation. The S π -parameter of the top antenna 2 is plotted in Fig. 5, where the non-controlled side antenna 3 is terminated by different resistors. The power consumption of the top antenna 2 is highest when there is resonance, if the side antenna 3 is terminated via "short". The power consumption is lowest when there is resonance, if the side antenna 3 is terminated via „"open". Fig. 6 shows the Sn-parameters of the side antenna 3, if the top antenna 2 is short-circuited by different resistors, open, short or 50 ohms. The side antenna 3 consumes the most power when there is resonance if the top antenna 2 is terminated with 50 ohms. There are several options to realize such a circuit. One option is shown in Fig. 10 and consists in connecting a switch 11 between the high-frequency feeding lines 34 of the individual antennae and the high-frequency feeding line 34 of the inactive antenna being left open. Furthermore, as depicted in Fig. 11 , two more switches 30 can be provided behind this switch 11, which are each implemented in the high-frequency feeding lines 34 of the individual antennae. Such switches are available in various embodiments. In the simplest case, the switch has only two switching states, so that switching can be effected as a standard switch configuration between connecting through, switch 11 connects to the HF line to this antenna, or an open termination, switch 11 connects to the HF line to the other antenna and for example 50-ohm termination. If the switch has 3 switching states, so that it is possible to switch between connecting through or an open termination, termination with a 50-ohm resistor 19 and a short circuit to the metalization 5 of the application. Measurements in an antenna measuring chamber show that the maximum total efficiency of the antenna array, which is between 70 and 75 % in case of a termination of the inactive antenna with a 50-ohm resistor, and the radiation efficiency between 75 and 80 %, can each be improved by 10 to 15 %, if the inactive antenna is short-circuited or remains open. The open configuration is slightly superior here to the one with the short-circuited antenna. The table below shows the radiation efficiency and the absolute efficiency of the antennae, where the non-controlled antenna is terminated by different resis

For top antenna active, the maximum power consumption is found at "short" and for side antenna active the maximum power consumption is found at 50 ohms. These differences are not recognizably reflected in the total efficiency for the following reasons. It should be considered that the adaptation of the antenna and thus the power consumption is already good. An adaptation of-10 dB means that 90 % of the energy can actually be coupled in the antenna; at -15 dB about 97 % is coupled in so that everything else beyond that lies already in the range of measurement errors of the measuring chamber. It can be seen from Fig. 6 that the adaptation for the "short" and "open" configurations is very good. The differences in the radiation efficiency can thus also be seen in the total efficiency. If the top antenna is active, Fig. 5 shows that the "short" configuration is much better suited than the "open" configuration. The higher radiation efficiency is covered in this case by the better adaptation. In the "short" configuration, there is simply so much more energy reaching the top antenna that there is more radiation in spite of greater losses. In the implementation of such a solution, improvement in the adaptation of the top antenna in the "open" configuration can achieve that the improved radiation efficiency and thereby the objective criterion for assessing the losses in an antenna really become effective. Though the adaptation of the side antenna with the 50-ohm termination of the top antenna is very good indeed, but the energy transferred through the radio section from the active to the passive antenna is completely converted to heat in the resistor. In both the other configurations, this energy, apart from the Ohmic and dielectric losses within the antenna, is reflected and is thus not lost. The power consumption of the top antenna 2 is plotted in Fig. 7 for the different positions 1 to 3 shown in Fig. 1, where the side antenna 3 was terminated with 50 ohms during the measurements. A structure and an arrangement, as shown in Figs. 1 to 3, have been used. The resonant frequency of the top antenna is around 2020 MHz. Furthermore, the power consumption of the side antenna is shown in the positions 1-3 in Fig. 7, where the top antenna was terminated with 50 ohms during the measurements. The resonance of the side antenna 3 has shifted in comparison with the top antenna 2, due to the different coupling of the antenna 3 with the metalization 5 of the PCB 4 and the differing effect of the absorber 12 on higher resonance frequencies due to the mutual angular setting of the antennae. In general, the resonant frequency of the side antenna is somewhat higher than that of the top antenna. The reason for this is that the metalization of the PCB 4 also radiates. The different position of the two antennae relative to the PCB 4, however, causes a somewhat different current distribution in the metalization of the PCB 4, which for one thing affects the radiation pattern and, for the other, also has an effect on the resonant frequency. In the case described here, now there is the added effect of the head. This is present in the reactive near field and thus has an effect on the current distribution in the antenna metalization and the metalization on the PCB 4, on account of its electrical and dielectric properties. Within the framework of trial measurements in an antenna measuring chamber, which essentially corresponds to the arrangement illustrated in Fig. 7, it could be shown that depending on the position of the PCB relative to the head trial sample 24 the radiation efficiency of the antennae 2, 3 increases with increasing distance to the head trial sample. In case of the rotation of the PCB shown in Fig. 7, the radiation efficiency for the top antenna 2 increases by almost 10% from position 1, in which the top antenna is near the head trial sample 24, to the position 3, in which the top antenna has the greatest distance to the head trial sample 24. In the position 1 , which is the most remote from the head trial sample 24 the radiation efficiency of the side antenna 3 is also almost 10% higher than in position 3. The maximum change in the distance from the antennae to the head trial sample 24 was less than 1 cm in these trials. This means that less energy that is radiated from the antenna that is the most remote from the head trial sample 24 is absorbed by the head trial sample 24. At the same time, the actually radiated power (total efficiency) for the antenna the most remote from the head trial sample 24, increases by between 5 and 10 %. It can thus be shown that the selection of the active antenna can reduce the power consumption of the mobile application as well as the user's exposure to radiation.

In the total radiation efficiency both the power consumption and the radiation efficiency of the antenna play a role. An operating situation is described in Fig. 8, in which both antennae 2, 3 are operated simultaneously. The power is equally divided over the two antennae 2, 3 by a power divider 11, such as one of the Wilkinson type. It is possible to make a specific shift in the resonant frequency of the radiated signals by using a phase shifter 10. If both antennae 2, 3 are operated with a 180 ° phase offset, then the resonant frequency is shifted to an about 270 MHz lower frequency. This effect is the result of the reciprocal influence of the antennae. When the two antennae are operated simultaneously and have a small distance between them, the near-field of one antenna affects the current distribution in the metalization of the other antenna and vice versa. This changes the effective flow length of the current. This also changes the resonant frequency. In the example of embodiment shown, the resonant frequency is around 2200 MHz when the two antennae are operated and have a 0° phase offset. For a 180° phase offset the resonant frequency is about 1930 MHz. The resonant frequency can in principle be set steplessly between 2200 MHz and 1930 MHz by changing the phase shift, where the distance and the position of the antennae with respect to each other and the design of the antennae themselves must be taken into consideration. Thus, for every selection of a phase shift, the radiation field as well as the frequency is changed. The resonant frequency also changes in cases where only one antenna is operated and the other antenna is terminated by different resistors. This effect is, however, diminished because less than 10 % of the radiated power is coupled into the passive antenna and this portion is further reduced due to Ohmic and dielectric losses. The use of a short circuit or of an open termination affects, in principle, the phase position of the reflected signal. In case of a short circuit, the phase shift in the current is 0°; in case of open termination the phase jump in the current is 180°. The active antenna thus dominates the behavior of the antenna array. In another embodiment, not shown, an antenna array of the type described earlier is used, in which antennae of a different design were provided. These antennae can each be operated at several resonant frequencies. In the special embodiment, for example, the GSM 850 or GSM 900 systems were covered in the frequency range from 824 MHz to 960 MHz together with the DCS and PCS systems in the frequency range from 1710 MHz to 1990 MHz. Despite the great spatial proximity of the antennae, which were placed on a PCB of the same size as for the UMTS array described above, it was possible to restrict the coupling of the antennae to a little less than -9 dB with simultaneous operation. It was possible that the shorter distance due to larger antennae with a size of 24 x 11 xl mm³ and the more than twice the wavelength in the frequency range between 824 MHz and 960 MHz diminishes the performance of the individual antenna consequent to a strong coupling between the individual antennae. At the same time, the advantages shown for the UMTS array described earlier could also be proved. This means the antennae can be operated individually and also together in the same frequency bands. The weak coupling of the antennae also makes it possible to operate the individual antennae in neighboring frequency bands by making small changes in the design, such that a compact multi-band antenna array is obtained, which covers GSM850 and GSM 900 together with GSM 1800 (DCS) and GSM 1900 (PCS) as well as parts of the UMTS system e.g. TD-SCDMA. . REFERENCE LIST

1 Antenna array 2 First antenna 3 Second antenna 4 PCB 5 Metalization 6 Border area 7 Mobile station 8 Control device 10 Phase shifter 11 Power divider / switch 12 Absorber 13 Base station 16 Position detector 17 Memory 18 Power source, Power supply 19 50-ohm resistor 30 Switch 32 Substrate 34 High-frequency feeding line 36 Excitation line 38 Ground metalization 40 Ground terminal

Claims

CLAIMS:

1. An antenna array (1) comprising a first antenna (2) and a second antenna (3), which are designed for the transmission and reception operation in a same frequency range, where the signals assigned to the individual antennae are fed to a control device (8) for selecting an operating mode.

2. An antenna array as claimed in claim 1, characterized in that the first antenna (2) and the second antenna (3) are supported by a housing of an application.

3. An antenna array as claimed in claim 1, characterized in that a detector is assigned to the antennae (2, 3) for determination of the shortest distance from the respective antenna (2 or 3) to an absorber (12).

4. An antenna array as claimed in claim 1, characterized in that the first antenna (2) is arranged perpendicular to the second antenna (3).

5. An antenna array as claimed in claim 1 , characterized in that a phase shifter (10) is provided.

6. An antenna array as claimed in claim 1, characterized in that a power divider ( 11 ) is provided.

7. An antenna array as claimed in claim 1 , characterized in that the control device has a memory (17) provided for storing operating modes or for storing routines for determining operating modes.

8. An antenna array as claimed in claim 1, characterized in that one switch (30) each is assigned to the antennae (2,3) for terminating the antennae with different resistors.

9. A method of operating an antenna array comprising an first antenna (2) and a second antenna (3), where signals received by the antennae (2, 3) are fed to a control device (8) and, depending on these signals, an operating mode is selected for determining the transmission and reception characteristics.

10. A method as claimed in claim 9, characterized in that it is defined by the operating mode that when transmission takes place by means of one of the antennae, the other antenna is optionally short-circuited or connected to a metalization of an application through a resistor or connected directly to a metalization of an application.

11. A method as claimed in claim 9, characterized in that power consumption is defined by the operating mode.

12. A method as claimed in claim 11, characterized in that the power consumption comprises the division of the power between the antennae.

13. A method as claimed in claim 11, characterized in that the power consumption comprises the control of power made available by a power source.

14. A method as claimed in claim 9, characterized in that a spatial alignment of the transmission and reception behavior of the antenna array (1) is carried out by the selection of an operating mode.

15. A method as claimed in claim 9, characterized in that a fine tuning of the antenna array (1) to a frequency is carried out by the selection of the operating mode.

16. A mobile device, preferably a mobile telephone, comprising an antenna array which is operated by means of a method as claimed in one of the claims 9 to 15.

17. A mobile device with an antenna array as claimed in one of the claims 1 to 8.