Patent Publication Number: US-6909401-B2

Title: Antenna device

Description:
This is a nationalization of PCT/SE01/01600 filed Jul. 10, 2001 and published in English. 

   FIELD OF INVENTION 
   The present invention relates generally to antenna devices and more particularly to an antenna device for use in a radio communication device, such as a mobile phone. 
   BACKGROUND 
   In a portable radio communication device the space for an internal antenna arrangement is limited. With the growing need for greater functionality and better radio channel quality it is often necessary to utilise more than one antenna element in a portable radio communication device, such as a mobile telephone. Because of the limited space in a portable radio communication device, internal antennas tend to be closely spaced. With closely spaced antenna elements unwanted coupling between the antennas can arise. 
   Low coupling between closely spaced antennas is necessary for various applications. These can be for example: separate RX and TX antenna system which eliminates the need for a diplexer, antenna diversity systems (both receiver and transmitter diversity), antennas for different systems (e.g. GSM-Bluetooth). 
   In WO 9013152 is described the case of separated RX/TX antennas. WO 9013152 mentions only the case of two antennas of the same type (two similar patches). In WO 9013152 a solution for eliminating the need for a diplexer is provided. Further, it is disclosed that separated transmit and receive antennas are elevated above a grounding surface by a conductive pedestal, wherein the pedestal is placed between the antennas and electrically isolates the antennas. 
   The above-described document only describes reduced coupling between separated transmit and receive antennas. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an antenna device for use in a radio communication device, preferably a portable radio communication device, wherein the electrical coupling between transmit and receive antenna elements is minimised. 
   The invention is based on the realisation that when one antenna element has a balanced feed and another antenna element has an unbalanced feed, it is possible to design the two antennas in a way in which the coupling between the antennas can be lower than with two antennas both having unbalanced or balanced feed. 
   An advantage with a balanced and unbalanced antenna pair is that with a balanced input to receiver electronic circuits and an unbalanced feeding from an output amplifier, lower losses and improved matching to the receiver/transmitter are achieved. 
   According to the present invention there is provided an antenna device as defined in appended claim  1 . 
   There is also provided a radio communication device comprising such an antenna device. 
   Further preferred embodiments are defined in the dependent claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic to view of a first embodiment of an antenna device according to the invention; 
       FIG. 1A  is an elevated sectional view of the first embodiment of an antenna device according to the invention taken along lines  1 A— 1 A of  FIG. 1 ; 
       FIG. 2  is a schematic top view of a second embodiment of an antenna device according to the invention; 
       FIG. 3  is a schematic top view of a third embodiment of an antenna device according to the invention; 
       FIG. 4  is a schematic top view of a fourth embodiment of an antenna device according to the invention; 
       FIG. 5  is a schematic perspective view of a fifth embodiment of an antenna device according to the invention; 
       FIG. 6  is a schematic top view of a sixth embodiment of an antenna device according to the invention; 
       FIG. 7  is a block diagram showing a preferred layout of the receiving RF chain of an antenna device according to the invention; and 
       FIG. 8  is a block diagram showing a preferred layout of the transmitting RF chain of an antenna device according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following, a detailed description of preferred embodiments of an antenna device according to the invention will be given. In the several embodiments described herein, the same reference numerals are given to identical parts of the different embodiments. 
   A balanced feed is defined as when a transmission line, comprising two conductors in the presence of ground, is capable of being operated in such a way that when voltages of the two conductors at all transverse planes are equal in magnitude and opposite in polarity with respect to ground, currents in the two conductors are essentially equal in magnitude and opposite in direction. An unbalanced feed is defined as a feed that does not fulfil the above criteria. 
   Reference is first made to  FIG. 1 , wherein an antenna device or module, generally designated  1 , comprises a printed circuit board (PCB)  10 , having mounted thereon circuits for the transmitter portion  24  and the receiver portion  34  of the electronic circuitry of a mobile phone (not shown in the figures). The PCB  10  also functions as a ground plane for a Planar Inverted F Antenna (PIFA), the radiating element of which is designated  20 . The radiating element  20  is positioned spaced apart from the PCB  10  and essentially parallel thereto and comprises a feeding portion  22  connected to a feed element of the transmitter portion  24  and a grounding portion  23  connected to a ground element of the transmitter portion. Thus, the PIFA  20  functions as a transmitting antenna of the device  1 . 
   The PIFA  20  is an unbalanced fed electric antenna where the grounding portion  23  is connected directly to ground. The connection may alternatively be via a matching network comprising lumped or distributed inductors and/or capacitors. 
   The antenna device  1  also comprises a loop antenna designated  330 . The loop antenna is made of a conducting wire forming a loop in a plane parallel to the PCB  10  and also to the radiating element  20  and essentially in the plane of the PIFA element, surrounding the PIFA element. 
   The coupling between the PIFA antenna  20  and the balanced fed loop antenna  330  is low because the PIFA  20  gives an essentially electric field perpendicular to the antenna plane and the balanced fed loop antenna  330  gives an essentially magnetic field with direction perpendicular to the antenna plane. Therefore the antennas have a low coupling between each other. 
   The loop antenna  330  is connected by its ends  332 ,  333  to a respective input of the receiver portion  34 . In the figure, one input is labelled as positive and the other as negative. The loop antenna  330  is balanced, i.e. it is fed by opposing signals, whereby it functions as a magnetic antenna. 
   Alternatively the loop antenna  330  could be replaced with an open loop, i.e. a dipole antenna. 
   A preferred layout of the receiving RF chain of the receiver portion  34  will now be described with reference to FIG.  7 . The RF chain comprises a balanced filter  334 , the inputs of which, labelled “+” and “−” in the figure, are connected to the receiving antenna  330  by its ends  332 ,  333 . The outputs of the balanced filter  334  are connected to a low noise amplifier (LNA)  335  having a balanced input, labelled “+” and “−”. The LNA  335  is in turn connected to RF electronics (not shown) of the receiver portion  34 . This connection can be balanced or unbalanced. 
   The provision of a balanced filter  334  gives the following advantages. Firstly, the balun found in conventional arrangements for converting the received signal from unbalanced to balanced is omitted, thereby decreasing signal losses, manufacturing costs and the space required by the RF electronics. Secondly, the isolation between the transmitter and receiver circuits is increased because the transmitter chain is unbalanced and the receiver chain is balanced. This results in less crosstalk between the circuits. 
   Yet another advantage of having a balanced LNA is that it can then be made by Application Specific IC (ASIC) technology, which is preferred. ASIC applications are always balanced. 
   A preferred layout of the transmitting RF chain of the transmitter portion  24  will now be described with reference to FIG.  8 . The TX chain comprises a balanced power amplifier (PA)  25  having an input connected to the transmitter electronics (not shown). This input can be balanced or unbalanced. The balanced output of the amplifier is connected to the input of a balanced filter  26 , the output of which feeds the transmitter antenna. 
   The advantage of providing the transmitter chain in this way is the decreased coupling between the transmitting and receiving circuits, resulting in less crosstalk etc. 
   In an alternative second embodiment shown in  FIG. 2 , the loop antenna has been replaced by a dipole antenna  230  comprising two strand like portions  231   a ,  231   b . These portions surround the PIFA element  20  on either side thereof and in level with the essentially planar element  20 . As in the first embodiment, the dipole antenna  230  has a balanced feed portion. 
   The two strand like portions  231   a  and  231   b  are shown as two portions of equal length, but preferably the length of one of the elements, e.g.  231   b , is adapted to adjust the impedance and/or the resonance frequency of the dipole antenna  230 . 
   In an alternative third embodiment shown in  FIG. 3 , the receiving antenna is a loop antenna  30  with its ends  32 ,  33  connected to respective input of the receiver portion. However, in contrast to the embodiment described hereinabove with reference to  FIG. 1 , this loop antenna is provided in a plane perpendicular to the PCB  10  and the PIFA element  20 . 
   The three embodiments shown in  FIGS. 1-3  are all basic configurations of the antenna device according to the invention. They all have in common that there is provided an unbalanced PIFA antenna functioning as a transmitting antenna and a balanced fed receiving antenna, which is either a loop or dipole antenna. In  FIGS. 1 and 2  the balanced fed antenna  330 ,  230  further surrounds the PIFA antenna  20 . 
   In  FIGS. 4-6 , variants of the basic configurations are shown. The embodiment shown in  FIG. 4  is similar to the one of  FIG. 1  but with the difference that the connections  432 ,  433  to the loop antenna  430  are made on a side of the PIFA element  20  opposite of the feeding and grounding portions  22 ,  23  of the PIFA. This provides the advantage that the coupling to the PCB is affected and may be adapted to a desired design. 
   The embodiment shown in  FIG. 5  is similar to the one of  FIG. 3  but with the difference that the loop antenna  530  is provided on a side of the PIFA element  20  opposite of the feeding and grounding portions  22 ,  23  of the PIFA. This provides the advantage that the coupling to the PCB is affected and may be adapted to a desired design. 
   The embodiment shown in  FIG. 6  is similar to the one of  FIG. 2  but with the difference that the connection portions of the dipole antenna  630  are provided on a side of the PIFA element  20  opposite of the feeding and grounding portions  22 ,  23  of the PIFA. This provides the advantage that the coupling to the PCB is affected and may be adapted to a desired design. 
   In order to minimise coupling between the unbalanced and balanced antenna, the unbalanced antenna is preferably miniaturised. By loading the unbalanced antenna with a high dielectric material, such as ceramic or a mixture of ceramic and plastic, the minimum distance between the unbalanced and balanced antenna is increased, whereby the coupling there between is further reduced. 
   Preferred embodiments of an antenna device according to the invention have been described. However, the person skilled in the art realises that these can be varied within the scope of the appended claims without departing from the inventive idea. Thus, although a transmitting and a receiving antenna have been shown, the inventive idea is not limited to that. As an example, two receiving antennas could be provided, wherein one is balanced and the other is unbalanced. In that way, the coupling there between is minimised. Also, antenna diversity is obtained. The different antennas could be operating for different communication systems, e.g. Bluetooth, GSM, and UMTS. They could also be operating at different bands, e.g. GSM900 and GSM1800. There could also be provided further combinations such that the possible combinations are: two transmitting/receiving antennas; a transmitting/receiving antenna and a receiving antenna; a transmitting/receiving antenna and a transmitting antenna; two receiving antennas; two transmitting antennas; a transmitting antenna and a receiving antenna; wherein each combination each antenna could be unbalanced or balanced. 
   Specific antenna patterns have been shown. However, the person skilled in the art realises that the unbalanced antenna is not necessarily a PIFA, but can be e.g. a patch, modified PIFA, meander PIFA, or slot. 
   In the figures feeding devices  24 ,  34  have been illustrated, which should be interpreted as: a feed device for a transmitting antenna, a receiver device for a receiving antenna, and a feed/receiver device for a transmitting/receiving antenna. 
   It will be realised that the receiver RF chain described with reference to FIG.  7  and the transmitter RF chain described with reference to  FIG. 8  are applicable to the receiver and transmitter portions of all the above described embodiments.