Patent Publication Number: US-7218282-B2

Title: Antenna device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of copending International Application No. PCT/EP04/004482, filed Apr. 28, 2004, which designated the United States and was not published in English, and is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an antenna device and, in particular, to an antenna device suitable for multi-band operation. The present invention relates to an antenna for wireless data transmission, which may also include voice transmission. 
   2. Description of Related Art 
   For a wireless connection of mobile data processing devices, such as, for example, in wireless local area networks (WLAN), compact small antennas which often need to be dual-band- or multi-band-capable are required. 
   For this purpose, separate antennas may be used in practice for each frequency range. These separate antennas are, for example, connected to a diplexer in the form of a directional filter or to a multiplexer by means of which the signals to be transmitted are distributed to the respective individual antennas corresponding to the frequency ranges used. The disadvantage of using separate antennas for each frequency range is the size of the individual antennas, the area required for the antennas increasing with an increasing number of antennas required. Additionally, the required distributing circuit in the form of a diplexer or a multiplexer consumes a considerable amount of space. 
   Another known approach is to use antennas which have a very broad band or are multi-band-capable. In Kin-Lu Wong “Planar Antennas for Wireless Communications”, John Wiley and Sons, Inc., Hoboken, N.J., USA, 2003, pp. 26 to 53, several dual-/multi-band antennas in particular for being used in wireless local area networks are explained. Integrated IFAs (IFA=inverted F antenna) and PIFAs (PIFA=planar inverted F antenna) are, among other things, described there. 
   Dual-band PIFAs described in the above-mentioned document include, on a main surface of a substrate, different antenna patches realized by slots in an electrode formed on the surface, the antenna patches being fed via a common feeding point and connected to ground via a common short-circuited point. Antennas of this kind are also described in Zi Dong Liu et al., “Dual-Frequency Planar Inverted F Antenna”, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, pp. 1451 to 1458. 
   This document by Kin-Lu Wong (pages 226 ff.) also describes an integrated dual-band antenna in the form of a stacked IFA antenna. Two IFA antennas are “stacked” and galvanically excited via a microstrip line. This antenna may also be employed for wireless local area networks. 
   Additionally, dual-band PIFAs in which an antenna patch is galvanically fed by a feeding point, whereas a second antenna patch is fed by a capacitive coupling to the galvanically fed antenna patch, is described in the document mentioned. Antenna patches of this kind having capacitive coupling are also described in Yong-Xin Guo et al., “A Quarter-Wave U-Shaped Patch Antenna With Two Unequal Arms for Wideband and Dual Frequency Operation”, IEEE Transactions on Antennas and Propagation, Vol. 50, No. 8, August 2002, pp. 1082 to 1087. 
   Another way of implementing a dual-band antenna in which the antenna patch is lengthened or shortened in a frequency-selective way via an LC resonator or a chip inductor connected therebetween, is also known from the above-mentioned document by Kin-Lu Wong and also described in Gabriel K. H. Lui et. al., “Compact Dual-Frequency PIFA Designs Using LC Resonators”, IEEE Transactions on Antennas and Propagation, Vol. 49, No. 7, July 2001, pp. 1016 to 1019. 
   A non-planar broad-band antenna using a radiation coupling technique is described in Louis F. Fei et al., “Method Boosts Bandwidths of IFAs for 5-GHz WLAN NICs, Microwaves and RF”, September 2002, pp. 66 to 70. The bandwidth of the antenna is extended in a non-planar integrated IFA antenna by means of the radiation-coupled resonating of another IFA antenna. 
   It can be denoted in general that IFA antennas most often have a greater bandwidth compared to PIFA antennas, wherein most integrable dual-band concepts are of disadvantage due to a smaller bandwidth or due to an increased area demand. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an antenna device having a simple setup and a dual-band or multi-band capability or a great bandwidth. 
   In accordance with a first aspect, the present invention provides an antenna device having a first radiation electrode having an open end and a short-circuited end connected to ground and being coupled to a feed line at a feeding point, wherein the feed line and a portion of the first radiation electrode between the feeding point and the short-circuited end define an exciter loop; a second radiation electrode having an open end and a short-circuited end connected to ground, wherein a portion of the second radiation electrode is part of a conductor loop through which an alternating current may flow, wherein the exciter loop and the conductor loop are arranged spatially adjacent to each other such that an alternating current through the feed line to the short-circuited end of the first radiation electrode, for feeding the second radiation electrode, induces an alternating current into the conductor loop via magnetic coupling, wherein the second radiation electrode is arranged oh a surface of a substrate on which, additionally, a ground area to which the short-circuited end of the second radiation electrode is connected is arranged, wherein, additionally, a coupling point of the second radiation electrode is connected to the ground area via a coupling conductor such that the part of the second radiation electrode between the short-circuited end and the coupling point, the coupling conductor and the ground area define the conductor loop through which an alternating current may flow. 
   In preferred embodiments of the inventive antenna device, the first radiation electrode and the feed line are arranged on a first main surface of a substrate, whereas the second radiation electrode is arranged on a second surface of the substrate opposite the first surface. The second electrode is preferably part of a conductor loop, through which an alternating current may flow, which can be infiltrated by a magnetic field generated by an alternating current through the feed line to the short-circuited end of the first radiation electrode, such that the feeding current for the second radiation electrode is induced into the conductor loop. In further preferred embodiments of the present invention, the first radiation electrode and the feed line define an exciter loop such that the conductor loop to which the second radiation electrode contributes is fed by a mutual induction of two spatially neighboring conductor loops. 
   The two radiation electrodes of the inventive antenna device preferably comprise different lengths and thus different resonant frequencies so that the inventive antenna device may also be used as a dual-band antenna. The radiation electrodes, however, may also comprise such resonant frequencies that an antenna having an increased bandwidth compared to an antenna with only one radiation electrode is obtained. The inventive antenna device may also comprise more than two radiation electrodes and thus be employed as a multi-band antenna. 
   The inventive antenna or antenna device may be integrated in a planar way, which is of advantage due to its small size in particular with transmission frequencies in the centimeter and millimeter wave range. Preferred fields of application of the inventive antenna are in mobile transmitters and receivers utilizing two or more frequency bands or requiring a high bandwidth. Thus, the present invention is, for example, extraordinarily suitable for a wireless LAN connection of mobile data processing devices, since frequency ranges from 2400 to 2483.5 MHz and 5150 to 5350 MHz are for example used there (Europe). Furthermore, frequency ranges from 5470 to 5725 MHz and the ISM band from 5725 to 5825 MHz may also be used (USA). In addition, the inventive antenna is also suitable for being employed in dual-band or multi-band mobile phones (900 MHz/1800 MHz, etc.). Due to its small size and the capability of being integrated on planar circuits, the inventive antenna is, among other things, suitable for being integrated on PCMCIA-WLAN adapter cards for laptop computers. 
   In a preferred embodiment, the inventive antenna for wireless data transmission is an integrated dual-band antenna which is, for example, provided for being used in the WLAN ranges of 2.45 GHz and 5.2 GHz. The inventive principle, however, may also be extended to more than two bands and different frequencies. 
   The inventive antenna device is preferably implemented as an integrated IFA antenna in which, in contrast to conventional integrated IFAs, only a single element, i.e. the first radiation electrode, is fed galvanically. The other element or the other elements (the second and further radiation electrodes) are coupled inductively. The result is a decrease in manufacturing cost and area demand, in particular when the antenna is implemented using a multi-layered concept. The area demand of the entire antenna is only determined by the size of the antenna element for the lowest frequency. As is typical in IFA antennas, the inventive antenna is also characterized by a high bandwidth which is above average for planar antennas. 
   The inductive coupling and the characteristic wave impedance of the antenna elements, i.e. of the radiation electrodes, can be optimally adjusted by the substrate thickness, the substrate material (the permittivity thereof), the shape of the feed line and a displacement of the feeding point. 
   The inventive antenna stands out from multi-band concepts known up to now by optimal adjustability, minimum area demand, high bandwidth and small manufacturing cost. The antenna can be integrated in a completely planar way on a substrate (dual-band) or on a multi-layered substrate (multi-band). In preferred embodiments of the present invention, the only thing required is a ground through-connection at the short-circuited side of the radiation electrodes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
       FIG. 1  is a schematic illustration of a first embodiment of an inventive antenna device; 
       FIGS. 2   a  and  2   b  are schematic illustrations for explaining the embodiments shown in  FIG. 1 ; 
       FIG. 3  is a schematic illustration of an alternative embodiment of an inventive antenna device; 
       FIG. 4  is a schematic illustration of two antenna devices realized according to the invention; and 
       FIGS. 5   a  and  5   b  show characteristics measured of the antenna devices of  FIG. 4 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   An embodiment of an inventive antenna device implemented on a double-sided substrate  10  is shown in  FIG. 1 . It is to be pointed out here that the substrate is illustrated in a transparent manner in  FIG. 1  for reasons of clarity. The inventive antenna device illustrated in  FIG. 1  principally includes two integrated IFAs (inverted F antennas), one of the antennas being formed on a top side  10   a  of the substrate  10 , the other one being formed on a bottom side  10   b.    
   A first radiation electrode  12  comprising an open end  12   a  and a short-circuited end  12   b  is formed on the main surface  10   a  of the substrate  10  corresponding to the top side. Additionally, a supply line  14  for galvanically feeding the first radiation electrode  12  is provided on the main surface  10   a . The supply line  14  is connected to the first radiation electrode  12  at a feeding point  16 . With regard to the structure of the metallizations provided on the main surface  10   a , i.e. the electrodes and lines provided there, reference is made to  FIG. 2   a  representing a top view of the top side  10   a  of the relevant part of the substrate  10 . 
   The short-circuited end  12   b  of the first radiation electrode  12  is connected to a ground electrode  22  (in  FIG. 1  illustrated in a hatched manner) formed on the main surface  10   b  of the substrate  10  opposite the main surface  10   a , via a through-connection  20 . This opposite main surface  10   b  (the back side in  FIG. 1 ) is illustrated in  FIG. 2   b  as a “shine-through image” from above, wherein the metallizations provided on the front side  10   a  are omitted for reasons of clarity and the substrate is transparent. As can best be seen in  FIG. 2   b , a second radiation electrode  24  comprising an open end  24   a  and a short-circuited end  24   b  is formed on the main surface  10   b . The short-circuited end  24   b  is connected to the ground electrode  22 . Additionally, a coupling conductor  26  comprising a first end connected to the ground electrode  22  and a second end connected to the second radiation electrode  24  at a coupling point  28  is formed on the main surface  10   b.    
   The ground electrode is provided as a back side metallization on the bottom side of the substrate and also serves as a ground level for the microstrip line  14  and the antennas. The galvanically fed, longer first radiation electrode  12  is provided for the lower frequency band, whereas the inductively fed, shorter antenna  24  is provided for the upper frequency band. 
   The antenna shown in  FIG. 1 , in principle, consists of two integrated IFAs, the first one of the two antennas for the first frequency band being fed by the supply line  14  in the form of a microstrip line. The second antenna for the second frequency band comprising the second radiation electrode  24  is inductively excited via a current loop. In particular, in the embodiment illustrated, the supply line  14  and the portion of the first radiation electrode  12  between the short-circuited end  12   b  and the feeding point  16  form an exciter current loop generating a magnetic flux. Additionally, the coupling line  26 , the area of the second radiation electrode  24  between the short-circuited end  24   b  and the coupling point  28 , and the ground electrode  22  form an electric circuit. This electric circuit, in the inventive antenna device, is arranged such that it is infiltrated by the magnetic flux generated by the exciter current loop such that a current is induced into this current loop. The second radiation electrode  24  is fed by this induced current. 
   In order to obtain the best possible magnetic coupling, in the embodiment illustrated, the dimensions of the excited current loop formed on the back side  10   b  roughly corresponds to the dimensions of the exciter loop formed on the front side  10   a . The thickness of the substrate  10  may, for example, be 0.5 mm so that the spacing of the current loops on the top side and bottom side of the substrate, respectively, is small (compared to the wave length at the resonant frequency of the radiation electrode  24 ) such that good magnetic coupling can be achieved. 
   In the embodiment shown, the radiation electrode  24  is thus excited inductively by magnetic coupling, the intensity of the coupling depending on the mutual inductivity between the excitation conductor and the excited conductor. The size and form of the exciter current loop and of the excited current loop can be adjusted to obtain a desired coupling. Additionally, the coupling depends on the mutual distance of the loops. 
   It is to be pointed out here that the exciter current loop and the excited current loop need not be closed current loops formed on the substrate but may be formed as conductor regions which, together with conductors not formed on the substrate, form an alternating current circuit or current loop. The exciter current loop need only have one course to generate a sufficient magnetic field or a sufficient magnetic flux such that a current sufficient for a feeding current can be induced into the part of the electric circuit of the second antenna element which is arranged in the magnetic field or the magnetic flux. Additionally, it is to be pointed out that the respective current loops or electric circuits are formed in a way suitable for enabling an alternating current flow such that capacitive couplings may be provided within these current loops or electric circuits. 
   The feeding point  16  is selected to obtain impedance matching between the microstrip line  14  and the radiation electrode  12 . The respective position for the feeding point  16  must be determined when designing the antenna, wherein the antenna impedance may be diminished by shifting the feeding point  16  to the left, whereas it can be increased by shifting the feeding point  16  to the right, as is indicated in  FIG. 2   a  by an arrow  30 . The antenna impedance can thus be adjusted to the impedance of the galvanic supply by correspondingly selecting the feeding point  16 . 
   In the same way, matching between the antenna impedance of the second radiation electrode  24  and the coupling line  26  can be obtained by suitably selecting the coupling point  28 , as is shown in  FIG. 2   b  by an arrow  32 . It can be achieved by this matching that the current induced may be utilized optimally for feeding the second radiation electrode. 
   Even though in the embodiment shown in  FIGS. 2   a  and  2   b  the supply line  14  and the coupling line  26  are coupled to the part of the respective radiation electrode parallel to the edge of the ground electrode  22 , each of these lines could also be coupled to that part of the respective radiation electrode perpendicular to the edge of the ground electrode  22 , depending on how it is necessary to obtain impedance matching. 
   The entire geometry of the inventive antenna device may be reduced to obtain, for example, a minimization of the area demand by, for example, forming the radiation electrodes or at least the longer one thereof in a meandering shape. 
   The shape of the feed line  14   a  and the coupling line  26  and the selection of the feeding point and the coupling point  26  may differ for obtaining impedance matching for the two radiation electrodes to allow optimum matching for the two individual antenna elements. The bend  14   a  in the supply line  14  and the bend  26   a  in the coupling line  26  may, for example, be provided in the embodiment shown in  FIGS. 1 and 2  to obtain impedance matching. 
   A schematic illustration for an embodiment of an inventive multi-band antenna is shown in  FIG. 3 . 
   The multi-band antenna is implemented in a multi-layered substrate  50  which in turn is shown in a transparent manner for reasons of illustration and comprises a first layer  52  and a second layer  54 . A first antenna element basically corresponding to the antenna element formed on the top side  10   a  of the substrate  10  comprising the first radiation electrode  12 , is formed on the top side of the first layer  52 , wherein, in contrast to the embodiment shown in  FIG. 1 , only the supply line  14  is connected to the part of the radiation electrode  12  perpendicular to the edge of the ground area  22  and thus has a corresponding portion  14   b.    
   In analogy to the embodiment described above, the second radiation electrode  24  is formed on the bottom side of the first layer  52  (and on the top side of the second layer  54 , respectively). A third radiation electrode  56  having an open end  56   a  and a short-circuited end  56   b  is formed on the bottom side of the second layer  54 . The short-circuited end is connected to the ground electrode  22  via a through-connection  58  provided in the second layer  54 . In addition, another through-connection  60  is provided in the second layer  54 , via which a first end of a coupling line  62  is connected to the ground electrode  22 . A second end of the coupling line  62  is connected to the third radiation electrode  56  at a coupling point  64 . 
   The third antenna element comprising the radiation electrode  56  thus has a setup comparable to the setup of the second antenna element comprising the radiation electrode  24 . 
   In the embodiment shown in  FIG. 3 , the third radiation electrode  56  is fed by at first inducing a current into the electric circuit of the second antenna element and by inducing a current into the electric circuit of the third antenna element by the current induced into the electric circuit of the second antenna element. This electric circuit of the third antenna element is formed by a conductor loop comprising the through-connection  60 , the coupling line  62 , the portion of the third radiation electrode  56  arranged between the coupling point  64  and the short-circuited end  56   b , the through-connection  58  and the ground electrode  22 . 
   As can be seen in  FIG. 3 , the respective feeding points and coupling points for the different antenna elements may be arranged at different positions to obtain matching for the respective different elements. 
   Alternatively to the embodiment shown in  FIG. 3 , the galvanically fed antenna element could be arranged between two inductively fed antenna elements so that no double magnetic coupling would be required for feeding the third antenna element. 
   In the embodiment shown in  FIG. 3 , instead of providing the through-connection  60 , the first end of the coupling line  64  could be connected to the short-circuited end of the third radiation electrode  56  via a conductive track (not shown) provided on the bottom side of the second layer  54  to implement the electric circuit of the third antenna element. In such a case, only one respective through-connection would be required in both the first layer  52  and the second layer  54  of the multi-layered circuit board. 
   According to the invention, the several antenna elements can be used for producing a dual-band or multi-band antenna. Alternatively, respective additional antenna elements may be used for expanding the bandwidth of an individual frequency band by, for example, selecting the resonant frequencies of two antenna elements to be adjacent to each other. 
   Prototypes of inventive antenna devices have been simulated by means of HFSS and then formed on an Ro4003 substrate having an effective permittivity ε r ≈3.38. An Ro4003 substrate is a high-frequency substrate by Rogers Corporation and is made of a glass-reinforced cured hydrocarbon/ceramics laminate. HFSS is an EM field simulation software by Ansoft Corporation for calculating S parameters and field configurations, which is based on the finite elements method. 
     FIG. 4  purely schematically shows photographies of two prototypes of this type in which the respective microstrip supply line is fed by a coaxial line. To illustrate size proportions, a 20 cent coin is also shown in  FIG. 4 . As can be seen in  FIG. 4 , the left antenna has a somewhat narrower radiation electrode, whereas the right antenna has a wider radiation electrode. 
     FIG. 5   a  shows the characteristics obtained in input reflection measurements of the left antenna of  FIG. 4 , whereas  FIG. 5   b  shows the characteristics obtained with the right antenna of  FIG. 4 . As can be deduced from the graphs of  FIGS. 5   a  and  5   b , a change in bandwidth can be obtained by varying the geometry. 
   Even though setups having only two or three radiation electrodes have been described before, it is obvious that the inventive concept may also be extended to more than three radiation electrodes to obtain a corresponding multi-band capability or broad-band capability. For this purpose, a multi-layered substrate having more than two layers can be used in a suitable way. In addition, the present invention is not limited to the embodiments of antenna devices described but rather also includes single-sided printed antennas (where two or more radiation electrodes are provided on one surface of the substrate) or wire antenna assemblies. 
   While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.