Abstract:
An antenna including: a first substantially planar ground plate; a first substantially planar resonator positioned in a plane substantially parallel to the first ground plate; a second substantially planar ground plate positioned in a plane substantially parallel to the first ground plate; two or more connectors for electrically connecting the second ground plate to ground; and one or more connectors for electrically connecting the first resonator to the second ground plate; wherein the first resonator and the second ground plate are connected to at least one of receiver means and transmitter means by antenna feeding means.

Description:
This application claims priority to GB 0330052.2 filed on Dec. 24, 2003. 
   FIELD OF INVENTION  
   The present invention relates to radio frequency antennas for use in mobile communication terminals. More particularly, it relates to providing multiple-band resonances and/or wider bandwidth resonances in mobile terminals. 
   BACKGROUND OF THE INVENTION  
   It is known in the field of mobile communications to use PIFA&#39;s (planar inverted-F antennas) to achieve a relatively large bandwidth at particular frequencies that may be used for the transmission and reception of mobile communications. Such a prior art PIFA is shown in  FIG. 1 .  FIG. 1A  shows a ground plane  1  with a conducting plate  2  mounted above it by means of a short circuit plate  4 .  FIG. 1B  shows a side view of the PIFA and  FIG. 1C  shows a top view of the conducting plate  2 . The frequencies at which a PIFA produces resonances, and the bandwidths of generated frequencies depend on the geometry of the PIFA. Relevant parameters include the length and width of the conducting plate  2  (x and y); the position of the connecting wire  3  (I x , I y ); the width of the short circuit plate  4  (w); the height of the conducting plate  2  above the ground plane  1  (h); and the radius of the wire  3 . These parameters can be adjusted to provide optimal bandwidth at a particular frequency. The plate  2  will typically be a quarter wave structure. 
   It is also known to use a double PIFA antenna, in which an additional resonator, or parasitic resonator, is positioned between the ground plane and the main resonator and parallel to them. Such a configuration is shown in  FIG. 2 . Above the ground plane  21  is a parasitic resonator  25 , and above that is a main resonator  22 . These three components are electrically connected together by means of a short circuit plate  24 . In addition, a feed cable  23  passes through a hole in the ground plane  21  and a hole in the parasitic resonator  25 , and the inner conductor of the feed cable (“source+”) makes electrical contact with the main resonator  22 . The outer conductor (“source−”, or ground) is connected to the ground plane  21 . This arrangement provides a greater bandwidth than is achievable with a single PIFA. In addition, the parasitic resonator can provide an extra resonance. The parasitic resonator is excited indirectly by the main resonator, rather than directly by the feeding cable. This geometry ( FIG. 2 ) is a variation on the well known ‘PIFA with parasitic element’ design, where the parasitic element is placed largely in the same plane and adjacent to the main resonator. In the structure of  FIG. 2 , the main resonator  22  and the parasitic resonator  25  are both quarter wave plates. 
   The frequency bands used in GSM mobile communication systems are currently USGSM850, EGSM900, DCS1800, and PCS1900. USGSM is a frequency band commonly used in North America; EGSM is used in Europe and ranges from around 880 to 960 MHz; DCS1800 is a common “digital cellular service” band ranging from 1710 to 1880 MHz; and PCS1900 is a common “personal communications service” frequency band. Mobile telephones that are capable of transmitting and receiving signals at all of these frequency bands are known as “quad-band”. It is known in the field of mobile communications to use a slotted PIFA pair antenna (see U.S. Pat. No. 6,621,455 entitled “Multi-band Antenna”) to achieve resonance at frequencies within the four GSM bands. 
   A slotted PIFA pair antenna is shown in  FIG. 3A . This configuration behaves as two adjacent PIFA&#39;s. A printed circuit board  31  acts as a ground plane of the antenna. A planar conductive layer  32 B, comprised of copper or any other highly conductive material with slots  35  etched out it, forms a radiating element. The geometry of this layer dictates the resonant frequencies obtainable by the antenna and their bandwidths. The conductive layer  32 B is typically supported by an insulating substrate  32 A. 
   Conducting pins  30  are used to ground the conductive layer  32 B, and a feed is provided on the underside of the supporting substrate  32 A.  FIG. 3B  shows a detailed view of the feeding structure of this slotted PIFA pair. Typically, a coaxial cable would be used to feed the antenna. The inner and outer conductors of the cable would be connected to different points on the antenna structure. In  FIG. 3B ,  34 A is a conductive strip connected to “source+” and this strip is positioned along the centre of the substrate  32 A, preferably extending parallel to the slot  35 .  34 B is connected to “source−” and protrudes from the printed circuit board  31 . There is a small gap between the feed strips  34 A and  34 B. The coaxial cable connects the antenna to transmitter and receiver circuitry. 
   It has been found that such a slotted PIFA pair antenna is less effective for telecommunications handsets whose covers are made substantially of metal. In the case of metal mobile terminals, the bandwidth achievable by a slotted PIFA pair antenna such as that shown in  FIGS. 3A and 3B  is significantly worse compared with non-metal terminals at the lower GSM band (USGSM850 and EGSM900). 30-40% bandwidth reduction is experienced with metal handsets due to the large metal blocks used therein. However, metal handsets have an appearance of quality and luxury and are aesthetically appealing. It is therefore desirable to produce a quad-band mobile handset that may be made of metal. 
   SUMMARY OF THE INVENTION  
   According to one aspect of the present invention there is provided an antenna comprising: a first substantially planar ground plate; a first substantially planar resonator positioned in a plane substantially parallel to the first ground plate; a second substantially planar ground plate positioned in a plane substantially parallel to the first ground plate; two or more connectors for electrically connecting the second ground plate to the first ground plate; and one or more connectors for electrically connecting the first resonator to the second ground plate; wherein the first resonator and the second ground plate are connected to at least one of receiver means and transmitter means by antenna feeding means. 
   According to a second aspect of the present invention there is provided a mobile communication terminal comprising an antenna, the antenna comprising: a first substantially planar ground plate; a first substantially planar resonator positioned in a plane substantially parallel to the first ground plate; a second substantially planar ground plate positioned in a plane substantially parallel to the first ground plate; two or more connectors for electrically connecting the second ground plate to the first ground plate; and one or more connectors for electrically connecting the first resonator to the second ground plate; wherein the first resonator and the second ground plate are connected to at least one of receiver means and transmitter means by antenna feeding means. 
   According to a third aspect of the present invention there is provided an antenna for use in a mobile communications terminal having a first substantially planar ground plate, the antenna comprising: a first substantially planar resonator positioned in a plane substantially parallel to the first ground plate; a second substantially planar ground plate positioned in a plane substantially parallel to the first ground plate; two or more connectors for electrically connecting the second ground plate to the first ground plate; and one or more connectors for electrically connecting the first resonator to the second ground plate; wherein the first resonator and the second ground plate are connected to at least one of receiver means and transmitter means by antenna feeding means. 
   Preferred features of the present invention are set out in the dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     The present invention will now be described by way of example with reference to the accompanying drawings in which: 
       FIG. 1  shows a prior art planar inverted-F antenna; 
       FIG. 2  shows a prior art double PIFA antenna (PIFA); 
       FIG. 3  shows a prior art slotted PIFA pair antenna; 
       FIG. 4  shows an antenna in accordance with an embodiment of the present invention; 
       FIG. 5  shows possible locations for connectors on a ground plane of an embodiment of the present invention; 
       FIG. 6  is a graph of simulated return loss against frequency for various connector positions; 
       FIG. 7  is a graph of simulated radiation efficiency against frequency for a particular combination of connector positions. 
   

   In the drawings, like reference numerals are used to refer to like parts. 
   DETAILED DESCRIPTION  
   The inventors of the present invention have found that significant improvements in antenna performance may be achieved, compared with a standard PIFA, by introducing an additional ground plane between the ground plane  1  of  FIG. 1A  and the conducting sheet  2 . 
   In general, if two or more ground connections are made between a conducting plate and ground, then the plate will act as an additional ground plane rather than as a parasitic resonator such as that shown as  25  in the prior art structure illustrated in  FIG. 2 . As discussed further below, the positions of the ground connections affect the frequency at which the additional ground plane will provide resonance. 
   An exemplary arrangement is shown in  FIG. 4A , where a circuit board (for example, a printed wiring board)  5  acts as the antenna&#39;s main ground plane. Alternatively, an RF shielding can or any metal part of mobile handset may be used in place of the printed wiring board (PWB) as the antenna&#39;s ground plane. Positioned above the PWB  5  of  FIG. 4A  is an additional plane  7 , which is connected to the PWB  5  by means of conducting pins  10 B. 
   The structure of the plane  7  need not be a PIFA since, as in the embodiment shown in  FIG. 4A , it could be connected to the PWB at multiple points. Using multiple ground connections  10 B provides the advantage that the current distribution across the PWB can be controlled. Conversely, the known double PIFA has just one ground connection ( 24  in  FIG. 2 ), and this could result in very high current concentrated in some areas. 
   With multiple ground connections spread around the plane  7 , the plane would not act as a quarter wave structure, but rather as a half wave structure, and to obtain resonance at the frequencies of interest the size of the plane would need to be doubled. Due to size constraints associated with mobile handsets, doubling the area of an antenna structure would not be desirable. 
   Instead, the inventors of the present invention have realised that if the plane  7  is connected to the “source−” of a feed cable, the plane  7  will act as an extra ground plane in addition to the main ground plane  5 , thereby providing the advantages discussed below. 
   The extra ground plane  7  will not be resonant in its own right, but the combined structure of resonators  6 A and  6 B (described in detail below) together with the ground plane  7  acts as a resonator structure which can produce up to four resonant frequencies. 
   By varying the position and number of the connecting pins  10 B the resonances produced by the combined structure may be tuned to desired frequencies. The extra ground plane  7  may also have slots  9  cut into it in order to modify the frequency band(s) of these resonances. 
   Positioned above the ground plane  7  is a conducting antenna track  6 A whose shape determines the frequency band(s) at which the antenna track  6 A resonates. In a preferred embodiment of the invention, the antenna track  6 A comprises one or more resonators (which may, for example, be conventional PIFAs), each exhibiting one or more resonances. The resonances generated are dependent upon the antenna track geometry. 
   The track  6 A is electrically connected to the ground plane resonator  7  by means of a connecting element  10 A. A coaxial cable could suitably be used to feed the antenna  20 . As in the antenna of  FIG. 3 , strip  8 A could conveniently be connected to the inner conductor of the coaxial cable (source+), and  8 B could be connected to the outer conductor (source−), thus connecting the antenna to transmitter and receiver circuitry. 
   A further resonator, parasitic element  6 B may be positioned adjacent to the antenna track  6 A, and in substantially the same plane. This parasitic resonator is connected directly to the ground plane  7  and acts as a PIFA without a feeding pin. It is excited indirectly by the main resonator  6 A rather than by a feeding cable. Its resonant frequency is determined by its dimensions. 
     FIG. 4B  shows a detailed view of the feed connections of the antenna shown in  FIG. 4A . 
   Conveniently, the track  6 A and the parasitic element  6 B can be mounted on an insulating substrate  40 , as shown in  FIG. 4B . Suitably, the track  6 A and element  6 B can comprise conductive layers applied to the substrate and subsequently etched to define the resonator geometries. 
   In one embodiment, the antenna is made up of a dual band resonator (antenna track  6 A) which comprises one part  27  for providing a resonance at one of the lower GSM bands (USGSM850 or EGSM900) and one part  26  for providing a resonance at one of the upper GSM bands (DCS1800 or PCS1900), and a parasitic resonator  6 B which provides a third resonance at a third GSM band (either DCS1800 or PCS1900). The antenna described so far is a conventional tri-band antenna, suitably covering either the USGSM850, DCS1800 and PCS1900 bands or EGSM900, DCS1800 and PCS1900. 
   However, by adding a ground plane resonator  7  in accordance with the present invention, a further low band resonance can be created, such that the conventional tri-band antenna combined with the ground plane resonator can provide performance over all four GSM bands (USGSM850, EGSM900, DCS1800 and PCS1900). As discussed above, the ground plane alone does not add a further resonance, but rather it acts as a part of the whole resonator comprising  6 A,  6 B and  7  in order to add an extra resonance. The size of the ground plane is typically too small for a USGSM850 or EGSM900 resonance to be created by the ground plane alone, but the combined structure comprising  6 A,  6 B and  7  allows a lower band resonance within either USGSM850 or EGSM900 to be produced. Thus, in a preferred embodiment of the present invention, the antenna shown in  FIG. 4  can provide resonance at each of the four GSM bands. This arrangement is advantageous compared with a conventional double-PIFA configuration in that the volume of antennas in accordance with embodiments of the invention can be smaller due to the use of a ground plane positioned below the main resonators ( 6 A,  6 B) to contribute to the obtainable frequency bands. 
   As an alternative to the embodiment shown in  FIG. 4A  in which several connecting pins are provided between the plate  7  and the main ground plane  5 , the plate  7  would also act as a ground plane if a single connecting pin were provided between the plate  7  and the main ground plane  5  and additionally the “source−”, or ground, of a feeding cable were connected to the plate  7 . In this alternative embodiment, the plate  7  would have two ground connections—one directly to the main ground plane  5  and one via the feeding cable—and it would thus function as a ground plane. 
   The inventors of the present invention have found that an arrangement such as that shown in  FIG. 4  can provide performance in each of the four GSM bands even within a metal mobile terminal. Whereas a known slotted PIFA pair provides bandwidths reduced by 30-40% when used in a metal handset as opposed to a conventional plastic handset, antennas in accordance with embodiments of the present invention can achieve wide bandwidths for quad-band performance. The antenna  20  of  FIG. 4  is less sensitive to metallic blocks surrounding the antenna compared with conventional, slotted PIFA pair antennas, and thus has improved performance in metal handsets compared with other quad-band antenna structures. The antenna of embodiments of the present invention could suitably be used in a communications handset whose cover consists substantially of metal. The cover could suitably comprise at least 80%, at least 50% or at least 20% metal. Alternatively, embodiments of the present invention could be used in handsets having entirely plastic covers. 
   In an exemplary embodiment of the present invention, an antenna has the following configuration: the PWB is 35 mm×105 mm; the ground plane  7  and the antenna track  6 A have length 35 mm and width 20 mm, and the antenna track  6 A is positioned 10 mm above the ground plane resonator  7 . It should be noted that various functional components of a mobile communications terminal may be placed between the ground plane and the PWB, and thus the relevant height of the antenna is the distance between the ground plane  7  and the track  6 A. 
   In the exemplary embodiment, no slots are cut in the antenna&#39;s ground plane, and only two conducting pins are used to connect the ground plane and the PWB for simplicity of discussion. 
   The tuning of the resonant frequencies and their bandwidth is affected by adjusting the number and positions of connecting pins. The current distribution across the handset can also be controlled with careful selection of the pin positions to provide an even current distribution as opposed to high current concentrations. This can yield better antenna performance. Preferred numbers and positions of connecting pins can be determined by means of trial and error by simulating antennas having particular configurations of pins. Tests carried out by the inventors suggest that the optimum configuration is to use a single pin at the centre of each of the two side edges of the ground plane  7 , and optionally further pins along the two side edges. 
     FIG. 5  shows various locations of conducting pins  10  for connecting the ground plane  7  to the PWB  5 . Four specific cases are considered, in each of which different pin locations are used. These cases are not intended to be limiting, and it will be clear to the skilled person that any position or combinations of positions in addition to those discussed may be used within the scope of the invention. 
     FIG. 6  shows a simulated return loss against frequency for each of the four considered cases. The return loss data was obtained using an IE3D simulator. 
   In case one, pin locations C and D are used. The simulated return loss for this case is shown as  11  in  FIG. 6 . 
   In case two, pin locations B and D are used, and the resulting simulated return loss is shown as  12  in  FIG. 6 . 
   In case three, pin locations F and G are used, and the resulting simulated return loss is shown as  13  in  FIG. 6 . 
   In case four, pin locations B and C are used, and the resulting simulated return loss is shown as  14  in  FIG. 6 . 
   In  FIG. 6 , the frequencies at which the antenna radiates are denoted by dips in the graphs. Thus, the number of dips in a graph indicates the number of obtainable frequencies with the respective antenna, and the greater the width of the dips, the wider the frequency ranges at which the antenna radiates. It can be seen that for each of cases two, three and four an additional resonance is generated at the lower GSM band (USGSM850 and EGSM900). The bandwidths generated can be seen to be wide enough to allow performance at both these bands. 
     FIG. 7  shows the simulated radiation efficiency for case two. It is desirable to obtain high efficiency across the frequency bands of interest, and it is not surprising that the additional resonance generated can be seen from the graph to produce strong radiation. 
   It should be appreciated that resonances at different frequency bands may be generated by adjusting the number, the shape and the location of the conducting pins  10  in conjunction with the slots  9  cut out of the antennas ground plane. Thus, the antenna shown in  FIG. 4  can in general achieve multi-band and/or wide bandwidth performance, with high radiation efficiency and with controllable current distribution on the PWB. Embodiments of the invention provide antennas that are less affected by metal blocks compared with known antennas, and provide a more flexible feeding structure compared with slotted PIFA pairs, as discussed further below. 
   The structure of the known slotted PIFA pair antenna requires the feed to be positioned at or near the central line of the antenna so as to excite both elements of the PIFA pair (as shown in  FIG. 3B ). However, there is no such restriction on the feeding structure of the antenna of the present invention. The feed could potentially be moved to any points of the antenna track  6 A if an appropriate modification were made to the shape of the antenna track. 
   It can be appreciated that antennas of embodiments of the present invention are suitable not only for GSM frequencies but also for any other frequencies desired for mobile communications. 
   The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any definitions set out above. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.