Abstract:
A method of independently modifying the ¼ and/or the ¾ wavelength resonant frequency in an open-ended slotted PIFA antenna, an open-ended slotted PIFA antenna comprising
       an antenna feed and an antenna ground wherein the antenna ground is associated with the antenna short-circuit end, and an open-ended slot having an open-end associated with the antenna open-circuit end, and   wherein the antenna ground and the slot are mutually arranged to provide operational variations in the current density between the open and short circuit ends of the antenna and around the perimeter of the slot, and an operational mean current path length between the open and short circuit ends of the antenna and around the perimeter of the open-ended slot, the mean current path length determining the ¼ and ¾ wavelength resonant frequencies for the open-ended slotted PIFA antenna,
 
the method comprising determining operational variations in current density around the perimeter of a pre-modified open-ended slotted PIFA antenna and modifying the mean current path length around the perimeter of the pre-modified open-ended slot in regions of comparatively high current density.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuing application of Ser. No. 10/020,197, entitled Multiband Antenna filed on Dec. 18, 2000, now U.S. Pat. No. 6,621,455 which application is incorporated hereby by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to open-ended slotted PIFA antennas having a ¼ wavelength resonance mode at a first frequency and a ¾ wavelength resonance mode at a second frequency and a method of adjusting the frequency ratio between the ¼ and ¾ wavelength resonant frequencies while maintaining independent control of the ¼ wavelength and ¾ wavelength resonant frequencies. The method can be used in the design/manufacture of open-ended slotted PIFA antennas with ¼ and ¾ wavelength resonance modes which can have resonance frequencies which vary from the normal 1:3 ratio. The present invention also relates to multi-band antennas. 
   BACKGROUND TO THE INVENTION 
   In recent years there has been a move towards harmonising mobile phone systems throughout the world. For instance, many countries have GSM900 systems enabling users from one country to use their mobile phones in another country. However, this harmonisation has not yet been completed. For instance, spectrum availability has let to the introduction of DCS1800 which is similar to GSM900 but operates in a band in the region of 1800 MHz rather than 900 MHz as in the case of GSM. Additionally, national spectrum management authorities do not necessarily decide to allocate the same bands to the public land mobile network service. For instance, in the United States of America a DCS1800-like system (PCS1900) is implemented in a band in the region of 1900 MHz. Further incompatibilities arise during transitional periods when a new system is being introduced and an old one phased out. 
   Accordingly, there is a need to provide a mobile phone antenna which can operate at various frequencies. 
   SUMMARY OF THE INVENTION 
   The invention provides methods and antennas according to the claims, and also as described with reference to specific embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a prior art perspective view of what is known in the art as a slotted PIFA antenna with an indirect feed (not shown); 
       FIG. 2  is a plan view of  FIG. 1 , illustrating the indirect feed; 
       FIG. 3  is a schematic representation of the current flow around the prior art antenna of  FIG. 2  at the ¼ wavelength resonant frequency; 
       FIG. 4  is a return loss v frequency plot illustrating the ¼ wavelength resonant frequency of the prior art antenna of  FIG. 2 ; 
       FIG. 5  is a schematic representation of the current flow around the prior art antenna of  FIG. 2  at the ¾ wavelength resonant frequency; 
       FIG. 6  is a return loss v frequency plot illustrating the ¾ wavelength resonant frequency of the prior art antenna of  FIG. 2 ; 
       FIG. 7  is a drawing of a first embodiment of an antenna according to the present invention; 
       FIG. 8  is a schematic representation of the current flow around the antenna of  FIG. 7  at the ¼ wavelength resonant frequency; 
       FIG. 9  is a return loss v frequency plot illustrating the ¼ wavelength resonant frequency of the antenna of  FIG. 7 ; 
       FIG. 10  is a schematic representation of the current flow around the antenna of  FIG. 7  at the ¾ wavelength resonant frequency; 
       FIG. 11  is a return loss v frequency plot illustrating the ¾ wavelength resonant frequency of the antenna of  FIG. 7 ; 
       FIG. 12  is a drawing of a second embodiment of an antenna according to the present invention; 
       FIG. 13  is a schematic representation of the current flow around the antenna of  FIG. 12  at the ¼ wavelength resonant frequency; 
       FIG. 14  is a return loss v frequency plot illustrating the ¼ wavelength resonant frequency of the antenna of  FIG. 12 ; 
       FIG. 15  is a schematic representation of the current flow around the antenna of  FIG. 12  at the ¼ wavelength resonant frequency; 
       FIG. 16  is a return loss v frequency plot illustrating the ¾ wavelength resonant frequency of the antenna of  FIG. 12 ; 
       FIG. 17  is a drawing of a third embodiment of an antenna according to the present invention; 
       FIG. 17   a  is a drawing of the third embodiment of the antenna with a direct feed arrangement; 
       FIG. 18  is a schematic representation of the current flow around the antenna of  FIG. 17  at the ¼ wavelength resonant frequency; 
       FIG. 19  is a return loss v frequency plot illustrating the ¼ wavelength resonant frequency of the antenna of  FIG. 17 ; 
       FIG. 20  is a schematic representation of current flow around the antenna of  FIG. 17  at the ¾ wavelength resonant frequency; 
       FIG. 21  is a plot illustrating the ¾ wavelength resonant frequency of the antenna of  FIG. 17 ; 
       FIGS. 22 to 26  illustrate alternative slot forms of the slotted PIFA antenna according to the present invention; 
       FIG. 27  is a perspective view of a multi-band antenna comprising two slotted PIFA antennas according to the present invention; 
       FIG. 28  is a plan view of the antenna illustrated in  FIG. 27 ; 
       FIG. 29  is a return loss v frequency plot illustrating the ¼ wavelength and the ¾ wavelength resonant frequencies of the antenna illustrated in  FIG. 27 ; 
       FIG. 30  is a plan view of the antenna illustrated in  FIG. 27  comprising a single feed structure. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A prior art drawing of what is known in the art as a quarter wavelength resonant slotted PIFA antenna  1  is illustrated in  FIG. 1  disposed on a substrate  2  mounted (mounting not shown) to the main Printed Circuit Board (PCB)  3  of a radio communication device. The antenna/substrate ½ are generally rectangular in shape and lie above and parallel to a major face  3   a  of the larger rectangular main printed circuit board  3 . Such an antenna is configured to resonate at a ¼ wavelength resonant frequency (e.g. 980 MHz) and a ¾ wavelength resonant frequency (e.g. 2.8 GHz) by virtue of its geometry (overall size/shape, slot size/shape/position). It will be appreciated that the frequencies quoted against specific antenna geometries throughout the text are provided for guidance purposes and do not necessarily reflect actual frequencies for specific geometries. 
   The antenna  1 , which is disposed on the away facing (with respect to the underlying PCB  3 ) surface  5  of the substrate  2 , is formed from copper (a conductive material). Furthermore, the antenna  1  comprises an inverted L-shaped slot  4  which is defined by the absence of copper from a L-shaped region ( 4   a ,  4   b ) of the conductive layer  5 . The slot  4  comprises a first section  4   a  which extends perpendicularly from approximately a third of the way down the right hand side of the substrate  2  and extends to approximately midway across the surface  5  to a first distal end  6 . The slot  4  has a second section  4   b  extending at a right angle from the first distal end  6  towards the lowermost edge of the surface  5  to a second distal end  13  ( FIGS. 1–2 ). 
   The copper conductor is also absent from a margin  7  along the upper edge of the surface  5 , save for a branch  8  situated towards the right hand side of the surface  5  and extending to the upper edge of the substrate  2  ( FIGS. 1–2 ). The branch  8  is electrically grounded so as to define a fixed electrical short circuit (minimum E field position). 
   The antenna&#39;s feed  9 ,  10  is provided on the underside (with respect to surface  5 ) of the substrate  2 , this underside facing the major face  3   a  of the PCB  3 . The feed comprises a coaxial cable  9  and a conductive strip  10  (indicated with dashed lines in  FIG. 2 ) aligned with the right hand edge of the substrate  2 . The feed  9 ,  10  does not form a conductive path to the surface  5  and will be recognised by those skilled in the art as an indirect feed arrangement. The conductive strip  10  starts at the edge of the aforementioned margin  7  and extends until it coincides with the electrically open circuit of the slotted PIFA antenna  1 , which in  FIG. 2 , is approximately midway down the right hand edge of the substrate  2 . This position is also known as the maximum E field position  11 . 
   The surface  5  of the antenna  1  acts as a ¼ wavelength resonant element at a first frequency ( FIG. 3 ,  4 ). This antenna  1  is also resonant at a second frequency which will be approximately three times the first frequency ( FIG. 5 ,  6 ) i.e. also acts as a ¾ wavelength resonant element. It should be noted that what is known in the art as a ¼ wavelength resonant PIFA antenna would be resonant at frequencies which are odd integer multiples of a quarter wavelength, e.g. ¼, ¾ etc. The antenna  1  may have resonant frequencies which are not exact integer multiples of a quarter wavelength due to antenna coupling effects, which can occur when the distal end  12  of the conductive layer  5  is in close proximity to the branch  8 . However, such coupling effects apply to all the resonant frequencies at the same time (to varying degrees). 
   The current density around the prior art antenna  1  will now be described by way of example at the ¼ and ¾ wavelength resonant frequencies. For clarity and simplicity, the current densities will be treated separately but it will be appreciated by those skilled in the art that the current densities, as shown in  FIGS. 3 and 5  of the antenna  1  can occur simultaneously when the antenna  1  is excited by both the ¼ and ¾ wavelength resonant frequencies. 
   The antenna  1  of  FIG. 2  is illustrated with the electrical current flow around the structure at the ¼ wavelength resonant frequency in  FIG. 3 . The electrical current flow is substantially located around the perimeter of the slot  4 , flowing in a clockwise direction from the maximum E field position  11  of the antenna  1  to the short circuit end of the antenna, which coincides with the branch  8  (minimum E field position). The maximum electrical current density occurs at the short circuit end of the antenna  1  at the branch  8  (minimum E field position), which is electrically grounded. The minimum electrical current density occurs at the open circuit end of the antenna  1  which coincides with the maximum E field position  11  which is approximately midway down the right hand side of the substrate  2 . There is only one occurrence each of the maximum and minimum electrical current densities. The mean path length taken by the current around the slot  4  determines the ¼ wavelength of the antenna. The resultant ¼ wavelength resonant frequency in this case is 980 MHz and is illustrated in  FIG. 4 . 
   The antenna  1  of  FIG. 2  is illustrated with the electrical current flow around the structure at the ¾ wavelength resonant frequency in  FIG. 5 . Again, the electrical current flow is substantially located around the perimeter of the slot  4 , flowing in a clockwise direction from the maximum E field position  11  of the antenna  1  to the short circuit end of the antenna  1 , which coincides with the branch  8  (minimum E field position). However, in this case, there are two occurrences each of the maximum and minimum electrical current density. A first maxima  15  of the electrical current density occurs at the short circuit end of the antenna  1  which coincides with the branch  8 , which is electrically grounded. A second maxima  16  occurs at a position which is electrically ½ wavelength away in a counter-clockwise direction around the slot  4  from the first maxima  15 , coinciding with a position which is towards the lowermost edge of the section  4   b.  The first minima  17  of the electrical current density occurs at a position which is electrically a ¼ wavelength away in a counter-clockwise direction around the slot  4  from the first maxima  15  and coincides with a position which is approximately mid-way down the left hand edge of the section  4   b.  The second minima  18  of the electrical current density occurs at the open circuit end of the antenna which coincides with a position approximately mid-way down the right hand side of the substrate  2  and coincides with the maximum E field position  11 . The mean path length taken by the current around the slot  4  determines the ¾ wavelength of the antenna. 
   The resultant ¾ wavelength resonant frequency is shown at 2800 MHz in  FIG. 6 . As mentioned earlier, and as in this case, the ¼ and ¾ wavelength resonant frequencies of the prior art slotted PIFA antenna  1  (980 MHz and 2800 MHz) do not have a numerical ratio of exactly 1:3. As indicated already this is due to coupling effects between the distal end  12  of the surface  5  being in close proximity to the branch  8 . 
   An antenna according to the present invention will now be described by making modifications to the antenna  1  of  FIG. 2  by altering the current path length around the slot  4  to realise three embodiments of the present invention ( FIGS. 7 ,  12  and  17 ). In each case, the mean current path length, with respect to the prior art of  FIG. 2 , has been changed by altering the perimeter/shape of the slot  4 . The figure references  1 – 13  of  FIGS. 1 and 2  have corresponding reference numbers  101 ,  201 ,  301  to  113 ,  213 ,  313  within  FIGS. 7 ,  12  and  17  respectively. 
   The antenna  101  shown in  FIG. 7  is the same as the antenna  1  of  FIGS. 1 and 2  except that the inverted-L shape slot  4  of antenna  1  has a third section  104   c  to form a substantially r-shaped slot. The third section  104   c  extends at a right angle towards the uppermost edge of the substrate  102  from the distal end  106  of the first section  104   a,  orientated about the same vertical axis as the second section  104   b.    
   The electrical current flow around the structure of the antenna  101  of  FIG. 7  is illustrated in  FIG. 8  at the ¼ wavelength resonant frequency. It is similar to  FIG. 3 , with corresponding features appropriately labelled. The mean path length taken by the current around the slot  104  determines the ¼ wavelength of the antenna and this arrangement provides a ¼ wavelength resonant frequency at 950 MHz ( FIG. 9 ). Comparing  FIGS. 3 and 8 , it can be seen that the antenna  1  has been modified by the addition of the third section  104   c  to the slot  4 . This modification has resulted in a change in the mean current path length in an area where the current density is large (c.f. the position of the maximum current density in  FIG. 3 and 8  is largely unchanged given the small change in the perimeter of the slot  4 ). Changing the perimeter of the slot  4  where the current density is large and thereby changing the mean current path length where the current density is large changes the ¼ wavelength resonant frequency of the antenna  1 . By increasing the current path length we have changed the resonant frequency from 980 MHz to 950 MHz. 
   Correspondingly, it will also be appreciated that a reduction in the slot perimeter where the current density is large and therefore a reduction in the mean current path length where the current density is large would result in the ¼ wavelength resonant frequency increasing. 
   The antenna  101  of  FIG. 7  is illustrated with the electrical current flow around the structure at the ¾ wavelength resonant frequency in  FIG. 10 . It is similar to  FIG. 5 , with corresponding features appropriately labelled. The mean path length taken by the current around the slot  104  determines the ¾ wavelength of the antenna. The resultant ¾ wavelength resonant frequency is 2780 MHz and is shown in  FIG. 11 . Comparing  FIGS. 5 and 10 , it can be seen that the antenna  1  has been modified by the addition of the third section  104   c  to the slot  4 . This modification has resulted in a change in the mean current path length in an area where the current density is small (c.f. the position of the minimum current density in  FIG. 6 and 11  is largely unchanged). Comparing  FIGS. 6 and 11 , it can be seen that changing the perimeter of the slot  4  where the current density is small and thereby changing the mean current path length where the current density is small has a modest change on the ¾ wavelength resonant frequency of the antenna  101 . By increasing the current path length we have changed the resonant frequency from 2800 MHz to 2780 MHz. Correspondingly, it will also be appreciated that a reduction in the slot perimeter where the current density is small and therefore a reduction in the mean current path length where the current density is small would result in the ¾ wavelength resonant frequency increasing. 
   In summary, the addition of the slot  104   c  to the antenna  1  substantially changes the ¼ wavelength resonant frequency of the antenna  101  and has a minimal effect on the ¾ wavelength resonant frequency. 
   A second embodiment is shown in  FIG. 12  in which the antenna  201  is the same as the antenna  1  of  FIGS. 1 and 2  except that the inverted-L shape slot  4  of antenna  1  has an additional section  204   d  to form a substantially C-shaped slot. The additional section  204   d  has been added at a right angle to the distal end  213  of the section  204   b  and extends in a direction towards the right hand edge of the substrate  202 .  FIG. 13  illustrates with the electrical current flow around the structure at the ¼ wavelength resonant frequency. The current flow is similar to  FIG. 3 , with corresponding features appropriately labelled. The mean path length taken by the current around the slot  204  determines the ¼ wavelength of the antenna, and is 970 MHZ in this case ( FIG. 14 ). 
   Comparing  FIGS. 3 and 13 , it can be seen that the antenna  1  has been modified by the addition of the section  204   d  to the slot  4 . This modification has resulted in a change in the mean current path length in an area where the current density is low. Comparing  FIGS. 4 and 14  it can be seen that changing the perimeter of the slot  204  where the current density is low and thereby changing the mean current path length where the current density is low will change the ¼ wavelength resonant frequency of the antenna  1 . By increasing the current path length we have changed the resonant frequency from 980 MHz to 970 MHz. Correspondingly, it will also be appreciated that a reduction in the slot perimeter where the current density is low and therefore a reduction in the mean current path length where the current density is low would result in the ¼ wavelength resonant frequency increasing. 
   The antenna  201  of  FIG. 12  is illustrated with the electrical current flow around the structure at the ¾ wavelength resonant frequency in  FIG. 15  It is similar to  FIG. 5 , with corresponding features appropriately labelled. The mean path length taken by the current around the slot  204  determines the ¾ wavelength of the antenna, and in this case is 2700 MHz ( FIG. 16 ). 
   Comparing  FIGS. 5 and 15 , it can be seen that the antenna  1  has been modified by the addition of the section  204   d  to the slot  4 . This modification has resulted in a change in the mean current path length in an area where the current density is large. Referring to  FIGS. 6 and 16  it can be seen that changing the perimeter of the slot  204  where the current density is large and thereby changing the mean current path length where the current density is large will change the ¾ wavelength resonant frequency of the antenna  201 . By increasing the current path length we have changed the resonant frequency from 2800 MHz to 2700 MHz. It will also be appreciated that a reduction in the slot perimeter where the current density is large and therefore a reduction in the mean current path length where the current density is large would result in the ¾ wavelength resonant frequency increasing. 
   In summary and with reference to  FIGS. 3 to 16  it will be appreciated that the addition of the slot  204   d  to the antenna  1  substantially changes the ¾ wavelength resonant frequency of the antenna  201  and has a minimal effect on the ¼ wavelength resonant frequency. 
   A third embodiment is shown in  FIG. 17 , in which the antenna  301  is the same as the antenna  1  of  FIGS. 1 and 2  except that the inverted-L shape slot  4  of antenna  1  has an additional third section  304   c  and fourth section  304   d  to form a substantially t-shaped slot  304 . The third section  304   c  extends at a right angle towards the uppermost edge of the substrate  302  from the distal end  306  of the first section  304   a  and is oriented about the same vertical axis as the second section  304   b.  The fourth section  304   d  has been added at a right angle to the distal end  313  of the section  304   b  and extends in a direction towards the right hand edge of the substrate  302 . 
   The antenna  301  of  FIG. 17  is illustrated with the electrical current flow around the structure at the ¼ wavelength resonant frequency in  FIG. 18 . It is similar to  FIG. 3 , with corresponding features appropriately numbered. The mean path length taken by the current around the slot  304  determines the ¼ wavelength of the antenna and in this case is 940 MHz ( FIG. 19 ). 
   Comparing  FIGS. 3 and 18  it can be seen that the antenna  1  has been modified by the addition of the sections  304   c  and  304   d  to the slot  4 . Comparing  FIGS. 4 and 19  it can be seen that changing the perimeter of the slot  304  will change the ¼ wavelength resonant frequency of the antenna  1 . By increasing the current path length we have changed the resonant frequency from 980 MHz to 940 MHz. Correspondingly, it will also be appreciated that a reduction in the slot perimeter where the current density is large and therefore a reduction in the mean current path length where the current density is large would result in the ¼ wavelength resonant frequency increasing. 
   The antenna  301  of  FIG. 17  is illustrated with the electrical current flow around the structure at the ¾ wavelength resonant frequency in  FIG. 20 . It is similar to  FIG. 5 , with corresponding features appropriately numbered. The mean path length taken by the current around the slot  304  determines the ¾ wavelength of the antenna and in this case is 2680 MHz ( FIG. 21 ). 
   Comparing  FIGS. 5 and 20  it can be seen that the antenna  1  has been modified by the addition of the sections  304   c  and  304   d  to the slot  4 . Comparing  FIGS. 6 and 21  it can be seen that changing the perimeter of the slot  304  and thereby changing the mean current path length will change the ¾ wavelength resonant frequency of the antenna  301 . By increasing the current path length we have changed the resonant frequency from 2800 MHz to 2680 MHz. Correspondingly, it will also be appreciated that a reduction in the slot perimeter where the current density is large and therefore a reduction in the mean current path length where the current density is large would result in the ¾ wavelength resonant frequency increasing. 
   In summary, it will be appreciated that the addition of section  304   c  to the antenna  1  has a substantial effect on the change in ¼ wavelength resonant frequency of the antenna  1 , whereas the addition of section  304   c  to the antenna  1  has a minimal effect on the change in ¾ wavelength resonant frequency of the antenna  1 . Furthermore, it will also be appreciated that the addition of section  304   d  to the antenna  1  has a substantial effect on the change in ¾ wavelength resonant frequency of the antenna  1 , whereas the addition of the section  304   d  to the antenna  1  has a minimal effect on the change in ¼ wavelength resonant frequency of the antenna  1 . It will be appreciated that the addition of sections  304   c  or  304   d  has the effect of independently controlling the ¼ wavelength or ¾ wavelength resonant frequency respectively while the other ¾ wavelength or ¼ wavelength resonant frequency respectively is substantially fixed. It will also be appreciated that the rate of change of ¼ and ¾ wavelength resonant frequency is determined by the extent to which the geometry (overall size/shape, slot size/shape/position) of the slot  4  is altered and also where the geometry is altered with respect to the ¼ and ¾ wavelength maximum current densities around the slot  4 . 
   The embodiments described in  FIGS. 7 ,  12  and  17  illustrate the use of an indirect feed structure,  109 ,  110 ,  209 ,  210  and  309 ,  310  respectively. A direct feed arrangement ( FIG. 17   a ) could be used in preference to an indirect feed arrangement. The use of either feed structure does not change the functionality of the described invention. 
   Antenna  351  ( FIG. 17   a ) is the same as antenna  301  ( FIG. 17 ) except that the indirect feed structure  309 ,  310  has been replaced by a direct feed structure  359 ,  360 . The features  301 – 308  and  311 – 314  of  FIG. 17  are corresponding numbered  351 – 358  and  361 – 364  in  FIG. 17   a.  The direct feed arrangement comprises a conductive branch  359  which is adjacent to and to the right hand side of the grounded branch  358 . The conductive branch has similar dimensions to the branch  358  and is electrically connected to the surface  355 . In an alternative arrangement (not shown) the position of the grounded branch  358  and the conductive branch  359  may be swapped or their positions relative to one another may be adjusted. A co-axial cable  360  is connected to the conductive strip  359  at one end and the other of the co-axial cable  360  is connected to radio circuitry (not shown). 
   The present invention is not restricted to the slot forms  104 ,  204  and  304  shown in  FIGS. 7 ,  12  and  17  respectively.  FIGS. 22–26  illustrate alternative slot forms  404 – 804 . Each of the antennas  401 – 801  illustrated in  FIGS. 22–26  has a short circuited branch (not shown) along the top edge of the surface  405 ,  505 ,  605 ,  705  and  805  respectively and an indirect feed (not shown) similar to that illustrated by  9 ,  10  in  FIG. 2  towards the right hand edge of the surface. 
   Slot  404  is a T-shaped slot comprising slotted sections  404   a  and  404   b.  It has an open-ended cross piece  404   a  extending horizontally across the surface  405  of the substrate  402 , ( FIG. 22 ). A second slot  404   b  extends vertically downwards from a position midway along the slot  404   a.    
   Slot  504  is an I-shaped slot comprising slotted sections  504   a ,  504   b  and  504   c.  It has an open-ended cross-piece  504   a  extending horizontally across the surface  505  of the substrate  502  ( FIG. 23 ). A second slot  504   b  extends vertically downwards from a position midway along the slot  504   a.  A third slot  504   c  lies parallel to slot  504   a  and is connected midway along its length to slot  504   b.  Slot  504   c  is shorter in length than slot  504   a.    
   Slot  604  is a substantially L-shaped slot comprising slotted sections  604   a  and  604   b.  Slot  604  has an open-ended slot  604   a  extending horizontally across the surface  605  of the substrate  602  to a distal end  606  ( FIG. 24 ). A second slot  604   b  extends downwards at a right angle from the distal end  606  of the slot  604   a  to a distal end  613 . The slot  604   b  is substantially rectangular except at the distal end  613  where the perimeter is semi-circular. 
   Slot  704  is a substantially y-shaped slot comprising slotted sections  704   a  and  704   b.  The open-ended slot  704   a  extends diagonally from the upper right hand edge of the surface  705  towards the lower left hand edge of the surface  705  ( FIG. 25 ). A second slot  704   b  extends vertically upwards from a position approximately midway along the slot  704   a    
   Slot  804  is a substantially T-shaped slot comprising slotted sections  804   a  and  804   b.  It has an open-ended cross-piece  804   a  extending horizontally across the surface  805  of the substrate  802 , ( FIG. 26 ). A second slot  804   b  extends downwards from a position midway along the slot  804   a  and at a right angle thereto. The second slot  804   b  is terminated at the distal end  813  with a non-uniform width caused by the distal end of the slot  804   b  being wavy. 
   Slot  404  ( FIG. 22 ) illustrates a means of adjusting the ¼ wavelength resonant frequency of the antenna  401  by the addition of section  404   aa  (highlighted by the cross hatched section) when compared to the antenna  1  of  FIG. 2 . Slot  704  ( FIG. 25 ) illustrates a means of adjusting the ¼ wavelength resonant frequency of the antenna  701  by the addition of section  704   b  (highlighted by the cross hatched section) when compared to a prior art slotted PIFA antenna (not shown). 
   The slots  504   aa  and  504   c  of  FIG. 23  and slots  804   aa  and  804   bb  (highlighted by the cross hatched sections) of  FIG. 26  illustrate alternative means of adjusting the ¼ wavelength and ¾ wavelength resonant frequencies respectively when compared to the antenna  1  of  FIG. 2 . 
   Slot  604   bb  (highlighted by the cross hatched section) shown in  FIG. 24  illustrates a means of adjusting the ¾ wavelength resonant frequency when compared to the antenna  1  of  FIG. 2 . 
   In an alternative arrangement, re-locating the short circuit branch  8  to alternative positions on the surfaces  405 ,  505 ,  605 ,  705  and  805  would result in the slotted forms shown in antennas  401 ,  501 ,  601 ,  701  and  801  having different effects on the ¼ and ¾ wavelength resonant frequencies when compared to the antenna  1  of  FIG. 2 . For example, if the short circuited branch  8  as shown in  FIG. 2  were moved to a position mid way down the right hand edge of the surface  705  of  FIG. 25  (not shown) then the addition of the slot  704   b  would have a minimal effect on the ¼ wavelength resonant frequency but would alter the ¾ wavelength resonant frequency. 
   The present invention provides a means of adjusting the frequency ratio between the ¼ and ¾ wavelength resonant frequencies while maintaining independent control of the ¼ wavelength and ¾ wavelength resonant frequencies. In an application where more than two resonant frequencies are required, for example in a multi-band mobile handset, the use of two quarter wavelength resonant slotted PIFA planar elements according to the present invention would give up to four resonant frequencies at the ¼ and ¾ wavelength resonant frequencies. A further implementation of the present invention illustrating a multi-band antenna with up to four resonant frequencies will now be described by way of example. 
   The antenna  1001  of  FIG. 27  comprises two slotted PIFA antennas  1001   a ,  1001   b  disposed on a substrate  1002  mounted (mounting not shown) to a PCB  1003  of a radio communication device. The antenna/substrate  1001 / 1002  are generally rectangular in shape and lie above and parallel to the major face  1003   a  of the larger rectangular main PCB  1003 . 
   The first slotted PIFA antenna  1001   a  is the same as the antenna  101  of  FIG. 7  and the antenna reference numbers  103 – 113  have corresponding reference numbers  1003 – 1013 . The second slotted PIFA antenna  1001   b  is the same as the mirror image of the antenna  201  in  FIG. 12  about a vertical axis. The antenna reference numbers  203 – 213  have corresponding reference numbers  1103 – 1113 . The substrates  1002   a  and  1002   b  of the antennas  1001   a  and  1001   b  respectively are connected via a non-conductive strip  1019  ( FIGS. 27 and 28 ) to form a single unitary substrate  1002 . The open ends of the slots  1004  and  1104  open into the non-conductive strip  1019  and face one another. The antennas feed circuits  1009 ,  1010  and  1109 ,  1110  are indicated with dashed lines ( FIG. 28 ) and may be combined using matching circuitry (not shown) to provide impedence matching between the radio circuitry and the antennas  1001   a  and  1001   b.  Alternatively it may be advantageous for the feeds to be kept separate and fed directly to suitable radio circuitry, e.g. a switch (not shown). 
   It will be appreciated that in a further embodiment the substrates  1002   a  and  1002   b  need not be joined by the non-conductive strip  1019  so that the antennas  1001   a  and  1001   b  exist as separate structures (not shown). In another embodiment it will be appreciated that the open ends of the slot need not face one another but may be offset from one another (not shown). 
   The antenna  1001  has ¼ wavelength resonant frequencies at 950 MHz ( 1001   a ) and 970 MHz ( 1001   b ) and ¾ wavelength resonant frequencies at 2700 MHz ( 1001   b ) and 2780 MHz ( 1001   a ), as shown in  FIG. 29 . The ¼ wavelength resonant frequencies of antennas  1001   a  and  1001   b  are close enough so that they overlap to form a single wider bandwidth resonant frequency, centred at 960 MHz ( FIG. 29 ). The antenna  1001  will therefore have three distinct resonant frequencies. It will be appreciated that altering the geometry of the slots  1004  and  1104  can result in the antenna  1001  having both the ¼ wavelength and the ¾ wavelength resonant frequencies overlapping to form two wider bandwidth resonant frequencies (not shown) (in the case where the geometries of  1001   a  and  1001   b  are substantially similar). It will also be appreciated that altering the geometry of the slots  1004  and  1104  can result in the antenna  1001  having no overlapping resonant frequencies and therefore having four distinct resonant frequencies (not shown) (in the case where the geometries of  1001   a  and  1001   b  are substantially different). 
   In a further embodiment an antenna  1201 , which is the same as the antenna  1001  of  FIG. 27  except that the feed structures  1009 ,  1010  and  1109 ,  1110  have been removed and replaced by a single feed structure  1209 ,  1210 . References  1001 – 1019  and  1102 – 1119  have corresponding references  1201 – 1219  and  1302 – 1319 . The antenna  1201  has a single feed  1209 , 1210  positioned midway between the antennas  1201   a  and  1201   b  and lying beneath the non-conductive strip  1219  ( FIG. 30 ). In each of the cases shown, the open-ended slot geometry forms a polygon determined so as the sum of the interior angles excluding the open end is not 540 degrees. 
   It will be appreciated that many modifications may be made to the preferred embodiment described above. For instance, the antenna could be made symmetrical to give a reduced bandwidth but better matching characteristics. In addition, it will be appreciated that one or more of the various embodiments may be combined.