Abstract:
An antenna has at least one slotted planar element. The slot in the planar element is open at one end and configured such that the planar element has a quarter wave resonance mode at a first frequency and there is a second resonant frequency at which the element has a ¾ wave resonance mode and/or the element&#39;s slot has a quarter wave resonance mode. The second frequency is not substantially three time the first frequency. Multiple slotted elements may be employed to achieve increased bandwidth.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a multiband antenna. 
     BACKGROUND TO THE INVENTION 
     In recent years there has been a move towards harmonizing 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 harmonization has not yet been completed. For instance, spectrum availability has lead 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 DCS1SOO-like system (DCS1900) 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. 
     SUMMARY OF THE INVENTION 
     The present invention provides a multiband antenna that is compact. 
     According to the present invention, there is provided a multiband antenna comprising a doubly resonant slotted substantially planar element having an open-ended slot therein, wherein said element is slotted such that said element has a quarter wave resonance mode at a first frequency and the element has a ¾ wave resonance mode at a second frequency or the slot has a quarter wave resonance mode at a second frequency, the second frequency not being substantially three time the first frequency. The planar element need not be flat but will have a large area to thickness ratio. For instance, the planar element may be curved to conform to an overriding structural limitation. Furthermore, the planar element may have small auxiliary portions, such as supporting and/or structures 
     Greater bandwidth may be obtained by including a further doubly resonant, slotted planar element having an open-ended slot therein. 
     Preferably, said further element is slotted such that said further element has a quarter wave resonance mode at a third frequency and the further element has ¾ wave resonance mode at a fourth frequency or its slot has a quarter wave resonance mode at a fourth frequency, the fourth frequency not being substantially three time the third frequency, More preferably, the first and third frequencies are sufficiently close to provide a single unbroken usable bandwidth and/or the second and fourth frequencies are sufficiently close to provide a single unbroken usable bandwidth. This gives a broader bandwidth. 
     Preferably, a feed circuit comprising an elongate signal line extends past the open ends of the or each slot. 
     Preferably, said planar elements are separated by a non-conductive strip into which said slots open. 
     Preferably, an antenna according to the present invention includes an insulating substrate, and the or each planar element is on one side of said substrate and said signal line comprises a strip on the other side of the substrate and is aligned with said non-conductive strip. 
     Preferably, there is a ground connection to the or each slotted planar element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an antenna according to the present invention; 
     FIG. 2 is a top plan view of the antenna of FIG. 1; 
     FIG. 3 is a bottom plan view of the antenna of FIG. 1; 
     FIG. 4 is an end view of the antenna of FIG. 1; 
     FIG. 5 illustrates the return loss characteristic of the antenna of FIG. 1; and 
     FIGS.  6 ( a ) to  6 ( f ) illustrate alternative embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings. 
     Referring to FIG. 1, an antenna  1  according to the present invention is mounted to the main printed circuit board (PCB)  2  of a radio communication device. The antenna  1  lies parallel to and is spaced perpendicularly from a major face of the main printed circuit board  2 . 
     Referring to FIGS. 2 and 3, the antenna  1  comprises a substrate  3 . First and second angular, substantially C-shaped, slots  4 ,  5  are on one side of the substrate  3 . The slots  4 ,  5  extend in opposite directions from a central strip  6 , extending across the printed circuit board  3 . Both the central strip and the slots  4 ,  5  comprise regions from which copper of a conductive layer on the substrate  3  has been removed. The copper conductor has also been removed from a margin  7  of the printed circuit board  3  which runs perpendicular to the central strip  6 , except for two branches  8 ,  9  reaching to the edge of the substrate  3  on respective sides of the central strip  6 . 
     The antenna&#39;s feed is provided on the other side of the substrate  3  and comprises a conductive strip  10  aligned with the central strip  6 . The conductive strip  10  starts at the edge of the aforementioned margin  7 . 
     The first slot  4  comprises a first section  4   a  extending parallel to the aforementioned margin  7 , a second section  4   b  extending from a point, about two thirds of the way along the first section  4   a , parallel to the central strip  6  and a third portion  4   c  extending towards the central strip  6  from the distal end of the second section  4   b  and at a right angle thereto. The first section  4   a  is 12 mm long, the second section  4   b  is 17 mm long from the edge of the first section  4   a  and the third section  4   c  is 8 mm long including the width of the second section  4   b.    
     The second slot  5  comprises first, second and third sections  5   a ,  5   b ,  5   c  arranged as a mirror image of the first slot element  4  except that the third section is shorter, being only 1 mm long. 
     The sections  4   a ,  4   b ,  4   c ,  5   a ,  5   b ,  5   c  of the slot elements  4 ,  5  are all 2.5 mm wide. 
     The branches  8 ,  9  are 1 mm wide. The conductive strip  10  is 1.5 mm wide and 15.5 mm long and the central strip  6  is slightly wider. The substrate  3  is 1.5 mm thick and comprises LCP with a relative permittivity of 3.8. 
     Referring additionally to FIG. 4, first and second copper strips  11 ,  12  are soldered to respective branches  8 ,  9  at one end and to the ground plane of the main circuit board  2  at the other end. The core of a coaxial cable  13  is soldered to the end of the conductive strip  10 , adjacent the aforementioned margin  7 . The shield of the coaxial cable  13  is soldered to the ground plane of the main circuit board  2 . The coaxial cable  13  connects the antenna to transmitter and receiver circuitry (not shown). 
     The first of the antenna  1  containing the second slot  5  resonates in a similar manner but at slightly higher frequencies. Consequently, the antenna  1  is resonant at four frequencies. However, the differences between the respective lower and upper resonant frequencies are such that the antenna  1  has two operational, unbroken 6 db bandwidths as shown In FIG.  5 . In the present example, with the antenna  1  in free space, it has a lower band centered on 948 MHz with a bandwidth of 155 MHz and an upper band centered on 2038 MHz with a and with of 509 MHz. Thus, the antenna  1  provides a single structure that can be used for GSM900, DCS1SOO, DCS 1900 and WCDMA frequency bands. Furthermore, it is far smaller with a volume of 5 cm 3  than an equivalent using conventional planar inverted-F antennas which would require a volume of 10 cm 3 . 
     The side of the antenna  1  containing the second slot  5  resonates in a similar manner but at slightly higher frequencies. Consequently, the antenna  1  is resonant at four frequencies. However, the differences between the respective lower and upper resonant frequencies are such that the antenna  1  has two operational, unbroken 6 db bandwidths as shown in FIG.  5 . In the present example, with the antenna  1  in free space, it has a lower band centred on 948 MHz with a bandwidth of 155 MHz and an upper band centred on 2038 MHz with a bandwidth of 509 MHz. Thus, the antenna  1  provides a single structure that can be used for GSM900, DCS1800, DCS1900 and WCDMA frequency bands. Furthermore, it is far smaller with a volume of 5 cm 3  than an equivalent using conventional planar inverted-F antennas which would require a volume of 10 cm 3 . 
     The present invention is not restricted to the slot forms shown in FIGS. 1 to  3 . 
     Referring to FIG.  6 ( a ), the first slot  4  is L-shaped and the second slot  5  has an angular J shape with its shank extending perpendicularly from the central strip  6 . 
     Referring to FIG.  6 ( b ), the first slot  4  is T-shaped, with its cross-piece extending perpendicularly from the central strip  6 , and the second slot  5  has an angular J shape with its shank extending perpendicularly from the central strip  6  and a stub extending its shank. The openings of the two slots  4 ,  5  into the central strip are offset a short way with respect to each other. 
     Referring to FIG.  6 ( c ), the first slot  4  comprises a first portion perpendicular to the central strip  6  and a T-shaped branch and the second slot  5  has a squared Z-shape. 
     Referring to FIG.  6 ( d ), the first slot  4  comprises a straight portion perpendicular to the central strip  6  and a T-shaped portion crossing the straight portion, and the second slot  5  comprises a straight portion perpendicular to the central strip  6  and a long cross shape crossing the straight portion. 
     Referring to FIG.  6 ( e ), the first slot  4  is L-shaped and accompanied by a third slot  14  which is also L-shaped but oppositely arranged. The second slot  5  is L-shaped with a small stub at its heel and accompanied by a fourth slot  15  which is T-shaped with its cross piece connected to the central strip  6  and it leg extending towards the second slot  4 . 
     Referring to FIG.  6 ( f ), the first slot  4  is straight and extends diagonally from the central strip and has a short branch parallel to the central strip  6 . The second slot  5  is curved and generally C-shaped. 
     It will be appreciated that many modifications may be made to the preferred embodiment described above. For instance, one half of the antenna can be dispensed with if a less broad bandwidth is required. Alternatively, the antenna could be made symmetrical to give a reduced bandwidth but better matching characteristics.