Abstract:
A resonant monopole slot antenna comprising a ground plane, having a radiating slot which is dimensioned such that the slot is equivalent electromagnetically to an odd number of quarter wavelengths at the antenna&#39;s operating frequency, wherein the antenna&#39;s feed is arranged at the open end of the radiating slot.

Description:
This application is a continuation of Ser. No. 10/020,195, Dec. 18, 2001 now U.S. Pat. No. 6,618,020. 

   FIELD OF THE INVENTION 
   The present invention relates to a slot antenna. 
   BACKGROUND OF THE INVENTION. 
   Slot antennas have found wide application in the field of radio communication. Conventional slot antennas comprise halfwave elements. This has put them at a disadvantage, with regard to size, compared with patch or wire antennas, such as the PIFA (planar inverted-F antenna), which can be constructed with quarterwave elements. 
   Ideally, a wire monopole antenna or the like comprises a quarterwave radiating element perpendicular to an infinite ground plane. This configuration is in practice impossible to achieve. However, in some circumstances, such as a mobile phone, it is impossible even to approximate this configuration well because of other design constraints. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a slot antenna that is not at a size disadvantage to PIFA antennas. 
   According to the present invention, there is provided a resonant monopole slot antenna including a radiating slot which is dimensioned such that the slot is equivalent electromagnetically to an odd number of quarter wavelengths at the antenna&#39;s operating frequency, wherein the antenna&#39;s feed is arranged at the open end of the radiating slot. Feeding the slot at its open end provides a broader usable bandwidth than feeding at a position towards the closed end. 
   Preferably the antenna&#39;s feed is provided at a position at which the maximum E-field occurs. 
   The radiating slot may be straight or not straight. If the slot is not straight, it may be, for example, L-shaped or meander. 
   Preferably, said odd number is 1. 
   Preferably, the radiating slot comprises an area of a printed circuit board which is free of conductor. More preferably, said area extends to an edge of the printed circuit board. 
   Preferably, said feed comprises a conductor extending along or parallel to the longitudinal axis or transversely across the radiating slot at its open end. 
   The feed may comprise a conductor, which could be any transmission line structure, but more preferably, said conductor comprises a signal line of a stripline or microstrip transmission line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the front ( FIG. 1(   a )) and back ( FIG. 1(   b )) of a PCB carrying a first antenna according to the present invention; 
       FIG. 2  shows the front ( FIG. 2(   a )) and back ( FIG. 2(   b )) of a PCB carrying a second antenna according to the present invention; 
       FIG. 3  shows the front ( FIG. 3(   a )) and back ( FIG. 3(   b )) of a PCB carrying a third antenna according to the present invention; and 
       FIG. 4  shows the front ( FIG. 4(   a )) and back ( FIG. 4(   b )) of a PCB carrying a fourth antenna according to the present invention; and 
       FIG. 5  shows metal ( FIG. 5(   a )) and plastic ( FIG. 5(   b )) radiotelephone casings having a slot antenna according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. 
   Referring to  FIGS. 1(   a ) and  1 ( b ), a slot antenna  1  is formed on a double-sided printed circuit board  2 . The slot antenna  1  is formed by removing a strip of copper from a margin of the front side  2   a  of the printed circuit board  2 . The front side  2   a  ( FIG. 1(   a )) of the printed circuit board  2  is otherwise an unbroken ground plane. 
   The back side  2   b  ( FIG. 1(   b )) of the printed circuit board  2  is devoid of copper save for a microstrip feed  3  to the slot antenna  1  and the tracks of a radio transmitter circuit  4 . 
   The slot antenna  1  is open at the edge of the printed circuit board  2 . In the present example, the length of the slot antenna is 12 mm and its width is 2 mm and the slot antenna resonates at 2451 MHz. This is approximately the same resonant frequency that would be expected for a closed slot antenna 24 mm long and 2 mm wide. Such a closed slot antenna is analogous to a halfwave dipole wire antenna and the present antenna can be viewed as analogous to a quarterwave monopole wire antenna. Consequently, the dimensions of slots with hereinafter be referred to by reference to the analogous wire antenna length. 
   The microstrip feed  3  to the slot antenna  1  extends along the edge of the printed circuit board  2 , perpendicular to the slot antenna  1 ,  FIG. 1 . It is spaced apart from the groundplane,  2   a . The microstrip feed  3  terminates behind the slot antenna  1 . In this example, the microstrip feed  3  feeds the slot antenna  1  at its high impedance end. Feeding the antenna at the high impedance end in this way provides a good match over a larger bandwidth than can be achieved by feeding the slot at its low impedance end. 
   Referring to  FIGS. 2(   a ) and  2 ( b ), the straight slot of the antenna  1  of  FIGS. 1(   a ) and  1 ( b ) can be replaced by an L-shaped slot. 
   Referring to  FIGS. 3(   a ) and  3 ( b ), the straight slot of the antenna  1  of  FIGS. 1(   a ) and  1 ( b ) can be replaced by a meandering slot. The feed  3 , is shown positioned at the maximum E-field position. 
   Referring to  FIGS. 4(   a ) and  4 ( b ), the “quarterwave” slot of the antenna  1  of  FIGS. 1(   a ) and  1 ( b ) can be extended by units of a “quarterwave”, for instance to three “quarterwaves” as shown. In this case the longer length of slot is 36 mm. With the feed point at the open end of the slot, the antennas feed impedance will be high for lengths which are odd numbers of “quarterwaves” and low for even numbers of “quarterwaves”. 
   Radiotelephone handsets  10  may have largely metal  11  ( FIG. 5(   a )) or largely plastic  13  ( FIG. 5(   b )) casings.  FIG. 5(   a ) shows the outer casing  11  of a handset  10  made from a metal such as steel (conductive material). The side of the handset  10  has a ‘T’ shape area  12  removed from it; (the shape of the removed area does not have to be a ‘T’ shape but may for example, be a meander shape). The base of the ‘T’ shape defines a slot in the metal casing  11  which can be used to provide a slot antenna, subject to arranging the feed section as described previously. In an alternative embodiment, the empty ‘T’ shape  12  in the side of the handset  10  may be filled in by a plastic (non-conductive) insert. Correspondingly, a side of a radio telephone  10  with a plastic casing  13  ( FIG. 5(   b )) may have a ‘U’ shaped metal insert  14  placed therein to again provide a slot antenna subject to appropriately arranging the feed sections as described previously. 
   Although reference to an outer casing is made the antenna could form part of an inner casing which is then covered by an exterior handset casing. 
   It will be appreciated that many modifications can be made to the above-described embodiments without departing from the spirit and scope of the claims appended hereto.