Abstract:
An antenna device for a radio communications apparatus operating in dual frequency bands and having one antenna element ( 18, 23 ) for each of the frequency bands. The antenna elements are included in a rod-shaped arrangement and are disposed in line with one another. The antenna element ( 18 ) which is intended for the higher frequency band is galvanically coupled to the circuits in the radio communications apparatus and is designed as a rod or bar antenna ( 18 ). The antenna element ( 23 ) for the lower frequency and is a helix, whose one end portion ( 24 ) surrounds an end portion of the rod or bar antenna ( 18 ). The antenna elements ( 18, 23 ) are capacitatively coupled to one another so that the helix constitutes a capacitative top load to the rod or bar antenna ( 18 ).

Description:
This application is a continuation of PCT/SE98/02032 having an International filing date of Nov. 11, 1998 and which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an antenna device for a radio communications apparatus operating in dual frequency bands, and comprising one antenna element for each frequency band. 
     BACKGROUND ART 
     In radio communications apparatuses, in daily parlance known as mobile telephones, use is made of different antennae for different operational states in the apparatus. In the talk position of the apparatus, use is made of an antenna in which the most important requirements involve a high degree of efficiency. 
     Mobile telephones operate within different frequency bands, for example approx. 450 MHz in the NMT band, approx. 900 MHz in the GSM band and approx. 1800 MHz in the DCS band. There are also mobile telephones which are designed to be able to operate in more than one frequency band, and such constructions require antenna capacity in both frequency bands. 
     As regards the switching between stand-by position (or paging mode) and talk position, use is often made of mechanical switches which are based on the retraction or protraction of an antenna rod. To employ, in addition to such a switch, also a switch for switching between different frequency bands would appear to be clumsy and difficult to operate. 
     PROBLEM STRUCTURE 
     The present invention has for its object to design the antenna device intimated by way of introduction such that it obviates the requirement on the use of switches for switching between different frequency bands. The present invention further has for its object to design the subject matter of the invention such that this will have a high degree of efficiency in both frequency bands, and may be manufactured simply and economically and with the requisite mechanical strength. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The present invention will now be described in greater detail hereinbelow, with reference to the accompanying Drawings. In the accompanying Drawings: 
     FIG. 1 shows, in the talk position, a complete, dual band antenna device in which the subject matter of the present invention is included; 
     FIG. 2 shows the antenna device of FIG. 1 in the stand-by position, or paging mode; 
     FIG. 3 is a cross section through the upper end portion of the antenna device according to FIG. 2; 
     FIG. 4 shows a lower end portion of an antenna rod included in the antenna device according to FIGS. 1 and 2; 
     FIG. 5 shows a modified embodiment of the lower end of the antenna rod according to FIG. 2; 
     FIG. 6 is a cross section through an antenna rod included in the antenna device according to FIGS. 1 and 2; and 
     FIG. 7 shows, on a larger scale, a lower end portion of the antenna rod according to FIG.  6 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The present invention will be described as included in an antenna device which, in addition to antenna elements for talk position, also includes antenna elements for stand-by position, so called paging mode. The antenna arrangement in which the device according to the present invention is included is, therefore, a complete dual band antenna switchable between stand-by position and talk position. 
     From FIGS. 1 and 2 taken together, it will be apparent that the antenna device includes a first antenna unit  1  which is designed as a rod arrangement  3 , and a second antenna unit  2  which is designed as a helix arrangement  4 . The helix arrangement  4  is secured in the radio communications apparatus, in daily parlance the mobile telephone, which the antenna arrangement is intended to serve. 
     The first antenna unit  1  is of dual band design in order to be able to operate within both frequency bands for which the mobile telephone is rehearsed. Correspondingly, the second antenna unit  2  is also of dual band design for the same frequency bands as the first antenna unit. The first antenna unit is intended for the talk position of the mobile telephone, while the second antenna unit  2  is intended for the stand-by position, the paging mode, of the mobile telephone. Switching between the talk position and stand-by position is realised by displacement of the first antenna unit  1  in its longitudinal direction through the second antenna unit  2  between both of the states which are shown in FIGS. 1 and 2. It should be observed that, in the state for stand-by position (FIG.  2 ), it is only the second antenna unit  2  which is visible on the outside of the outer casing of the mobile telephone. 
     The antenna device includes means for electric switching between stand-by position and talk position, and the switching between these two positions is realised by the above-mentioned displacement movement of the first antenna unit  1 . 
     The means for switching between stand-by position and talk position may include either an electric switching device or a device for bringing one of the antenna units to a resonance position which is located a safe distance outside the selected frequency bands when the relevant antenna unit is not to be employed. 
     The antenna device further includes means for switching the two antenna units  1  and  2  between the relevant frequency bands. Such means are designed in such a manner that no mechanical switching arrangement is included therein. Instead, the switching is realised between the frequency bands in that radiating antenna elements included in the antenna units are capacitatively or inductively engaged and disengaged, respectively. 
     In FIG. 1, it is intimated that the rod arrangement  3  has an upper portion  17  which is produced from electrically insulating and non-magnetic material. The longitudinal extent of the upper portion  17  is such that, when the antenna device is located in the position according to FIG. 2, the upper portion  17  is to fill out in the longitudinal direction at least the helix arrangement  4  and its lower mounting and connection parts and fittings. The reason for this is that, in the stand-by position, FIG. 2, where the helix arrangement  4  is located in operation, there cannot be any electrically conductive magnetic material interiorly in the helix arrangement  4  without seriously disrupting its function. 
     The rod arrangement  3  has further a third antenna element  15  intended for the lower frequency band and a fourth antenna element  16  intended for the higher frequency band. 
     As is apparent from FIG. 6, the fourth antenna element  16 , i.e. the antenna element of the rod arrangement  3  for the high frequency band, has an elongate, metallic connector  18  which serves the function of a rod antenna which is set to the higher frequency band. Around the rod antenna  18 , the rod arrangement has an insulating tube  19 , and outside this a spacer  20  designed as an insulating tube which is surrounded by a surface coating  21  of insulating material. In its lower end, the straight rod antenna  18  is galvanically connected to a metal sleeve  22  via which the rod arrangement  3  is supplied. 
     The third antenna element  15 , which is designed for use in the low frequency band in the talk position of the antenna device, is designed as a helix antenna  23  which is disposed on the outside of the insulating tube  19  of the rod arrangement. Within the longitudinal extent of the helix  23 , the spacer  20  is absent, since the helix  23  takes up its place. Possibly however, parts of the spacer  20  may be disposed between the winding turns in the helix  23 . The helix  23  is set for use in the lower frequency band and is, in terms of supply, capacitatively coupled to the metallic conductor  18  which, on operation in the lower frequency band, functions as an extension of the supply lead to the rod arrangement  3 . The capacitative coupling is realised in that the helix  23  in the longitudinal direction overlaps the metal conductor  18  within a region which is marked by reference numeral  24  in FIG.  6 . It will be apparent that the helix  23  terminates with its lower end at the lower end of the region  24  and that the metallic conductor  18  terminates with its upper end at the upper end of the region  24 . Within the overlap zone, there is naturally the insulating tube  19  between the helix  23  and the conductor  18 . 
     While not being apparent from FIG. 6, the rod arrangement  3  has, over the helix  23 , the upper portion  17  which is electrically insulating. Within this region, the rod arrangement includes the insulating tube  19 , the spacer  20  and the surface coating  21 . 
     On operation of the rod arrangement  3 , this functions as follows. 
     On operation of the rod arrangement  3 , this is galvanically connected to the circuits of the mobile telephone in that the metal sleeve  22  at the lower end of the rod arrangement is galvanically connected to these circuits in a manner which will be described in greater detail hereinbelow. 
     In operation in the higher frequency band, the conductor  18  functions as a quarter wave rod antenna, while the helix  23  constitutes a capacitatively coupled top load to the rod antenna. The presence of this top load implies that the rod antenna may be made physically shorter than its electric length. 
     In operation in the lower frequency band, the metallic conductor  18  functions only as a coupling lead between the connection of the antenna device to the circuits of the mobile telephone and the capacitative coupling zone  24  between the conductor  18  and the helix  23 . 
     FIG. 3 shows a section through the upper portion of the antenna device in the stand-by position. It will be apparent from the figure that the antenna device is built-up around a bushing portion  5  produced from metal, which is intended for securing the antenna device in a casing (not shown) for the mobile telephone and which is galvanically connected to the circuits of the mobile telephone. The bushing portion  5  has a central, through-going channel for accommodating the rod arrangement  3 , and further an upper, substantially cylindrical portion  6 , which serves as an anchorage for the helix arrangement  4 . Further, the bushing portion has a lower portion  8  with a thread  9  which serves for securing the antenna device on the mobile telephone. 
     Interiorly in the bushing portion, there are provided a number of resilient contact tongues  25 . In the retracted position of the rod arrangement, FIGS. 2 and 3, these rest on the insulating outer layer of the rod arrangement, for which reason the rod arrangement is electrically disconnected from the bushing portion  5  and thereby also from the circuits of the mobile telephone. In the protracted position of the rod arrangement, FIG. 1, the contact tongues  25  rest on the metal sleeve of the rod arrangement, for which reason the rod arrangement is galvanically coupled to the circuits of the mobile telephone. 
     A summary description of the helix arrangement  4  will be given below. 
     The helix arrangement is built-up around a carrier portion  7  producted from insulating, non-magnetic material and secured on the cylindrical portion  6 . 
     The carrier portion  7  has a central, substantially cylindrical channel  10  which is open in the upper end of the carrier portion. Interiorly in the channel  10  of the carrier portion  7 , there is provided a first antenna element  11  which is designed as a helix. The first antenna element  11  is set for the lower frequency band for the antenna device and is galvanically connected to the electric circuits of the mobile telephone. 
     On the outside of the carrier portion, there is disposed a second antenna element  12 , also designed as a helix, and designed for the higher frequency band. The second antenna element  12 , i.e. the helix, is galvanically discrete from the electric circuits of the mobile telephone and is supplied inductively/capacitatively from the first antenna element  11 . 
     A hood  13  of electrically insulating, non-magnetic material is disposed around the second antenna element  12  and the carrier portion  7 . The hood  13  has, in its upper end (facing away from the bushing portion  5 ), an opening  14  for permitting the passage and longitudinal displacement of the rod arrangement  3 . 
     Electrically, the helix arrangement  4  functions as follows. The first antenna element  11 , i.e. the helix disposed interiorly in the carrier portion  7 , is galvanically connected to the circuits of the mobile telephone. When the antenna element  11  is supplied with or receives electromagnetic radiation within the lower frequency band, this radiation will pass through the second antenna element  12  without being influenced to any appreciable degree thereby, since it has a resonance frequency a safe distance from the resonance frequency of the first antenna element  11  and further, because it is so “scant” that it has no appreciable screening-off effect. The second antenna element, i.e. the outer helix, is set to the higher frequency band, for which reason it must have a considerably fewer number of turns and greater pitch than is the case with the first antenna element. For this reason, the second antenna element  12 , seen from the first antenna element  11 , will, as intimated above, be considered as transparent. If transmission takes place within the higher frequency band, the second antenna element  12 , i.e. the outer helix, will function as radiating element which is capacitatively/inductively coupled to the inner helix  11 . Correspondingly, on receiving within the higher frequency band, a capacitative/inductive coupling will be in place between the two antenna elements  12  and  11  so that received energy is transmitted between them. 
     In the foregoing, the rod arrangement  3  has been described in detail and the helix arrangement  4  has been described schematically, individually both electrically and mechanically. With reference to FIG. 3, it will now be disclosed how cooperation between them takes place. 
     The bushing portion  5  has, as was mentioned above, a through-going channel through which the rod arrangement  3  is displaceable in the longitudinal direction. In order to prevent play and give the desired electric contact, there are disposed interiorly in the bushing portion  5 , metallic spring elements which slide along the surface coating  21  of the rod arrangement  3  when the rod arrangement is displaced. In the protracted position of the rod arrangement, in the talk position as shown in FIG. 1, the bushing portion  5  with its spring elements  25  realises electric contact with the lower contact sleeve  22  of the rod arrangement, whereby the rod arrangement will be directly galvanically couple to the circuits of the mobile telephone. 
     In the position of the rod arrangement according to FIG. 2, i.e. the stand-by position, the lower end of the inner helix  11  rests on an electrically conductive washer  26  which in turn rests on the upper side of the cylindrical upper portion  6  of the bushing portion  5 . Hereby, an electrical galvanic connection prevails between bushing portion and the lower end of the inner helix  11 . 
     On protraction of the rod arrangement from the position illustrated in FIG. 2 to the position illustrated in FIG. 1, the rod arrangement will slide through the helix arrangement  4  until such time as its projecting edge region  27  (FIGS. 4 and 7) on the upper end of the metal sleeve  22  comes into abutment against the underside of the washer  26 . On continued lifting, the washer  26  will accompany the rod arrangement  3  in its upwardly directed movement, whereby the inner helix  11  is compressed in the axial direction. Granted, the helix  11  is still galvanically coupled to the electric circuits of the mobile telephone but will, as a result of this axial compression, first have a misadptation as regards frequency and thereafter wholly cease to exist as a radiating antenna element. Hereby, the outer helix  12  is naturally also electrically disconnected. 
     In one alternative embodiment, it is possible, on the upper side of the metallic sleeve  22  on the lower end of the rod arrangement  3 , to place a ring  28  of insulating material. On protraction of the rod arrangement  3 , the ring  28  will come into abutment against the washer  26  and lift this out of electric contact with the bushing portion  5 . As a result, the inner helix  11  will be electrically disconnected from the circuits of the mobile telephone as soon as the washer  26  has been lifted out of electric contact with the bushing portion  5 . 
     In the rod arrangement, the helix  23  has a length of approx. 40 mm, containing 40-42 turns and with a pitch of 0.8 mm. The overlap length  24  amounts to approx. 9 mm. The total length of the entire rod arrangement is approx. 116 mm. 
     The present invention may be modified without departing from the scope of the appended claims.