Abstract:
The present invention relates to a small microstrip antenna device, mountable inside a hand-held radio communication device, for receiving and transmitting RF signals in one or more frequency bands. The microstrip antenna comprises a ground plane means ( 101 ), at least a first feeding means ( 107 ) and N radiating elements where N is an integer greater than zero. The microstrip antenna structure also has a first conductive path ( 104 ). The feeding means is arranged on the first patch for feeding radio frequency signals to the N radiating elements, wherein at least a first of the N radiating elements has a second patch ( 106 ). The second patch is inductively coupled ( 108 ) to the first patch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on the Swedish patent application SE  9702659-5  ‘Compact Antenna Device’ which is hereby incorporated by reference and the international patent application PCT/SE98/00899 ‘Compact Antenna Device’ which is hereby incorporated by reference. Both applications have the same applicant as the present invention. 
     1. Technical Field of Invention 
     The present invention relates in general to an antenna structure and more specifically to a microstrip antenna structure. 
     2. Description of Related Art 
     With the recent advances in mobile communication, there has been tremendous interest in development of small size and low profile antennas for the further miniaturization of mobile radio communication equipment. Goals include small size, low profile, low cost and ease of manufacturing. Frequencies of interest can for instance be 900 MHz band antennas for applications in cellular handheld radio devices such as GSM (890-935 MHz), indoor cordless telephones such as the European CT1+ (886-931 MHz) and 1.9 GHz band antennas for applications in DECT (1.89 GHz) and PCS (1.8 GHz). These systems have their own requirements in antenna characteristics, such as resonant frequency, bandwidth, gain etc. 
     Existing antennas used in mobile phones include the most common whip antennas (monopole), microstrip patch antennas and planar inverted-F antennas. Microstrip patch antennas and planar inverted-F antennas are typically low-profile antennas. Although the microstrip patch antenna previously has had the shortcoming of narrow bandwidth and low efficiency, its advantages of low profile, small size and light weight are attractive properties. 
     However both planar inverted-F antennas and microstrip patch antennas exhibit size problems when they should be adjusted for the specific frequencies and fit into the newer generation of miniature mobile radio communication devices. This is particular problematic when modern mobile phone design calls for multiple antennas to be placed into one handset to be able to simultaneously communicate in two different systems, in a very broad frequency band or more generally to take advantage of antenna diversity. 
     EP 749 176‘Planar and non-planar double C-patch antennas having different aperture shapes’ discloses a patch antenna. The C-patch antenna includes a truncated ground plane, a layer of dielectric material having a first surface overlaying the ground plane and an opposing second surface, and an electrically conductive layer. The conductive layer forms a radiating patch and has a non-rectangular aperture. 
     Wo 96/27219‘Meandering inverted-F antenna’ discloses an inverted F-antenna with a meandering pattern. The antenna is a planar radiating structure having alternating cutouts along a longitudinal dimension of a planar radiating element or patch which is parallel to a nearly coextensive ground plane. 
     SUMMARY OF INVENTION 
     The object of the present invention is thus to achieve a small microstrip antenna device, mountable inside a hand-held radio communication device, for receiving and transmitting RF signals in one or more frequency bands. 
     The problems described above, with how to achieve an antenna which is mountable inside and hand-held radio communication device is solved by providing a microstrip antenna comprising a ground plane, at least a first feeding means and N radiating elements where N is an integer greater than zero. The micro strip antenna structure having a first conductive patch. The feeding means being arranged on the first patch for feeding radio frequency signals to the N radiating elements, at least a first of the N radiating elements having a second substantially rectangular patch. The second patch being inductively coupled to the first patch and the second patch having a free end. 
     In more detail the objects of the present invention according to one embodiment, is achieved by providing the above mentioned microstrip antenna structure wherein at least one of the N radiating elements having a capacitive coupling to ground in the free end. 
     In more detail the objects of the present invention according to one embodiment, is achieved by providing the above mentioned microstrip antenna wherein, the first and second patch being thin conductive layers on a dielectric substrate. The substrate comprising at least first and second protrusions arranged for retaining a component in electric contact with the first and second patch. 
     An advantage with the present invention is that a small microstrip antenna structure is achieved which is suitable for mounting inside a hand-held radio communication device. 
     Another advantage with the present invention is that the antenna structure can be tuned to be responsive to multiple frequencies. 
     An advantage, according to one embodiment of the invention, is that the antenna structure can be achieved with a choice of using discrete components or not. 
     An advantage, according to one embodiment of the invention, is that the antenna structure may be implemented directly on the inside of a back cover of a hand-held radio communication device. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only,- since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention and wherein, 
     FIG. 1 shows a schematic, perspective view according to a first preferred embodiment of the invention, 
     FIG. 2 shows a schematic, perspective view according to a second preferred embodiment of the invention, 
     FIGS. 3 a ,  3   b  and  3   c  shows schematic views of a retainer arrangement according to a preferred embodiment of the invention, 
     FIGS. 4 a  and  4   b  show diagrammatic views according to a second embodiment of the invention, 
     FIG. 5 shows a diagrammatic view according to a third embodiment of the invention, 
     FIGS. 6 a ,  6   b  shows diagrammatic views of different variants according to a fourth embodiment of the invention, 
     FIGS. 7 a ,  7   b ,  7   c ,  7   d  shows diagrammatic views of different variants according to the fourth embodiment of the invention, 
     FIGS. 8 a ,  8   b ,  8   c  shows diagrammatic views of different variants according to a fifth embodiment of the invention, 
     FIGS. 9 a ,  9   b  shows diagrammatic views of different variants according to a sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic, perspective view according to a first preferred embodiment of the invention. A ground plane is denoted  101  and applied to the backside of a printed circuit board  102 . A dielectric substrate is denoted  103  and is acting as a carrier for a radiating structure  104 . The radiating structure  104  is, in this preferred embodiment, a conductive pattern, which can be achieved with for instance MID-technique (Molded Intrusion Design) which is a technique well known to the skilled man in the art. Another possibility is to use a conductive pattern, screen printed on an adhesive flexible film. 
     The radiating structure  104  comprises a first patch  105  and a second patch  106 . The first patch  105  comprises feeding means  107  for feeding an RF signals to the radiating structure. The first patch  105  is connected to the second patch  106  through a meandering pattern  108 . The meandering pattern  108  acts as a inductive connection between the first and second patches  105  and  106 . The inductance is determined by the number of turns and the width of the meandering pattern  108 . The second patch  106  is folded over the edge  109  and continues towards the ground plane  101  to effectively achieve a capacitive coupling between the second patch  106  and the ground plane  101 . A capacitive coupling is, of course, also existing between the first patch  105  and the second patch  106 , and the capacitance is determined by the distance between the two patches. 
     FIG. 2 shows a schematic, perspective view according to a second embodiment of the invention. A ground plane is denoted  201  and a dielectric substrate is denoted  202 , a first patch is denoted  203  and a second patch is denoted  204 . A feeding means in the form of a coaxial cable is denoted  205 . The shield of the coaxial cable is connected to the first patch  203  at a first connection point  206  and the feed of the coaxial cable is connected to the first patch  203  at a second connection point  211 . The distance between the first and second connection point is determining the input reactance. The dielectric substrate comprises first, second, third and fourth protrusions denoted  207 ,  208 ,  209  and  210 , respectively. The first patch  203  comprises a conductive strip folded over the first protrusion  207  and the second patch comprises a conductive strip folded over the second protrusion  208 . The first and second protrusions are arranged for retaining a discrete component, such as for instance a coil or more generally an inductance, in electrical contact with the first and second patch. The discrete component is not shown in FIG. 2 for sake of clarity. The protrusions are somewhat flexible or resilient so a contact force is established between the folded strip and the discrete component on respective side. It is of course also possible to solder the discrete component to achieve even better retaining capabilities. 
     The second patch  204  comprises a second conductive strip folded over the third protrusion  209  and the ground plane also comprises a conductive strip folded over the protrusion  210 . Thus can a discrete component, such as a capacitor (not shown), be retained between the third and fourth protrusions in electric contact with the second patch  204  and the ground plane  210 . 
     FIGS. 3 a ,  3   b  and  3   c  show schematically in a closer view different variants of the retainer arrangement in FIG.  2 . In FIG. 3 a  a discrete component is illustrated which can be a coil, active inductor, tunable inductor or other inductive means, denoted  301 . The first patch  203  with the conductive strip folded over the first protrusion  207  is soldered to the component  301 . The second patch  204  is folded over the second protrusion  208  and soldered to the component  301 . 
     In FIG. 3 b  a different retainer arrangement is disclosed where the resilient or flexible characteristics of the dielectric substrate are fully used. In this embodiment, no soldering is required. When the discrete component  302  is pushed down in the retainer the first and second protrusion  303  and  304  flexes back so as to let the component  302  to pass. Once the component is in the retainer the protrusions resumes their original positions effectively retaining the component  302  through the small cutouts in the protrusions. In FIG. 3 b , a ground plane  305  is folded over an edge and again over the second protrusion  303  to achieve electrical contact between the ground plane  305  and the component  303 . 
     In FIG. 3 c  the electrical contact between a component  305  and a ground plane  307  is achieved through a connector means  308 . 
     FIG. 4 a  shows a first diagrammatic view and FIG. 4 b  a second diagrammatic view according a preferred embodiment of the invention. A first patch is denoted  401 , a second patch is denoted  402  and an inductive coupling between the first and second patch is denoted  403 . A first capacitive coupling between the second patch and ground is denoted  404  and a second capacitive coupling between the first and second patch is, in FIG. 4 b , denoted  405 . A signal generator is denoted  406  and a ground connection is, in FIG. 4 a , denoted  407 . The radiating structure is adjusted to have first resonance frequency f 1  for which the inductance  403  and the second capacitance  405  effectively act as an open circuit where substantially only the first patch is radiating FR signals. For a second resonance frequency f 2  the inductance  403  and the second capacitance  405  effectively act as a short circuit and substantially both the first and second patches radiate RF signals as one antenna element. Thus, the combined inductive and capacitive coupling between the first and second patch act as a trap preventing signals within a specific frequency band to pass the coupling. 
     FIGS. 5 a  and  5   b  shows diagrammatic views according to a third preferred embodiment of the invention where no top capacitance is used. 
     FIGS. 6 a  and  6   b  show diagrammatic views according to a fourth embodiment of the invention. A first patch is denoted  601  having first and second protruding parts  602  and  603  respectively. Feeding means for feeding RF signals to the radiating structure is denoted  604  and a ground feed is denoted  605 . A second patch is denoted  606  and a third patch is denoted  607 . The second patch is coupled through a first inductance  608  to the first protruding part  602  and the third patch  607  is coupled through a second inductance to the second protruding part  603 . Thus is two separate, parallel radiating arms achieved which each can be tuned to different resonance frequencies as described. FIG. 6 b  disclose the arrangement in a more schematic view. 
     FIG. 7 a  shows a variant of the fourth preferred embodiment where first and second top capacitances,  701  and  702 , are coupled to the first and second radiating arms,  703  and  704 . In FIG. 7 b , the second radiating arm is an elongated conductive strip  705  and no top capacitances are used. In FIG. 7 c , the first radiating arm  703  comprises a top capacitance  701  and in FIG.  7   d , the second radiating arm  704  comprises a top capacitance  702 . 
     FIG. 8 a  shows a diagrammatic view according to a fifth preferred embodiment of the invention where three radiating arms are used. The arms are arranged in parallel and each radiating arm comprises inductive coupling. In FIG. 8 b , the radiating arms are arranged in an Y-form, and in FIG. 8 c , the arms are arranged in a T-form. Even though not shown, each individual radiating arm may or may not comprise a top capacitance to ground and even though each arm is shown comprising a inductive coupling, it is also possible to have individual arms as elongated conductive strips. Also the feeding is left out for sake of clarity as well as a possible short connecting the antenna to ground in FIGS. 8 b  and  8   c.    
     FIG. 9 a  shows a diagrammatic view according to a sixth preferred embodiment of the invention where four radiating arms are used. The radiating arms are arranged in a cross form and each radiating arm comprises inductive coupling. In FIG. 9 b  the radiating arms are arranged in a H-form. Also in this embodiment it is possible to use elongated conductive strips and/or top capacitances. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.