Coupling element for a radio telephone antenna

0001The invention relates to an arrangement for connecting a radio communication device to an external antenna. The invention can be advantageously applied for connecting a mobile phone to a car antenna.

0002A common way of improving the car use characteristics of a mobile phone is to install outside the body of the car a so-called car antenna to which the mobile phone is connected via a coaxial conductor and a RF connector at the end of the conductor. Therefore, a prior art mobile phone must include a counterpart for said RF connector. Then the antenna of the mobile phone must be made inoperative either by an automatic switch connected to said counterpart for the RF connector or by the user with a separate switch, lest simultaneous operation of the both antennas cause interference and losses.

0003Said RF connector and its counterpart may constitute a standard coaxial interface based on galvanic contact, but such a solution is susceptible to oxidization, dirt and wear. A solution is known from Finnish Patent No. 84536 and corresponding EPO application EP-0 399 975, wherein the RF coupling between the mobile phone and the connector part in the car installation set is made capacitively by means of metal plate counterparts. This arrangement avoids said oxidization, dirt and wear problems, but it does not eliminate the need for a separate switch to disconnect the mobile phone antenna for the duration of car use. In addition, the switching means and the transmission lines connected to the means require space inside the mobile phone.

0004There also exist prior art methods and devices for electromagnetically coupling a car antenna to a mobile phone antenna. The simplest electromagnetic coupler is an induction loop placed around the mobile phone antenna. Patent document GB-2 266 997 discloses a solution according to Figure 1, wherein a connector part B comprising a resonator element C is attached with adhesive tape or the like to the casing of a mobile phone A to be used in a car. By attaching the connector part B to the mobile phone A said resonator element C is brought so close to the mobile phone antenna D that it is electromagnetically coupled to the antenna and starts to resonate, whereby RF power radiated by the mobile phone antenna is coupled to it and that power is further taken via a coaxial conductor E to the car antenna. A disadvantage of this arrangement is that the coupling between the mobile phone antenna D and the resonator element C depends on their positions relative to each other. An external resonator element also puts an electric load to the mobile phone antenna, thereby decreasing its resonating frequency and weakening the coupling in the uppper part of the frequency band used.

0005The publication US 4,220,955 to Frye discloses a coupling device which is meant to be slid over a rubber-insulated helix antenna. The coupling device comprises a central insulating sleeve, the outer surface of which comprises a loosely wound helical conductive element. The upper end of the helical element is galvanically coupled to the inner conductor of a coaxial connector, and the cylindrical outer conductor of the coaxial connector is coupled to a conductive, hollow cylindrical chamber which is also the outer cover of the coupling element. The cavity between the sleeve and the chamber is filled with air.

0006The publication WO 95/07556 discloses an inductive loop which is to be placed around the protruding antenna of a radio device with the help of a dielectric support plate. There is an impedance matching circuit that couples the inductive loop to both the inner and outer conductors of a coaxial cable.

0007The object of the invention is to provide an arrangement for connecting a radio communication device, advantageously a cellular mobile phone, to an external antenna, partly avoiding and partly reducing the aforementioned disadvantages related to prior art solutions. The invention may provide an arrangement with which said connection to an external antenna is done without a separate antenna switch and with small losses. The invention may be applied in large scale production and may be durable and easy to use.

0008The invention is based on the idea that dielectric material with a high permittivity attracts electromagnetic energy. Therefore a dielectric piece, together with conductors, can be used to direct the electromagnetic field radiated by the antenna of a radio communication device. The field can be guided e.g. to the feed cable of a external antenna with relatively small losses. A dielectric coupling element is placed near the antenna of a radio communication device so that the electromagnetic waves leaving the antenna refract on the boundary surface in an optimal manner, i.e. so that significant part of the energy radiated by the antenna of the radio communication device is directed to the coupling element.

0009The electrical length of the coupling element may be e.g. half or a quarter of the wavelength of the carrier of the radio communication device. The coupling element has a port through which the electromagnetic field is taken to a coaxial cable and through this to external antenna. Similarly, the field received by the external antenna is taken via the coaxial cable and the signal port to the coupling element wherefrom it is guided to the antenna of the radio communication device.

0010According to the present invention there is provided a coupling arrangement for connecting a radio telephone to an external antenna, comprising means for contacting said radio telephone detachably to it, a cable for feeding the external antenna, and a coupling element for transferring electromagnetic field between the fixed antenna of radio telephone and said cable. The coupling element is wholly beside the fixed antenna and the angle between the longitudinal axis of the coupling element and the axis of the fixed antenna deviates substantially from zero. The coupling element has dielectric body with conductive coating on at least two longitudinal surfaces and a coupling hole. Said contacting means to the radio telephone have conductive and grounded coating, which is connected to outer of said conductive coatings of the coupling element, the inner of said conductive coatings being arranged for connection to the external antenna.

0011The invention is described in more detail with reference to the preferred embodiments, presented for the sake of illustration, and to the accompanying drawings wherein <dl id="dl0001" compact="compact"><dt>Fig. 1</dt><dd>shows an arrangement according to the prior art to connect a mobile phone to an external antenna,</dd><dt>Fig. 2</dt><dd>shows a preferred embodiment of the coupling element according to the invention,</dd><dt>Fig. 3</dt><dd>shows an example of the arrangement according to the invention,</dd><dt>Fig. 4</dt><dd>shows another example of the coupling element according to the invention.</dd></dl>

0012Fig. 1 was already explaned in connection with the description of the prior art.

0013Fig. 2 is a schematic illustration of a piece 10 in the shape of a cylinder segment having an inner surface 1 in the shape of a cylinder part and an outer surface 2 in the shape of a cylinder part concentric with the aforementioned cylinder part but having a longer radius. The drawing also shows the imaginary longitudinal axis 3 of the circular cylinders that include surfaces 1 and 2. The cylinder segment determined by the inner and outer surfaces is bounded by a first end surface 4 and a second end surface 5 perpendicular to the axis 3. In addition, the piece is bounded by plane surfaces 6 and 7 parallel to the axis 3 and intersecting all the above-mentioned surfaces.

0014A "longitudinal axis" means in this description and in patent claims above-explaned straight line and also a straight line parallel with a longitudinal edge of dielectric piece shaped in another way. Correspondingly "longitudinal" means the direction of the longitudinal axis and at the same time the propagation direction of electromagnetic field. The "crosswise direction" means in patent claims a direction perpendicular to the longitudinal axis.

0015According to the invention, the piece shown in Fig. 2 is made of a material having a high dielectric constant, advantageously ε_r = 36 or ε_r = 82 or a corresponding value in that order of magnitude. Suitable materials include zircon tin titanate and barium neodymium titanate which are commonly used as materials for body blocks of so-called ceramic filters. Ceramic filters are radio frequency filters used in mobile phones, for example. The inner surface 1 and outer surface 2 are treated with a metal-bearing coating material, said material and its use being known to one skilled in the art as they are applied in the manufacture of the ceramic filters. On the inner surface 1 thus is an inner conductor and on the outer surface 2 is an outer conductor. The structure constitutes a dielectric resonator the electrical length of which in the direction of the axis 3 is half of the wavelength of the resonating frequency as the end surfaces 4 and 5 are electrically non-conductive. Furthermore, if the inner conductor and outer conductor are short-circuited by forming on the second end surface 5 an electrically conductive coating which is in galvanic contact with the coatings of both the inner surface 1 and outer surface 2, the electrical length of the structure in the direction of the axis 3 will be a quarter of the wavelength of the resonating frequency.

0016The electromagnetic field can penetrate into the piece shown in Fig. 2 through open end surfaces 4 and 5 or if end surface 5 is coated, through the open end surface 4. The electromagnetic field is fed to the coupling element shown in Fig. 2 through the open end surface 4 (and/or 5 if surface 5 is open). The open end surface 4 can also be called a coupling hole. If necessary, it is possible to form on plane surfaces 6 and 7 electrically conductive coupling patterns, or so-called parasitic elements, the principle of which is known from ceramic filter technology and which affect, among other things, the coupling bandwidth, ie. the width of the frequency band on which the electromagnetic field can be fed to the coupling element without significant coupling losses.

0017The matching between the antenna and the coupling element generally refers to the quality of the electromagnetic coupling between them. The antenna is in tune on a particular frequency band. The design of the coupling element and possible parasitic elements are chosen such that when the coupling element is brought in the vicinity of the antenna, the coupling element is in tune essentially on the same frequency band as the antenna. Then, the coupling loss between the antenna and the coupling element on the frequency band in question is small, advantageously about -1 to -3 decibels. The tuning-in of a dielectric coupling element on a desired frequency band e.g. by changing its design and/or parasitic elements is a technique known to a person skilled in the art.

0018Fig. 3 shows an apparatus for connecting a mobile phone to an external antenna, the apparatus including a coupling element according to Fig. 2. The drawing shows in schematic format a mobile phone A and its fixed antenna D. The coupling element 10 according to the invention is attached to the body 9 which advantageously comprises means for mechanically fastening the mobile phone into it for the duration of a ride, for example. The body 9 is advantageously made of plastic and there is connected to it a ground plane 11 which can be a metal plate or an electrically conductive coating section, for example, and which in the embodiment illustrated is located on the bottom surface of the body 9. The ground plane 11 intensifies coupling to the frame of the mobile phone and prevents the occurrence of radiation losses in the direction of the ground plane as seen from the antenna D. The apparatus also comprises a coaxial cable 12 having a middle conductor 12a and a skin conductor 12b. The middle conductor 12a is connected to the inner conductor of the coupling element 10, and the skin conductor 12b is connected to the outer conductor of the coupling element 10 which is in galvanic contact with the ground plane 11, too. At the other end of the coaxial cable 12 there is an external antenna, such as a car antenna (not shown).

0019In the embodiment illustrated in Fig. 3, the location of the coupling element 10, relative to the mobile phone antenna D, is such that the axis D1 of the antenna and the coupling element are on one and the same plane (the paper surface in Fig. 3) but there is between them an angle <b>a</b>. The purpose of this is that the electromagnetic field radiated by the antenna D is fed to the coupling element 10 through the open end surface 4 and, upon passing the surface, it is refracted such that, having penetrated into the dielectric body material, it propagates in it in an optimal manner. In the embodiment shown, angle a is about 40 degrees. It has been chosen empirically by measuring the power of the mobile phone antenna D and the power of the external antenna, calculating the coupling loss between them and changing the angle <b>a</b> such that the least coupling loss is achieved. Since different antenna types have different direction patterns at a distance of a few millimeters from the antenna, the right value for angle a should be determined separately for each antenna type. The angle value also depends on the dielectric constant of the body material of the coupling element 10 because the greater the difference between the dielectric constants of two materials, the more an electromagnetic wave refracts at their boundary surface. Furthermore, the value of angle a depends on the amount and shape of conductive patterns formed on the surface of the coupling element. Determining the optimum position for the coupling element empirically is a technique known to a person skilled in the art and requires no special inventional action.

0020As the coupling element is attached to the body 9 and the mobile phone A is attached to the body always in the same way, the position of the coupling element 10 relative to the mobile phone antenna D is always the same when the user places the mobile phone in the body for the duration of a ride, for example.

0021In the embodiment of Fig. 3, the middle conductor 12a of the coaxial cable is connected to the inner conductor of the coupling element at the second end 5 of the element, and the first end 4 of the element points towards the mobile phone antenna. This positioning requires that both end surfaces of the element be uncoated, ie. the element be dimensioned according to half the wavelength. If the second end of coupling element is short-circuited so that the coupling element functions as a quarter-wavelength resonator, the middle conductor 12a of the coaxial cable must be connected to the inner conductor of the coupling element at the first end of the coupling element.

0022The cylindrical shape of the coupling element presented above as an example does not restrict the invention to only certain shapes of coupling elements. Fig. 4 shows another possible coupling element shape. Here the coupling element 10 is shaped as a rectangular prism which has, referring to the positioning in the drawing, an upper surface 13, a lower surface 14, two end surfaces 15 and two side surfaces 16. The names of the surfaces serve illustrative purposes only and do not restrict the positioning of the piece. The upper and lower surfaces are coated with conductive layers in the same way as the inner surface 1 and outer surface 2 in the embodiment in Fig. 2. The end surfaces 15 are both uncoated, in which case the electrical length of the coupling element is half the wavelength, or one of them is coated, in which case the electrical length of the coupling element is a quarter of the wavelength.

0023The dielectric body block of the coupling element can be shaped in many ways. Variations to the illustrative embodiments described above are e.g. different bevellings and groove shapes on the surfaces confining the body block in order to control the electromagnetic field in the body block. The arrangement according to the invention may also comprise more coupling elements arranged on different sides of the antenna of the mobile phone attached to the rack, in which case it is possible to further decrease the coupling losses caused by stray radiation.

0024According to measurements, the dielectric coupling element causes a smaller coupling loss than prior art solutions. In addition, it has a very simple construction and includes no fragile parts or parts susceptible to wear or dirt. The arrangement for connecting a radio communication device to an external antenna requires no special fastening means or awkward threading movements to attach or unattach a mobile phone, so it is extremely easy to use. The invention is well applicable to large-scale series production and its manufacturing costs are reasonable. The invention is not limited to be used only in conjunction with a mobile phone or a car antenna but can be applied to any object where one wishes to connect a radio communication device having an antenna of its own to an external auxiliary antenna.