The invention relates to an apparatus for sending and receiving electromagnetic waves and relates in particular to an antenna arrangement with circular polarization.
Radio-based access systems have how become the standard method of making controlled access to motor vehicles possible. These access systems primarily serve to enable the doors or trunk of a motor vehicle to be locked in a convenient manner, as well as allowing the immobilizer present in a motor vehicle to be activated and deactivated.
By integrating bidirectional communication into the radio transmission between the mobile radio station of the access system and distant end embodied in the motor vehicle as the on-board station, remote-control and remote-interrogation functions can be realized. It is thus possible to retrieve data relating to the status of the motor vehicle by means of the mobile station. Typically this is information about the fill level of the fuel tank, the tire pressure or the like. In addition bidirectional communication usually also offers the option of calling up further functions of the motor vehicle, such as enabling vehicle windows, sun roofs and sliding doors, but also any static heating present in the motor vehicle, to be controlled from longer distances.
There are a number of frequency bands available for the radio connection between mobile station and on-board station, which are predominantly to be found in the ISM (Industrial, Scientific and Medical) band. The frequency bands reserved for radio communication are not identical in all countries in such cases, so that the radio stations mostly have to be optimized for a number of frequency bands.
The services supported by the radio-based access systems require coverage ranging from a few meters (e.g. for closing the vehicle doors) up to a few hundred meters and possibly kilometers for remote interrogations. Specific services, such as the opening of the motor vehicle doors for example, can sometimes only be called up in such cases when the distance to the motor vehicle is less than a certain amount. Others, such as requesting the current time for which the vehicle has been parked, should be able to be performed over a distance that is as great as possible. The propagation conditions for the radio waves between the two stations of the access system are characterized in such cases by different parameters. Apart from the frequency range, said parameters are primarily the distance between the radio stations, the direction of polarization of the electromagnetic wave used for radio transmission, the type of antenna(s) fitted in or on the motor vehicle, the type of antenna(s) used in the mobile station, the orientation of the mobile radio station in the space as well as its position in the hand or on the body of the user and finally also the environment in the area of the radio connection path which can lead to a multipath propagation of the radio signals.
The antenna(s) of the radio station located in the motor vehicle is (are) generally designed so that a specific polarization of the radio wave is preferred for the sent and received signals. Mostly this is the vertical polarization, i.e. the direction of polarization in which the E-vector is aligned vertically. The reason for this is the shortened vertical monopole antennas predominantly used in motor vehicles.
In the mobile radio stations it is mostly loop antennas or monopole antennas or a combination of the two that are used. In the case of monopole antennas helical antennas are preferred above all.
Loop antennas are characterized by their low body effect but generally have a low efficiency and create a purely linear polarization.
The efficiency of monopole antennas is greater as a rule, however because of the smaller mass counterweight the power transmitted via the antenna is very sensitive to contact however (body effect) This type of antenna too only supports one direction of polarization and also has only one additional zero point in the directional diagram. In mobile radio devices with a lower range monopole antennas which are printed directly onto the circuit board of the device are sometimes used. In this case the body effect is even greater since the entire antenna is generally covered by the hand when the device is being used.
Although antenna arrangements with a combination of loop and monopole antennas make a compromise possible, the characteristic of the one or the other type of antenna predominates depending on where there is contact between the body and the device. In practice the two antennas are connected in parallel, which means that an unbalance of one of the two antennas always also has an effect on the send and receive characteristic of the other antenna in each case. Radiation and reception of electromagnetic waves also occurs in a largely linear polarized manner with these antenna combinations.
For antennas with high efficiency options include structures with a monopole or dipole character. With the conductor dimensions that are acceptable for mobile radio stations, loop structures usually have losses that are too high to be suitable for the coverage required.
With all the previously described types of antenna and possible combinations thereof there are always areas in the directional diagram in which no connection or only an inadequate connection is possible. Apart from the body effect and the so-called zero points in the directional diagram, linear polarization is also above all a problem here. Since as a rule a user decides how to hold the mobile radio station, it is not possible for the manufacturer to match the relative directions of polarization of mobile station and on-board station to each other. Instead it should be assumed that the directions of polarization of the two stations can be in any given orientation to each other if need be. Thus, depending on the direction of polarization, the transmission conditions can differ greatly between mobile radio station and motor vehicle. In the extreme case the directions of polarization of mobile station and on-board station can be at right angles to one another, which means that, despite transmission power being sufficient under usual conditions, there is no communication over relatively small distances.
Using this as its starting point, the object of the invention is to specify an antenna arrangement that makes possible a more reliable transmission of radio signals between a mobile radio station and another radio station.
The object is achieved in accordance with the independent claims of the invention.
The invention comprises an antenna apparatus with a first antenna branch and a second antenna branch, with both the first and the second antenna branch having the form of a non-closed conductor loop and the first antenna branch being arranged in an essentially perpendicular direction to the surface enclosed by the respective conductor loop, spaced in relation to the second antenna branch so that the loop direction determined from the foot point to the free end of the first antenna branch is arranged opposite to the second loop direction determined from the foot point to the free end of the second antenna branch.
The invention further comprises a radio station having said antenna apparatus.
In this context it is pointed out that the terms used in this description and the claims to enumerate features “comprise”, “feature”, “encompass”, “include” and “with” as well as their grammatical variations are generally to be understood as a non-exclusive enumeration of features such as for example method steps, devices, ranges, variables and the like which in no way excludes the presence of other or additional features or groupings of other or additional features.
A corresponding antenna apparatus exhibits a circular-polarized radiation and reception characteristic which makes possible a reliable radio connection independently of the alignment to a radio distant end and has a low body effect. Because of the compact design of the antenna branches in the form of conductor loops, the antenna apparatus is especially suitable for use in small mobile radio devices, the dimensions of which are less than a quarter of the wavelength used to the transmission (e.g. motor-vehicle keys).
The invention is developed in its independent claims.
Preferably the form of the first antenna branch essentially corresponds to the form of the second antenna branch, whereby a defined the embodiment of the E field can be achieved.
The first antenna branch can advantageously be arranged in relation to the second antenna branch so that the location of the first antenna branch is produced from a 180° rotation of the second antenna branch around an axis lying in the longitudinal direction of the device in order to obtain an E-field which is essentially parallel to the H-field.
A compact antenna structure is obtained by the first and the second antenna branch together delimiting a parallelepiped-shaped hollow space, with the parallelepiped-shaped hollow space especially also able to be embodied in the shape of a cuboid. An advantageous reduction in the size of the antenna structure can be achieved in this case if the loop ends of the first and of the second antenna branch each project into one of the delimiting surfaces of the hollow space, or if the end sections of the first and of the second antenna branch are embodied folded back so that they are arranged at a distance from and essentially in parallel to another section of the respective antenna branch.
Alternatively a compact antenna structure can also be achieved when the first and the second antenna branch together delimit a cylindrical hollow space with the first antenna branch and the second antenna branch each being able to be embodied in the form of a helix or a spiral.
The distance between the first antenna branch and the second antenna branch is expediently essentially constant. If necessary the inclination of the first antenna branch and/or of the second antenna branch can be non-constant. The space between the first antenna branch and the second antenna branch can vary in this case along the direction of the loop for optimizing an adaptation of the antenna structure to the housing geometry.
It is useful for the first and the second antenna branch to each be connected at an angle to a foot section. The angle in this case is preferably ninety degrees.
To compensate for non-symmetrical electrical or magnetic stresses on the antenna structure in the device, the strictly symmetrical arrangement of the antenna parts can be departed from, for which purpose the embodiment and/or arrangement of the first antenna branch in relation to the second antenna branch advantageously exhibits at least one deviation from the symmetry.
Further features of the invention emerge from the following description of inventive exemplary embodiments in conjunction with the claims and also with the figures. The individual features can be realized for an exemplary embodiment in accordance with the invention individually or in combination. In the explanation given below of exemplary embodiments of the invention, the reader is referred to the enclosed figures, of which
Between the foot points of the two foot sections 3 and 4 and electrically connected to these points is arranged the RF feed 5 (not shown in the figure). In the exemplary embodiment shown in
The purpose of the foot section 3 and 4 is to keep the antenna branches 1 and 2 at a specific distance from and in a specific location relative to the circuit carrier 7 as well as to each other. Preferably the antenna branches are arranged as shown symmetrically to that surface of the circuit carrier 7 on which the foot points of the antenna structure are located.
Basically each of the two antenna branches 1 and 2 forms a non-closed conductor loop. The two conductor loops are arranged in this case in opposite directions to each other, so that in the plan view (in the direction of or opposed to the z-axis), an apparently closed loop structure is produced. In the exemplary embodiments depicted this “closed” loop structure delimits a rectangular surface. If the two antenna branches 1 and 2 are arranged vertically above one another (in the z-direction) as shown in
The current distributions to the conductor structures of the antenna apparatus from
The RF feed 11 charges the two antenna branches 1 and 2 with an opposing polarity, by feeding in the antenna current 10 at the foot points of the conductor structures. The current I flowing in this case in the two branches 1 and 2 adopts different amplitude values along the conductor. The opposing arrangement of the conductor structures means that the magnetic fields created by the current flow in the lower antenna branch 2 and the upper antenna branch 1 are aligned in the same direction, so that the course of the H-field within the hollow space enclosed by the conductor loops has the direction illustrated in
In the area of the hollow space enclosed by the conductor loops 1 and 3 as well as 2 and 4, the two fields generated by the current flow I in the antenna branches 1 and 2, i.e. the electrical E field and the magnetic H field, are essentially arranged in parallel to one another. This parallel alignment is also provided in the far field of the antenna arrangement. This means that the resulting E-vectors are at right angles to each other and in accordance with their creation are offset by π/2 in their phase. As a result the antenna structure depicted in
The radiation characteristic or the overall gain 12 respectively of the antenna structure 10 from
The direction dependencies of the two wave radiations in the x-z plane are shown in
The embodiment of the antenna apparatus 20 from
In the exemplary embodiment depicted in
For smaller frequencies with longer wavelengths the spiral-shaped embodiment of the antenna branches 51 and 52 in accordance with the symbol 50 of
The open ends of the antenna structures shown are very sensitive to being touched because of the high E field. Touching, because of its primarily capacitive effect, causes an imbalance of the antenna The antenna structure 60 shown in
The common aspect of the exemplary embodiments presented above is that, like a dipole structure, they have two antenna branches, or including the associated foot sections, two conductor loops which essentially possess the same form. The two conductor loops are arranged in opposite directions to each other in that one is in a position which is produced from a rotation of the other by 180 degrees around an axis lying between the two antenna branches, with this axis preferably coinciding with the longitudinal direction of the device which is represented in the figures by the x axis. To compensate for non-symmetrical electrical or magnetic loads on the antenna structure in the device, the design can deviate from the strictly symmetrical arrangement of the antenna parts by the first antenna branch being embodied or arranged slightly non-symmetrical in relation to the second antenna branch.
In this case a non-symmetrical design can be combined with a non-symmetrical arrangement.
Number | Date | Country | Kind |
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10 2007 037 614.8 | Aug 2007 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/059987 | 7/30/2008 | WO | 00 | 3/12/2010 |