The invention relates to a device for transmitting and receiving electromagnetic waves and relates, in particular, to an antenna array having a circular polarization.
Radio-based access systems have now become standard for controlled access to motor vehicles. Said access systems are primarily used for easy unlocking and closing of the vehicle doors and trunk, as well as activating and deactivating an engine immobilizer present in the vehicle.
By integrating bi-directional communication into the radio transmission between the mobile radio station of the access system and the remote station formed as an on-board station in the vehicle, further radio services, such as for example remote control and remote request functions may be implemented. Thus it is possible to retrieve data relating to the status of the vehicle by means of the mobile station. For example, this includes information about the filling level of the fuel tank, the tire pressure, a possible alarm state, the engine temperature or the like. Moreover, the bi-directional communication generally also provides the possibility of accessing further functions of the vehicle so that, for example, vehicle windows, sun roofs and sliding doors, but also a heating system which is possibly present in the vehicle, may be operated from a greater distance.
For the radio connection between the mobile station and the on-board station of the access system, a plurality of frequency ranges are available which are predominantly in the ISM band (Industrial, Scientific and Medical band). The frequency ranges which may be used for the bi-directional communication are in the range of a few megahertz (MHz) up to several Gigahertz (GHz). These frequency bands are, however, not identical in all countries, so that the radio stations generally have to be optimized for a plurality of frequency bands.
The services supported by the radio-based access systems require an operating range of a few meters (for example for unlocking the vehicle doors), through a few hundred meters, up to the kilometer range for some remote requests. Specific services, such as for example opening of the vehicle doors, may thus occasionally only be activated when coming within a specific distance of the vehicle. Other requests, such as for example the request for the current parking time, should be able to be carried out over distances which are as large as possible. The propagation characteristics for the radio waves between the two stations of the access system are thus characterized by different parameters. Apart from the frequency range, these are primarily the distance between the radio stations, the polarization direction of the electromagnetic wave used for radio transmission, the type of antenna and/or antennae attached in or on the vehicle, the type of antenna and/or antennae used in the mobile station, the spatial orientation of the mobile radio station and the position thereof in the hand or on the body of the user and finally also the environment in the region of the radio connection path, which determines the propagation characteristics.
The antenna and/or antennae of the radio station located in the vehicle is and/or are generally designed so that for the transmitted and received signals, a specific polarization of the radio wave is preferred. Generally, this is vertical polarization, i.e. the polarization direction in which the E-vector is vertically aligned. This is required by the shortened vertical monopole antenna which is predominantly used in vehicles.
Loop antennae or monopole antennae, as well as combinations of both types of antennae, are generally used in mobile radio stations. In the case of monopole antennae, helical antennae are predominantly preferred.
Loop antennae are characterized by their low hand sensitivity, but generally are less efficient and produce a purely horizontal polarization.
The efficiency of monopole antennae is generally greater but, due to the smaller ground counterpoise, the power transmitted via the antenna is very sensitive to contact (hand sensitivity) and to influences from the remaining immediate environment of the radio device. Also, this type of antenna only supports one polarization direction and, moreover, also has an additional zero point in the direction of the longitudinal axis of the device in the directional diagram. In mobile radio devices with a smaller operating range, monopole antennae have hitherto been used which are directly printed onto the printed circuit board of the device. In this case, the hand sensitivity is even greater since, when using the device, generally the entire antenna is covered by the hand.
Antenna arrays constructed from a combination of loop antennae and monopole antennae do allow a compromise to be made but, depending on the contact, the characteristic of one or other type of antenna predominates. In practice, the two antennae are arranged in parallel, whereby tuning of one of the two antennae always has an effect on the radiation characteristic and/or receiving characteristic of the respective other antenna. The radiation and reception of electromagnetic waves even in this combination of antennae is also substantially linearly polarized.
For antennae with a high degree of efficiency, structures with monopole or dipole characteristics are considered, amongst others. Loop structures with conductor dimensions acceptable for mobile radio stations generally have losses which are too great in order to be suitable for the required operating ranges.
In all the types of antenna described above, and possible combinations thereof, regions are always present in the directional diagram in which no connection and/or only an inadequate connection is possible. Apart from the hand sensitivity and the so-called zero points in the directional diagram, in this connection the linear polarization is predominantly a problem. As it is generally the user who decides how the mobile radio station is held in the hand, it is not possible for the manufacturer to match the relative polarization directions of the mobile station and on-board station to one another. Instead, it is assumed that the polarization directions of both stations may be oriented, if required, in any manner relative to one another. Depending on the polarization direction, therefore, quite different transmission conditions may prevail with the same distances between the mobile radio station and the vehicle. In an extreme case, the polarization directions of the mobile station and the on-board station may be perpendicular to one another, whereby instead of generally sufficient transmission power, no communication is achieved even with relatively small distances.
By using an antenna structure with circularly polarized radiation, corresponding faulty alignments of the polarization directions may be avoided. In order to achieve a circularly polarized radiation with conductor dimensions acceptable for mobile radio stations, a folded dipole structure with two antenna branches may be used which are configured in the form of two winding elements oriented in opposing directions and arranged on top of one another. If the HF feed line is located between the two antenna branches, then the current directions run parallel to one another in the antenna branches, whereby in combination with the loop shape of the antenna branch an H-field is produced. Due to the potential difference between the two antenna branches arranged on top of one another, an E-field is produced which is aligned parallel with the H-field. As this alignment of the fields is also provided in the far field, the E-vector produced from the H-field is located perpendicular to that produced from the E-field, from which a circular polarization is produced. Such antenna structures, however, require a matching network for matching the input impedance to the output impedance of the HF feed line. The radiation power of such structures is limited by the complex matching circuit of the HF feed line.
It is, therefore, the object of the present invention to provide an antenna structure having a circularly polarized radiation characteristic and receiving characteristic with high radiation power and easy adaptability.
The object is achieved according to the independent claims of the invention.
The invention comprises an antenna device comprising a first antenna branch and a second antenna branch, both the first and the second antenna branch having the form of a non-closed conductor loop and being connected to one another so that they form a cohesive conductor loop and the antenna branches are arranged spaced apart from one another in a direction which runs substantially perpendicular to the surface enclosed by the respective conductor loop, and the antenna device comprising at least two feed points which are arranged in the center section of the cohesive conductor loop at a distance from each other.
The invention further comprises a radio station which comprises such an antenna device and an HF feed line, which is connected via the two feed points electrically or via an HF coupler to the antenna device.
In this connection, reference is made to the fact that the terms “comprise”, “have”, “contain”, “include” and “with”, as well as the grammatical variants thereof used in this description and the claims for listing the features, are generally to be understood as an inconclusive list of features, such as for example method steps, devices, areas, sizes and the like, which by no means excludes the presence of other or additional features or groups of other or additional features.
A corresponding antenna device represents a resonator structure activated by the HF feed line, the antenna currents thereof being a multiple of the feed current to be activated and producing high transmitting field strengths. The particular geometry of the antenna branches produces a circularly polarized far field, which in connection with the high radiation power permits a reliable radio connection even over long distances, irrespective of the alignment with a radio counter station. Due to the small and/or intrinsic ground counterpoise, the antenna structure has low hand sensitivity. The input impedance of the array may be freely selected by the choice of feed points, so that a matching network for matching the impedance to the HF feed line is not necessarily required. Due to the compact design of the antenna branches in the shape of the conductor loop, the antenna device is suitable, in particular, for use in small mobile radio devices, such as for example in vehicle locks, the device dimensions thereof falling below a quarter of the wave length used for transmission.
The invention is developed in the dependent claims thereof.
For a simple HF feed line the at least two feed points are in each case advantageously designed in the form of a connector or as part of an HF coupler.
Preferably, the shape of the first antenna branch substantially corresponds to the shape of the second antenna branch, whereby a defined configuration of the E-field may be achieved.
The first antenna branch may be arranged relative to the second antenna branch such that the position of the first antenna branch substantially results from a 180° rotation of the second antenna branch about an axis of symmetry. As a result of this symmetry of the array, the E-field is configured perpendicular to the conductor loop parts, so that it is aligned parallel to the antenna current flowing through the conductor loop.
A compact antenna structure is achieved by the first and the second antenna branch together defining a parallelepiped hollow space, the parallelepiped hollow space, in particular, also being able to be configured as cuboidal. For an advantageous reduction in size of the antenna structure, the conductor structure may be configured so that the loop ends of the first and the second antenna branch in each case protrude into one of the defining surfaces of the hollow space.
Alternatively, a compact antenna structure may also be achieved if the first and the second antenna branch together define a cylindrical hollow space, the first and the second antenna branch preferably together defining a hollow space in the shape of a half-pipe.
Expediently, the spacing between the first antenna branch and the second antenna branch is substantially constant. If required, the spacing between the first antenna branch and the second antenna branch may vary in the direction of the loop, whereby an optimization may be undertaken when matching the antenna geometry to a predetermined housing geometry.
In an advantageous development, the spacing between the two connectors is selected so that the impedance between the two connectors in the region of the supplied frequency band corresponds to the source impedance of the HF feed line. As a result, matching networks are superfluous and thus the manufacturing costs are reduced.
Further features of the invention are revealed from the following description of exemplary embodiments according to the invention in combination with the claims and the figures. The individual features may be implemented separately or in combination in an embodiment according to the invention. In the following description of several exemplary embodiments of the invention, reference is made to the accompanying figures, in which:
In
Each of the two antenna branches shown in
In the embodiment shown, the “closed” loop structure defines a rectangular surface. If the two antenna branches 1 and 2 as shown in
In
The direction of current on the conductor structures is indicated by arrowheads. The direction of current provided is only valid for one of the two half-waves of the guided wave. In the other half-wave, the direction of current and thus also the directions of the electrical and magnetic fields produced are reversed. The physical relationships are, however, the same for both half-waves.
The feed current Is generated by the HF feed line 8 is introduced into the conductor structure formed by the antenna branches 1 and 2 together with the web connection 3 via the two connectors 4 and 5. As a result of the current flow, the two antenna branches 1 and 2 adopt an opposing polarity. The antenna current I has different amplitude values along the conductor structure. As the web connection 3 combines the two antenna branches 1 and 2 to form a continuous winding, the antenna current I runs in the upper antenna branch 1 in the same direction parallel to the antenna current in the lower antenna branch 2. Thus the magnetic fields produced by the current flows in the two antenna branches are added together in phase, so that the path of the H-field inside the hollow space enclosed by the conductor loops, in a first approximation, has the directional path illustrated in
As a result, therefore, the antenna structure shown in
In contrast to dipole antennae, in which the antenna current flows through the HF feed line and/or the matching network, which connect the two antenna branches in series, the HF feed line in the antenna array shown in
The antenna structure shown in
The radiation characteristic and/or the total gain 11 of the antenna structure 10 of
The directional dependencies of both wave emissions in the x-z plane are shown in
In
A further embodiment with a modified shape of the antenna branches relative to
In
Even if the invention has been described hitherto with reference to specific types of antenna branches, it is obvious for a person skilled in the art that shapes of the antenna branches deviating therefrom may also be used with the same or a similar result. In particular, the arrangement of the antenna structure shown in the exemplary embodiments of
In the examples set forth above, the loop length of an antenna branch was less than a complete winding. In an alternative embodiment, an antenna branch may also have the shape of a conductor loop with a plurality of windings. The possible shapes of the cross-sectional geometries of the windings are only limited by an H-field which is substantially parallel to the E-field being produced via the current flow through the two antenna branches. Thus a completely circular polarization does not have to be achieved, as the antenna structure even operates satisfactorily if the field strengths of the two polarization components differ from one another by a few dB. If antenna branches with a plurality of windings are used, said windings may be arranged both adjacent to one another for forming the resonator system, and interwoven with one another similar to a double helix.
Moreover, the spacing between the antenna branches does not have to be constant. Instead, in order to adapt the antenna structure to the available space, for example, the spacing profile may have almost any path. Also, the two antenna branches do not necessarily have to be designed symmetrically. Instead, by suitable dimensioning the structure may also deviate from a symmetrical design, a radiation characteristic similar to the symmetrical arrangement being able to be achieved.
The HF power is coupled in the above described exemplary embodiments by means of metal connectors. Alternatively, the HF feed line may, however, also be coupled via HF couplers, which similar to a directional coupler do not have to have galvanic contact with the antenna structure. Naturally, in this case the connection of the two antenna branches may also be designed such that it is of low impedance at the high frequency used.
The antenna array may be fastened and stabilized in very different ways. For example, support and/or fastening elements may be provided on the antenna branches themselves and/or on the connection web, which are designed and/or arranged such that they are almost at zero current during use of the antenna, and thus exert practically no negative influence on the current distribution of the emitting structure. For example, the ability to mount the antenna on a circuit carrier or within a housing may be improved if the ends of the antenna branches are folded back in the opposing direction to form a support. A further possibility for simplifying the mounting and for stabilizing the antenna structure is provided by the attachment of the antenna structure to a carrier, for example to a plastics carrier formed as a support structure. The antenna may thus, amongst others, be printed as an electrically conductive coating, applied by means of metallized films, or be produced by the structuring of PCB-metallizing.
An antenna array according to the invention may also be produced as a stamped-bent part or as a combination of a plurality of different components, for example a printed structure or the like continued with wire elements or housing parts.
It is essential that the geometry of the antenna structure is suitable for producing an H-field which is substantially parallel with the E-field, and the two antenna branches are connected to one another with low impedance at the frequency used, so that an externally-activated resonator system is formed. The disclosed antenna arrays are primarily suitable for use in mobile radio stations of, for example, vehicle access systems with a bi-directional communication interface.
Number | Date | Country | Kind |
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10 2007 061 305 | Dec 2007 | DE | national |
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PCT/EP2008/066498 | 12/1/2008 | WO | 00 | 6/21/2010 |
Publishing Document | Publishing Date | Country | Kind |
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WO2009/077312 | 6/25/2009 | WO | A |
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