The invention relates to an antenna with a dielectric (or permeable) substrate and at least one resonant conductor track structure, designed in particular for use in the high-frequency and microwave range, for example for mobile dual-band or multiband telecommunication devices (cellular and cordless telephones), as well as for devices which communicate in accordance with the Bluetooth standard. The invention further relates to a circuit board and to a telecommunication device having such an antenna.
To follow the trend towards ever smaller electronic components, in particular in the field of telecommunication technology, all manufacturers of passive and/or active electronic components enhance their activity in this field. Particular problems then arise in the use of electronic components in the field of high-frequency and microwave technology, because numerous properties of the components are dependent on their physical dimensions, and because the wavelength of the signal becomes smaller as the frequency rises, which in its turn leads to an interference with the supplying signal source in particular owing to reflections.
This relates to a particularly high degree to the structure of the antenna of such an electronic device, for example of a mobile telephone, which is more strongly dependent on the desired frequency range of the application than are all other HF components. This is because the antenna is a resonant component which is to be adapted to the respective application, i.e. the operating frequency range. In general, wire antennas are used for transmitting the desired information. Certain physical lengths are absolutely necessary in order to achieve good radiation and reception characteristics with these antennas.
Optimum radiation characteristics are found in so-called xcex/2 dipole antennas whose length corresponds to half the wavelength (xcex) of the signal in free space. These antennas are each formed from two wires of xcex/4 length which are rotated through 180xc2x0 with respect to one another. These dipole antennas, however, are too large for many applications, in particular for mobile telecommunication (the wavelength is approximately 32 cm in the GSM900 band), which is why alternative antenna structures are used. A widely used antenna in particular for the field of mobile telecommunication is the so-called xcex/4 monopole. This consists of a wire having a length of one fourth the wavelength. The radiation characteristic of this antenna is acceptable while at the same time its physical length (approximately 8 cm for the GSM band) can be accommodated. Furthermore, antennas of this kind distinguish themselves by a high impedance and radiation bandwidth, so that they can also be used in systems which require a comparatively large bandwidth. To achieve an optimum power adaptation to 50 ohms, a passive electrical adaptation is chosen for this kind of antennas, as indeed for most xcex/2 dipoles. This adaptation usually consists of a combination of at least one coil and one capacitance, which adapts the input impedance of the xcex/4 monopole different from 50 xcexa9 to the connected 50 xcexa9 component, given a suitable dimensioning.
Even though antennas of this kind are widely used, they still have considerable disadvantages. The latter are found on the one hand in the passive adaptation circuit mentioned above.
On the other hand, for example, mobile telephones are usually fitted with a pull-out wire antenna. Such xcex/4 monopoles cannot be soldered directly to a circuit board. The result of this is that expensive contacts are necessary for the signal transmission between the circuit board and the antenna.
A further disadvantage of this kind of antennas is the mechanical instability of the antenna itself as well as the adaptation of the housing to the antenna made necessary by this instability. If, for example, a mobile telephone is dropped on the floor, the antenna will usually break off, or the housing is damaged in the location where the antenna can be pulled out.
To avoid these disadvantages, antennas were developed in which one or several resonant metal structures are provided on a dielectric substrate having a dielectric constant xcex5r greater than 1. Since the wavelength in the dielectric is smaller than that in vacuum by a factor 1/√xcex5r, antennas reduced in size by that same value can be manufactured.
A further advantage of these antennas is that they can be directly provided on a printed circuit board (PCB) by means of surface mounting (SMD technology), i.e. through planar soldering and contacting on the conductor tracksxe2x80x94possibly together with other componentsxe2x80x94, without additional retention means (pins) for the supply of the electromagnetic power being necessary.
It is an object of the invention to provide an antenna with a dielectric (or permeable) substrate and at least one resonant conductor track structure which is further improved as regards its radiation properties.
In addition, such an antenna is to be provided which has as small a weight as possible and which can be provided on a printed circuit board in particular through surface mounting (SMD technology), i.e. through planar soldering and contacting on the conductor tracksxe2x80x94possibly together with other componentsxe2x80x94, without additional retention means (pins) for supplying the electromagnetic power being necessary.
These antennas should in particular be configured such that they are suitable for use in the high-frequency and microwave ranges, that they have a bandwidth which is as large as possible and/or tunable, and that they are capable of miniaturization to a high degree and mechanically particularly stable.
This object is achieved according to claim 1 by means of an antenna formed by a dielectric (or permeable) substrate and at least one resonant conductor track structure, which is characterized in that the substrate comprises at least one cavity.
It was surprisingly found that the radiation efficiency, and accordingly the radiation properties of the antenna are or can be considerably increased and improved by means of such a cavity. Depending on the shape, size, and number of the cavities, said efficiency may be increased by approximately 15% or more. A particular advantage of this solution is that the weight of the antenna becomes substantially lower at the same time.
This solution is particularly advantageous for miniaturized microwave antennas for single-band applications (for example the GSM900 band) as described in DE 100 49 844.2, as well as for dual- and triple-band antennas for the frequency ranges of the GSM900 and the DCS 1800 standards, and also for Bluetooth systems, as disclosed in DE 100 49 845.0. The contents of these publications should accordingly be deemed included in the present disclosure by reference.
It should be noted here that antennas with U-shaped dielectric substrates are known from EP 0 923 153 and U.S. Pat. No. 5,952,972. This, however, relates to substrates which are shaped for the purpose of increasing the impedance bandwidth without measures being taken for increasing the efficiency of the radiated electromagnetic waves. Moreover, said two publications relate to antennas with shell electrodes, U.S. Pat. No. 5,952,972 exclusively describing dielectric resonator antennas (DRA). In these antennas, the operating modes are determined by the bulk resonance, whereas in the antennas according to the invention (PWAxe2x80x94printed wire antennas) without mass electrodes the operating modes are defined by the resonances of the conductor track structure on the substrate. The operating principles are accordingly fundamentally different from one another.
The dependent claims relate to advantageous further embodiments of the invention.
The embodiment of claim 2 relates in particular to substrates made of foam-type materials into which it is not absolutely necessary to provide separate cavities.
In contrast thereto, the embodiments of claims 3 to 5 are to be used first and foremost where solid substrates are provided into which the cavities are introduced in the form of corresponding depressions.
The claims 6 and 7 relate to antennas which can be used in particular for the high-frequency and microwave ranges, the embodiment of claim 6 having a particularly great impedance and radiation bandwidth, and the embodiment of claim 7 being tunable.