The present invention relates to an antenna device for a radio communications apparatus operating in the frequency range of 800-3000 MHz and including a radiator, a dielectric resonator that is galvanically connected to a feed line of the antenna and that is operative to feed it.

On small portable radio communications apparatus, so-called mobile telephones, use has previously been made of a plurality of different antenna types, including rod antennae of the quarter wave or half wave type, helix antennae, etc. One common denominator for all of these prior art designs and constructions is that attempts have been made to keep the physical dimensions of the antenna as small as possible, the degree of efficiency as high as possible and the band width as large as possible. Certain antenna types have proved to be successful in one or two of these respects, but no truly optimum design and construction has yet been developed.

In particular in such situations where transmission and reception take place at great distances in terms of frequency, it is important that the antenna is of the wide band type.

WO-A-9509454 discloses an antenna device designed for transmitting RF energy through a dielectric barrier, such as a window.

This antenna device has on the "indoor" side of the window a dielectric resonator galvanically connected to a feed line via an exciter strip. On the "outdoor" side of the window there is a second dielectric resonator coupled to the first resonator by a resonance mode coupling. The second resonator is galvanically connected to a radiator via an exciter strip.

Due to its physical size this antenna device could not be used in the same technical field as this invention i.e. as an antenna in a mobile phone.

The present invention has for its object to realise an antenna of the type disclosed by way of introduction which obviates the drawbacks inherent in prior art models. In particular, the present invention has for its object to realise an antenna device which has a good band width, which has small physical dimensions and which is insensitive to changes in the earth plane. Furthermore, the present invention has for its object to realise an antenna device which is simple and economical in manufacture and which displays a high level of mechanical strength.

The objects forming the basis of the present invention will be attained if the antenna disclosed by way of introduction is characterized in that the radiator is galvanically discrete from the resonator, that it is connected to earth and that it is disposed to be fed by the resonator capacitively and/or inductively.

Further advantages will be attained if the antenna device is moreover characterized in that the radiator is a rod radiator which is connected in its one end to a coil, that the coil surrounds at least a part of the dielectric resonator and that the end of the coil facing away from the rod radiator is connected to earth.

These characterising features will realise an antenna which satisfies the objects established in the present invention.

The present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawing which shows a vertical part cross section through an antenna device designed according to the invention.

There is a proliferation on the market of different so-called dielectric resonators which, for instance, are employed as active elements in wide band filters. One supplier of such resonators is the Japanese company Murata.

In principle, such a dielectric resonator is constructed as a hollow body which may possibly be cylindrical and which has a through channel provided with a thin metal layer extending out on one end surface of the resonator in order also to extend to the circumferential surface of the resonator. Otherwise, the material in the resonator is a ceramic substance with high dielectricity constant, of the order of magnitude of between 40 and 200, but preferably between 80 and 100.

The dielectric resonator which is employed in the device according to the present invention has been given reference numeral 5 in its entirety and encompasses a cylindrical body of insulating, preferably ceramic material of high dielectricity constant, preferably of the order of magnitude of between 80 and 100. The higher the dielectricity constant, the smaller will be the physical dimensions of the resonator at the same resonance frequency, while a high dielectricity constant entails a slightly poorer degree of efficiency and possibly also a slightly reduced band width. Values as high as 200 may possibly be employed.

The antenna device has a connection terminal 1 with a metallic centre conductor 2, a metallic outer sleeve 3 and an insulation 4 in between. The centre conductor 2 is galvanically connected to the resonator 5.

The resonator 5 has a central channel 7 which is coated with a thin metal layer, a metalisation 8. The metal layer 8 of the channel extends up on the upper end surface of the resonator in the Drawing figure and, thereby, one end metal layer 9 covers the entire end surface. Further, the resonator also has a metal layer 10 on its outer surface or circumferential surface. The metal layers 8, 9 and 10 are galvanically interconnected.

The upper end of the centre conductor 2 of the connection terminal 1 is, as was intimated above, galvanically connected to the metal layer 8 of the channel 7 via a contact device 11, for example in the form of a sock soldered in the metal layer 8 and the conductor 2.

The outer sleeve 3 of the connection terminal 1 is, via a short connection lead 12, in galvanic communication with a coil 13 which, in its upper end, merges in a radiator in the form of a rod 14. The connection lead 12, the coil 13 and the radiator 14 may be made of an enamelled copper wire or an insulated metal wire of other, preferably durable metal alloy. Between the inner surface of the coil 13 and the outside of the outer metal layer 10 of the resonator 5, there is a small gap 15. There is thus disposed between the inside of the coil and the outside of the resonator a non-conductive dielectric which may consist of air, but which may also consist of an insulating layer of plastic or rubber-like type.

It is entirely possible to realise a dielectric resonator for approx. 1 GHz in the form of a cylinder of a length of 9 mm and a diameter of 3.5 mm. In a prototype antenna, use has been made of 0.88 mm wire diameter in both the coil 13 and the radiator 14, in which event the coil had approx. 3 turns and the rod length was approx. 45 mm. The antenna is set to a central frequency of 900 MHz and operates as a quarter wave radiator.

The dielectric resonator has a well-defined natural resonance frequency which manifests itself as a very narrow and high "peak" in scan transmission measurement. The resonance frequency is selected to be at a level just above the desired upper operating frequency for the antenna. The inductive portion of the antenna, i.e. the coil 13, is dc-earthed, which will have as a consequence that the resonator is gradually coupled over to the coil, either capacitatively or inductively, but preferably both. By optimisation of the number of turns and/ or pitch in the coil, as well as the distance between the coil and the resonator, the transfer between the resonator and the coil may be made adequate. At the same time as the transfer is optimised, an increase in the band width is also ensured. A band width of as much as 15 percent of the central frequency of the antenna device is possible.

An antenna of this type is also less sensitive to variations in the earth substrate, whereby such chassis currents as may be induced in a resonant chassis can be reduced. This property may further be improved if the end of the coil 13 facing away from the radiator 14 is provided with a body 16, possible a hollow cylinder, of absorbent ferrite.

In the foregoing, the antenna has been described as a rod radiator of quarter wave length. However, the present invention may also be applied to other types of radiators, of both the quarter wave and half wave type. For example, helix antennae may be selected.