A MICROSTRIP ANTENNA ARRANGEMENT IN A COMMUNICATION DEVICE

Abstract: The invention relates to a microstrip antenna arrangement in a communication device comprising at least two resonance antenna elements (11, 12) resonating at mutually different wavelengths, saidantenna elements being arranged in relation to at least one shared ground plane (14), each of the at least two antenna elements (11, 12) establishing a directional radiation pattern.

Description: A MICROSTRIP ANTENNA ARRANGEMENT IN A COMMUNICATION DEVICE Background of the invention The invention relates to a microstrip antenna arrangement in a communication device according to claim 1 and a communication device according to claims 7 and 8. When dimensioning communication devices, one of many important design parameters is the size of the end product. Especially within the art of designing mobile communication devices, the size of the product has turned out to be an important competition parameter. A further important design parameter is related to obtaining a robust construction. When designing mobile handheld communication devices, it is of great importance that a robust device be obtained as mobile devices are often accidentally more or less damaged during daily use of the device. The most vulnerable part of a mobile device is typically an external antenna, e.g. a whip antenna . As an attempt to avoid this serious problem, some mobile device designers have chosen to rely on an internal antenna design, such as patch antennas. This type of antenna has the advantage that all active antenna elements may be incorporated in the housing of the mobile device and consequently improve the robustness of the antenna elements significantly. A disadvantage of an internal patch antenna is that it requires very high-skilled designers, as the internal arrangement of the antenna elements occupies more space than that which is available inside a device. Despite this, it has nevertheless been possible to design very compact and small high-quality devices. During the past few years, a general problem related to mobile telecommunication is the heavy expansion of cellular telecommunication, as the ever growing number of subscribers requires a corresponding growth in transmission capacity. Among others things, network operators have chosen to establish dual band cellular networks, such as GSM and DCS1800. Consequently, a mobile subscriber would benefit fully from these measures if his or her cellular phone was able to transmit and receive both cellular networks. During the past few years, many types of dual band cellular phones have entered the market. The availability of the dual band cellular phones on the market has meant that the phones have been fitted with external wire antennas, which have basically have been developed by adding an additional wire to the original wire instead of developing cellular phones having an internal antenna. This type of antenna design has made it possible to maintain a traditional external antenna design, as the internal structure of the cellular phones may be more or less maintained with respect to space consuming elements. So far, it appears that the new multi-band strategies, paired with the market tendency towards ever smaller cellular phones, have turned the strategies of the mobile manufacturing companies even more in the direction of external antennas, as internal antennas, and in particular directional microstrip antennas, require a significant part of the internal space of a cellular phone . It is the object of the invention to provide a multi-band cellular phone having directional radiation. The invention When a microstrip antenna arrangement in a communication device comprises at least two resonance antenna elements resonating at mutually different wavelengths, said antenna elements being arranged relative to at least one shared ground plane and each of said at least two antenna elements establishing a directional radiation pattern, a multi- band antenna arrangement suitable for multi-band operations has been obtained. In particular, the obtained antenna arrangement is suitable for utilisation in applications offering very little space for the mechanical arrangement. The above-mentioned arrangement tolerates a certain divergence between the radiation patterns of the directional antennas of the arrangement . The minimum dimension of the ground plane should preferably be greater than 0.095 times the wavelength of the primary operating resonance frequency, thus ensuring sufficient directionality and minimised mutual divergence in the direction between the antennas. The invention teaches that directional performance of the antennas may be obtained if the ground plane is shared by two or more antennas, as a relatively broad ground plane is required in order to obtain directional characteristics for both antennas. In the above-mentioned embodiment, the price of a dual band directional radiation pattern is a small inclination between the directions of the two characteristics. When, as stated in claim 2, the directions φl, φ2 of said radiating patterns define an angle φ at other than 0°, an advantageous embodiment has been obtained, as mutual spacing between the radiating elements should be kept as low as possible. It should be noted that the angle φ primarily defines a horizontal angle between the directions of the radiating patterns and the normal of the ground plane, as φ is the more critical of the two angles with respect to the normal, see fig. 4. Nevertheless it should be noted that the angle φ should be interpreted broadly as incorporation of the angle θ as an inclination in the longitudinal direction of the ground plane may be tolerated, too, according to the invention. In terms of literature, both θ and φ should be kept as low as possible in the transverse and the longitudinal directions of the ground plane, respectively. When, as stated in claim 3, the angle θ is between 0° and 170°, preferably between 0° and 50°, a further advantageous embodiment of the invention has been obtained. When, as stated in claim 4, the radiation pattern of each antenna at φ = 0° is at least 3 dB greater than φ = 180°, a further advantageous embodiment of the invention has been obtained, as the combined performance of all antennas should be kept as high as possible in the positive half -plane of the antenna and as low as possible in the negative half-plane of the antenna. When, as stated in claim 5, the antenna arrangement comprises two antenna elements, each adapted to frequency bands at substantially 900 MHz and substantially 1800 MHz, respectively, a preferred and advantageous embodiment of the invention has been obtained, as a dual band version of the physical requirements GSM and the DCS systems may be combined, benefiting from the fact that existing GSM directional patch arrangements may be modified without making substantial changes in the design of the mechanical system of e.g. cellular phones. When, as stated in claim 6, each antenna performs as a TM 1 0 mode antenna, i.e. having one dominating excitation frequency, a further preferred embodiment of the invention has been obtained. It should be noted that many kinds of microstrip radiator shapes are possible within the scope of the invention, such as the above-mentioned and preferred rectangular or substantially rectangular patches, circular patches or ring patches. Larger bandwidths can be obtained by increasing the electrical thickness of the substrate of a patch antenna and choosing a substrate with a lower dielectric constant . Other variations within the scope of the invention with respect to microstrip designs include a stacked microstrip antenna in which a parasitic patch is added load a double-layer structure. When, as stated in claim 7, the direction of the at least two radiation patterns is away from a user, a further preferred embodiment of the invention may be obtained by establishing radiation directed away from the user, thus obtaining minimal SAR values and maximised efficiency. When, as stated in claim 8, a communication device has a multi -band antenna, said multi -band antenna having at least two radiator antenna elements sharing a ground plane, each of said at least two radiator elements being adapted to radiation of an electromagnetic wavelength λ χ said radiator antenna elements and said ground plane comprising an electronically conductive material, the minimum effective dimension W of said shared ground plane being greater than or equal to 0.095 λ m , with λ m as the longest wavelength of the wavelengths λ 1 , and preferably greater than 0.1 λ m , an advantageous embodiment of the invention has been obtained. A shared ground plane is a ground plane of which at least part of the ground plane is shared by the radiators. An important aspect of the invention is that the shared ground plane of the multi -band antenna provides directional radiation characteristics of the radiator antenna elements. It should be noted that the different radiators are adapted to different wavelengths and do consequently have different physical dimensions. Even when sharing the same ground plane of one common physical dimension adapted for operation as a resonance element for each of the multi -band radiators, it has been recognised that it is possible to obtain directional characteristics for each of the radiators. It should be noted that the desired directional characteristics are obtained, even if the different radiation elements are mutually spaced, as a small divergence in the directional characteristics may be tolerated according to the invention. When sharing the ground plane, a compact antenna arrangement is obtained, as the effective size of the ground plane is minimised, especially when compared with separate antenna arrangements. According to the invention, more antenna elements may be arranged on top of the same ground plane, and need not necessarily be symmetrical with respect to the relative positioning between the ground plane and the antenna elements in order to provide multi-directional radiation patterns. Still, all radiation patterns may point in the same direction, i.e. often away from the head of the user. When, as stated in claim 9, the antenna radiator elements are fed by means of at least one proximity coupling, a further advantageous embodiment of the invention has been obtained. When, as stated in claim 10, the antenna radiator elements are fed by means of one shared proximity coupling, a very compact version of the antenna is obtained, as the proximity coupling may be implemented by means of only one feeding line. When, as stated in claim 11, the antenna radiators comprise a multi -band antenna comprising at least two TM 1 0 mode antenna radiator elements, each antenna element having substantially one dominating resonance frequency, said multi -band antenna having at least two radiator antenna elements having a shared ground plane, each radiator element being adapted to radiation of an electromagnetic wavelength, said radiator antenna elements and said ground plane comprising an electrically conductive material, at least two of said radiator antenna elements (11, 12) being grounded to said ground plane (14) , and the antenna elements (11, 12) having free resonance ends being extended in such a way that said free resonance ends (11A, 12A) are substantially facing each other, an advantageous embodiment of the invention has been obtained. Especially the combination of height above the ground plane and the width of the free resonance ends are advantageous . When, as stated in claim 12, a coupling from at least one of the radiator antenna elements to said ground plane has a width smaller in size than that of said radiator antenna element, it is possible to reduce the length of the radiator antenna element . Generally, the length controls the frequency of the antenna element where a longer antenna means a lower frequency radiation. By partly separating the ground plane and the antenna element electrically, the lower frequency can be obtained from the antenna element without extending the length. The drawings The invention will be described below with reference to the drawings, in which fig. 1 shows a preferred patch antenna arrangement according to the invention, fig. 2 shows the principles of inclination of the patch antennas according to the invention, fig. 3 is an illustrative view of the arrangement of some key components according to an embodiment of the invention, and fig. 4 shows the directionality between two radiation patterns of the invention in the cross-section plane of the antenna arrangement . Detailed description Referring to fig. 1, a dual band patch antenna arrangement according to the invention is shown in a perspective view. The antenna arrangement comprises a ground plane 14 incorporated in a PCB and two patch antennas 11 and 12 galvanically coupled to the ground plane 14 via two groundings 17A and 17B. The width W of the ground plane 14 is preferably at least 0.095 times the wavelength of the signal at the lowest frequency required by the antennas . The patch antenna 12 is connected to the ground plane 14 in its entire width, i.e. the grounding 17B must be just as wide as the antenna. The antenna 11 is only connected to the ground plane 14 by parts of its width, i.e. the grounding 17A only covers a fraction of the width of the antenna. The rest of the width is at a small distance 19 from the antenna 11 to the ground plane 14. This distance 19 could preferably be made by milling off an appropriate part of the patch antenna 11. Each antenna 11, 12 has a free resonance end 11A or 12A. These ends face each other and are separated by a channel of dielectric 18B. The width of the dielectric 18B, i.e. the distance from 11A to 12A, is in the range 0.1 mm to 10.0 mm, with 2.0 mm as the preferred width. Both antennas 11, 12 are fed through a feeding line 13 which establishes a shared proximity coupling to both antennas 11, 12. The feeding line 13 is distanced from the antennas 11, 12 by a channel of dielectric 18A. The feeding line is connected to a receiving and transmitting circuit (not shown) through the ground plane 14 via a feeding pin 16. The above-mentioned antenna elements are arranged on a solid dielectric 15. The shape of the solid dielectric 15 is a wedge, giving the two antennas 11, 12 an inclination. Each of the antennas 11, 12 is adapted to a specific frequency band. According to the shown preferred embodiment, antenna 11 is adapted to 900 MHz GSM signals, and antenna 12 is adapted to 1800 MHz DCS signals, and the free resonance ends 11A and 12A act as the active radiators . The inclination of the antennas ensures that the thickness of the dielectric 15 is sufficient to ensure the desired bandwidth for each antenna 11, 12. When sharing both ground plane 14 and the wedge of dielectric 15, a combination of improved bandwidth and improved signal performance of the antenna is obtained for both antennas, even though the capacitive coupling of the antenna front edge 12A in this embodiment is a little less than the capacitive coupling of the antenna front edge in 11A, due to the further distance between the front edge 12A and the ground plane 14. The described microstrip antenna arrangement is a patch antenna arrangement benefiting from the regular forms of antennas 11, 12, i.e. the front resonance edges 11A and 12A are the primary radiating sources. In fig. 2, the principles of the above-mentioned inclination of the patch antennas are illustrated. Basically, fig. 2 illustrates the inclination of the antennas 11 and 12 in fig. 1. The resonance ends 11A and 12A of the antennas 11, 12 are shown relatively to the ground plane 14 and the corresponding vertical distance to the ground plane 14 are shown as DH and D . It should be noted that the capacitive coupling of the free resonance ends 11A and 12A, providing the aforementioned improvement in signal quality, is different due to the difference between DH and DL. Nevertheless, the coupling between the ground plane 14 and DH and DL, respectively, is comparable and the capacitive coupling of the antenna 12A is influenced by the fact that the further distance, i.e. the lower capacitance, will belong to the antenna having the highest frequency, thus resulting in an improvement of the effective coupling to the ground plane 14. According to the shown preferred embodiment of the invention, the lower capacitive coupling will always belong to the antenna having the highest frequency. In some embodiments, a modified design of the antenna arrangement would be possible as the shorter antenna 12 could be further inclined than the antenna 11, thus resulting in an improved capacitive coupling of the front end of the short-wave antenna 12. In fig. 3, other important features of the invention are shown and illustrated by some key components of the above-mentioned dual band patch antenna. Fig. 3 is a cross section view of the antenna in fig. 1, in which a cross section 311 of the antenna 11 and a cross section 312 of the antenna 12 are shown relative to a cross section 314 of the shared ground plane 14. Furthermore, a cross section 313 of the feeding line 13 is shown. Each antenna establishes directional radiation patterns φl, φ2 having corresponding directions 111 and 112. The components and the shown directions are a little distorted for the purpose of explaining the basic properties of the antenna arrangement. Fig. 4 shows the above-mentioned inclination, of which direction 111 represents the main direction of the radiation pattern of the antenna 11 and direction 112 represents the main direction of the radiation of the antenna 12. It should be noted that the angle φ in this illustration is only a two-dimensional angle between the directions of the radiating patterns in a plane defined by the transverse cross-section of the antenna arrangement of fig. 1. Experiments have shown that the inclination represented by the angle φ between the radiation direction of the two antennas is a relatively small price to pay in exchange for the desired dual band directional pattern, as a dual band antenna according to the invention provides a satisfying homogeneous radiation pattern in the sense that the two radiation patterns are comparable with respect to both receiving and transmitting quality as well as the directional performance. The invention teaches that directional performance of the antennas may be obtained if the ground plane is shared by two or more antennas, as a relatively broad ground plane is required in order to obtain directional characteristics for both antennas. In the above-mentioned embodiment, the price for a dual band directional radiation pattern is a small inclination between the directions of the two characteristics. Those skilled in the art will appreciate that other embodiments of the invention are possible. One embodiment includes the possibility of having each of the resonance ends face the ends of the radiator antenna element. The coupling to the ground plane will then be shared by the radiator antenna elements and divide the wedge of solid dielectric in two. A further embodiment includes the possibility of having each resonance end face the same way, either by facing the back end or the front end of the wedge of solid dielectric. The coupling to the ground plane will then be made at the opposite end.

Claims: Claims 1. Microstrip antenna arrangement in a communication device comprising at least two resonance antenna elements (11, 12) resonating at mutually different wavelengths, said antenna elements being arranged relative to at least one shared ground plane (14) , each of said at least two antenna elements (11, 12) establishing a directional radiation pattern. 2. Microstrip antenna arrangement according to claim 1, wherein the directions φl, φ2 of said radiating patterns define an angle φ other than 0°. 3. Microstrip antenna arrangement according to claim 1 or 2, wherein the angle θ is between 0° and 170°, and preferably between 0° and 50°. 4. Microstrip antenna arrangement according to claims 1 - 3, wherein the radiation pattern of each antenna at φ = 0° is at least 3 dB greater than φ = 180°. 5. Microstrip antenna arrangement according to claims 1 - 4, wherein the antenna arrangement comprises two antenna elements (11, 12), each adapted to frequency bands at substantially 900 MHz and substantially 1800 MHz, respectively. 6. Microstrip antenna arrangement according to claims 1 - 5, in which each antenna performs as a TM 1 0 mode antenna, i.e. having one dominating excitation mode. 7. Communication device, preferably a cellular phone, having a multi -band antenna according to claims 1 - 6, wherein the direction of the at least two radiation patterns is away from a user. 8. Communication device having a multi-band antenna, said multi -band antenna having at least two radiator antenna elements (11, 12) having a shared ground plane (14), each of said at least two radiator elements (11, 12) being adapted for radiation of an electromagnetic wavelength λ 1( said radiator antenna elements (11, 12) and said ground plane (14) comprising an electrically conductive material, the minimum effective dimension W of said shared ground plane (14) being greater than or equal to 0.095 λ m , λ m being the longest wavelength of the wavelengths λ 1 and preferably greater than 0.1 λ m . 9. Communication device according to claim 8, wherein the antenna radiator elements are fed by means of at least one proximity coupling. 10. Communication device according to claim 8 or 9, wherein the antenna radiator elements are fed by means of one shared proximity coupling. 11. Communication device according to claims 8 - 10, wherein the antenna radiators comprise a multi-band antenna comprising at least two TM 1 0 mode antenna radiator elements, each antenna element having substantially one dominating resonance frequency, said multi-band antenna having at least two radiator antenna elements (11, 12) having a shared ground plane (14), each radiator element (11, 12) being adapted to radiation of an electromagnetic wavelength λ ιr said radiator antenna elements (11, 12) and said ground plane (14) comprising an electrically conductive material, at least two of said radiator antenna elements (11, 12) being grounded to said ground plane (14), and the antenna elements (11, 12) having free resonance ends being extended in such a way that said free resonance ends (11A, 12A) are substantially facing each other. 12. Communication device according to claim 11, wherein a coupling (17A) from at least one of the radiator antenna elements (11, 12) to said ground plane (14) with a width smaller in size than that of said radiator antenna element (11, 12) .

Inventors: AMTOFT TORBEN, JACOBSEN KATRIN A FLOETTI, AMTOFT, TORBEN, JACOBSEN, KATRIN A FLOETTI
Applicants: AMTOFT TORBEN, JACOBSEN KATRIN A FLOETTI, TELITAL R & D DENMARK A S, AMTOFT, TORBEN, JACOBSEN, KATRIN A FLOETTI, TELITAL R & D DENMARK A/S
Application year: 2000
Main classification: H01Q1
IPC codes: H01Q1, H01Q5, H01Q9, H01Q21