|
(11) | EP 1 471 599 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||
| (54) | Multiband planar antenna |
| (57) A multiband planar antenna comprising at least one radiant element constituted by
a triangular or polygonal-shaped metallic patch (1) with one or two coplanar projecting
parts (2) with strips, printed on a substrate of thin dielectric material (3). The
planar radiant element is preferably vertical with respect to a ground plane (5-7)
such as the roof of a vehicle. The lower vertex (8) of the metallic patch (1) is the
entry of the antenna and it can be directly connected to the middle wire of the coaxial
feeder or by means of a matching network. |
[0001] The present invention refers to a multiband planar antenna. Various kinds of dual band antennas to be used on vehicles for cellular telephone services are already known. Most of them are of the monopole or asymmetric dipole, wire or strip type to be installed on vehicle roofs near their front or rear edge. The radiant element of said antennas is usually protected by a plastic material casing having also aesthetic and aerodynamic functions, it can be of various shapes and configurations, even though the "dolphin fin" one is the most commonly used.
[0002] The positioning of the antenna on the roof of vehicles allows obtaining an isotropic diagram on the horizontal plan as the radiation is not influenced by the presence of structural barriers in near sound field.
[0003] Said printed monopole or dipole planar antennas with constant width strip radiant arms, have a common drawback consisting in the intrinsic limitation of the impedance band, therefore the only way to obtain a dual frequency operation is their association to a suitable matching network causing however a further bandwidth reduction.
[0004] The most relevant consequence of these solutions is substantially the difficulty in covering more services with the same antenna and this condition does not comply with the needs of the car manufacturers which are more and more frequently requiring compact antennas at low costs being able to carry out multiple services on different frequencies. The typical need is the covering of all cellular telephone services including the most recent UMTS (Universal Mobile Telecommunications System) with one antenna.
[0005] Object of the present invention is to carry out a simple compact multiband planar antenna at low cost being able to cover the present cellular telephone services and optimized for the operation on vehicle roof or on any broad enough metallic surfaces, such as the vehicle rear hood or the doors.
[0006] Another object consists in obtaining an "intrinsic" bandwidth operation using a planar radiant structure that conceptually derives from the "pressure" of one pseudo-conical radiant element loaded with dielectric material and equipped with ground-connected strip of the type correspondent to the one present in one conical antenna loaded with dielectric material, said antenna is suitable for operation in vertical position on vehicle roof and is shown and described in the European patent no. 01121539.9 having title "Low profile, cord-less aerial" filed on 10.09.2001 in the name of Zendar S.p.A. - 42020 Montecavolo , RE (Italy).
[0007] Another object is the structuring of the radiant element in such a way to integrate a further commercial "patch" in it for the reception of the GPS (Global Positioning System) satellite signals with the relevant amplification and filtering circuitry.
[0008] A further object consists in designing the antenna for an intrinsically bandwidth operation that can be centered on two separate bands through the insertion of a matching network, a first narrower band centered around 900 MHz and a second broader band centered on a frequency that simultaneously covers both the GSM (Global System Mobile) and the UMTS (around 1800 MHz) cellular telephone services.
[0009] These objects according to the invention are attained in the following way:
a) using a thin dielectric material plate as a support of a radiant element constituted by a triangular-shaped, or more generally, polygonal printed metallic patch oriented in such a way that its lower vertex constitutes the power supply point;
b) using, as an alternative to the single dielectric material plate, a multilayer comprising also magnetic material if a further reduction of the radiant element is required;
c) inserting one or two coplanar strips with one end connected to the above-mentioned printed metallic patch radiant element and the other one being directly connected or through a concentrated impedance;
d) introducing a slit in order to extend a coplanar strip so that it can reach the dimensions required to obtain a good impedance pattern to the lower end of the band;
e) placing the dielectric material plate in an orthogonal position with respect to the ground plane (ex. the vehicle roof) so that the polygonal metallic patch that constitutes the radiant element with its projection parts shows a radiant behaviour which is similar to the one of a typical vertical folded monopole operating in presence of a metallic infinite reflector;
f) considering the removal of one of the coplanar strip line sections in order to allow the insertion of a further patch perpendicular to the previous one, for the GPS signal reception;
g) considering the possibility of using a couple of V-placed dielectric material patches with the feeding vertexes in common, in order to reduce the overall dimension in height and give the antenna the utmost aerodynamic properties and also a pleasant appearance;
h) considering the optimization of shape, sizes, configuration and position of the planar radiant element with respect to the ground plane in order to obtain the utmost bandwidth being physically consistent with the overall dimensions of the antenna;
i) considering the possibility of using the multilayer of the windscreen as a support for the polygonal-shaped thin metallic patch constituting the radiant element;
j) considering the insertion of a matching network with concentrated, distributed or mixed constants, in order to simultaneously cover the old and the new bands of cellular telephone services.
[0010] The advantages obtained by the present invention essentially consist in that the antenna keeps its main flatness and thinness features together with the electromagnetic behaviour of a monopole on a finite size ground plane compared with the wavelength such as the roofs, the hoods, the doors the wing mirrors, the windscreen of the vehicles and can take an arbitrary flat or bent triangular, rectangular, trapezoid or polygonal shape in order to fit the manufacturing requirements and the installation constraints.
[0011] Another advantage consists in that the shape of the metal conductor patch that constitutes the radiant element and the shape of the dielectric material plate on which the same patch is printed can be configured in order to allow the integration of a patch for the GPS satellite signal reception using the same ground plane as the radiant element.
[0012] A further advantage consists in that the metal conductor patch that constitutes the radiant element can be embedded into a multilayer constituted by thin layers of dielectric and/or magnetic material.
[0013] The invention is described in detail here below according to embodiments given only by way of non-limitative example with reference to the attached drawings, wherein:
Figure 1 is a perspective view of the radiant element for multiband planar antenna according to the invention in a basic embodiment constituted by a symmetric polygonal metallic patch and by two strips that are symmetrically placed with respect to the vertical axis and ground-connected;
Figure 2 is a perspective view of the radiant element for multiband planar antenna according to the invention in another embodiment constituted by an asymmetric polygonal metallic patch comprising one strip;
Figure 3 is a section view of the dielectric material plate constituting the radiant element in the monolayer embodiment;
Figure 4 is a section view of the dielectric material plate constituting the radiant element in the double layer embodiment;
Figure 5 shows the trend of the input reflection coefficient in function of the frequency for an antenna with the radiant element of Figure 1, without matching network;
Figure 6 shows the trend of the input reflection coefficient in function of the frequency for an antenna with the radiant element of Figure 1, comprising a matching network;
Figure 7 shows the trend of the radiation diagram on the horizontal cut of an antenna with the radiant element of Figure 1, at the frequency of 900 MHz;
Figure 8 shows the trend of the radiation diagram on the horizontal cut of an antenna with the radiant element of Figure 1, at the frequency of 1800 MHz;
Figure 9 is a perspective view of the radiant element for multiband planar antennas according to the invention in a different embodiment comprising a couple of V-placed dielectric material plates with the feeding vertexes and the two ends of the strips in common;
Figure 10 is a front view of a radiant element for multiband planar antenna according to the invention in an embodiment wherein the polygonal metallic patch comprises an extension slit of the correspondent strip;
Figure 11 is a perspective view of an example of radiant element for multiband planar antenna according to the invention associated to a patch for the GPS reception;
Figure 12 is a perspective view of an example of association and connection between a radiant element for multiband planar antenna according to the invention and a circuit containing a matching network and
Figures 13 and 14 are perspective views of embodiment examples of the multiband planar antenna according to the invention that are preset for the installation on vehicle roof and are equipped with a circuit containing a matching network and a patch for the GPS reception.
[0014] With reference to the figures, it can be noticed that the multiband planar antenna according to the invention, in its basic embodiment, is substantially constituted by a radiant element obtained by a triangular or polygonal-shaped thin metal conductor patch 1 with one or two coplanar projecting parts with ground-connected strips 2.
[0015] The metallic patch 1 is generally printed on a single substrate of thin dielectric material 3, by means of the well known selective etching technique which is commonly used for the production of printed circuits. Other suitable methods to obtain the metallic patch can also be used without leaving the scope of the invention, for example the serigraphy with conductive paint.
[0016] In a second embodiment, the metallic patch 1 with the relevant strip/strips 2 is obtained on a thin plate of dielectric material 3 associated to a multilayer comprising also thin layers of magnetic materials 4; by means of this solution an appreciable reduction of the multiband planar antenna sizes is obtained; this allows installing the antenna into narrower areas other than the vehicle roofs, for example into wing mirrors, bumpers, doors, spoilers and the like to the detriment of the bandwidth of the antenna.
[0017] The upper end of strip/strips 2 is connected to the above said metallic patch radiant element 1 and the lower one 8 is directly connected to an underlying ground plane 5 or by means of a concentrated or distributed impedance. Strip/strips 2 allow the in phase electric currents forming the low frequency radiation diagram flowing towards the ground.
[0018] A further patch 6 for the reception of the GPS satellite signals using the same ground plane 5 without exceedingly compromising the low frequency performance of the antenna can be inserted with an appropriate configuration of the shape of the metal conductor patch that constitutes the radiant element 1, of the shape of the dielectric material plate 3 on which the same patch is printed and removing one of the strips (Figures 11, 13, 14).
[0019] The planar radiant element 1 is preferably orthogonally placed with respect to a lower ground plane constituted by a base 5 having the required dimensions compatible with the space available and connects for example with the underlying roof 7 of a vehicle.
[0020] The input electrical connection of the antenna is constituted by the connection between the lower vertex 8 of the planar radiant element 1 and the middle wire of the coaxial feeder whose earth braid is ground-connected. The connection can be directly obtained or through a circuit containing a concentrated, distributed, or mixed constant matching network 9.
[0021] The substantially orthogonal position of the thin dielectric material plate 3 with respect to the base - ground plate 5, presets the metallic patch that constitutes the radiant element 1 with the strips 2 in such a way to show a radiative behaviour similar to the one of a typical folded monopole operating in presence of a metallic infinite reflector.
[0022] The possible insertion of a slit 10 allows extending the strip 2 (Figures 10, 13) so that it can reach the required dimensions in order to obtain a good impedance pattern at the lower end of the strip (below 900 MHz).
[0023] In another embodiment two dielectric material plates 3 can be used instead of one; these plates are V-placed with the lower feeding vertexes 8 of the metallic patches that constitute the relevant radiant elements 1 in common (Figure 9) and with the lower ends of the strips being jointly or severally ground-connected; this layout allows reducing the overall dimension in height and obtaining a greater possibility of aerodynamic configuration of its external covering.
[0024] In a further embodiment, the multilayer that constitutes the windscreen, the rear window or any other casing or plastic support for accessories such as wing mirrors, spoilers, bumpers and the like can be used as a support for the polygonal-shaped thin metallic patch that constitutes the radiant element 1.
[0025] The configuration and layout of the plate/plates of dielectric material supporting the radiant element 1 with respect to the ground plane 5-7 and the optimization of shapes and dimensions allow obtaining the utmost bandwidth consistent with the overall dimensions available for the application of the antenna.
[0026] In brief, even though the antenna keeps its main flatness feature together with the typical electromagnetic behaviour of a monopole on a ground plate with limited dimensions, preferably of big dimensions, with respect to the wavelength, such as roofs, hoods, doors and windscreens of vehicles, it can also have any arbitrary flat bent triangular, rectangular, trapezoidal shape in order to fit, also from the aesthetical point of view, the requirements and the installation constraints on any position and/or any kind of vehicle. Apart from the patch shape that constitutes the radiant element 1 and the thin dielectric material plate 3 supporting it, the radiative behaviour of the antenna is similar to the one of a monopole operating on the same ground plane.
[0027] The circuit with concentrated, distributed or mixed constant integrated matching network 9, associated to the input electrical connection of the antenna is used to optimize the performances on two separate bands; the first narrower band is centered around 900 MHz and the second broader band is centered around a frequency that simultaneously covers the GSM and UMTS cellular phone services (between 1700 and 2200 MHz). In the comparison between Figures 5 and 6 showing the trend of the input reflection coefficient in function of the frequency of an antenna of the type shown in Figure 1, with and without matching network, it can be noticed that the function of the matching network is the centering of the two minimums of the chart on the frequency values required.
[0028] The present invention has been described and illustrated according to its embodiments that are given only by way of non limitative example, it will be clear to people skilled in the art that various changes in shapes, details, orientations, components and assembly can be made without leaving its field and scope.