FR3033449A1 - BROADBAND OMNIDIRECTIONAL ANTENNA STRUCTURE - Google Patents

Abstract

The invention relates to an antenna structure (10) with a high frequency polarization, with a polarization in a preferred, so-called vertical direction, adapted for transmitting and / or receiving signals of wavelength between a minimum wavelength. and a maximum wavelength. The antenna structure comprises a plane (1) of mass, extending in a plane perpendicular to said vertical direction, said horizontal plane, and a radiating structure comprising a first strip (21) metal and a second strip (22) metal arranged vertically, substantially parallel to each other, the second metal strip (22) being connected to the ground plane (1) and substantially perpendicular to the ground plane (1), and a radiant loop comprising a plurality of radiating strips a first end of the loop being connected to the first metal strip (21) and a second end of the loop being connected to the second metal strip (22).

Description

FIELD OF THE INVENTION The invention relates to a broadband antennal structure. In particular, the invention relates to a vertically polarized broadband antenna array with horizontal omnidirectional radiation, in particular for mobile application, for frequencies between the low frequency bands LF (for Low Frequency in English) and ultra high frequency UHF (for Ultra High Frequency in English). 2. Background Art Vertical-polarized broadband antennas are used in various telecommunication or broadcasting applications, particularly in the context of mobile applications, for example by being placed on vehicles. The antennas currently used for these types of mobile applications are generally broadband monopole antennas (conical monopole, planar variable geometry) integrated under a radome, or damped bullets, right or inclined. For fixed applications, the antennas used are generally biconical antennas or whip antennas equipped with an adaptation cell for the frequencies between the low frequencies LF and the high frequencies HF (for High Frequency).

These antennas have the main disadvantage of having a vertical footprint near or more than a quarter of the wavelength of the lowest frequency of operation of the antenna. The solutions currently proposed to reduce this vertical space are inefficient and have a large reduction in gains for radiation in the horizontal plane, called azimuthal radiation. In addition, the azimuthal radiation is generally not stable in frequency on the frequency band of the antennas. OBJECTIVES OF THE INVENTION The invention aims at overcoming at least some of the disadvantages of known antenna structures.

In particular, the invention aims to provide, in at least one embodiment of the invention, a vertical polarization antenna structure of small vertical dimensions. The invention also aims to provide, in at least one embodiment, an antenna structure whose performance is stable over a wide frequency band. The invention also aims to provide, in at least one embodiment of the invention, an antenna structure with omnidirectional radiation in a horizontal plane. The invention also aims to provide, in at least one embodiment, an antenna structure whose azimuth gain is substantially constant over the entire frequency band of operation. 4. DESCRIPTION OF THE INVENTION To this end, the invention relates to an antennal structure with a wide frequency band, polarized according to a preferred direction, referred to as a vertical direction, adapted for transmitting and / or receiving signals of length a wave between a minimum wavelength and a maximum wavelength, said antenna structure comprising: a ground plane, extending in a plane perpendicular to said vertical direction, said horizontal plane, a radiating structure, characterized in that said radiating structure comprises a first metal strip and a second metal strip, arranged vertically, spaced from each other and substantially parallel to each other, the second metal strip being connected to the ground plane and substantially perpendicular to the ground plane, a radiant loop, comprising a plurality of radiating strips, a first end of the radiating loop e being connected to the first metal strip and a second end of the radiating loop being connected to the second metal strip.

The maximum wavelength corresponds to signals of minimum frequency in the frequency band, and the minimum wavelength corresponds to signals of maximum frequency in the frequency band. The part of the frequency band close to the minimum frequency is called the lower part of the band, and the part of the frequency band close to the maximum frequency is called the upper part of the band.

The minimum and maximum frequencies correspond to the terminals of the frequency band in which the antenna structure is intended to be used without degrading performance. The antennal structure is adapted to operate outside this frequency band, but without guarantee of performance. Metal strip and radiating strip are metal elements 10 or a combination of metallic elements extending mainly over two dimensions, a width and a length, and having a negligible thickness with respect to said width and length. In particular, the metal strips and the radiating strips are distinguished from a wire which is mainly characterized by a single dimension, its length, and a three-dimensional element of which none of the dimensions is negligible compared to the other two. In particular, the metal strips and the radiating strips have a width greater than one twelfth of the maximum wavelength. An antenna structure according to the invention has a small footprint and allows transmission or reception of a signal over a wide frequency band and linear polarization. The antenna structure also allows omnidirectional radiation in the horizontal plane, called azimuthal radiation, and substantially constant gain over the entire operating frequency band in this plane. The radiation of the antennal structure is optimized in the horizontal plane. The use of the antennal structure in a wide band of frequency of use is notably permitted thanks to the width of the metal strips and the radiating strips. The radiating loop comprises an upper portion substantially parallel to the horizontal plane, of greater length than the distance between the first metal strip and the second metal strip, and the two ends of which are connected to the first metal strip and the second metal strip by substantially vertical lateral portions and then substantially horizontal connecting portions.

Advantageously, the ground plane extending horizontally and the metal strips extending vertically, the polarization is vertical, the radiation is omnidirectional in the horizontal plane, and the azimuth gain is substantially constant over the entire operating band. Vertical polarization allows a better efficiency of the mobile antenna structure, for example if it is mounted on the roof of a moving vehicle, and more particularly at a height near the ground level. Advantageously and according to the invention, the radiating loop is a folded radiating loop 10, in which the radiating strips form at least one U-shaped cross-section fold. According to this aspect of the invention, the U-shaped fold or folds to reduce the length of the radiating strips parallel to the horizontal plane, in order to reduce the zenith radiation, that is to say the radiation that does not propagate in the horizontal plane, in the upper part of the frequency band. U-folds result in a distribution of radiating bands parallel to the horizontal plane in several parallel and non-coincidental planes. In addition, the U-shaped bend or recesses improve the impedance matching of the antenna structure, and reduce the bulk of the antenna structure while maintaining the same total length of the metal strips and radiating strips of the radiating structure. . Advantageously, an antenna structure according to the invention has a vertical space between the ground plane and the highest point of the radiating structure, less than one-tenth of the maximum wavelength.

According to this aspect of the invention, the reduction of antennal congestion below one tenth of the maximum wavelength makes it possible to avoid performance degradation in the upper part of the strip and allows radiation without loss of gain. on the horizon. In addition, the antenna structure is thus less cumbersome than conventional monopole quarter wave antennas, while having equal or greater performance in terms of gain and radiation in the horizontal plane.

Advantageously and according to the invention, the radiating structure comprises at least one short-circuit element, electrically connecting the first metal strip and the second metal strip in their parts furthest from the ground plane.

According to this aspect of the invention, the short-circuit element makes it possible to improve the adaptation of the antenna structure. In addition, the omnidirectional radiation of the antennal structure is improved in the central part of the frequency band, called the band medium.

Advantageously and according to the invention, the first metal strip is adapted to be connected to a positive terminal of a transmitter / receiver and the ground plane is adapted to be connected to a negative terminal of said transmitter / receiver. Transmitter / receiver is either a single transmitter or a single receiver, or a device adapted to both transmit and receive signals.

Advantageously, the transmitter / receiver is adapted to be connected to the antenna structure via a coaxial cable whose inner conductor connects the positive terminal of the transmitter / receiver to the first metal strip and an outer conductor connects the negative terminal of the transmitter / receiver to the second metal strip and / or the ground plane.

According to this aspect of the invention, the coaxial cable allows a better impedance matching of the antenna structure. Advantageously, an antenna structure according to the invention comprises a metal box disposed on the ground plane and delimiting a cavity adapted to contain the transmitter / receiver, said metal box being electrically connected to the ground plane and the second metal strip. The cavity formed by the metal case makes it possible to board the transceiver in the antenna structure, thus reducing the disturbances between the antenna structure and the transmitter / receiver, while maintaining a weak connection length between the transmitter / receiver. receiver and the first metal strip and the ground plane. In order not to increase the vertical bulk of the antenna structure, the lengths of the first metal strip and the second metal strip can be reduced so that the space released is occupied, in part of its height, by the box. metallic: the height of the metal case is preferably less than one sixth of the total height of the antenna structure.

Advantageously, the metal housing is adapted to receive processing elements of the signal emitted or received by the antenna structure, for example amplification, filtering elements, etc. Advantageously and according to the invention, the first metal strip is connected to the transmitter / receiver via a metal connection surface substantially parallel to the horizontal plane. According to this aspect of the invention, the metal bonding surface connects an end of the first metal strip to the transmitter / receiver to adjust the impedance match to the desired frequency band. Preferably, the metal connection surface is trapezoidal in shape, with a large base of the trapezium connected to the first metal strip and a small base of the trapezium connected to the transmitter / receiver. According to several variants of the invention, the connecting surface extends from the first metal strip and towards the second metal strip, or extends from the first metal strip and in the opposite direction to the second metal strip. . Advantageously and according to the invention, the length of the radiating structure between the positive terminal of the transceiver and the connection of the ground plane and the second metal strip is between half of the maximum wavelength and the minimum wavelength. The length of the radiating structure is understood to mean the sum of the length of the metal strips and the radiating strips forming said radiating structure. According to this aspect of the invention, this structural length allows an improvement of the adaptation and control of the azimuthal radiation over the entire frequency band.

Advantageously and according to the invention, the width of the radiating structure is between one eighth of the maximum wavelength and one third of the minimum wavelength. According to this aspect of the invention, the width of the radiating structure, corresponding to the width of the radiating band composing the widest radiating structure, is large enough to allow transmission / reception in a wide frequency band, and sufficiently small so that the congestion of the antennal structure is limited. In addition to the frequency band, the width of the radiating structure also influences the standing wave ratio, which is all the lower in the lower part of the frequency band as the width of the radiating structure is high. According to another variant of the invention, the width of the radiating structure is less than one-eighth of the maximum wavelength. Such a width is less advantageous than a width greater than one-eighth of the maximum wavelength, in particular in terms of adaptation of the antenna structure, but makes it possible to obtain a reduced-size antenna for practical or aesthetic reasons. when the antenna structure is used in applications in which the adaptation of the antennal structure is not critical.

Advantageously and according to the invention, the width of the radiating strips is variable along the radiating loop. According to this aspect of the invention, the radiating strips have a variable width and therefore a variable surface in order to allow a homogenization of the surface density of the current passing through the radiating strips. This homogenization of the current surface density makes it possible to improve the radiation of the antenna structure and in particular to homogenize the gain of the antenna structure in the azimuthal plane. Advantageously and according to the invention, the ground plane has a width and a length greater than the maximum wavelength. According to this aspect of the invention, the standing wave ratio of the antennal structure is improved. In practice, either the ground plane has a real length and width greater than the maximum wavelength, or the ground plane is electrically connected to a metal surface having a length and a width greater than the maximum wavelength .

Advantageously, an antenna structure according to the invention comprises a radome surrounding the radiating structure. According to this aspect of the invention, the radome allows a protection of the radiating structure, for example against the weather, and allows to hide the antenna structure. In addition, the radome is designed not to degrade the radiation of the antennal structure. The invention also relates to a vehicle, characterized in that it is equipped with an antenna structure according to the invention, the ground plane of the antenna structure being fixed in electrical continuity to a surface extending in a substantially plane. parallel to the horizontal plane. A vehicle according to the invention is adapted to transmit and receive signals via the antennal structure, for telecommunications applications in particular. Fixing the ground plane on a conductive surface extending in a plane substantially parallel to the horizontal plane, for example the roof of the vehicle, makes it possible to easily enlarge the surface acting as a ground plane. The invention also relates to an antenna structure and a vehicle characterized in combination by all or some of the features mentioned above or below. 5. BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention will appear on reading the following description given solely by way of nonlimiting example and which refers to the appended figures in which: FIG. 1 is a diagrammatic view of In a perspective view of an antenna structure according to a first embodiment of the invention, FIG. 2 is a diagrammatic sectional view of an antenna structure according to the first embodiment of the invention, FIG. schematic perspective view of an antenna structure according to a second embodiment of the invention, Figure 4 is a schematic sectional view of an antenna structure according to the second embodiment of the invention, Figure 5 is a schematic perspective view of an antenna structure according to a third embodiment of the invention, Figure 6 is a schematic sectional view of an antenna structure according to the 10 tr In the third embodiment of the invention, FIG. 7 is a schematic perspective view of an antenna structure according to a fourth embodiment of the invention; FIG. 8 is a schematic perspective view of an antenna structure according to a fifth embodiment of the invention, FIG. 9 is a schematic perspective view of an antenna structure according to a sixth embodiment of the invention; FIG. 10 is a schematic perspective view of a vehicle equipped with An antenna structure according to one embodiment of the invention, Fig. 11 is a curve showing the impedance matching of an antenna structure according to an embodiment of the invention as a function of frequency, in the 470-700 MHz frequency band, Fig. 12 is an azimuthal radiation pattern of an antenna structure according to one embodiment of the invention for a frequency of 550 MHz, Fig. 13 is a curve representing the maximum azimuthal gains as a function of the frequency, in the frequency band 470-700 MHz, of an antenna structure according to one embodiment of the invention. 6. Detailed Description of an Embodiment of the Invention The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to one embodiment. Simple features of different embodiments may also be combined to provide other embodiments.

Figure 1 shows schematically and in perspective an antenna structure according to a first embodiment of the invention. FIG. 2 diagrammatically represents a section of the antenna structure according to this first embodiment, according to a section plane defined by the X-X and Z-Z axes as represented in FIG.

The antenna structure is adapted to transmit or receive signals in a wide frequency band between a minimum frequency associated with a maximum wavelength and a maximum frequency associated with a minimum wavelength. These minimum and maximum frequencies are the frequencies between which the antenna structure is intended to operate with optimum performance. Thus, the antenna structure can operate outside this frequency band, but the performance is not assured, since the design of the antenna structure is related to the desired frequency band. For example, the antenna structure may be configured for broadband application between a minimum frequency of 470 MHz and a maximum frequency of 700 MHz. These two values are therefore associated with a minimum wavelength of approximately 43 cm and a maximum wavelength of approximately 63 cm. The antenna structure comprises a ground plane 1 to which is connected a radiating structure 12. The ground plane 1 defines a plane, called a horizontal plane, comprising two axes X-X and Y-Y perpendicular to each other, and further defines an axis ZZ perpendicular to the horizontal plane. The radiating structure 12 comprises two metal strips, a first metal strip 21 and a second metal strip 22, the ends farthest from the ground plane 1 are connected to a radiating loop 14. The radiating loop 14 is composed of a plurality of radiating strips, here thirteen, referenced 231a, 233a, 234a, 235a, 236a, 237a, 231b, 233b, 234b, 235b, 236b, 237b and 232, making it possible to connect the ends two metal strips 21, 22. The antenna structure is intended for the transmission and / or reception of preferentially vertical polarization signals, ie oriented along the ZZ axis, with azimuthal omnidirectional radiation, that is, say that the signals propagate substantially parallel to the horizontal plane. The two metal strips 21, 22 are arranged substantially perpendicularly to the ground plane 1, and therefore arranged substantially vertically, and are parallel to one another. The second metal strip 22 is connected to the ground plane 1 so as to be electrically continuous, for example by welding, screwing, riveting, and the first metal strip 21 is connected to a positive terminal of a transmitter / receiver 4, a negative terminal of the transceiver 4 being connected to the ground plane 1, the positive and negative terminals being preferably located in the plane defined by the axis XX and the ZZ axis. The upper parts of the two metal strips 21, 22, that is to say the parts farthest from the ground plane 1, are connected by a short-circuit element 24. The lower part of the first metal strip 21, that is to say the part closest to the ground plane 1, to which the positive terminal of the transceiver 4 is connected, is at a distance from the plane 1 of mass less than one hundredth of the minimum wavelength. The distance separating the first metal band 21 from the second metal band 22 is less than one-tenth of the minimum wavelength. The length of the second metal strip 22 is between one-twelfth and one-tenth of the minimum wavelength, in order to provide optimum radiation over the entire frequency band. According to the embodiments, the transmitter / receiver 4 may be for example only a transmitter, only a receiver or a device grouping the transmitter and receiver functions. The plane 1 of mass is here represented of a size equivalent to the overall size of the radiating structure 12 along the X-X and Y-Y axes. Preferably, the ground plane 1 is electrically connected to a substantially horizontal surface of greater size, preferably of greater width and length than the maximum wavelength, for example the roof of a vehicle as shown with reference to FIG. Figure 10.

The radiating loop 14 comprises an upper portion, here composed of the radiating strips 231a, 231b, 232, 233a and 233b. This upper portion is connected to substantially vertical lateral portions, composed respectively of the radiating strips 235a and 235b, said lateral portions being connected to connecting portions respectively composed of the radiating strips 234a, 236a, 237a and the radiating strips 234b, 236b , 237b, said connecting portions being connected to the two metal strips 21, 22, thus closing the radiating loop 14. The radiating strips 232, 233a, 233b, 234a, 234b, 237a, 237b are substantially horizontal. The radiating strips 231a, 231b, 235a, 235b, 236a, 236b are substantially vertical. In this first embodiment, the radiating loop 14 is thus symmetrical on either side of the plane defined by the Y-Y and Z-Z axes.

The radiating strips each have a defined width along the Y-Y axis and a length defined either along the X-X axis for the radiating bands oriented substantially horizontally, or along the Z-Z axis for the radiating bands oriented substantially vertically. By extension, the width of the radiating loop 14 is defined by the width of the widest radiating band among those forming the radiating loop 14, and the length of the radiating loop 14 is defined by the sum of the lengths of the radiating bands forming the radiant 14 loop. In this first embodiment, the radiating strips all have the same width. The length of the radiating structure 12 is the sum of the length of the radiating loop 14 and the lengths along the ZZ axis of the first metal strip 21 and the second metal strip 22, that is to say the length of the radiating structure 12 between the positive terminal of the transmitter / receiver 4 and the connection of the ground plane 1 and the second metal strip 22. In this case, in this embodiment, the length of the radiating structure 12 is between half of the maximum wavelength, ie 63/2 = about 31.5 cm, and the minimum wavelength. about 43 cm. The width of the radiating structure 12 is between one eighth of the maximum wavelength, ie 63/8 = 7.9 cm approximately, and one third of the minimum wavelength, ie 43/3 = 14.3 about cm. According to another embodiment of the invention, the width of the radiating structure 12 is less than one-eighth of the maximum wavelength. The adaptation of the antennal structure is thus less optimized, but makes it possible to reduce the size of the antenna structure when the use of the antennal structure is little sensitive to the degradation of the adaptation. The radiating loop 14 is a folded loop, comprising at least one U-shaped fold, here three folds 16a, 16b, 16c. A fold is composed of three radiating strips, a radiating strip being connected at each of its two ends by a radiating strip perpendicular thereto, so as to form a U. The folds make it possible in particular to reduce the size of the parallel radiating strips. in the horizontal plane, thus limiting a zenith radiation of the antenna structure, that is to say a radiation substantially oriented in the direction of the ZZ axis. For example, a first U-shaped fold 16a is located on the upper portion of the radiating loop 14 and is formed of the radiating strips 231a and 231b each connected to one end of the radiating strip 232 and perpendicular thereto. The ends of the radiating strips 231a and 231b not connected to the radiating band 232 are respectively connected to and perpendicular to the radiating strips 233a and 233b. Without this first U-fold, the radiating strips 233a and 233b would be directly connected to form a single long radiating band. The length of this long radiating band parallel to the horizontal plane would result in excessive zenith radiation. With this first U-fold, the radiating strips 233a and 233b are located on the same plane, and the radiating band 232 is located on a parallel plane and not coincident with the latter plane.

In the same way, the lateral portions and the connecting portions form a second fold 16b U and a third fold 16c U. The side portions comprise radiating strips 235a and 235b substantially vertical and perpendicular to the radiating strips 233a and 233b. The connecting portions comprise radiating strips 234a and 234b that are substantially horizontal and perpendicular to the metal strips 21, 22. The connection portions each further comprise a U-shaped fold acting as a connection with the lateral portions. More specifically, the second U-fold 16b comprises the radially band 236a perpendicular to the radiating band 234a, and the radiating band 237a perpendicular to the radiating band 236a and the radiating band 235a, thereby forming a U. The radiating strips 234a and 237a are thus located on two planes parallel to the horizontal plane and not merged. In a symmetrical manner, the radiating strips 236b, 237b and 235b form the third U-fold 16c. The length of the radiating strips 236a and 236b are preferably between a quarter and a third of the length of, respectively, the radiating strip. 235a and the radiating band 235b.

As for the first U-shaped fold 16a, the second U fold 16b and the third U fold 16c in particular make it possible to reduce the size of the horizontal radiating strips to reduce the zenith radiation of the antenna structure. In addition, the U-shaped folds, in particular the first U-shaped fold 16a, make it possible to improve the adaptation of the antenna structure and the azimuthal radiation, in particular for frequencies in the upper part of the frequency band. The U-shaped folds also make it possible to reduce the overall size of the antenna structure, in particular along the XX axis (termed lengthwise space) and along the ZZ axis (known as vertical space), while maintaining a length. radiant loop 14 sufficient for the intended application. In particular, the vertical dimension 15 of the antenna structure 10 is thus less than one-tenth of the maximum wavelength. For a minimum frequency of 470 MHz, associated with a maximum wavelength of 63 cm, the antenna structure will therefore have a vertical footprint of less than 6.3 cm, in practice about 6 cm. By comparison, antennas of the prior art for the same frequency band have a vertical footprint of about one quarter of the maximum wavelength, in practice between 14 and 16 cm. In order to improve the adaptation of the antenna structure and the azimuth radiation for frequencies in the central part of the frequency band, the two metal strips 21, 22 are connected by a short-circuit element 24. The short-circuit element 24 is composed of a metal strip, as shown in FIG. 1, with a width between one-hundredth of the maximum wavelength and the width of the radiating strip 232, or a plurality of strips distributed over the width of the antenna structure, symmetrically on either side of the plane defined by the axes XX and ZZ. In this embodiment, the short circuit element 24 is electrically connected to the radiating strip 232, for example by welding. As shown in FIG. 2, the short-circuit element 24 may consist solely of two small strips connecting firstly the first metal band to the radiating band 232 and, on the other hand, the radiating band 232 to the radiating band 232. the second metal strip 22, the radiating band 232 then partly acting as a short-circuit element. In order to improve the rigidity of the antenna structure, it is possible to add dielectric spacers (not shown) of low relative permittivity (less than 4) and low tangent of loss in the antenna structure, especially between radiant strips 237a, 237b and the plane 1 of mass, between the radiant strip 232 and the ground plane 1, between the first metal strip 21 and the ground plane 1, between the radiating strips 237a and 233a, between the radiating strips 10 237b and 233b, between the radiating strips 234a and 233a and / or between the radiating strips 234b and 237b. Figures 3 and 4 show respectively a schematic perspective view and a schematic sectional view of an antenna structure according to a second embodiment. The antenna structure according to this second embodiment is distinguished from the first embodiment by the presence of a metal housing 6 disposed on the ground plane 1 and electrically connected thereto. The metal housing 6 delimits a cavity adapted to contain the transmitter / receiver. The metal housing 6 is arranged at the level of the first metal strip 21 and the second metal strip 22: the first metal strip 21 is connected to the positive terminal of the transmitter / receiver 4 via an orifice formed in the metal box 6 allowing access to the cavity; the second metal strip 22 is connected directly to the metal housing 6, the latter being connected to the ground plane 1. The length along the X-X axis and the width along the Y-Y axis of the metal housing 6 are less than the length and width of the ground plane 1. The height, along the Z-Z axis, of the metal housing 6 is less than one-sixth of the vertical dimension of the antenna structure. The height of the housing is limited so as not to significantly modify the radiation performance and the adaptation of the antenna structure. The vertical bulk of the antenna structure in this second embodiment is the same as in the first embodiment, the height of the metal housing being compensated for by a decrease in the length of the metal strips 21. The metal housing 6 furthermore makes it possible to contain signal processing elements, for example a filter 7 and an amplifier 8, as shown in FIG. 4. For use of the antennal structure in reception, the amplifier 8 may be a preamplifier . Figures 5 and 6 show respectively a schematic perspective view and a schematic sectional view of an antenna structure according to a third embodiment. The antenna structure according to this third embodiment is distinguished from the second embodiment in particular by a first and a second asymmetry of the antenna structure with respect to the plane defined by the Y-Y and Z-Z axes. The first asymmetry appears at the level of the first metal strip 21: at its end closest to the mass plane 1, the first metal strip 21 is connected to a connecting surface 211 substantially parallel to the horizontal plane and facing the second metal strip 22. This connection surface 211 is connected to the positive terminal of the transceiver 4, directly or via signal processing equipment such as the filter 7 and the amplifier 8. The connection surface 211 has a substantially triangular shape or is trapezoidal, whose long side is connected to the first metal band 21 and a vertex, if the shape is triangular, or a small side, if the shape is trapezoidal, is connected to the positive terminal of the transceiver 4, here through the orifice of the metal case 6. This connecting surface 211 makes it possible to improve the adaptation of the antenna structure in the frequency band.

The second asymmetry is present on the radiating loop 14. In the first and second embodiments of the invention, because of the connection of the positive terminal of the transmitter / receiver 4 to the first metal strip 21 and the connection of the second metal strip 22 to ground, a first pan of the metal structure lying, with respect to the axes YY and ZZ, on the side of the first metal strip 21, has a surface density of current greater than a second panel of the structure lying on the side of the second strip 22 metallic. This difference in surface current density generates a gain difference in the azimuthal radiation of the antenna structure, the gain being lower on the second side of the structure. Thus, in this third embodiment, the surface density is homogenized by reducing the width, and therefore the area, of the radiating and metallic strips lying in the second part of the metal structure, in particular here the radiating strips 233b 235b, 237b, 236b, 234b and the second metal strip 22. The width of the radiating strips is progressively reduced at the level of the radiating strip 233b, which comprises a trapezoidal portion 26 whose base is of the same width as the radiating bands of the first panel and whose width decreases until it reaches a reduced width. . The trapezoidal portion 26 is then followed by a rectangular portion 28 of reduced width and the second metal strip 22 and the radiating strips 235b, 237b, 236b and 234b are of the same reduced width. The reduced width allows a decrease in the surface area of the radiating strips for the same current passing therethrough, thereby increasing the current surface density which is homogeneous with the current surface density of the elements of the first section of the antenna structure, thereby improving the omnidirectionality of azimuthal radiation. Figures 2, 4 and 6 schematically show in section the first, the second and the third embodiments. In these three embodiments, the antenna structure comprises a parallelepipedal radome surrounding the radiating structure 12, made of a material of low permittivity, for example fiberglass, polyamide or ABS polymer. The radome is designed so as not to disturb the radiation performance of the antenna structure, to protect it from possible damage, and allows a camouflage thereof. The radome may also be cylindrical, hemispherical, or any other suitable form that does not degrade the performance of the antenna structure. For the sake of clarity, the radome is not shown in the perspective views of FIGS. 1, 3, 5 and 8. In other embodiments, the antenna structure may not include a radome.

Fig. 7 is a schematic perspective view of an antenna structure according to a fourth embodiment of the invention. The antenna structure according to this fourth embodiment is distinguished from the third embodiment in particular by the connecting surface 211, which is fixed to the first metal strip 21 and which is here oriented in a direction opposite to the third embodiment, that is to say in a direction opposite to the second metal strip 22. In addition, the short circuit element is composed of two strips 24a, 24b. In addition, the second pan of the antenna structure is slightly modified.

The radiating strip 233b is composed of a single trapezoidal portion and does not include a rectangular portion as was the case in the third embodiment. Then, the rectangular strip 237b is trapezoidal in shape, its width increasing from the radiating strip 235b to the strip 236b. Due to the shape of this second panel, the radome 3 is not parallelepipedal but has a shape close to the contours of the antenna structure, thus reducing its bulk. Likewise, the shape of the metal box 6 and the ground plane 1 is adjusted to the shape of the radome 3. The modifications made by the successively described embodiments each make it possible to improve the performance of the antenna structure, the performance being increasing between the first, second, third and fourth embodiments. In particular, the possible frequency band of use of the antenna structure is the widest for the fourth embodiment and decreases for the other modes. However, the first embodiment 25 is also the least complex to produce, and the manufacturing complexity increases with the following embodiments, until the fourth embodiment which is the most complex of embodiments presented, for performance higher. The radiating strips of the previously described embodiments are composed of metal surfaces. Figures 8 and 9 schematically show antenna structures respectively in fifth and sixth embodiment, in which radiating strips and metal strips are composed of a plurality of radiating strips. These radiating strips are metallic and are distributed so as to occupy the same length and the same width as the metal surfaces of the preceding embodiments.

The fifth embodiment, shown in FIG. 8, is based on an antenna structure according to the first embodiment, in which the metal strips and the vertically oriented radiating strips are composed of a plurality of radiating strips, here three strips 28 radiating by radiating band and metal band. Horizontally oriented radiating strips 10 take the form of a metal surface, as in the previous embodiments. In the sixth embodiment, shown in FIG. 9, all the radiating strips and the metal strips are composed of radiating strips 28. In addition, the ground plane 1 is composed of conductor wires arranged in a star pattern starting from the antenna structure. The use of radiating strips is particularly useful for the use of an antenna structure adapted for low frequencies, i.e. for high wavelengths, the dimensions of the antennal structure making it difficult to use the antenna. use of large metal surfaces, for reasons of difficulty of manufacture, cost, resistance of the antennal structure to physical stresses, bad weather, etc. The radiating strips have a width that can vary between a few thousandths and a few hundredths of the maximum wavelength. Similarly, in the case where the antenna structure is large, the ground plane used depends on the nature of the ground on which the antenna structure, said ground plane, is disposed. When the ground plane is composed of a medium of low electrical conductivity (sand, earth, rock, etc.), a ground plane is added, for example by means of the star conductors as shown in FIG. The star conductor wires used vary according to the electrical conductivity of the medium and can reach 120 wires for a medium of very low electrical conductivity. When the ground plane is composed of a highly conductive medium (sea, salt marsh, etc.), the ground plane forms the ground plane of the antenna structure.

FIG. 10 shows a vehicle 32, here an automobile, equipped with an antenna structure according to one embodiment of the invention. The ground plane 1 of the antenna structure 10 is electrically connected to a metal roof 34 of the vehicle 32, thereby extending the effective surface of the ground plane 1. Fig. 11 is a graph showing the impedance matching of an antenna structure according to the fourth embodiment of the invention, as a function of frequency, in the frequency band 470-700 MHz. The impedance matching is represented by the Standing Wave Ratio (VSWR) of the antenna structure. The stationary wave ratio of an antenna structure is perfect if it is equal to 1. The antenna structure according to the invention aims to obtain preferably a standing wave ratio of between 1 and 1.5. The curve of FIG. 11 shows that in the frequency band 470-700 MHz, the standing wave ratio is less than 1.5 and that it is 1.5 at the 470 MHz and 700 MHz terminals. The impedance matching is thus good for all the frequencies of the frequency band, thus allowing use of the antennal structure for transmission and reception.

Fig. 12 is a far-field azimuth radiation pattern of an antenna structure according to one embodiment of the invention. The radiation pattern is represented for a frequency of 550 MHz, that is to say within the frequency band of 470-700 MHz. The radiation is represented in the azimuthal plane, i.e. in the plane defined by the axes XX and YY, in a configuration where the antenna structure is placed on a circular metal plane 1.5 m in diameter and in an angular position whose angular values are in the range -180 °, 180 °]. The angles 0 ° and 180 ° correspond to angular positions on the axis XX, the angle 0 ° being located on the side of the second metal strip 22 and the angle 180 ° being located on the side of the first metal strip 21 . The 90 ° and - 90 ° angles correspond to angular positions on the Y-Y axis.

The radiation is represented in dBi, which corresponds to the decibel gain of the antenna structure relative to an isotropic antenna. On the curve, the radiation varies gradually between about -2 dBi for an angle of 0 ° to a value slightly lower than 0 dBi for an angle of 180 °. The variation is identical over the interval -180 °, 01, with a radiation close to OdBi for an angle close to -180 °. The radiation difference of the antennal structure between the 0 ° angle and the 180 ° angle is due to the change in surface density of the current on the first and second panes of the antennal structure, due to the presence the positive terminal of the transmitter at the first metal band 21.

Radiation for all frequencies between 470 MHz and 700 MHz have radiation curves, not shown for reasons of clarity, similar to the radiation curve for a frequency of 500 MHz, with slight variations of less than 1 dB .

Fig. 13 is a graph showing the maximum azimuth gains as a function of the frequency, in the frequency band 470-700 MHz, of an antenna structure according to one embodiment of the invention. The measurement is the same as the curve of FIG. 12, the radiation being expressed in dBi. As can be seen in the radiation diagram of FIG. 12, the maximum gain is generally the gain measured on the axis XX of the antenna structure, on the side of the first metal strip, that is to say at the level of the angular value 180 ° in FIG. 12. As can be seen in FIG. 13, the maximum azimuthal gain is stable between -1 dBi and 0 dBi over the entire frequency band 470-700 MHz.

The invention is not limited to the described embodiments. In particular, the embodiments presented describe an antenna structure with vertical polarization, but a different orientation of the antenna structure can allow its use for transmission and reception in a different linear polarization, for example oblique or horizontal. In addition, although the antenna structure has been described for use in a frequency band between 470 MHz and 700 MHz, an antenna structure according to the invention can be used in other frequency bands, the dimensions of it being adapted accordingly. The use of the antenna structure with dimensions adapted to other frequency bands makes it possible to obtain the same advantages as the embodiments described in these frequency bands. 5

Claims (13)

REVENDICATIONS1. Antenna structure with a wide frequency band, with a polarization in a preferred direction, called a vertical direction, adapted for transmitting and / or receiving signals of wavelength between a minimum wavelength and a maximum wavelength , said antenna structure comprising: a plane (1) of mass, extending in a plane perpendicular to said vertical direction, said horizontal plane, a structure (12) radiating, characterized in that said structure (12) radiating comprises - a first metal strip (21) and a second metal strip (22), arranged vertically, spaced from each other substantially parallel to each other, the second strip (22) being connected to the plane (1) of mass and substantially perpendicular to the plane (1) of mass, - a loop (14) radiating, comprising a plurality of radiating strips, a first end of the loop (14) radiating being connected to the prem first strip (21) and a second end of the loop (14) radiating being connected to the second strip (22) metal. 2. Antenna structure according to claim 1, characterized in that the loop (14) radiating is a folded radiating loop, wherein the radiating strips form at least one fold (16a, 16b, 16c) U-shaped cross section. 3. Antenna structure according to one of claims 1 or 2, characterized in that it has a vertical space between the plane (1) of mass and the highest point of the structure (12) radiating, less than one tenth of the maximum wavelength. 4. Antenna structure according to one of claims 1 to 3, characterized in that the radiating structure comprises at least one element (24) of short circuit, electrically connecting the first strip (21) and the second metal strip (22) metal in their parts farthest from the plane (1) of mass. 5 Antenna structure according to one of claims 1 to 4, characterized in that the first metal strip (21) is adapted to be connected to a positive terminal of a transmitter / receiver (4) and the plane (1) of mass is adapted to be connected to a negative terminal of said transmitter / receiver (4). 10 Antenna structure according to claim 5, characterized in that the length of the radiating structure (12) between the positive terminal of the transmitter / receiver (4) and the connection of the ground plane (1) and the second band (22) metal is between half the maximum wavelength and the minimum wavelength. 15 7. Antenna structure according to one of claims 5 or 6, characterized in that it comprises a housing (6) arranged on the metal plane (1) of mass and defining a cavity adapted to contain the transmitter / receiver (4). ), said housing (6) being electrically connected to the ground plane (1) and the second (22) metal strip. 20 8. Antenna structure according to one of claims 5 to 7, characterized in that the first strip (21) is connected to the metal transmitter / receiver (4) via a surface (211) of metal connection substantially parallel to the horizontal plane . 9. Antenna structure according to one of claims 1 to 8, characterized in that the width of the radiating structure (12) is between one eighth of the maximum wavelength and one third of the minimum wavelength . 10. Antenna structure according to one of claims 1 to 9, characterized in that the width of the radiating bands is variable along the loop (14) radiating. 3033449 25 11. Antenna structure according to one of claims 1 to 10, characterized in that the plane (1) of mass has a width and a length greater than the maximum wavelength. 5 12. Antenna structure according to one of claims 1 to 11, characterized in that it comprises a radome (3) surrounding the structure (12) radiating. Vehicle, characterized in that it is equipped with an antenna structure (10) according to one of claims 1 to 12, the plane (1) of mass of the antenna structure (10) being attached to a surface (34) extending in a plane substantially parallel to the horizontal plane.