EP0714151A1 - Broadband monopole antenna in uniplanar printed circuit technology and transmit- and/or receive device with such an antenna - Google Patents

Abstract

The antenna includes a feed line (2) which is positioned on the first surface of a substrate. A conductor (3) is positioned on the second surface to form a ground plane (4) for the RF feed. A radiating stub (6) extends at least partially along the length of the side of the ground plate. A coupling slot (7) is formed between the stub and the plate. The coupling slot crosses the supply line at a crossing point beyond which the supply line has a series stub of a first adaptable length and the slot forms a parallel stub of a second adaptable length. The slot can be rectangular and the stub have variable width.

Description

The field of the invention is that of radio transmissions. More specifically, the invention relates to transmit and / or receive antennas, in particular for equipment of reduced size, such as portable devices.

The invention thus applies, in particular, to radio communication systems with mobiles. In fact, the extension of radiocommunication networks with land mobiles requires the development of portable autonomous stations having the dual functionality of transmitting and receiving microwave signals. These stations must therefore include an integrated antenna.

The frequencies currently used for these applications (of the order of 2 GHz), as well as various constraints linked to the ergonomics and aesthetics of the handsets (integration of the antenna in the design of the device, ease storage and use, fragility of large antennas, ...) lead to the use of antennas of very small dimensions. Several types of antenna are thus known, the dimensions of which are less than the wavelength of the microwave signal.

These antennas are generally in the form of a radiating element located outside of a metal case, for example of rectangular shape, constituting the shielding of one or more electronic cards ensuring in particular modulation and demodulation functions. microwave signals, in transmission and reception respectively.

A first known type of antenna is the half-wave doublet, that is to say a doublet of length λ / 2, with λ the operating wavelength.

The half-wave doublet, which generally consists of elements of two-wire lines (that is to say of conductive cylindrical rods) supplied by a supply line, has relatively broadband performance, which makes it usable in many applications.

However, several drawbacks are linked to its use. In fact, the supply lines (for example coaxial lines) are generally asymmetrical, while the radiating elements are symmetrical. Therefore, so that the radiation half-wave doublet is acceptable, a balun should be used. A balun is traditionally presented as a transformer involving localized or distributed impedances, and allowing, when placed between a symmetrical radiating element and an asymmetrical supply line, to make the currents symmetrical on the radiating structure. Such a balun has the major drawback of requiring always delicate focusing.

Half-wave doublets are also known which are self-symmetrized, so that they can be used without a balun. However, due to the use of conductive cylindrical rods, such an autosymmetry characteristic can only be obtained at the cost of increased complexity of the antenna structure.

Finally, in general, the half-wave doublets with cylindrical rods have difficult mechanical handling as well as a space which is still too large (although reduced), the minimum length of the antenna being imposed by the length of the main strands. , or about λ / 2.

As stated before, reducing clutter has become an essential goal for antenna designers.

A second type of antenna, even more compact than the half-wave doublet, was therefore designed. This is the inverted F antenna, which consists of a horizontal rectangular conductive element and a vertical rectangular conductive element. The vertical element performs a short-circuit function on the horizontal element, by connecting one of its ends to a ground plane. The length of the horizontal element is generally L = λ / 4. In other words, the horizontal element is placed in a plane parallel to the ground plane and at a height h thereof.

Thus, for frequencies of the order of 2 GHz, these dimensions are of the order of a few centimeters. The antenna obtained is therefore very compact (its minimum length is λ / 4 instead of λ / 2 for the half-wave doublet).

On the other hand, this antenna has very frequency dispersive characteristics, and therefore, consequently, a very low bandwidth, and for example of the order of 2 to 3%. This is due to the fact that this antenna structure behaves substantially like a λ / 4 resonator.

The bandwidth of an antenna is defined here as the frequency band over which the Standing Wave Ratio (ROS) is less than 2. This last parameter represents the ability of the antenna to transmit the active power which is which is most critical for small antennas.

This quantity is directly linked to the input impedance of the antenna, which must be adapted to the impedance of the transmission line carrying the microwave signal to be transmitted and / or received. For an optimal functioning of the antenna, it is necessary that this impedance remains appreciably constant (that is to say that the ROS remains lower than 2, a ROS equal to 1 corresponding to a perfect adaptation) on a large band of frequency. A bandwidth of 2 to 3% as obtained using an inverted F antenna is generally insufficient.

The invention particularly aims to overcome the drawbacks of the various known types of antenna, and in particular those of half-wave dipoles and antennas in inverted F.

More specifically, an objective of the invention is to provide a compact antenna having a large pass. Thus, the object of the invention is in particular to provide such an antenna, the bandwidth of which is at least of the order of 20 to 30% and having a reduced size, in particular compared to an antenna in inverted F.

The invention also aims to provide a self-symmetrized antenna, and therefore requiring no balun.

The invention also aims to provide such an antenna, which can operate over a wide range of input impedances, and in particular for input impedances between 10 and 200 Ohms.

• une plaque de substrat ;
• au moins une ligne d'alimentation située sur une première face de ladite plaque de substrat ;
• un dépôt conducteur situé sur une seconde face de ladite plaque de substrat de façon à définir :
  • une surface principale qui constitue un plan de masse pour ladite ligne d'alimentation ;
  • au moins un brin rayonnant possédant une première extrémité reliée à ladite surface principale et une seconde extrémité libre s'étendant au moins partiellement le long d'au moins un côté de ladite surface principale ;
  • au moins un espace longitudinal formant une fente de couplage entre chacun desdits brins rayonnants et ladite surface principale.

These objectives, as well as others which will appear subsequently, are achieved according to the invention using an antenna for transmitting and / or receiving microwave signals, comprising:

• a substrate plate;
• at least one supply line located on a first face of said substrate plate;
• a conductive deposit located on a second face of said substrate plate so as to define:
  • a main surface which constitutes a ground plane for said supply line;
  • at least one radiating strand having a first end connected to said main surface and a second free end extending at least partially along at least one side of said main surface;
  • at least one longitudinal space forming a coupling slot between each of said radiating strands and said main surface.

The antenna of the invention is therefore produced in printed technology, which allows a considerable saving of space and a much easier mechanical maintenance.

Furthermore, the main surface of the conductive deposit, by constituting a ground plane for the supply line, ensures that the supply is self-symmetrized. In other words, the antenna according to the invention does not require the joint use of a balun.

The supply line feeds the radiating strand through the coupling slot.

The antenna according to the invention is notably based on a new and inventive adaptation of the inverted F antenna. In fact, the two-dimensional configuration of the inverted F antenna was projected into a single plane containing the entire antenna. In other words, the radiating strand and the ground plane are no longer in two separate parallel planes, but in the same plane. Compared with the inverted F antenna, the antenna of the invention is therefore much more compact since the height h is overcome between the radiating strand (or horizontal conductive element) and the ground plane.

In addition, the antenna of the invention has a much wider bandwidth than that of an inverted F antenna. This is explained in particular by the fact that for the inverted F antenna, the radiating strand is located just above the ground plane and forms with it a cavity which is very selective in frequency (generally 2 to 3% bandwidth). On the other hand, in the case of the invention, the ground plane and the strand radiant are located in the same plane, so that the cavity effect is much less marked. This makes it possible to reach bandwidths close to 25%, and to simultaneously cover the transmission band and the reception band.

Advantageously, said supply line and said coupling slot intersect at a point called cross point, said supply line having an end portion, or series stub, extending beyond said cross point of a first adaptable length, and said coupling slot having an end portion, or parallel stub, extending beyond said crossing point of a second adaptable length.

Thus, it is possible to implement the known principle of the adaptation double stubs (series and parallel). A suitable choice of these series and parallel stubs, and possibly other parameters (width of the radiating strand, width of the coupling slot, thickness of the conductive deposition connecting part connecting the radiating strand to the main surface, position of the feed line relative to the conductive deposition link part) makes it possible to adapt the antenna over a wide bandwidth.

Preferably, at least one of the elements belonging to the group comprising said radiating strand, said main surface, and said coupling slot, is of substantially rectangular shape.

Advantageously, said conductive deposit comprises at least two radiating strands, the longitudinal space between each of said radiating strands and said main surface forming a separate coupling slot.

• une diversité de polarisation, en associant la ligne d'alimentation à un diviseur ;
• une polarisation circulaire, en associant la ligne d'alimentation à des diviseurs et des déphaseurs.

Thus, we can obtain:

• a diversity of polarization, by associating the supply line with a divider;
• circular polarization, by associating the supply line with dividers and phase shifters.

Advantageously, the antenna comprises at least two supply lines, each of said radiating strands cooperating with one of said supply lines.

In this way, a duplex multiband antenna can be obtained.

Preferably, said radiating strand has at least one bend, so that said radiating strand extends at least partially along at least two sides of said main surface.

In this way, the overall size of the antenna is limited since the minimum dimension of the antenna is no longer linked to the total length of the radiating strand but only to the length of the sides of the main surface of the conductive deposit.

Preferably, said radiating strand has a variable width. Thus, the bandwidth of the antenna is increased.

Advantageously, said radiating strand has at least one recess on at least one of the longitudinal edges and / or at least one lumen on its surface. The light on the surface of the radiating strand is for example a slit.

In an advantageous embodiment of the invention, the antenna also comprises a ground plane placed at a predetermined distance from said feed line.

If the ground plane is without radiating element, it makes it possible to remove the parasitic radiation from the supply line and to obtain radiation in a half-space only.

According to an advantageous variant, said ground plane is a conductive deposit of the same shape as that located on the second face of said substrate plate, comprising a main surface and at least one radiating strand.

In this case, the ground plane makes it possible to obtain symmetrical radiation on each side of the antenna.

Preferably, said supply line has an impedance substantially between 10 Ohms and 200 Ohms.

Advantageously, the length of said radiating strand is substantially between λ / 8 and λ / 4, λ being the wavelength of said microwave signals.

The invention also relates to a device for transmitting and / or receiving microwave signals, comprising at least one antenna as described above.

• les figures 1A et 1B présentent chacune une vue, respectivement de dessus et de côté, d'un premier mode de réalisation d'une antenne selon l'invention ;
• la figure 2 est une vue détaillée partielle de l'antenne présentée sur la figure 1A ;
• la figure 3 présente une courbe de variation, en fonction de la fréquence, du rapport d'onde stationnaire pour un exemple d'antenne selon l'invention ;
• la figure 4 est un diagramme de Smith présentant la courbe d'impédance correspondant à un exemple d'antenne selon l'invention ;
• les figures 5, 6 et 7 présentent chacune une vue de dessus d'un mode de réalisation distinct (second, troisième et quatrième respectivement) d'une antenne selon l'invention.

Other characteristics and advantages of the invention will appear on reading the following description of several preferred embodiments of the invention, given by way of non-limiting example, and the accompanying drawings, in which:

• FIGS. 1A and 1B each show a view, respectively from above and from the side, of a first embodiment of an antenna according to the invention;
• Figure 2 is a partial detailed view of the antenna shown in Figure 1A;
• FIG. 3 presents a variation curve, as a function of frequency, of the standing wave ratio for an example of antenna according to the invention;
• FIG. 4 is a Smith diagram showing the impedance curve corresponding to an example of an antenna according to the invention;
• Figures 5, 6 and 7 each show a top view of a separate embodiment (second, third and fourth respectively) of an antenna according to the invention.

The invention therefore relates to a reduced-size antenna with wide bandwidth. This antenna is in particular intended to equip portable devices, and for example transmitters / receivers of radiocommunication networks with land mobiles.

FIGS. 1A and 1B, which are respectively a top view and a side view, illustrate a first embodiment of the invention.

In this embodiment, the antenna comprises a substrate plate 1 (not shown in FIG. 1), a supply line 2 and a conductive deposit 3.

The substrate plate 1 is for example a low loss Duroid substrate of the Teflon glass type having a relative permittivity ε _r = 2.2 and a reduced thickness of 0.76 mm.

The supply line 2 is located on a first face (the lower face for example) of the substrate plate 1. It is for example a microstrip line.

The conductive deposit 3, for example of copper, is located on a second face (the upper face for example) of the substrate plate 1 and can decompose (fictitiously, since it is in practice made in one piece) into three parts: a main surface 4, an intermediate part 5 and a radiating strand 6.

The main surface 4 (rectangular in this example) of the conductive deposit 3 constitutes a ground plane for the supply line 2 situated on the other face of the substrate plate 1. The antenna therefore generates symmetrical currents on the radiating strand 6. In other words, the antenna of the The invention is self-symmetrized.

In this example, the radiating strand 6 is rectangular and has a first end connected to the main surface 4 of the conductive deposit 3 by the intermediate part 5, and a second free end extending partially along one side of the main surface. 4 from driver's depot 3.

The length of the radiating strand 6 is close to λ / 4, with λ the operating wavelength of the antenna.

Thus, the antenna of the invention, which is planar and whose maximum length is λ / 4, has a smaller footprint than that of a dipole of length λ / 2 or even that of an antenna in inverted F of length λ / 4 but whose radiating strand is spaced a height h from the ground plane.

The antenna of the invention has not only a very small footprint but also a large bandwidth. Indeed, the main surface 4 of the conductive deposit 3 behaves like a ground plane especially with respect to the supply line 2 and the coupling slot 7, and very little with respect to the radiating strand 6, which greatly reduces the selectivity of the antenna. In addition, the cavity effect (and therefore the selectivity of the antenna) is much less marked than for an inverted F antenna since the ground plane (that is to say the main surface 4 of the conductive deposit 3 ) and the radiating strand 6 are located in the same plane.

In general, the antenna according to the invention has a bandwidth of 20 to 30% and can be easily incorporated inside an ultra-light portable handset.

The longitudinal space between the radiating strand 6 and the main surface 4 of the conductive deposit 3 forms a coupling slot 7 by means of which the supply line 2 supplies the radiating strand 6.

In the example presented in FIG. 1A, the coupling slot 7 is also rectangular.

Figure 2 is a partial detailed view of the antenna shown in Figure 1A.

• la longueur l₁ d'un stub série, le stub série étant la portion d'extrémité de la ligne d'alimentation 2 qui dépasse du point de croisement 9 entre la ligne d'alimentation 2 et la fente de couplage 7 ;
• la longueur l₂ d'un stub parallèle, le stub parallèle étant la portion d'extrémité de la fente de couplage 7 qui dépasse du point de croisement 9 ;
• la largeur e₁ du brin rayonnant 6 ;
• la profondeur p de la fente de couplage 7 ;
• la largeur g de la fente de couplage 7 ;
• l'épaisseur e₂ de la partie intermédiaire 5 reliant le brin rayonnant 6 à la surface principale 4 ;
• la distance e_p entre la ligne d'alimentation 2 et la partie intermédiaire 5.

In order to fine-tune the antenna and adjust its bandwidth in particular, several parameters can be modified, and in particular:

• the length l₁ of a series stub, the series stub being the end portion of the supply line 2 which protrudes from the crossing point 9 between the supply line 2 and the coupling slot 7;
• the length l₂ of a parallel stub, the parallel stub being the end portion of the coupling slot 7 which protrudes from the crossing point 9;
• the width e₁ of the radiating strand 6;
• the depth p of the coupling slot 7;
• the width g of the coupling slot 7;
• the thickness e₂ of the intermediate part 5 connecting the radiating strand 6 to the main surface 4;
• the distance e _p between the supply line 2 and the intermediate part 5.

Thus, although produced in printed technology, the antenna of the invention comprises a serial stub and a parallel stub. These series and parallel stubs allow the adaptation of the antenna according to the known principle of double stub adaptation, over a wide frequency band.

FIG. 3 shows a variation curve, as a function of frequency, of the standing wave ratio (or ROS) for an example of antenna according to the first embodiment of FIGS. 1A and 2.

• l₁=13mm ;
• l₂ = 22,6 mm ;
• e₁ = 5 mm ;
• e₂ = 6 mm ;
• g = 5 mm ;
• e_p = 1,65 mm ;
• p = 24,25 mm.

In this example, the antenna parameters have the following values:

• l₁ = 13mm;
• l₂ = 22.6 mm;
• e₁ = 5 mm;
• e₂ = 6 mm;
• g = 5 mm;
• e _p = 1.65 mm;
• p = 24.25 mm.

This curve is used to calculate the passband [f1, f2], defined here as the frequency band for which the ROS remains below 2. This passband can also expressed as a percentage, obtained by dividing the width (f2, f1) of the passband by the center frequency f3 of this band.

In the above example, the bandwidth is substantially between f1 = 1.823 GHz and f2 = 2.333 GHz.

With a central frequency f3 = 2.078 GHz, this bandwidth is approximately equal to 25%. The antenna according to the invention therefore has a pass band wide enough to simultaneously cover the transmit band and the receive band.

Figure 4 shows a variation curve, in a Smith chart, of the input impedance for the previous antenna example. Note the presence of a loop around the center of the abacus (which is the perfect adaptation point compared to a 50 Ω supply line). This loop guarantees low frequency dispersion and translates the efficiency of the adaptation.

It should be noted however that the antenna is not, in this example, perfectly optimized. Indeed, better centering of the loop relative to the center of the Smith chart would increase the performance of the antenna.

In this example, the impedance of the supply line carrying the HF signal to be transmitted was fixed at 50 Ω, but this value does not constitute a determining characteristic, because the input impedance of the antenna according to the invention can take any value between 10 and 200 Ω.

Figure 5 shows a top view of a second embodiment of the antenna according to the invention. This second embodiment differs from the first in that the radiating strand 6 has a bend 51 and extends along two sides of the main surface 4 of the conductive deposit 3. Thus, the overall size of the antenna is further reduced. If the length of radiating strand 6 is equal to λ / 4, it is possible, by creating a bend 51 at mid-length, to reach dimensions close to λ / 8. It is clear that the elbow 51 is not necessarily in the center of the radiating strand 6, or that the radiating strand 6 may include more than one elbow, so as to extend along more than two sides of the surface. main 4.

FIG. 6 presents a top view of a third embodiment of the antenna according to the invention. This third embodiment differs from the first in that the radiating strand 6 has a variable width over its length. This variable width, when chosen appropriately, makes it possible to increase the bandwidth of the antenna. In the example shown in Figure 6, the radiating strand 6 has a recess 61, 62 on each of its longitudinal edges. It should be noted that in other embodiments, the radiating strand 6 may have a slit in the middle, or have several recesses on each of its longitudinal edges, or even have one or more recesses on only one of its longitudinal edges .

Figure 7 shows a top view of a fourth embodiment of the antenna according to the invention. In this fourth embodiment, the antenna comprises several radiating strands 6 _A , 6 _B , 6 _C , 6 _D (four in this example). Each radiating strand 6 _A , 6 _B , 6 _C , 6 _D is connected to the main surface 4 by an intermediate part 5 _A , 5 _B , 5 _C , 5 _D , and each longitudinal space comprised between a radiating strand 6 _A , 6 _B , 6 _C , 6 _D and the main surface 4 forms a separate coupling slot 6 _A , 6 _B , 6 _C , 6 _D.

Depending on the applications, the radiating strands 6 _A , 6 _B , 6 _C , 6 _D may or may not be identical.

Likewise, a single supply line can supply all the radiating strands 6 _A , 6 _B , 6 _C , 6 _D , or else several supply lines can be used. Thus, by increasing the number of feed lines and by associating each of the radiating strands with a separate feed line, it is possible to obtain a duplex multiband antenna.

In the example presented in FIG. 7, the antenna comprises means 71 for shaping the HF signals received from a main supply line (not shown) and to be transmitted on the various secondary supply lines 2 _A , 2 _B , 2 _C , 2 _D associated with the different radiating strands 6 _A , 6 _B , 6 _C , 6 _D.

• soit de la diversité de polarisation linéaire, si les moyens 71 comprennent un diviseur ;
• soit de la polarisation circulaire, si les moyens 71 comprennent des diviseurs et des déphaseurs.

These means 71 make it possible to obtain:

• either of the linear polarization diversity, if the means 71 comprise a divider;
• or circular polarization, if the means 71 comprise dividers and phase shifters.

The elements (dividers, phase shifters) constituting the means 71 for shaping the signals can be produced by different lengths of supply lines, by hybrid rings, or by any other solution known to those skilled in the art and realizing the desired function.

It is clear that many other embodiments of the invention can be envisaged. The antenna may for example comprise another ground plane, placed at a predetermined distance from the supply line and separated from the latter by air or by a dielectric. In the latter case, the antenna comprises the following successive layers: a ground plane, a dielectric, a supply line, a substrate plate and a conductive deposit. The role of the additional ground plane is for example to suppress stray radiation from the supply line and to obtain radiation in only half a space.

It is also possible to provide that the additional ground plane is produced in the form of a conductive deposit also comprising a main surface and a radiating strand associated with a slot. In this case, symmetrical radiation is obtained on each side of the antenna.

The characteristics of the various embodiments presented above can also be combined in multiple ways, in order to provide still other embodiments of the antenna of the invention. Thus, by way of example, a radiating strand may have a variable width and extend on two sides of the main surface of the conductive deposit.

The invention also relates to any device for transmitting and / or receiving microwave signals equipped with an antenna according to the invention. Optionally, such a device can comprise several antennas, and in particular a transmitting antenna and a receiving antenna.

Claims (13)

Antenna for transmitting and / or receiving microwave signals, characterized in that it comprises: - a substrate plate (1); - at least one supply line (2; 2 _A to 2 _D ) located on a first face of said substrate plate; - a conductive deposit (3) deposited on a second face of said substrate plate (1) so as to define: - a main surface (4) forming a ground plane for said supply line (2; 2 _A to 2 _D ); - at least one radiating strand (6; 6 _A to 6 _D ) comprising a first end connected to said main surface (4) and a second free end extending at least partially along at least one side of said main surface (4); - At least one longitudinal space forming a coupling slot (7; 7 _A to 7 _D ) between each of said radiating strands (6; 6 _A to 6 _D ) and said main surface (4). Antenna according to claim 1, characterized in that said feed line (2; 2 _A to 2 _D ) and said coupling slot (7; 7 _A to 7 _D ) intersect at a point called crossing point, in that said supply line (2; 2 _A to 2 _D ) has an end portion, or series stub, extending beyond said crossing point of a first adaptable length, and in that said coupling slot (7; 7 _A to 7 _D ) has an end portion, or parallel stub, extending beyond said crossing point of a second adaptable length. An antenna according to any one of claims 1 and 2, characterized in that at least one of the elements belonging to the group comprising said radiating strand (6; 6 _A to 6 _D ), said main surface (4), and said slot coupling (7; 7 _A to 7 _D ), is of substantially rectangular shape. Antenna according to any one of claims 1 to 3, characterized in that said conductive deposit (3) comprises at least two radiating strands (6 _A to 6 _D ), the longitudinal space comprised between each of said radiating strands and said main surface forming a separate coupling slot (7 _A to 7 _D ). Antenna according to claim 4, characterized in that it comprises at least two supply lines (2 _A to 2 _D ), each of said radiating strands (6 _A to 6 _D ) cooperating with one of said supply lines. Antenna according to any one of claims 1 to 5, characterized in that said radiating strand (6; 6 _A to 6 _D ) has at least one bend (51), so that said radiating strand extends at least partially the along at least two sides of said main surface (41). Antenna according to any one of claims 1 to 6, characterized in that said radiating strand (6; 6 _A to 6 _D ) has a variable width. Antenna according to claim 7, characterized in that said radiating strand (6; 6 _A to 6 _D ) has at least one recess (61, 62) on at least one of the longitudinal edges and / or at least one light on its surface. Antenna according to any one of claims 1 to 8, characterized in that it also comprises a ground plane placed at a predetermined distance from said feed line. Antenna according to claim 9, characterized in that said ground plane is a conductive deposit of the same shape as that located on the second face of said substrate plate, comprising a main surface and at least one radiating strand. An antenna according to any one of claims 1 to 10, characterized in that said supply line (2; 2 _A to 2 _D ) has an impedance substantially between 10 Ohms and 200 Ohms. Antenna according to any one of claims 1 to 11, characterized in that the length of said radiating strand (6; 6 _A to 6 _D ) is substantially between λ / 8 and λ / 4, λ being the wavelength of said microwave signals. Device for transmitting and / or receiving microwave signals, characterized in that it comprises at least one antenna according to any one of claims 1 to 12.

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