FR2699740A1 - Broadband antenna with reduced space, and corresponding transmission and / or reception device. - Google Patents

Abstract

The invention relates to a broadband antenna with reduced bulk, in particular for portable stand-alone stations used in radiocommunication networks with land mobiles, comprising a substantially flat element (33), called a horizontal element, and a short-circuit element ( 35) substantially perpendicular to said horizontal element (33), said vertical element (35), said vertical element connecting a first end (34) of said horizontal element (33) to the electrical ground of a processing unit, said microwave signals being conveyed between said processing unit and said horizontal element (33) by a coaxial cable (311) connected to said horizontal element (33), said horizontal element (33) comprising over at least part of its length at least two strands (37, 38 ) substantially parallel to each other and substantially perpendicular to said vertical element (35).

Description

Broadband antenna with reduced space requirement and emission device and / or

corresponding reception.

  The field of the invention is that of radio transmissions More specifically, the invention relates to transmitting and / or receiving antennas, especially for small equipment, such as portable devices.

  The invention thus applies, in particular, to telecommunication systems

  with mobile In fact, the extension of radio networks with land mobiles requires the development of portable autonomous stations

  having the dual functionality of transmitting and receiving hyperfre-

  These stations should therefore include an integrated antenna.

  The frequencies currently used for these applications (of the order of 2 GHz), as well as various constraints related to the ergonomics and aesthetics of the handsets (integration of the antenna in the drawings of the device, ease of storage and use, fragility of large antennas,) lead to the use of antennas of very small dimensions Several types of antennas are known, whose dimensions are smaller than the wavelength

of the microwave signal.

  These antennas are generally in the form of a radiating element implanted outside a metal housing, for example of parallelepipedal shape, constituting the shielding of one or more electronic cards including the functions of modulation and demodulation signals

  microwaves, in transmission and reception respectively.

  The book "Small Antennas" by K Fujimoto, A. Henderson, K Hirasawa and J R James (published by Research Studies Press Ltd and John Wiley & Sons Inc.) presents various types of small antennas, among

  which the so-called inverted F antenna seems the most effective.

  Such an inverted F antenna is shown in section in FIG.

  in perspective in FIG. 2 It consists of a rectangular conductive element

  horizontal line 11 and a vertical conductive element 12 The vertical element 12 provides a short-circuit function on the horizontal element 11, connecting one of

  its ends 13 to a ground plane 14.

  The other end 15 of the horizontal element 11 is open.

  The microwave signal is conveyed by an excitation coaxial 16, which is connected to the horizontal element 11 at a location 17 The choice of this location 17 between the short-circuited end 13 and the open end 15 of

  the horizontal element 11 determines the impedance of the antenna thus obtained.

  If) is the wavelength of the transmitted microwave signal, the clutter

  This type of antenna is usually:

L = X / 4;

1 =) / 8;

h = X / 25.

  Thus, for frequencies of the order of 2 G Hz, these dimensions are of the order of a few centimeters. The antenna obtained is therefore very compact.

  In addition, the radiation pattern of this antenna is sensitive

  omnidirectional, which is essential for portable devices (this

  This characteristic is generally verified by all low-frequency antennas.

  is lying). On the other hand, this antenna has characteristics very dispersive in frequency, 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

  substantially comprises an X / 4 resonator.

  The bandwidth of an antenna is here defined as the frequency band on which the ROS is less than 2 This last parameter represents the ability of the antenna to transmit the active power supplied to it. , which is the most critical for small antennas. This quantity is directly related 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.

  the antenna, it is necessary for this impedance to remain substantially constant (that is,

  that is, the R O S remains below 2) over a large frequency band A bandwidth of 2 to 3% as obtained using an inverted F antenna

is usually insufficient.

  The invention particularly aims to overcome this drawback of the prior art. More precisely, an object of the invention is to provide a reduced-size antenna having a wide bandwidth. Thus, one particular object of the invention is to provide such an antenna whose bandwidth is

at least on the order of 8 to 10%.

  Another object of the invention is to provide such an antenna, which is a reduced cost In other words, the invention aims to provide such an antenna that is easy to achieve, and which n ' does not use expensive material. Another objective of the invention is 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.

  Another object of the invention is to provide such an antenna, the tuning frequency of which can be precisely adjusted. In particular, an object of the invention is to provide such an antenna, whose tuning frequency can be

  permanently and rapidly modified, for example to

alternately.

  These objectives, as well as others which will appear subsequently, are achieved according to the invention with the aid of an antenna for transmitting and / or receiving microwave signals, comprising a substantially plane element, called a horizontal element, and a short-circuit element substantially perpendicular to said horizontal element, said vertical element, said vertical element connecting a first end of said horizontal element to the electrical ground of a processing unit, said microwave signals being conveyed between said processing unit and said horizontal element by a coaxial cable connected to said horizontal element, said horizontal element comprising on at least a part of its length at least two strands substantially parallel to each other and substantially perpendicular

to said vertical element.

  In this way, the useful volume of the antenna is increased, compared to the known antenna F inverted It results in an increase in the bandwidth In contrast, the overall size of the antenna is not changed. Moreover, such an antenna has a radiation pattern of omnidirectional type, which is essential, because it is intended in particular to equip portable devices, can therefore take all positions The terms "horizontal" and "vertical" are therefore only used to simplify the understanding of the invention, and should not be interpreted strictly In practice, the notions of horizontality and verticality will often be defined by

  to a ground plane on which the antenna will be fixed.

  Such an antenna may comprise two parallel strands The principle of

  the invention can also be generalized to more than two strands.

  Advantageously, said strands of the antenna are of sensitive shape.

  rectangular, and have a substantially identical width but different lengths Other geometric characteristics may also

  to be retained, depending on the characteristics desired for the antenna.

  Preferably, said horizontal element comprises a substantially rectangular intermediate surface whose first end corresponds to said first end of said horizontal element and whose second end

  corresponds to a first end of each of said strands.

  According to a first advantageous embodiment of the invention, each of said strands is open at its end farthest from said first

  end of said horizontal element.

  In this case, each of said strands is a resonant element.

  If two strands have slightly different lengths, their resonance frequencies are different, but close The coupling of the two strands then makes it possible to obtain, for the impedance, a resonance loop, and therefore, in

  Consequently, a large bandwidth, for example of the order of 10%.

  According to a second preferred embodiment, the end furthest from said first end of said horizontal element of at least one of said strands is connected to the electrical ground of said processing unit, by

  via an additional short circuit element.

  The shorted wire at both ends (or stub short circuit) plays the role of adaptation circuit Again, the combination of the antenna strands provides a resonance loop, which leads to a bandwidth of

the order of 10% for example.

  If the antenna comprises two strands, the strand short-circuited at its two ends may be, depending on the needs and characteristics desired, the longest strand or the shortest strand. If necessary, the two strands may be

the same length.

  If the antenna comprises more than two strands, it is possible to combine the advantages of the first and second embodiments One or more strands

  can then be short-circuited at both ends.

  According to a third advantageous embodiment of the invention, the

  most distant from said first end of said horizontal element of at least one of said strands is connected to the electrical ground of said processing unit,

  through a capacity.

  This operating mode corresponds to an intermediate position between the first embodiment (open circuit) and the second embodiment (short circuit). Preferably, all the strands of the antenna are connected to the electrical ground of said processing unit, via a capacitor.

  at the point of the antenna is thus facilitated.

  Advantageously, at least one of these capacities is an adjustable capacity

(or varactor).

  This allows, under the control of the processing unit, to vary the capacities between two distinct values, a first value corresponding

  operation of the antenna at a frequency of transmission of hyperfrequency signals.

  quences and a second value corresponding to the operation of the antenna at a

  frequency of reception of microwave signals.

  Thus, the same physical antenna can operate alternately in a transmission band and in a reception band, for example to operate in half-duplex This is how the economy of the implementation of a second antenna This technique can of course be generalized to more than two frequency bands. Advantageously, the coaxial cable carrying the microwave signals has an impedance substantially between 10 ohms and 200 ohms. In other words, the input impedance of the antenna can be chosen between 10 and 200.

  Ohms Classically, this impedance can be equal to 50 Ohms.

  Preferably, if A is the wavelength of said microwave signals, the essential dimensions of the antenna of the invention are: width of said first end of said horizontal element: of the order of A / 8; maximum length of said horizontal element, corresponding to the distance between said first end of said horizontal element and the end furthest from said first end of said horizontal element of the longest strand: of the order of / 4; height of said vertical element, corresponding to the distance between said first end of said horizontal element and said electrical mass: of the order of 2/25;

  width of at least one of said strands: of the order of 2/20.

  Preferably, this wavelength A of said hyperfrequency signals

  quences is between 100 and 200 mm In this case, the dimensions of

  the antenna are very small, of the order of a few centimeters.

  In a preferred embodiment of the invention, the antenna is implanted on a housing containing said treatment unit, said mass

  corresponding to the electromagnetic shielding of said housing.

  Advantageously, said vertical element and said horizontal element are formed in the same strip of a conductive material.

  the whole is particularly simple.

  Other features and advantages of the invention will become apparent

  reading the following description of several preferred embodiments of

  the invention, given by way of illustrative and non-limiting examples, and the accompanying drawings, in which: FIGS. 1 and 2 show an antenna of the prior art according to the invention referred to as an inverted F, respectively in section and in perspective These figures have already been discussed in the preamble of the

present description;

  FIG. 3 shows a first embodiment of an antenna according to the invention, comprising two resonant strands open at one of their ends; Figures 4 to 6 are three Smith diagrams respectively showing the frequency response corresponding to the first strand of the antenna of Figure 3, the second strand and the combination of the two strands; FIG. 7 illustrates a second embodiment of an antenna

  according to the invention, comprising a resonant strand and a short strand

  circled at both ends; Figures 8 and 10 are three Smith diagrams respectively showing the impedance curves corresponding to the first strand of the antenna of Figure 7, the second strand and the combination of the two strands; FIG. 11 presents a third embodiment of an antenna according to the invention, whose impedance of the two strands is

adjustable;

  FIG. 12 is a top view on a scale of one embodiment of an antenna according to the invention, in the case of an operating frequency of 2.5 G Hz;

  FIGS. 13 and 14 respectively show the adaptation curve

  and the impedance curve corresponding to the antenna of the

figure 12.

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

land mobiles.

  In general, the antenna of the invention comprises a horizontal element (with respect to a ground plane), connected at one of its ends to the ground by a vertical short circuit. The main characteristic of the The invention is to provide, for example by cutting, at least two substantially parallel antenna strands in the horizontal element. The geometric and connection characteristics of these strands are chosen so as to obtain for the antenna

  desired characteristics, such as large bandwidth.

  Subsequently, three preferred embodiments of the invention, comprising two antenna strands, are described in detail. However, it is clear that the antenna according to the invention may comprise more than two strands, by simple generalization of the two antenna strands.

examples described.

  FIG. 3 thus illustrates a first embodiment of the invention. In this figure, the antenna 31 (hatched) is implanted on a box 32, capable of containing electronic cards (in particular for the demodulation and / or modulation of the signals Microwave frequencies received and / or transmitted by the antenna) The dimensions and the shape of this housing 32 are of course purely indicative in

  the example shown, the base b of the housing is 60 mm, and its height hi is 150 mm.

  This housing 32 is shielded, and constitutes the mass to which is connected

the antenna 31.

  The antenna 31 comprises a horizontal element 33, one of the ends 34 of which is connected to ground (shielding of the housing 32) by a vertical element of

short circuit 35.

  The horizontal element 33 can decompose (fictitiously, since it is practically made in one piece) in three parts: an intermediate portion 36, or base, which is connected to the vertical element 35 and which is substantially the same width as this vertical element; a first strand 37 extending in the extension of the base 36, the outer edge of this strand 37 corresponding to a first edge of the base 36; a second strand 38, parallel to the first strand 37, whose outer edge corresponds to the extension of the second edge of the base

36.

  Subsequently, the first end of a strand will be referred to as the end connected to the base 36, and at the second end of one strand the opposite end, that is to say, in other words, the end farthest from the first end 34 of the horizontal element 33 Optionally, the base 36 can be removed, the

  strands 37 and 38 then being directly connected to the vertical element 35.

  In practice, the horizontal element 33 is obtained by cutting in a rectangular surface a space 39 between the two strands 37 and 38, until the

base 36.

  A second cutting of a surface 310 is then performed on the strand

  the shortest 38, to adapt its length.

  Moreover, the vertical element 35 and the horizontal element 33 may be formed in the same material, the angle of the end 34 being made for example

by folding.

  Advantageously, the vertical portion 35 can extend along the housing

  32, and be attached to this housing by any suitable means of attachment (not shown).

  The microwave signals are conveyed by an excitation coaxial 311, which connects the electronic card contained in the housing 32 and the horizontal element 33 The location of the connection 312 between the vertical element 35 and the seconds

  ends 313 and 314 of the two strands 37 and 38 defines the impedance of the antenna.

  This connection 312 may be on the base 36 or on one of the strands 37 or 38

  its position, the impedance can for example vary between 10 and 200 Ohms.

  Preferably, for an operating wavelength A (of the order of a few cm), the dimensions of the antenna are substantially: length of the longest strand 37: 11 = A / 4; length of the second strand 38:12 slightly less than X / 4 (for example: 91/40); height of the vertical element 35: h => / 24 width of the base 36 and of the vertical element 35: 1 = 1/8 length of the base 36: S = X / 30; distance between the connection 312 and the vertical element 35: d = / 24;

  width of the strands 37 and 38: w1 = W 2 = X / 20.

  In other embodiments, the strands may have different widths, ends of various shapes, In the embodiment of Figure 3, the two strands 37 and 38 have their second ends 313 and 314 open In this case, the strand 37 of length> / 4, resonates at the working frequency f (corresponding to the wavelength A) The second strand 38 is also a resonant element, but at a frequency f ', different but close to the

  In other words, this first embodiment is based on the introductory

  multiple resonance frequencies in the antenna Of course, more than two

strands can be used.

  The length of these strands (and therefore their resonant frequency) is chosen so as to obtain, for the impedance of the antenna, a resonance loop, and therefore to widen the corresponding bandwidth. This is illustrated in particular by

Figures 4 and 6.

  FIG. 4 shows the Smith diagram bearing the impedance curve 41 of the strand 37 (resonant at Q). The bandwidth corresponding to this strand 37 alone, that is to say the frequency band on which the ROS is less than 2, is defined by the frequencies f 1 and f 2 corresponding to the intersections of the impedance curve

  41 with the hatched disc 42 defining the area where the ROS is less than 2.

  This bandwidth is written (f 2 f 1) / fr 'and is conventionally between 2 and 4%. As already mentioned, such a bandwidth is insufficient.

in many applications.

  The element 38 behaves similarly, but at the frequency f'r its impedance curve 51 is illustrated in FIG. 5 The corresponding bandwidth (f 4 f 3) / fr is also worth approximately 2 to 4% However, the frequency band lf 3,

  4 l is substantially offset from the frequency band 1, f 2 j.

  The coupling of the two radiating strands makes it possible to obtain a resonance loop, if the frequencies f 1 and f'r are well chosen, as is illustrated in FIG. 6. The impedance curve 61 corresponding to the combination of the strands 37 and 38 has a resonance loop 62, centered on f O This loop 62 remains in

  the disk 63 defining the zone in which the ROS is less than 2.

  This gives a very wide bandwidth (f, f 4) / f 0, equal to

example 10%.

  FIG. 7 shows a second embodiment of the invention, in which one of the antenna strands is short-circuited at both ends. The general structure of this antenna is similar to that of FIG. The shape of the elements 33 and vertical 35 is not described again. The fundamental difference with the first embodiment is that the strand 72 is no longer open at its second end 313, but short-circuited by a vertical short-circuit element 71 connecting this end 313 to the shield of the

case 32.

  This strand 72 no longer plays the role of resonant element, but the role of a "stub" (or section) short circuit, which plays the role of adaptation circuit, to expand the band on which the The overall input impedance of the antenna remains close to the excitation coaxial impedance. If necessary, several stubs can be realized. Similarly, if the antenna comprises at least three strands, the

  Embodiments of Figures 3 and 7 may be combined.

  FIG. 8 shows the Smith diagram bearing the impedance curve 81 corresponding to the resonant strand 38. The corresponding bandwidth (f 2 -f 1) / fo

  is always around 2 to 4%.

  The Smith diagram of FIG. 9 for its part presents the impedance 91 of the "stub" short-circuit 72. This curve 91 is substantially symmetrical with the curve 81 of FIG. 8. The combination of the two strands 37 and 38 makes it possible to obtain the impedance curve 101 of Figure 10 This curve 101 has a resonance loop 102 which remains in the disk 103 of ROS less than 2 As a result, the resulting bandwidth (f 4 f 3) / f O is again enlarged, and for example

the order of 10%.

  It should be noted that, from the point of view of the bandwidth, we obtain

  similar results with the first two embodiments described.

  FIG. 11 shows a third embodiment of the invention; it is in fact a generalization of the antenna of FIGS. 3 and 7, in which the second ends of the strands are neither open nor short-circuited, but connected

  to the masses using abilities.

  Thus, the antenna 111 comprises a first strand 112, connected to the ground 113

  by a capacitor 114, and a second strand 115 connected to the ground by a capacitor 116.

  These capacities 114 and 116 make it possible to vary the equivalent length of the strands (which is therefore no longer fixed at 4). This allows a fine tuning of the tuning frequency. In this embodiment, the antenna strands can have the same physical length, the equivalent length being modified by the capacitors. It should be noted, moreover, that it is not mandatory that all the strands are associated with

  some of them can be opened or shortcircuited.

  Preferably, at least some of the capacitors 114 and 116 are adjustable (they are, for example, varactors, or several parallel capacitors capable of being selected independently), and controlled (118) by an electronic control circuit 117 placed in the housing 32 it is thus possible to vary at any instant and almost instantaneously the bandwidth of the antenna 111 This makes it possible to operate the same physical antenna in several

  frequency bands, selectively.

  For example, this antenna 111 allows operation in alternate mode in a transmission band (corresponding to a transmission frequency) and in a reception band (corresponding to a reception frequency).

  equipped with this antenna can therefore operate in "half duplex" '.

  As a detailed example, the geometric description is given in particular.

  The embodiment of an antenna according to the invention This embodiment corresponds to the best mode currently developed However, as will be seen later, the results are still perfectible Figure 12 shows, in top view,

  the horizontal element of an antenna as illustrated in Figure 7.

  This embodiment aims to operate in the band

  nominal frequency 2.4 GHz 2.5 G Hz.

  The dimensions used for the horizontal element are: width: 1 = 15 mm; length of the longest strand 122: 1 = 30 mm; length of shortest strand 123: 12 = 27 mm; width of the base 124: S = 4 mm;

  width of the strands 122 and 123: w 1 = W 2 = 6 mm.

  The height of the vertical element is: h = 5 mm.

  The connection location 121 of the coaxial cable is remote from

  d = 5 mm from the vertical element, to obtain an input impedance of 50 Ohms.

  This impedance can be modified between 10 and 200 Ohms, modifying

this distance d.

  In this embodiment, the longer strand 122 is open at its second end 125, and the shortest strand 123 is bypassed at its second end.

end 126.

  FIG. 13 shows the curve 131 of adaptation of this antenna, that is to say

  say the curve of the R O S (ordinate) as a function of the frequency (on the abscissa).

  As shown by the markers 132 and 133, the ROS is less than 2 between 2.37 GHz and 2.55 GHz. This corresponds to a bandwidth of the order of 8%, which is much greater than the bandwidths. obtained with the antennas of

the prior art.

  Moreover, on the working band, that is to say between 2.4 GHz (marker 134) and 2.5 G Hz (marker 135), the R O S is less than 1.6. The Smith diagram of FIG. 14 shows the impedance curve 141 of the antenna of FIG. 12, between 2 GHz and 3 GHz. The markers 142 and 143

  delimit the working area of the antenna (2.4 2.5 GHz).

  This curve shows that this antenna is not yet fully optimized, and that a better centering of the curve 141 with respect to the abacus

  would bring better performance. The invention also relates to any transmitting and / or receiving apparatus

  microwave signals equipped with an antenna according to the invention, as illustrated for example by the housing 32 of Figures 3, 7 and 11 Possibly, such an apparatus may comprise several antennas, and in particular an antenna

  transmission and reception antenna.

Claims (19)

  An antenna for transmitting and / or receiving microwave signals, comprising a substantially plane element (33), said horizontal element, and a short-circuit element (35) substantially perpendicular to said horizontal element (33), said vertical element, said vertical element (35) connecting a first end (34) of said horizontal element (33) to the electrical ground of a processing unit, said microwave signals being conveyed between said treatment unit and said horizontal element (33) by a cable coaxial connector (311) connected to said horizontal element (33), characterized in that said horizontal element (33) comprises on at least a part of its length at least two strands (37,38; 72; 112,115; 122,123) substantially   parallel to each other and substantially perpendicular to said vertical element.   Antenna according to claim 1, characterized in that said strands   (37,38) are of substantially rectangular shape.   Antenna according to one of Claims 1 and 2, characterized in that   what said strands (37,38) have different lengths.   Antenna according to one of Claims 1 to 3, characterized in that   said strands (37,38) have substantially identical widths.   Antenna according to one of Claims 1 to 4, characterized   said horizontal element (33) comprises a substantially rectangular intermediate surface (36) whose first end corresponds to said first end (34) of said horizontal element (33) and whose second end   corresponds to a first end (313, 314) of each of said strands (37, 38).   Antenna according to one of Claims 1 to 5, characterized   each of said strands (37,38) is open at its end (313, 314) the most   away from said first end (34) of said horizontal member (33).   Antenna according to one of Claims 1 to 5, characterized   that the end (313) farthest from said first end (34) of said horizontal member (33) of at least one of said strands (72) is connected to the electrical ground of said processing unit, via an element (71) of additional short circuit.   Antenna according to one of claims 1 to 5, characterized   the end farthest from said first end of said horizontal element of at least one of said strands (112, 115) is connected to the electrical ground of   said processing unit, via a capacitor (114, 116).   9 Antenna according to claim 8, characterized in that the end farthest from said first end of said horizontal element of each of said strands (112, 115) is connected to the electrical ground of said processing unit, by   through a capacity (114,116).   Antenna according to one of Claims 8 and 9, characterized in   said capacity (114,116) is adjustable, and in that said processing unit comprises means (117) for controlling the value of said capacity adjustable.   11 Antenna according to claim 10, characterized in that said adjustable capacitance (114, 116) can take at least two distinct values, a first value corresponding to the operation of said antenna at a microwave frequency transmission frequency and a second value corresponding to the operation of said antenna at a frequency of reception of microwave signals.   Antenna according to one of claims 1 to 11, characterized   said coaxial cable (311) has a substantially between 10 Ohms to 200 Ohms.   Antenna according to claim 12, characterized in that said impedance is substantially equal to 50 Ohms.   Antenna according to one of Claims 1 to 13, characterized in that   the width (1) of said first end of said horizontal element (33) is   substantially equal to 1/8, where A is the wavelength of said microwave signals these.   Antenna according to one of Claims 1 to 14, characterized   the maximum length (11) of said horizontal member (33), corresponding to the distance between said first end (34) of said horizontal member (33) and the end (313) furthest from said first end (34) of said horizontal element (33) of the longest strand (37) is substantially equal to A / 4, where A is the   wavelength of said microwave signals.   Antenna according to one of claims 1 to 15, characterized   the height (h) of said vertical element (35) corresponding to the distance between said first end (34) of said horizontal element (33) and said electrical mass is substantially equal to X / 25, A being the wavelength said microwave signals.   Antenna according to one of Claims 1 to 16, characterized   the width (w1, W2) of at least one of said strands (37, 38) is substantially   equal to 2/20, where A is the wavelength of said microwave signals.   Antenna according to one of Claims 1 to 17, characterized   that the wavelength λ of said microwave signals is between 100 and 200 mmnr.   Antenna according to one of Claims 1 to 18, characterized   it includes two strands (37,38).   Antenna according to one of Claims 1 to 19, characterized   it is implanted on a housing (32) containing said processing unit, said electrical mass corresponding to the electromagnetic shielding of said housing (32).   Antenna according to claim 20, characterized in that at least a part of said vertical element (35) and said horizontal element (33) are formed in   the same band of a conductive material.   Device for transmitting and / or receiving microwave signals, characterized in that it comprises at least one antenna (31, 111) according to one of   any of claims 1 to 21.