FR2784236A1 - FREQUENCY SWITCHED ANTENNA - Google Patents

Abstract

The invention relates to a selective frequency-switching antenna. It is an antenna with radiating slots (20) etched in a conductive surface (2) connected to ground, the radiating slots (20) having a perimeter (21) corresponding to a given radio length, the antenna comprising non-linear electronic circuits (70), an electronic circuit (70) being associated with each radiating slot (20) and comprising at least one electronic device (70a) with two states, a blocked state which does not modify the radio length of the slot (20), and a passing state which decreases the radio length of the slot (20) by shortening the perimeter (21) of the slot (20) by short circuit made between two points (74 and 75) of said perimeter (21), the electronic circuits (70) being synchronized so that the radiating slots (20) have substantially the same radio length between them. The antenna can be ap applied to an antenna array using several frequencies for which the antenna radiation patterns must remain substantially stable.

Description

The invention relates to the field of radiant slot antennas. A

  radiating slot antenna is a resonant antenna therefore highly selective in frequency. The frequency bandwidth of such an antenna is generally low. Bandwidth is the frequency range over which the operation of the antenna remains

  sufficiently stable, both in radiation pattern and in adaptation.

  In certain applications, such as the IFF ("identification friend or foe" which means "identification friend or foe"), it is necessary to be able to radiate the antenna according to two frequency bands close, but distinct. It is then possible either to use two very frequency-selective antennas each operating in a frequency band, or a single antenna with a wider passband encompassing the two preceding bands. These solutions can sometimes come up against congestion considerations, especially in the case of antennas

  on board a weapon plane, for example.

  It is known to reduce the size of the antenna by folding its slots. However, the forms of slits obtained generally reveal an often undesirable cross polarization, as well as possible parasitic modes of radiation, and then reduce the band.

bandwidth of the antenna.

  The invention proposes an antenna capable of operating at several distinct frequencies by switching, while retaining, according to a given rectilinear polarization, a similar given radiation pattern for each of the frequencies, while maintaining a level of

weak cross polarization.

  The developed length of a radiating slit is the sum of all the median lengths of each of the folded elements of the slit if it has several, otherwise the developed length merges with the median length of the slit. The finer the slit, i.e. the greater the ratio between its two dimensions, the greater the value of its

  developed length merges with that of its half-perimeter.

  According to the invention, there is provided an antenna with radiating slots etched in a conductive surface connected to ground, the radiating slots having a perimeter corresponding to a given radio length, characterized in that the antenna comprises non-linear electronic circuits, an electronic circuit being associated with each radiating slit and comprising at least one electronic device with two states, a blocked state which does not modify the radioelectric length of the slit, and a passing state which decreases the radioelectric length of the slit by shortening the perimeter of the slot by short-circuit produced between two points of said perimeter, in that the electronic circuits are synchronized so that the radiating slots switch from one state to another 10 at substantially the same time, and in that the decreases in radio lengths slots are small enough that the

  antenna radiation patterns remain substantially stable.

  According to a preferred embodiment of the invention, the electronic circuits are synchronized so that the radiating slots have substantially the same radio length between them, and in that the decrease in radio length is sufficiently low that the radiation pattern of the 'antenna remains substantially stable. Preferably, the radiating slots of the antenna are excited by coupling with a microstrip line which remains unchanged

  whatever the blocked or passing state of the electronic devices.

  Advantageously, the radiating slots being aligned in a first direction, each radiating slot is T-shaped. This form of radiating slot makes it possible to maintain a low cross-polarization. The invention will be better understood and other features and

  advantages will become apparent from the following description and the drawings

  seals, given as examples, o: - Figure 1 schematically shows a sectional view of an embodiment of an antenna according to the invention; - Figure 2A schematically shows a slot and the associated electronic circuit, of an embodiment of an antenna according to the invention; - Figures 2B and 2C schematically represent two states of an embodiment of an electronic device according to the invention, respectively the blocked state and the on state; - Figure 2D schematically represents a preferred embodiment of an electronic device according to the invention; - Figure 3 schematically shows a top view of an embodiment of an antenna according to the invention; - Figure 4 schematically represents the radiation diagrams of an embodiment of an antenna according to the invention, at two distinct frequencies; - Figure 5 shows a variant of an electronic circuit of

antenna according to the invention.

  FIGS. 1 to 5 describe the preferred embodiment of the invention in which the electronic circuits are synchronized so that the radiating slots have substantially the same between them

radio length.

  Figure 1 shows schematically a general structure

  preferential of an antenna according to the invention. The antenna is seen in profile.

  The antenna sequentially comprises, from the outside to the inside of the antenna, a radome 1, a conductive surface 2 connected to ground, a dielectric substrate 3, a conductive cavity 5, preferably metallic, also connected to ground. . Radiating slots 20 are etched on the conductive surface 2. In the cavity 5, on the face of the substrate 3, on the side opposite that of the conductive surface 2, is deposited a supply line 4 which is advantageously a microstrip line. This microstrip line 4 is preferably deposited in the form of a printed circuit. The electronic circuits, not shown here, associated with the radiating slots of the antenna, include electronic devices. The electronic devices are advantageously mounted on the surface of another printed circuit comprising the electronic circuits and etched on the same face of the substrate 3 as the line 4, but independently of the line 4. The antenna then comprises only three elements mechanically distinct which are the radome 1, the substrate 3 with on one of its faces the conductive surface 2 and on the other face a printed circuit comprising the

  electronic circuits and supply line 4, and cavity 5.

  In a variant of the invention using a waveguide structure with radiating slots for example, the slots can be machined in the mass of the metal structure of the waveguide, and the electronic circuits can then be etched on a substrate glued to the surface

conductive of the slots.

  The thickness and the dielectric constant of the substrate 3 and of the radome 1 have a significant influence on the resonance wavelength, and therefore on the radio slot length. Indeed, the higher these thicknesses and these dielectric constants, the smaller for a given resonant frequency of the slit, the physical dimensions of the slit, which allows a smaller bulk of each

  slot. However, the bandwidth of the antenna then also decreases.

  In a preferred example, for frequencies around 1 GHz, the substrate 3 has a large thickness, of the order of 2 mm, and a high relative dielectric constant, of the order of 10. The radome 1, protective element of the antenna preferably plated on the conductive surface 2, can also cause significant ohmic losses when its thickness is too high. In this preferred example, the radome has a

  4mm thick and its material is laminated glass-epoxy.

  The cavity 5 makes it possible to suppress the radiation from the antenna towards the rear, that is to say on the side opposite to the radome 1. The presence of a cavity increases the frequency selectivity of an antenna with radiating slits. The deeper the cavity 5, the more for a given resonant frequency of the slit, the dimensions of the slit increase, and the more the frequency bandwidth decreases. The space saving in terms of the thickness of the antenna is to the detriment of the space requirement of the antenna

  and at the expense of the width of its bandwidth.

  FIG. 2A schematically represents a radiating slot of the antenna and the non-linear electronic circuit 70 associated with the slot, of a preferred embodiment of an antenna according to the invention. The slot 20 is seen from above with respect to FIG. 1; it is etched in the conductive surface 2. The electronic circuit 70 in the example of FIG. 2A

  has only one electronic device 70a with which it merges.

  The electronic circuit 70 is controlled by a control device 6 which is preferably common to several electronic circuits 70. The control device 6 comprises for example a control line 61 and an inductor 62 for decoupling high frequencies whose slots 20 are headquarters. The electronic device 70a preferably comprises a wire connection 71 connecting two points 74 and 75 of the perimeter 21 of the slot 20. The wire connection 71 preferably comprises a diode 72 and a capacitor

  73 for decoupling the mass to which the conductive surface 2 is connected.

  In the preferred T-shape of the slot 20 shown in FIG. 2A, lo the slot 20, of perimeter 21 corresponding to a given radio length, comprises a central bar 22 and a transverse bar 23 having flared sides 24. The different elements will be described in more detail below. The electronic device 70a has two states: a blocked state and a passing state. The blocked state does not modify the radio length of the slot 20. The on state decreases the radio length of the slot 20 by shortening the perimeter 21 of the slot 20 by short circuit made between two points 74 and 75 of the perimeter 21 Therefore, the slot 20 has the greatest radio length when the electronic device 70a is in the blocked state. The change of state of the electronic device 70a achieves a

  switching between two geometrically different states of the slot 20.

  Figures 2B and 2C schematically represent the two states of the electronic device 70a according to the invention, the blocked state is shown in Figure 2B and the on state in Figure 2C. The effective perimeter 21 of the slot 20 is shown in solid lines in each of the states of the electronic device 70a. The distance d, shortening distance of the transverse bar 23, corresponds to a decrease in the radio length of the slot 20. The decrease in the radio length of the slot 20 must be small enough so that the radiation pattern of the antenna remains substantially stable, that is to say has a stability considered sufficient in the application considered, even in the case where the supply line 4 remains identical for the two states of the

electronic device 70a.

  The electronic circuits 70 are synchronized so that the slots 20, of an antenna with several slots, have substantially the same radio length between them, that is to say have radio lengths considered to be sufficiently close in the intended application, for example by having a control device 6 cc: nmun which then allows for example a common voltage control of all the electronic circuits 70. The value of substantially in the expression "substantially the same radio length" depends on the tolerances allowable in l application envisaged. Similarly in the more general case of radiating slits switching from one state to another "substantially at the same time>", the value lo of "substantially" depends on the tolerances allowable in the intended application. Preferably, the perimeters 21 and the distances d are

  substantially equal for all the slots 20 of the antenna.

  The conductive surface 2, connected to ground, establishes an electromagnetic protection screen against strong fields at frequencies below the resonance frequency of the slots 20. This screen protects all the electronic elements which are behind the conductive surface 2 and

  in particular behind the substrate 3.

  The slot 20 can have various shapes. The folded T shape shown in Figures 2A, 2B and 2C is a preferred shape. Let D1 be a first direction and D2 a second direction orthogonal to D1. The slots 20, only one of which is shown in FIGS. 2A to 2C, are aligned in the direction D1. The slot 20 has two bars 22 and 23: a transverse bar 23 parallel to the direction D1 connected at its center to a central bar 22 parallel to the direction D2. This folding reduces the size in the direction D2 compared to a straight slot in the direction D2 of the same radio length, to the detriment of a reduction in bandwidth. In addition, at equal resonant frequencies, it can be seen that the perimeter of a T-shaped slot is less than the perimeter of a straight slot, which can be explained by a coupling phenomenon between the transverse bar 23 on the one hand and the central bar 22 on the other hand. This leads to a further reduction in size. It is also noted that the T shape results in a radiated cross polarization, that is to say here a polarization in the direction D2, significantly lower than that resulting from other types of shape such as a conventional L shape for example, which can be explained by the presence of opposing currents in the two branches of the crossbar 23 located on either side of the center of the crossbar 23. The central bar 22 must be sufficiently thin, that is to say that is to say that the ratio between its width and its length must be sufficiently low, so that the slit 20 has a slit behavior radiating above all a rectilinear polarization parallel to the direction D1 perpendicular to its length. Preferably also, the ratio of the length of the central bar 22 to the length of the cross bar 23 is large enough for the radiating slot 20 to radiate with a rectilinear polarization substantially parallel to direction D1, that is to say comprising a sufficiently low cross-polarization level for the application considered. The term "substantially" depends on the intended application and

  allowable tolerances for cross-polarization level.

  i15 The more the radio currents of which the radiating slot 20 is the seat are located in the central bar 22 and the more the direction of polarization of the

  field radiated by the slot 20 will be parallel to the direction D1.

  Advantageously, the length of the central bar 22 is substantially equal to one

  third of the perimeter 21 of the slot 20.

  Preferably, the transverse bar 23 widens towards its ends, as shown in FIG. 2A the oblique side 24 of the transverse bar 23. It can be seen that this widening widens the bandwidth of the antenna. This flaring can also be carried out along the other side of the crossbar 23 or even simultaneously along the two sides of the crossbar 23, but in these two cases, the gain in overall dimensions in the direction D1 is a little more weak than in the case

shown in Figure 2A.

  When the electronic device 70a is in the on state as in FIG. 2C, the T-shaped slot is no longer symmetrical, the two branches of the crossbar 23 no longer have the same length; as explained above, the distance d is chosen to be sufficiently small with respect to the length of the crossbar 23 so that the radiation pattern of the antenna remains substantially stable. In an alternative embodiment, two electronic devices can be used such as the electronic device 70a, which could each be placed at a distance d / 2 from the

  ends of crossbar 23.

  The wire connection 71 shortens the crossbar 23 and therefore reduces the perimeter 21 of the slot 20. Preferably, the wire connection 71 is parallel to the direction D2 and is located sufficiently in the vicinity of the plane of the conductive surface 2 so that the short - circuit thus produced is efficient and the radio length of the slot is reduced without the appearance of parasitic inductive effects. The wire connection 71 can be made on either side of the substrate 3. A preferred embodiment 10 of wire connection 71 is given in FIG. 2D. The wire connection 71 is produced in the form of metallic etching on the substrate face 3 which is situated on the side opposite to the conductive surface 2. As in FIG. 2A, it preferably comprises a diode 72 and a capacity 73 for decoupling in series. The capacity 73 is for example 500 pF in the preferential frequency domain of use located around 1 GHz. Metallized holes 71a and 71b connect the wire connection 71 respectively to the short-circuit points 74 and

  located on the conductive surface 2.

  Diode 72 is a microwave diode with two polarization states. Diode 72, generally a PIN diode, advantageously has the following characteristics: a fast switching time, for example less than ps, a high reverse voltage withstand, from 500 to 1000 volts for example, an equivalent resistance in relatively low forward bias , for example 0.6 ohms, a very low reverse capacitance, for example 0.4 pF. The control line 61 carries control signals from this diode, for example a voltage of -50 volts for the state

  blocked and a current of 50 mA for the on state.

  FIG. 3 schematically represents a top view of a preferential antenna according to the invention. The antenna preferably comprises two slots 20 with associated electronic circuits 70 similar to that of FIG. 2A. The slots 20, the central bars 22 of which are parallel to the direction D2, are aligned in the direction D1 which is the direction of polarization of the radiation emitted by the slots 20. The alignment thus obtained has a small surface area and makes it possible to achieve undersized antennas whose dimensions are of the order of the fraction of slot resonance wavelength, for example of the order of half in the direction D1 and of a third according to the

direction D2.

  A partition 51, preferably located equidistant from the central bars 22 of the two slots 20, parallel to the direction D2 and perpendicular to the plane of Figure 3, separates the cavity 5 into two parts. The partition 51 is shown in Figure 3 in dotted lines. The partition 51 is also connected to ground, in order to reduce or even eliminate the intracavity couplings, that is to say inside the cavity 5, between the slots, for example by means of metallized holes passing through the substrate 10 3 and establishing an electrical contact between the conductive partition 51 and the conductive surface 2. The control line 61 is connected to the partition 51 by a decoupling capacitor 63. The partition 51 has a hole 52

  passing a supply line 4.

  Under the substrate 3 not shown in Figure 3, is located the

  line 4 for supplying the slots 20, advantageously a microstrip line.

  The supply line 4 is shown in dotted lines. The slots 20 are supplied in series. The difference in excitation phase imposed between the slots 20 by the supply line 4 makes it possible in part to spot, with respect to normal to the surface of the antenna, the main lobe of the beam radiated by the antenna. In the case where the antenna is not completely flat, the normal to be considered is the vertical to the mean plane at point 0 and passing through point 0. The depointing is also linked to an extracavity coupling effect between the slots 20, especially when these are very close to each other, which will be discussed later. The central bars 22 are spaced substantially a quarter of the resonance wavelength of the slits 20, which allows, by establishing an excitation phase difference of about 90 degrees between the two slits 20 of the antenna to greatly reduce the radiation losses towards the rear of the main lobe, that is to say towards the source of the supply line 4 described below. Such a combination of two slots 20 allows the radiation of a main lobe with a strong inclination relative to the surface

driver 2.

  The supply line 4 has different sections and ends with a termination 44 in open circuit. From the source, not shown in FIG. 3, the supply line 4 comprises intermediate sections 41, on the one hand between the source and the first slot 20, and on the other hand between the two slots 20. Between the intermediate sections 41 and the slots 20, the supply line 4 comprises sections 42 of impedance adaptation, to achieve an adaptation between the impedance of the intermediate sections 41, for example 50 ohms, and the impedance of slots 20 seen through the supply line 4. The impedance matching is obtained by playing preferably on the width of the different sections of the supply line 4. The adaptation of the supply line 4 results from a compromise between the adaptation to the different radio lengths of the slots 20, so that during changes of state of the electronic circuits 70, the supply line 4 remains adapted and that the deflection of the beam is substantially preserved so that the radiation pattern remains substantially stable. Adaptation allows

  to keep a relatively low TOS, standing wave rate.

  i5 The supply line 4 is coupled to the slots 20 by

  by means of coupling sections 43 which excite the slots 20.

  Each section 43 is preferably located near the end

  of the central bar 22 located on the side opposite to the transverse bar 23.

  The example in FIG. 3 shows an optimal excitation known as "maximum offset", that is to say at the end of the slit. At a given slit resonance wavelength, an excitation at the slit end requires a slightly smaller slit size than for an excitation produced elsewhere in the slit. Again, a gain in surface area for the antenna. The supply line 4 and the slots 20 being located on either side

  and on the other side of the substrate 3, the excitation is carried out by capacitive coupling.

  The phase shift imposed by the supply line sections 4 located between the two slots 20 is partially responsible for the pointing effect of the antenna beam. Another important effect which intervenes in the pointing is the extracavity coupling, that is to say external to the cavity, between the slots 20, the intracavity coupling having been practically eliminated by the partition 51 connected to the ground. In adapting the feed line 4, this coupling must be taken into account for each of the frequencies radiated by the antenna so that the aiming effect sought, corresponding to a specific radiation pattern depending on the intended application is obtained for all the states of the electronic circuits 70, which states correspond to given slot radio lengths and therefore to frequencies radiated by the antenna. The radiation patterns obtained with the preferred antenna shown in Figure 3 are shown in Figure 4. Figure 4 schematically shows the radiation patterns at two distinct frequencies F1 and F2 which correspond respectively to the blocked and passing states of electronic devices 70a . The radiation diagrams are represented in the plane P of FIG. 3, the plane P being represented in phantom and passing at the center of the central bars 22 of the slots 20, parallel to the direction D1, and perpendicular to the plane of the figure 3 which plane corresponds to the mean plane of the antenna at these media. Indeed, the antenna is not necessarily flat, it can also be shaped to a particular surface, as in a preferred application i5 examined later. The curves are in

  solid lines for frequency F1 and dashed lines for frequency F2.

  These curves represent a relative gain in decibels on the ordinate axis, as a function of the angle between the direction of observation in the plane P and the vertical to the plane of FIG. 3 passing through the point O visible in FIG. 3 , angle expressed in degrees. The point O is equidistant from the two central bars 22. The depointing angle obtained is significant, the depointing angle being the angle for which the value of the relative gain is maximum; he

can reach 40 to 50.

  The preferred field of application of the invention is the L band around 1 GHz. The frequencies of the preferred application example are the frequencies F1 = 1030 MHz and F2 = 1090 MHz, spaced 60 MHz apart. The bandwidth around each of these frequencies is approximately 20 MHz, or approximately 2%, which means that the appearance of the radiation patterns is substantially stable in these two bandwidths. The curves in FIG. 4 show that it is possible to obtain stable and similar radiation patterns over narrow bandwidths, around 2%, around frequencies that are substantially far apart, their relative distance is around 6%. To obtain this operating result only by continuously widening a bandwidth around a central frequency of around 1060 MHz, it would have been necessary to obtain an equivalent bandwidth of 8%, ie significantly more than the bandwidths of 2%. Statements of TOS, standing wave rate, show that the bandwidths in the radiation pattern are also the

  bandwidths in TOS for the antenna considered in FIG. 3.

  FIG. 5 represents a variant of electronic circuit 70 comprising several electronic devices, here for example three, the devices 70a, 70b and 70c being similar to that of FIG. 3 and respectively located at distances dl, d2, d3. The slots 20 then have a stable radiation pattern for several frequencies, the frequencies corresponding to the different perimeter sizes 21 of the slots being obtainable with the electronic devices 70a to 70c. This variant allows for example to operate the antenna in frequency evasion mode or in automatic frequency tuning correction mode. Frequency escape mode inherently requires a short frequency switching period. The automatic frequency tuning correction mode, linked to the drift of the antenna characteristics as a function of certain parameters, such as temperature for example, will generally be able to operate with a much longer switching period. In one application of the invention, the antenna is conformed to the surface of an aircraft, which allows it to have, taking into account its offset from 40 to 50, an angle of inclination of the beam in low elevation by compared to

  horizontal, angle which corresponds to an observation towards the front of the airplane.

  Several antennas of this type, for example four, can be aligned parallel to the direction D2, thus forming an array that can be electronically controlled in order to scan the beam in the azimuth plane of the aircraft. Each pointed beam of the network advantageously covers a limited sector in azimuth of the observation space which

  is thus covered by an electronic scan of all the beams.

  One or more antenna networks described above make it possible, for example, to produce an identification system, of the IFF type (IFF having been defined above) operating at two frequencies F1 and F2, using shaped antennas, and covering the space in front of the aircraft on a given angular sector in elevation and in azimuth. The identification system is

  both an air-to-air system and an air-to-ground system.

Claims (13)

  1. Antenna with radiant slots (20) etched in a conductive surface (2) connected to ground, the radiating slots (20) having a perimeter (21) corresponding to a given radio length, characterized in that the antenna comprises non-linear electronic circuits (70), an electronic circuit (70) being associated with each radiating slot (20) and comprising at least one electronic device (70a, 70b, 70c) o10 in two states, a blocked state which does not modify the radio length of the slot (20), and a passing state which decreases the radio length by   the slot (20) by shortening the perimeter (21) of the slot (20) by short-   circuit formed between two points (74 and 75) of said perimeter (21), in that the electronic circuits (70) are synchronized so that the radiating slots (20) switch from one state to another substantially at the same time, and in that the decreases in radio lengths are small enough that the antenna radiation patterns remain substantially stable.   2. Antenna according to claim 1, characterized in that the electronic circuits (70) are synchronized so that the radiating slots (20) have between them substantially the same radio length and in that the decrease (d, dl, d2, d3 ) of radio length is short enough that the   radiation from the antenna remains substantially stable.   3. An antenna according to any one of claims 1 to 2,   characterized in that the electronic device (70a, 70b, 70c) has   a microwave diode (72) with two polarization states.   4. An antenna according to any one of claims 1 to 3,   characterized in that the radiating slots (20) being aligned in a first direction (D1), each radiating slot (20) is T-shaped, the T having a transverse bar (23) in the first direction (D1) connected to the level from its center to a central bar (22) according to a second   direction (D2) orthogonal to the first direction (D1).   5. Antenna according to claim 4, characterized in that the electronic device (70a, 70b, 70c) comprises a wire connection 71 substantially parallel to the first direction (D1) and making, when the electronic device (70a, 70b, 70c) is in the on state, the short circuit   between two points (74 and 75) of the crossbar (23).   6. Antenna according to claim 5, characterized in that the ratio of the length of the central bar (22) to the length of the transverse bar (23) is large enough for the slot (20) to radiate in a substantially straight polarization parallel to the first direction (D1).   7. Antenna according to claim 6, characterized in that the length of the central bar (22) is substantially one third of the perimeter (21) of the radiating slot (20).   8. An antenna according to any one of claims 4 to 7,   characterized in that the antenna comprises a feed line (4) which feeds in series all the radiating slots (20) in the vicinity of the end of their central bar (22) situated on the side opposite their bar transverse (23).   9. An antenna according to any one of claims 4 to 8,   characterized in that the crossbar (23) flares towards its ends.   10. Antenna according to any one of claims 4 to 9,   characterized in that the antenna has two radiating slots (20), the central bars (22) of which are spaced approximately a quarter apart   resonance wavelength of the radiating slots (20).   11. An antenna according to any one of claims 1 to 10,   sequentially comprising a radome (1), the conductive surface (2) connected to ground, a dielectric substrate (3), a metal cavity (5), characterized in that a microstrip supply line (4) is located in the metal cavity (5) on the face of the substrate (3) opposite to that where the radiating slots (20) are located, and in that the cavity (5) is partitioned   so as to reduce the coupling between radiating slots (20).   12. An antenna according to any one of claims 1 to 11,   characterized in that each electronic circuit (70) comprises several electronic devices (70a, 70b, 70c) so that the antenna operates in frequency escape mode or in correction mode automatic frequency tuning.   13. Antenna network characterized in that the network includes   several antennas according to any one of claims 1 to 11, one   given observation space consisting of observation sectors being covered by electronic scanning of the antenna array, each observation sector being covered by a position of the scanning beam of the array.