FR2707399A1 - Device for simulating an intense electromagnetic field in free space. - Google Patents

Abstract

The invention relates to a device for simulating an intense electromagnetic field in free space, of the GTEM cell type, comprising a cell (10) of pyramidal shape with rectangular section and with metal walls connected to the earth, resting on one of its walls. lateral (18), a metal plate or septum (12) situated in an intermediate plane passing through the top of the pyramid and charged at its end by a load of 50 OMEGA, an absorbent wall (14, 15) closing this cell, and a source (11), located at the top of the pyramid, connected to the intermediate plate (12). The intermediate plate (12) comprises, in the transverse direction, a flat central part (22) and lateral parts (23) inclined downward at an angle alpha with respect to this central part (22) so that the 'impedance seen from the source is an impedance of 50 OMEGA.

Description

"Device for simulating an intense electromagnetic field in space

free"

DESCRIPTION

TECHNICAL AREA

  The present invention relates to a device for simulating an intense electromagnetic field in free space, of the GTEM cell type ("gigahertz

electromagnetic transverse ").

STATE OF THE PRIOR ART

  Testing laboratories are often asked to simulate electromagnetic fields due, for example, to the presence of radars, natural sources such as lightning, or sources

  such as nuclear explosions.

  These laboratories must then implement systems simulating this environment in all its aspects: signal form, bandwidth, homogeneity of the simulated field. They are thus led to simulate strong electromagnetic fields on the equipment of an airplane, or of a rocket, to reproduce the situation that such a mobile subject to a

radar beam.

  To simulate an electromagnetic field in free space, we generally use a line, crossed by a current, stretched above a ground plane at zero potential. Between this line and this ground plane develops an electromagnetic field whose homogeneity depends on the shape of the line. The best known lines are of the strip line or TEM cell type, but these means have limitations

  in frequency, due to their shape, to around 150 MHz.

  For a few years now, a new family of TEM cells has been marketed called GTEM ("transverse electromagnetic giga" cell) exhibiting good performance over the whole range of

  frequencies ranging from continuous to Gigahertz.

  Many documents of the known art describe the characteristics of TEM cells, and those of GTEM cells, and for example the following documents: - an article entitled:

  "Electromagnetic susceptibility measure-

  elements of vehicle components using TEM cells 14 kHz - 200 MHz "(SAE Information report, J 1448, January 1984), - an article entitled:" A new family of TEM-cells with enlarged bandwidth and optimized working volume "by D. Kônigstein and D. Hansen (Proc. 7th Int. Zurich Symp. and Tech. Exh on EMC, pages 127-132, March 1987), - an article entitled: "Simulating open area test site emission measurements based on data obtained in a novel broadband TEM-cell "by P. Wilson, D. Hansen and D. Kônigstein (1989, IEEE, pages

171-177),

  - an article entitled: "Vehicle electromagnetic radiated susceptibility testing using a large TEM-cells" (SAE Information Report, J 1407, March 1988), - an article entitled: "The GTEM-cell concept; application of this new EMC test environment to radiated emission and susceptibility measurements "by H. Garbe, D. Hansen (pages 152 to 156), - Patents CH-670 174 and US-4 837 581. An article entitled:" Comparison of analysis of a WTEM cell with standard TEM cells for generating EM fields "by Lorenzo Carbonini (IEEE Transactions on Electronic Compatibility, volume 35, number 2, May 1993) describes a WTEM cell which uses a network of wires that are parallel (in the longitudinal direction) and rounded (in the transverse direction) in place of the septum

  flat metal of TEM and GTEM cells.

  An article entitled: "Conformal mapping analysis of a modified TEM cell by Changhua Wan (IEEE Transactions on Electromagnetic Compatibility, volume, number 1, February 1993) describes a modified TEM cell, which includes a central conductor (septum) surrounded by a rectangular shielding, in which the septum is replaced by an H-shaped conductor to produce a uniform field just above and below the inner surface of the cell shielding and not in the central part between the septum and the shielding as in conventional TEM cells. Such a modification can also be carried out in a GTEM cell, but TEM or GTEM cells of this type cannot pass high voltages: Indeed, for such voltages, it there are local spikes that can

cause a breakdown.

  The object of the present invention is to improve the efficiency of such GTEM cells by providing a larger useful area in which the electric field is constant and can be more intense. SUMMARY OF THE INVENTION The invention proposes a device for simulating an intense electromagnetic field in free space, of the GTEM cell type, comprising a pyramid-shaped cell with rectangular section and metal side walls connected to the earth, resting on one from its side walls, a metal plate or septum located in an intermediate plane passing through the top of the pyramid and charged at its end by a load of 50 Q, an absorbent wall closing this cell, and a source, located at the top of the pyramid, connected to the intermediate plate, making it possible to generate an electromagnetic field between the intermediate plate and the side walls of the cell, characterized in that the intermediate plate comprises, in the transverse direction, a flat central part and lateral parts inclined towards the bottom of an angle a with respect to this middle part so that the impedance seen from the source is an impedance of 50 Q. Advantageously, this angle a is of the order of. The angle at the top of pyramid 0 is such that:

0 <20.

  In a particularly advantageous embodiment, the middle part and the two lateral parts of the intermediate plate are separated by curved intermediate zones. The free ends of the side parts end in rounded elements, which are cylinders or cones. The voltage source delivers a sustained signal or a pulse signal. The median plane between the intermediate plate and the bottom wall of the cell is a horizontal plane. The access door

  in the cell is placed at the bottom of the cell.

  Such an invention has many advantages: - the modification of the shape of the septum makes it possible to obtain a homogeneous area of greater constant electric field in the transverse direction: a high efficiency of the cell is obtained by enlarging the field area; the modification of the position of the cell of the invention, relative to that of the cells of the prior art, makes it possible to obtain better homogeneity in the longitudinal direction; - the addition of rounded elements allows this device to be used in high pulsed voltage; - the improvement of the geometry of this device makes it possible to have an electric field area

constant and bigger.

Brief description of the drawings

  - Figures 1 and 2 illustrate a GTEM cell of the known art respectively in a partially cutaway perspective view and a cross-sectional view, with representation of the isonilevel lines of electric field; - Figure 3 illustrates the device of the invention in a cross-sectional view, with representation of the isoniveaux lines of electric field; - Figures 4 to 6 illustrate different embodiments of the device of the invention respectively in cross-sectional views, with representation of the isoniveaux lines of electric field. Detailed description of embodiments FIG. 1 illustrates a GTEM 10 cell of the known art which has a pyramidal shape with rectangular section, a source 11 is placed at the top of the pyramid, a connection making it possible to establish at this point the passage of current between this source and a

  metal plate 12 called "septum".

  The field develops between this plate 12 and

  the rectangular structure of cell 10.

  The assembly has a characteristic impedance of 50 Q. This GTEM cell is closed by a wall 15 covered on its internal face by high-frequency absorbers in the form of contiguous pyramids which form between them an absorbent wall to prevent the transmission of a wave reflected in the volume of the cell. This absorbent wall is curved in the manner of a spherical segment whose center would be located in the region of the top of the pyramid. The need to carry out the tests in a homogeneous field area (more or less 10%) limits the working area to the rectangle 17 shown in FIG. 2, which is a sectional view along the plane P of the device of the invention , with representation of the isonelectric field lines whose values in volts / meter correspond to a voltage of 1 volt

on the septum.

  For this work area 17 there is a maximum difference of 0.30 volts / meter. In the absence of an object we manage to obtain an impedance

characteristic of 47.3 f.

  It can be seen that the useful area is small compared to the dimensions of the cell: approximately one third of the height and one fifth of the width. Knowing that these cells can reach eight meters in length, we can measure the interest of enlarging this useful area: we will thus be able to test larger objects in a larger cell.

small for a lower cost.

  In FIG. 1 are also shown the inlet 20 and the outlet 21 of air used for

cell cooling.

  Such a GTEM cell has characteristics of very wide bandwidth in a range of operating frequencies going from continuous to GHz. TEM mode is used to simulate an incident plane wave for susceptibility testing. The emission data radiated in the cell can be used to simulate open area test sites or space measurements

free.

  The basic idea of wave propagation in such a GTEM cell is to propagate a slightly spherical wave emitted by a source in a rectangular coaxial transmission line and its hybrid end distributed without geometric distortions of the TEM wave. As the opening angle of the waveguide is small (<20 degrees), the undistorted spherical wave can be considered to be

approaching a plane wave.

  As shown in FIG. 1, the main volume of the GTEM cell is a section of line of pyramidal shape of a rectangular 50 Q transmission line. The cell begins with a tip in which the transition from a standard 50 Q coaxial connector to an asymmetrical rectangular waveguide takes place. The distributed charge section uses absorbent material 14 for quenching the electromagnetic waves (as in an achenoic chamber) and a distributed charge 13 comprising, for example, a series of resistors 19

for switching off the current.

  The invention consists in modifying the geometry of such a GTEM cell to improve the quality of the

field obtained.

  In the device of the invention, the shape of the plate 12, or septum has been modified, as shown in FIG. 3, so as to obtain a more homogeneous electric field, while having an industrially feasible plate. Others

  elements of cell 10 are preserved.

  This plate 12 comprises, in the transverse direction, a planar middle part 22 and lateral parts 23 inclined downwards at an angle a of the order of 135 relative to this middle part so that the impedance seen from the source remains Q, the angle at the top of the pyramid being less than 20 In FIG. 3, the middle part 22 and the side parts 23 are separated by zones

curved intermediaries 24.

  To allow the passage of high voltages, rounded anticorona 25 elements, for example cylinders or cones, are arranged at the ends

  respective sides 23.

  The values in volts / meter of the electric field level lines are reported on

  this figure as if we had 1 volt on the septum.

  We obtain a homogeneous field area represented by the rectangle 17, for which there is a maximum difference of 0.09 volts / meter (area between 1.09 volts / m and 1.18 volts / meter). If there is no object,

  achieves a characteristic impedance of 50.

  In order not to disturb the field and not to be obstructed by the shape of the septum for the introduction of objects into the cell, the access door is placed in the bottom of the cell, while, in the devices of the known art, this door was placed on one side. Thus, one can have the edges of the septum lower without penalizing the accessibility to the zone of

test.

  The device of the invention, moreover, is such that the median plane between the intermediate plate 12 or septum and the bottom wall 18 of the pyramid 10 is located in a horizontal plane, whereas, in the devices of the known art , this wall 18 was located in a horizontal plane, which resulted in a difference in electric field amplitude between the

  top and bottom of a vertical wall of the same object.

  In the invention, an object, placed on this wall 18, is perpendicular to the direction of propagation and the electric field, in the same vertical plane,

remains homogeneous.

  In an exemplary embodiment for a pyramid cell 8 m long, we have, at the place where the pyramid forms a section 2 m wide: - width of the section: 2 m

- thickness of the septum: 5 mm.

  - section height: 1.36 m - height above the septum: 45.5 cm height below the septum: 90 cm - width of the middle part 22: 30 cm - width of the septum: 159.6 cm - radius of curvature of each curved intermediate zone 24: 66.4 cm - radius of the anticorona elements 25: 4 cm

- angle a = 135.

  We can then obtain a field of approximately

kV / m.

  Figures 4 to 6 show different embodiments of the septum 22, in cross-sectional views, with representation of the isoniveaux lines of electric field. Rounded anticorona elements can, of course, be added to this septum 22 as in FIG. 3. As shown in FIGS. 4 and 5, the

lower corners of the cell.

  The device of the invention, as shown in one of FIGS. 3 to 6, is designed to meet the following optimization criteria: the characteristic impedance of the vacuum cell is 50 Q. We accept a tolerance of 1, which still corresponds to an excellent adaptation; - the homogeneity of the field is as wide as possible. Unless otherwise indicated, this zone subsequently corresponds to the zone in which the module of the electric field is between -2dB and + 2dB of its value at mid-height under the septum; - the field has a vertical polarization inside the test area; - the presence of field spikes is minimized which could cause breakdown in the case of using the cell in a very high level pulsed field; - the ease of mechanical production as well as other practical aspects (weight of the septum, accessibility to the test area, etc.) have been taken into account.

Claims (9)

  1. Device for simulating an intense electromagnetic field in free space, of the GTEM cell type, comprising a cell (10) of pyramidal shape with rectangular section and metal side walls connected to the earth, resting on one of its side walls ( 18), a metal plate (12), or septum, located in an intermediate plane passing through the top of the pyramid and charged at its end with a 50 Q charge, an absorbent wall (14, 15) closing this cell, and a source (11), located at the top of the pyramid and connected to the intermediate plate (12) making it possible to generate an electromagnetic field between the intermediate plate (14) and the side walls of the pyramid, characterized in that the intermediate plate ( 12) comprises, in the transverse direction, a planar middle part (22) and lateral parts (23) inclined downward at an angle a with respect to this middle part (22) so that the impedance v eu of the source is an impedance of Q. 2. Device according to claim 1, characterized in that this angle a is of the order of 135.   3. Device according to claim 1, characterized in that the angle at the top of the pyramid e is such that: 0 <20.   4. Device according to any one of   claims 1 to 3, characterized in that the part   middle (22) and the two lateral parts (23) of the intermediate plate are separated by zones curved intermediates (24).   5. Device according to claim 5, characterized in that the free ends of the lateral parts end in elements rounded (25).   6. Device according to claim 6, characterized in that these rounded elements (25) are cylinders or cones. 7. Device according to any one of   previous claims, characterized in that the   voltage source (11) delivers a sustained signal or an impulse signal.   8. Device according to any one of   previous claims, characterized in that the   median plane between the intermediate plate (22) and the bottom wall (18) of the cell is a horizontal plane. 9. Device according to any one of   previous claims, characterized in that the   access door in the cell is placed in the bottom of the cell.