EP2888784B1 - Inductive surface element - Google Patents

Description

The present invention relates to an inductive surface element, which is adapted to modify propagation conditions of electromagnetic radiation. It also relates to a surface wave generation assembly and a surface wave detection assembly each comprising such an inductive surface element.

• sa direction de propagation est parallèle à l'interface entre les deux milieux ; et
• elle présente une atténuation exponentielle selon la direction perpendiculaire à l'interface entre les deux milieux.

A surface wave is a mode of propagation of electromagnetic radiation, which is linked to a separation interface between two different media. The two media may differ from each other by their respective values of dielectric permittivity and / or electrical conductivity. A surface wave has the following characteristics:

• its propagation direction is parallel to the interface between the two media; and
• it has an exponential attenuation in the direction perpendicular to the interface between the two media.

In addition, the wave vector that characterizes the propagation parallel to the interface is connected to the attenuation coefficient in the perpendicular direction and to the electromagnetic impedance of the interface. Such a mode of propagation on the surface is opposed to the mode of propagation in free space, which is sometimes called propagation in sky wave.

Surface waves are used in various applications, including radar systems or communication systems for which the transmitted waves used are surface waves. For these systems, the interface that is used is the boundary between the ground and the air environment above the ground. An advantage of these systems results from the reduction of the power of electromagnetic radiation which is lost towards the sky, since the transmitted waves remain concentrated close to the ground.

• des ondes à propagation en espace libre subsistent malgré l'élément de conversion, ce qui constitue une perte d'efficacité, entre la puissance qui est rayonnée par la source et la puissance qui est réellement diffusée sous forme d'ondes de surface ; et
• les ondes de surface qui sont produites par l'élément de conversion possèdent des directions de propagation dispersées parallèlement au sol, de sorte qu'une partie seulement de l'énergie qui est transportée par ces ondes de surface est transmise dans une direction désirée.

Many sources of electromagnetic radiation that are available produce free-space propagation waves. A wave of The surface can then be produced using a conversion element that couples some of the free-space propagation waves, as produced by the source, to surface waves. However, the following difficulties affect the existing conversion items:

• free-space propagation waves remain in spite of the conversion element, which constitutes a loss of efficiency between the power radiated by the source and the power that is actually diffused in the form of surface waves; and
• the surface waves that are produced by the conversion element have propagation directions dispersed parallel to the ground, so that only a portion of the energy that is transported by these surface waves is transmitted in a desired direction.

On the other hand, when such a conversion element is used in association with a detector for receiving a surface wave transmission communication, the conversion element also transmits to the detector free space propagation waves, more than the received surface wave. The detection of the signal that is carried by the surface wave is then scrambled and disturbed by the free space propagation waves which are transmitted involuntarily.

The document GB 788.824 discloses an antenna element that is adapted to transmit a wave that is focused perpendicularly to a base plate of that element, according to the free space propagation mode.

The document EP 1,594,186 describes an antenna which is formed by an open loop above a ground plane, the latter being intended to be buried. Two confined waves are formed between the loop and the ground plane, and the peripheral edge of the ground plane produces surface wave radiation to the outside. However, this system also produces a free space propagation wave.

The document WO 03/007426 discloses an antenna with a low form factor radiator, which is disposed above a high impedance surface itself on a metal ground plane. But Horizontal polarized surface waves of the electric field appear in the high impedance surface and then transform into free-space propagation waves.

Finally, the documents WO 2010/142946 and EP 1 608 037 describe antennas which have different structures, and which increase the wave emission efficiency whose propagation directions have low elevation angle values above the ground level. But it is not surface waves whose propagation is actually parallel to the ground and whose transported power remains concentrated close to the ground.

Under these conditions, a first object of the present invention is to produce surface waves with a minimum amount of power that is radiated as free-space propagation waves.

A second object of the invention is to produce surface waves which are concentrated in azimuth around a desired direction of propagation, parallel to the surface of the ground.

A third object of the invention is to provide a surface wave production system, which can use the available radiation sources to produce free-space propagation waves, in particular wired antennas.

A fourth object of the invention is to provide a surface wave production system that is compact, inexpensive and easy to implement.

Finally, a fifth object of the invention is to propose a surface wave production system which is adapted to operate in the frequency range between 0.2 MHz (MHz) and 3000 MHz, and in particular in the band of high frequencies, called HF band, between about 3 MHz and 30 MHz.

• une plaque de base conductrice électriquement, qui s'étend dans un plan non parallèle à la composante de champ électrique du rayonnement électromagnétique ; et
• une série de plaques secondaires conductrices électriquement qui s'étendent perpendiculairement à la plaque de base, dans un même demi-espace sur un côté de la plaque de base, et de façon symétrique par rapport à un plan perpendiculaire à la plaque de base selon une direction dite sensible.

To achieve these and other objects, the present invention provides an inductive surface element for electromagnetic radiation, which element comprises:

• an electrically conductive base plate, which extends in a plane not parallel to the electric field component of the electromagnetic radiation; and
• a series of electrically conductive secondary plates which extend perpendicularly to the base plate, in the same half-space on one side of the base plate, and symmetrically with respect to a plane perpendicular to the base plate according to a so-called sensitive direction.

According to a first characteristic of the invention, the secondary plates have the same height within ± 10%, this height being between 0.035 x λ and 0.35 x λ, where λ is a sizing parameter of the surface element inductive.

According to a second characteristic of the invention, the distance between the base plate and the edges of the secondary plates opposite is between a zero value and half the height of the secondary plates concerned.

According to a third characteristic of the invention, the secondary plates are arranged parallel to each other in a period such that the product of the height of the secondary plates by the period is between 0.001 x λ ² and 0.15 x λ ² to ± 10 % near.

According to a fourth characteristic of the invention, the secondary plates each extend over a total length of at least 0.0003 x λ perpendicular to the sensitive direction.

Such an inductive surface element is then adapted to modify conditions of propagation, in the half-space, of a projection of the electric field component which is perpendicular to the base plate, when a wavelength of the radiation electromagnetic energy is between λ - 10% and λ + 10%. The wavelength of the electromagnetic radiation that is considered is that which is associated with the propagation in the medium where the inductive surface element is located, and which is equal to the speed of light in that medium divided by the frequency of the radiation. In this way, the inductive surface element makes it possible to produce or detect surface waves which are concentrated in azimuth around a direction of propagation parallel to a ground surface, when this element is placed on the ground or semi-buried near the ground surface, the base plate being parallel to an average boundary surface between the ground and the half airspace.

The inductive surface element of the invention is therefore an electromagnetic conversion element, which is capable of coupling free space propagation modes with surface waves. This coupling is particularly effective thanks to the geometry and sizing characteristics of the element that are introduced by the invention. In other words, the element makes it possible to efficiently transfer a portion of the radiated power of free-space propagation waves which reach the element, to surface waves. It can therefore be used with sources of electromagnetic radiation that are available, including wired sources whose cost is low and the use is easy to produce sky waves. These free-space propagation waves are then transformed, at least partially, into surface waves by the inductive surface element of the invention.

In particular, the inductive surface element of the invention is effective in the electromagnetic radiation frequency range which is between 0.2 MHz and 3000 MHz, and especially in the HF band between 3 MHz and 30 MHz.

Conversely, the inductive surface element of the invention also makes it possible to effectively couple surface waves that reach this element with free-space propagation waves that can then be detected.

In addition, the surface waves that are produced by the inductive surface element of the invention have propagation directions that are concentrated in azimuth around the sensitive direction of the element. In this respect, the secondary plates are not necessarily flat. They can be adapted to modify the azimuth directionality of the element of inductive surface. For example, while remaining perpendicular to the base plate, they may have a circular curvature on both sides of the sensitive direction.

Finally, the inductive surface element of the invention is simple, inexpensive and easy to implement. In particular, it can be manufactured separately from the source or the electromagnetic radiation detector with which it is intended to be used, which simplifies its method of manufacture.

• chaque série de plaques secondaires peut comprendre au moins six plaques secondaires ;
• une largeur d'une au moins des plaques secondaires peut être comprise entre λ/2 et λ, mesurée parallèlement à la plaque de base et perpendiculairement à la direction sensible ;
• la hauteur des plaques secondaires peut être comprise entre λ/20 et λ/5, mesurée perpendiculairement à la plaque de base et à partir de celle-ci ;
• l'élément de surface inductif peut comprendre une série de plaques secondaires dont chaque plaque secondaire est constituée d'une paire de plaques ternaires. Dans ce cas, les deux plaques ternaires constitutives d'une plaque secondaire sont séparées d'une distance qui est comprise entre λ/100 et λ/50, mesurée selon la direction sensible ;
• l'une au moins des plaques parmi la plaque de base et les plaques secondaires ou ternaire peut comprendre une portion de tôle ou de grillage métallique, ou une combinaison d'au moins une portion de tôle et d'au moins une portion de grillage métallique. Eventuellement, une (des) portion(s) de tôle perforée ou à ouvertures peut (peuvent) être utilisée(s), pour réduire le poids de l'élément et la quantité de matière consommée ; et
• certaines au moins des plaques secondaires ou ternaires peuvent être connectées électriquement à la plaque de base.

In preferred embodiments of the invention, some of the following improvements may be used, separately or in combination of several of them:

• each series of secondary plates may comprise at least six secondary plates;
• a width of at least one of the secondary plates may be between λ / 2 and λ, measured parallel to the base plate and perpendicular to the sensitive direction;
• the height of the secondary plates can be between λ / 20 and λ / 5, measured perpendicularly to and from the base plate;
• the inductive surface element may comprise a series of secondary plates, each secondary plate of which consists of a pair of ternary plates. In this case, the two ternary plates constituting a secondary plate are separated by a distance which is between λ / 100 and λ / 50, measured along the sensitive direction;
• at least one of the plates of the base plate and the secondary or ternary plates may comprise a portion of sheet metal or wire mesh, or a combination of at least a portion of sheet metal and at least one portion of wire mesh . Optionally, a portion (s) perforated or perforated sheet may be used (s), to reduce the weight of the element and the amount of material consumed; and
• at least some of the secondary or ternary plates may be electrically connected to the base plate.

• une source de rayonnement, qui est adaptée pour produire au moins un rayonnement électromagnétique ayant un mode de propagation en espace libre ; et
• au moins un élément de surface inductif tel que décrit précédemment, qui est posé sur le sol ou semi-enterré ou enterré à proximité de la surface du sol, de sorte que la plaque de base soit parallèle à une surface moyenne de limite entre le sol et un demi-espace aérien.

The invention also proposes a surface wave production assembly, which comprises:

• a radiation source, which is adapted to produce at least one electromagnetic radiation having a free space propagation mode; and
• at least one inductive surface element as described above, which is placed on the ground or semi-buried or buried near the ground surface, so that the base plate is parallel to an average boundary surface between the ground and a half airspace.

The radiation source is oriented such that an electric field component of the electromagnetic radiation at the location of the inductive surface element is not parallel to the base plate.

In addition, the radiation source is adapted so that a wavelength of the electromagnetic radiation is between λ-10% and λ + 10%, where λ is the sizing parameter of the inductive surface element.

In the context of the present invention, it is considered that the inductive surface element is buried near the surface of the ground when the distance between the base plate and the average boundary surface between the ground and the half airspace is less than λ.

The radiation source may in particular be adapted to produce the electromagnetic radiation with a radiation frequency which is between 0.2 MHz and 3000 MHz, and more particularly between 3 MHz and 30 MHz.

• le segment d'antenne rectiligne est séparé de celle des plaques secondaires de l'élément de surface inductif qui est la plus proche de du segment d'antenne rectiligne, d'une distance qui est inférieure ou égale à 0,5 x λ, mesurée selon la direction sensible ; et
• un point du segment d'antenne rectiligne qui est le plus proche de la plaque de base est situé à une hauteur qui est inférieure à 1,5 fois la hauteur des plaques secondaires, ces hauteurs étant mesurées à partir de la plaque de base selon une direction perpendiculaire à cette dernière et du côté des plaques secondaires.

The radiation source may comprise a wired antenna, the transmission efficiency of this type of antenna being particularly high. In the case of a monopole or dipole wire antenna, the strand is preferably oriented such that a straight antenna segment thereof is perpendicular to the base plate. In addition, the wired antenna can be advantageously positioned with respect to the inductive surface element in accordance with at least one of the following arrangement characteristics:

• the rectilinear antenna segment is separated from that of the secondary plates of the inductive surface element which is closest to the rectilinear antenna segment, by a distance which is less than or equal to 0.5 x λ, measured according to the sensible direction; and
• a point of the rectilinear antenna segment which is closest to the base plate is located at a height which is less than 1.5 times the height of the secondary plates, these heights being measured from the baseplate according to a direction perpendicular to the latter and the side of the secondary plates.

The radiation source may comprise a loop-type wire antenna, the plane loop is oriented preferably parallel to the base plate. The requirement for the invention to operate is that electromagnetic radiation from the source has an electric field component not parallel to the base plate.

• un détecteur de rayonnement, qui est adapté pour détecter au moins un rayonnement électromagnétique ; et
• au moins un élément de surface inductif tel que décrit précédemment, qui est posé sur le sol ou semi-enterré ou enterré à proximité de la surface du sol, de sorte que la plaque de base soit parallèle à une surface moyenne de limite entre le sol et un demi-espace aérien.

Finally, the invention also proposes a set of surface wave detection, which comprises:

• a radiation detector, which is adapted to detect at least one electromagnetic radiation; and
• at least one inductive surface element as described above, which is placed on the ground or semi-buried or buried near the ground surface, so that the base plate is parallel to an average boundary surface between the ground and a half airspace.

The radiation detector is oriented to detect electromagnetic radiation when an electric field component of this radiation is non-parallel to the base plate, and is effective for detecting electromagnetic radiation when a wavelength of this radiation is included between λ - 10% and λ + 10%, where λ is the sizing parameter of the inductive surface element.

The characteristic that the inductive surface element is buried near the ground surface further means that the distance between the base plate and the average boundary surface between the ground and the half air space is less than λ.

The radiation detector may then be placed within the surface wave detection assembly in a relative position with respect to the inductive surface element which is identical to that of the radiation source within the surface wave production set.

• la figure 1a est une vue en perspective qui illustre une mise en oeuvre d'un élément de surface inductif selon un premier mode de réalisation de l'invention, dans un ensemble de production d'ondes de surface ;
• les figures 1b et 1c sont respectivement une vue en plan et une vue de côté de l'élément de surface inductif de la figure 1 a ;
• les figures 2a à 2c correspondent respectivement aux figures 1a à 1 c pour un second mode de réalisation de l'invention ;
• les figures 3a à 3c illustrent respectivement trois installations possibles de l'élément de surface inductif selon l'invention ; et
• les figures 4a et 4b sont des diagrammes de gain en transmission et de modification du coefficient de réflexion, respectivement, en fonction de la fréquence du rayonnement électromagnétique, pour un élément de surface inductif selon l'invention.

Other features and advantages of the present invention will become apparent in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

• the figure 1a is a perspective view which illustrates an implementation of an inductive surface element according to a first embodiment of the invention, in a surface wave production assembly;
• the Figures 1b and 1c are respectively a plan view and a side view of the inductive surface element of the figure 1 at ;
• the Figures 2a to 2c correspond to the Figures 1a to 1 c for a second embodiment of the invention;
• the Figures 3a to 3c respectively illustrate three possible installations of the inductive surface element according to the invention; and
• the Figures 4a and 4b are diagrams of transmission gain and of modification of the reflection coefficient, respectively, as a function of the frequency of the electromagnetic radiation, for an inductive surface element according to the invention.

For the sake of clarity, the dimensions of the elements or parts of elements that are shown in these figures do not correspond to actual dimensions or real size ratios. In addition, identical references which are indicated in different figures designate identical elements or which have identical functions.

1

plaque de base

2

plaques secondaires

2a, 2b

plaques ternaires

LS

ligne médiane de la plaque de base, préférablement selon sa dimension longitudinale

Lopm

longueur de la plaque de base mesurée parallèlement à la ligne médiane LS

Lpm

largeur de la plaque de base mesurée perpendiculairement à la ligne médiane LS

L, Ldl

largeur de chaque plaque secondaire ou ternaire mesurée perpendiculairement à la ligne médiane LS

H, Hdl

hauteur de chaque plaque secondaire ou ternaire mesurée perpendiculairement à la plaque de base

Nbl, Nbdl

nombre de plaques secondaires dans chaque série, de préférence supérieur ou égal à six

P

espacement entre les plaques secondaires mesuré selon la ligne médiane LS, et constituant une période d'agencement des plaques secondaires

D

décalage entre deux plaques ternaires d'un couple constituant une plaque secondaire, mesurée selon la ligne médiane LS

3

antenne d'émission de rayonnement électromagnétique

4

source de signal HF, notée GEN. HF

Hant

hauteur de placement de l'antenne 3 au dessus du plan de la plaque de base, mesurée pour un point inférieur de l'antenne 3

Dant

distance entre l'antenne 3 et la plaque secondaire la plus proche

z

axe perpendiculaire à la plaque de base

OL

onde électromagnétique avec mode de propagation en espace libre, produite par l'antenne 3 en direction de l'élément de surface inductif

E

champ électrique de l'onde électromagnétique OL, qui est perpendiculaire à la direction de propagation de l'onde OL

E_z

composante du champ électrique de l'onde électromagnétique OL selon l'axe z

θ

angle d'abaissement ou d'élévation de la direction de propagation de l'onde électromagnétique OL par rapport à un plan parallèle à la plaque de base

OS

onde de surface produite par l'élément de surface inductif à partir de l'onde électromagnétique OL

The Figures 1a to 1c show an inductive surface element according to the invention which comprises a series of secondary plates, and the element of Figures 2a to 2c have a series of secondary plates each by a pair of ternary plates. The references that are mentioned in the figures have the following meanings:

1

base plate

2

secondary plates

2a, 2b

ternary plates

LS

median line of the baseplate, preferably along its longitudinal dimension

LOPM

length of the base plate measured parallel to the center line LS

lpm

width of the base plate measured perpendicular to the LS centerline

L, Ldl

width of each secondary or ternary plate measured perpendicular to the LS centerline

H, Hdl

height of each secondary or ternary plate measured perpendicular to the baseplate

Nbl, Nbdl

number of secondary plates in each series, preferably greater than or equal to six

P

spacing between the secondary plates measured along the centerline LS, and constituting a period of arrangement of the secondary plates

D

offset between two ternary plates of a pair constituting a secondary plate, measured along the center line LS

3

electromagnetic radiation emission antenna

4

HF signal source, denoted GEN. HF

Hant

height of placement of the antenna 3 above the plane of the base plate, measured for a lower point of the antenna 3

Dant

distance between the antenna 3 and the nearest secondary plate

z

axis perpendicular to the base plate

OL

electromagnetic wave with propagation mode in free space, produced by the antenna 3 towards the inductive surface element

E

electric field of the electromagnetic wave OL, which is perpendicular to the direction of propagation of the OL wave

E _z

component of the electric field of the electromagnetic wave OL along the z axis

θ

angle of lowering or elevation of the direction of propagation of the electromagnetic wave OL relative to a plane parallel to the base plate

BONE

surface wave produced by the inductive surface element from the electromagnetic wave OL

All secondary 2 or ternary plates 2a, 2b may have dimensions that are identical. They are all perpendicular to the central line LS, and therefore all parallel to each other. In addition, they are all centered on the centerline LS, and the period P of the secondary plates is constant. When the inductive surface element comprises a series of secondary plates of which each secondary plate consists of a pair of ternary plates, the period T is identical between the ternary plates of two adjacent pairs.

By way of illustration, the base plate 1 may be rectangular, as may each secondary or ternary plate 2a, 2b.

Under these conditions, the median line LS is the direction of emission of the surface wave OS which is produced by the inductive surface element from the electromagnetic wave OL, when the antenna 3 is placed itself. at the right of the center line LS. When the inductive surface element produces a surface wave beam, this beam is centered in azimuth about the center line LS, in a plane that is parallel to the base plate 1. For this reason, the center line LS is called sensitive direction in the general part of the present description.

A surface wave generation assembly is formed by combining the inductive surface element of the FIGS. 1a-1c or 2a-2c with a source of electromagnetic radiation. Such a radiation source comprises the transmitting antenna 3 and the signal source 4, which is connected to supply the antenna 3 as a transmission signal. The antenna 3 may be of the wired antenna type, and in particular a quarter-wave monopole antenna. Such an antenna is known to those skilled in the art. It comprises a rectilinear antenna segment capable of producing electromagnetic radiation that propagates initially in free space, from the antenna segment. The antenna 3 and the source 4 can be adapted so that the electromagnetic radiation has a frequency f in the HF band between 3 MHz and 30 MHz. The wavelength of the wave OL that is produced by the antenna 3 is then λ = C / f, where C is the speed of light in the medium, equal to C ₀ / N where N is the index refraction of the medium and C ₀ the speed of light in the vacuum. The wavelength is then between 10 m (meter) and 100 m for the HF band mentioned above in the air.

• l'élément de surface inductif est placé à proximité de la surface du sol ;
• des dimensions de l'élément de surface inductif sont sélectionnées de façon appropriée par rapport à la longueur d'onde du rayonnement électromagnétique qui est produit par l'antenne 3 ; et
• le segment rectiligne de l'antenne 3 est placé et orienté de façon appropriée par rapport à l'élément de surface inductif.

To produce the surface wave OS which is intended to propagate on the ground surface, from the OL wave with a good power transfer efficiency between the two waves, the arrangement of the production set of surface waves satisfies the following conditions:

• the inductive surface element is placed near the surface of the ground;
• dimensions of the inductive surface element are suitably selected with respect to the wavelength of the electromagnetic radiation that is produced by the antenna 3; and
• the rectilinear segment of the antenna 3 is positioned and oriented appropriately with respect to the inductive surface element.

The inductive surface element can either be placed on the ground ( figure 3a ), or semi-buried ( figure 3b ), completely buried ( figure 3c ). In all cases, the base plate 1 is parallel or substantially parallel to an average surface S of boundary between the ground and the upper half airspace. The actual surface of the ground may be irregular, but the average surface S of the soil boundary is flat. The inductive surface element is further oriented with its secondary plates 2 which are vertical, and upwards above the base plate 1. The depth K of the base plate 1 below the average surface S of the ground boundary is preferably less than λ, or even less at λ / 2. When the inductive surface element is semi-buried, the upper edges of at least one of the secondary plates 2 overflow above the ground, in the half airspace. In the use of the invention which is reported hereinafter, the inductive surface element is placed on the ground ( figure 3a ).

By way of illustration, the inductive surface element used is in accordance with Figures 1a-1c with the following precise characteristics: the number Nbl of secondary plates 2 is equal to 21, the spacing P between two successive secondary plates 2 is constant and equal to 0.0697 × λ, the height H of each secondary plate 2 is equal to 0.161 x λ, the width L of each secondary plate 2 is equal to 0.6678 x λ, the width Lpm of the base plate 1 is equal to the width L of the secondary plates 2 increased by 0.5 x λ, and LOPM length of the base plate 1 is equal to 1.931 x λ. Deviations of ± 10% from these dimensions may be adopted, without the operation of the surface wave generation assembly being significantly altered, for the same frequency of the radiation that is produced by the antenna 3. Being Given the large dimensions that can have the entire inductive surface, at least some of the plates that compose it can be advantageously mesh or perforated sheet, to reduce the weight and the amount of raw material consumed. The thickness of the plates has no significant effect, as long as the plates can be considered as two-dimensional conductive surfaces. Each secondary plate is held fixed relative to the base plate 1, but it does not need to be electrically connected to the base plate 1. Thus each secondary plate can be electrically insulated from the base plate 1.

The antenna 3 is preferably oriented so that the rectilinear antenna segment is vertical, and therefore perpendicular to the base plate 1. The distance Dant can be equal to 0.5 x λ, and the height Hant of the segment of antenna above the base plate 1 may be zero.

An operating condition of the surface wave production assembly is that the component E _z of the electric field E of the electromagnetic wave OL which is produced by the antenna 3, is not zero. In other words, the electric field E of the wave OL is not parallel to the base plate 1. The inductive surface element then modifies the conditions of propagation of the component E _z , by converting a part of the wave OL in the surface wave OS. When the antenna 3 is placed plumb with the center line LS on one side of the inductive surface element, then the OS wave emerges above the inductive surface element, with a direction of propagation which is parallel to the centerline LS. The OS wave has a surface wave structure, with an amplitude of the electric field decreasing exponentially in the Z direction in the air half-space, from the average surface SI of the boundary of the inductive surface element.

The Figures 4a and 4b correspond to an inductive surface element as described above, which has been dimensioned for a wavelength value of the OL wave equal to about 27.3 cm (centimeter). In other words, the dimensioning parameter λ is equal to 27.3 cm. This wavelength value corresponds to a frequency f of electromagnetic radiation which is equal to 1.1 GHz (gigahertz).

In the diagram of the figure 4a , the solid line curve characterizes a transmission efficiency between the radiation source and a remote detector, using the inductive surface element in combination with the radiation source. The dashed line characterizes the same transmission efficiency, but in the absence of inductive surface element. The use of the inductive surface element allows a transmission gain of about 20 dB for the 1.1 GHz radiation frequency.

The diagram of the figure 4b shows the modification of the energy reflection coefficient of the antenna 3, comparing its operations with (continuous curve) and without the inductive surface element (dashed curve). A reduction in reflection of up to 10 dB is achieved at 1.1 GHz using the inductive surface element.

Moreover, radiation power measurements electromagnetic were performed near the end of the inductive surface element from which emerges the surface wave OS, by the method of measuring heating. These measurements were made for the 1.1 GHz radiation frequency, with the inductive surface element being sized for this frequency. They reveal that the inductive surface element produces a high concentration of electromagnetic energy that is transmitted near the ground surface and around the center line LS. Precise measurements confirm that the electromagnetic energy density at the output of the inductive surface element decreases exponentially as the elevation angle with respect to the ground increases. The surface wave nature of the OS wave is thus verified.

A possible dimensioning for the inductive surface element Figures 2a-2c can be: the number Nbdl of the ternary plates 2a and that of the ternary plates 2b are also equal to 21, the spacing P between two ternary plates 2a which are successive, or between two successive ternary plates 2b, is equal to 0.0953 x λ, the shift D between the ternary plates 2a and 2b is 0.0229 x λ, the height Hdl of each ternary plate 2a or 2b is equal to 0.1074 x λ, and the width Lpm of the base plate 1 is at least equal to the width Ldl of each ternary plate 2a or 2b, itself equal to 0.6678 x λ. However, deviations of ± 20% from these precise dimensions can be adopted without changing the source radiation frequency that is used with the inductive surface element.

The arrangement of such an inductive surface element with two series of ternary plates, with respect to the antenna 3 within the surface wave production assembly, may be identical to that described for the inductive surface element Figures 1a-1c .

An inductive surface element according to the invention can also be used within a surface wave detection assembly. For this, the inductive surface element is still placed on the ground, semi-buried or buried in the same way, but with its central line LS, or sensitive direction, which is oriented in a direction of reception of surface waves . A radiation detector can then be placed substantially in the same place as the antenna 3 with respect to the inductive surface element. Under these conditions, the inductive surface element partially transforms the received surface wave into a wave structure that converges on the radiation detector. The surface wave that is received can thus be detected with a high sensitivity.

It is understood that the invention may be reproduced by modifying some of the features which have been described by way of example. It is recalled that an inductive surface element that is in accordance with the invention can be dimensioned simply for any radiation frequency, using the dimensioning rules that have been given.

Finally, several inductive surface elements according to the invention may be arranged around the same source of electromagnetic radiation, in order to simultaneously transmit surface waves in several directions.

Claims (15)

1. Inductive surface element for an electromagnetic radiation, said element comprising:

   - an electrically conductive base plate (1) which extends in a plane not parallel to an electric field component of the electromagnetic radiation; and

   - a series of electrically conductive secondary plates (2), the secondary plates extending perpendicularly to the base plate and in a same half-space on one side of said base plate, symmetrically with respect to a plane perpendicular to said base plate in a direction referred to as the sensitive direction,

   said secondary plates (2) having a same height (H; Hdl) to within ±10%, said height being between 0.035 x λ and 0.35 x λ, where λ is a sizing parameter of the inductive surface element;
   the distance between the base plate and the facing edges of the secondary plates is between zero and half the height of the relevant secondary plates;
   said secondary plates (2) being arranged parallel to each other with a period such that the product of the height of the secondary plates times the period is between 0.001 x λ² and 0.15 x λ² to within ±10%;
   said secondary plates (2) each extending for a total length of at least 0.0003 x λ perpendicularly to the sensitive direction;
   said inductive surface element thus being adapted to modify propagation conditions, in said half-space, of a projection of the electric field component which is perpendicular to the base plate (1), when a wavelength of the electromagnetic radiation is between λ - 10% and λ + 10%,
   so as to allow generating or detecting surface waves concentrated in azimuth about a direction of propagation which is parallel to a ground surface, when said inductive surface element is placed on the ground or is partially buried near the ground surface, so that the base plate (1) is parallel to an average boundary surface between the ground and the air half-space.
2. Inductive surface element according to claim 1, wherein the series of secondary plates (2) comprises at least six secondary plates.
3. Inductive surface element according to claim 1 or 2, wherein a width (L; Ldl) of at least one of the secondary plates (2) is between λ/2 and λ, measured parallel to the base plate (1) and perpendicularly to the sensitive line.
4. Inductive surface element according to any preceding claim, wherein the height (H; Hdl) of the secondary plates (2) is between λ/20 and λ/5, measured perpendicularly to the base plate (1) and from said base plate.
5. Inductive surface element according to any preceding claim, wherein each secondary plate consists of a pair of ternary plates (2a, 2b).
6. Inductive surface element according to claim 5, wherein the ternary plates of each pair (2a, 2b) are separated by a distance of between λ/100 and λ/50, measured along the sensitive direction.
7. Inductive surface element according to any preceding claim, wherein at least one of the plates among the base plate (1) and the secondary (2) or ternary (2a, 2b) plates comprises a portion of sheet metal or metal grating, or a combination of at least one sheet metal portion and at least one metal grating portion.
8. Inductive surface element according to any preceding claim, wherein at least some of the secondary (2) or ternary (2a, 2b) plates are electrically connected to the base plate (1).
9. Surface wave generation assembly, comprising:

   - a radiation source (3), which is adapted to generate at least one electromagnetic radiation having a free space propagation mode; and

   - at least one inductive surface element according to any of claims 1 to 8, placed on the ground or partially buried or buried near the ground surface, such that the base plate (1) is parallel to an average boundary surface between the ground and an air half-space;

   the radiation source (3) being oriented so that an electric field component of the radiation at the location of the inductive surface element is not parallel to the base plate (1); and
   the radiation source (3) being adapted so that a wavelength of the electromagnetic radiation is between λ - 10% and λ + 10%, where λ is the sizing parameter of the inductive surface element.
10. Surface wave generation assembly according to claim 9, wherein the radiation source (3) is adapted to produce the electromagnetic radiation with a radiation frequency of between 0.2 MHz and 3000 MHz.
11. Surface wave generation assembly according to claim 9 or 10, wherein the radiation source (3) comprises a wire antenna.
12. Surface wave generation assembly according to claim 11, wherein the wire antenna comprises a rectilinear antenna segment, and said wire antenna is oriented such that the rectilinear antenna segment is perpendicular to the base plate (1).
13. Surface wave generation assembly according to claim 12, wherein the wire antenna is positioned such that the rectilinear antenna segment is separated from the one of the secondary plates (2; 2a, 2b) of the inductive surface element which is closest to said rectilinear antenna segment, by a distance (Dant) which is less than or equal to 0.5 x λ measured along the sensitive direction.
14. Surface wave generation assembly according to claim 12 or 13, wherein the wire antenna is positioned so that a point of the rectilinear antenna segment which is closest to the base plate (1) is located at a height (Hant) less than 1.5 times the height (H; Hdl) of the secondary plates (2; 2a, 2b), said heights being measured from the base plate (1) in a direction perpendicular to said base plate and on the side of the secondary plates.
15. Surface wave detection assembly comprising:

    - a radiation detector which is adapted to detect at least one electromagnetic radiation; and

    - at least one inductive surface element according to any of claims 1 to 8, placed on the ground or partially buried or buried near the ground surface, such that the base plate (1) is parallel to an average boundary surface between the ground and an air half-space;

    the radiation detector being oriented to detect the electromagnetic radiation when an electric field component of said radiation is not parallel to the base plate (1), and being effective for detecting the electromagnetic radiation when a wavelength of said radiation is between λ - 10% and λ + 10%, where λ is the sizing parameter of the inductive surface element.

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