EP4203185B1 - Improved wideband wire antenna - Google Patents

Description

The present invention relates to the field of wide frequency band wire antennas.

In an electromagnetic listening system, for example airborne, the antennas, which are used either individually or in a goniometric or interferometric network, must operate in a very wide frequency band and in a circular, linear or double linear polarization, because neither the frequency nor the polarization of the signal to be captured are known a priori. It should be noted that the characteristics of an antenna being the same in reception and in transmission, an antenna can be characterized both in transmission and in reception. In what follows, the behavior in transmission is more often presented.

These antennas must have the smallest possible size and, in particular, a low thickness. They must also have radiation performances (gain, quality of radiation patterns, etc.) reproducible from one antenna to another, for network applications or to facilitate their replacement during a maintenance operation.

In this context, it is known to use wire antennas. In such an antenna, the radiating element consists of a metal wire which is shaped to describe, in a so-called radiating plane, a spiral-shaped pattern for a spiral antenna, or a log-periodic-shaped pattern for a log-periodic antenna, or a hybridization of these two geometries for a sinuous antenna (as defined for example in the article by Crocker DA et al. “Sinuous Antenna Design for UWB Radar” 2019 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting, DOI: 10.1109/APUSNCURSINRSM.2019.8888630 ).

For a spiral type antenna, the metal wire is wound on itself so as to form, when viewed from above, a spiral. This spiral can for example be an Archimedean spiral, a logarithmic spiral, or other. Alternatively, several metal wires can be used to form as many spirals nested between each other.

In a log-periodic antenna, the wire is shaped so as to have, when viewed from above, several segments. Each segment is inscribed in an angular sector, extends radially and has indentations. The length of each tooth and the spacing between two successive teeth of a segment follow a logarithmic progression.

In the following we will talk about a strand of the radiating element, whether it is a turn of the wire of a spiral antenna or a tooth of a segment of a log-periodic antenna.

In practice, in planar technology, the radiating element is produced by etching a thin metal layer, for example a copper layer between 2 and 40 µm, for example equal to 17.5 µm or 35 µm, deposited on a thin support film.

In a known technology, the radiating plane is located above a metal reflecting plane. In such an antenna, the wave emitted by the radiating element towards the rear of the radiating plane is reflected forward by the reflecting plane. During this reflection, the wave is phase-shifted by an angle π. The reflected wave propagates forward and interferes, beyond the radiating plane, with the wave emitted by the radiating element towards the front of the radiating plane. This interference is constructive when, for a position of the wave front, the phases of the waves emitted forward and reflected forward are close. This occurs if the distance separating the radiating plane and the reflecting plane is close to λ/4, where λ is the wavelength in the dielectric medium between the radiating plane and the reflecting plane of the wave emitted by the radiating element.

The thickness of such an antenna is reduced compared to that of an antenna according to other known technologies, in particular an absorbing cavity antenna. In addition, its manufacture is greatly simplified and reproducible.

However, the frequency band of such an antenna is restricted because of the relationship between the operating frequency of the antenna (i.e. the wavelength of the emitted wave) and the fixed distance between the radiating plane and the reflecting plane (which is defined by construction).

To address this issue, the applicant proposed, in the application FR 3 003 702 , two embodiments of an antenna comprising, interposed between the radiating plane and the reflecting plane, a substrate having a relative electrical permittivity which varies as a function of the distance from the axis of the antenna, here called radius r. The fixed distance between the radiating plane and the reflecting plane is thus overcome by modifying the wavelength in the substrate as a function of the radius by varying the value of the permittivity. At a given frequency, only one ring of the antenna functions correctly, i.e. allows for constructive interference in front of the radiating plane in transmission.

In the first embodiment, a permittivity gradient along the radius r is obtained by producing, in a disk made of a first dielectric material, vertical and through vias (empty or filled with a second dielectric material).

However, the production of vias is delicate, especially in the center of the antenna where their density must be high to reduce the relative electrical permittivity. The mechanical strength of the substrate is then greatly reduced. In addition, it is difficult to produce vias with a diameter less than λ/10, a dimension below which the propagation of a wave in the substrate is not disturbed by the presence of these vias.

In the second embodiment, a permittivity gradient along the radius r is obtained by associating rings made of different dielectric materials, the lateral faces of the rings being beveled to obtain a continuous permittivity gradient along the radius r.

However, regardless of the embodiment, a first problem has been identified. It relates to the generation of creeping waves on the surface of the reflecting plane. Once generated, a creeping wave can disturb the reflection of the wave emitted backwards by the radiating element and consequently alter the constructive interference that one seeks to create with the wave emitted forwards in front of the radiating plane.

This first problem is caused by the substrate material in the immediate vicinity of the reflecting plane, which has too high a local relative electric permittivity. It should ideally be equal to or close to unity.

A second problem has been identified. It is related to the coupling established between two successive strands of the radiating element. Since each strand is associated with a specific operating frequency, such coupling degrades the accuracy of the antenna.

This second problem is caused by the substrate material in the immediate vicinity of the radiating plane, which has too high a local relative electric permittivity. It should ideally be equal to or close to unity.

A third problem has been identified. It lies in the disruption of the wave path when crossing the substrate.

In the first embodiment, in particular at the center of the antenna associated with high frequencies and therefore short wavelengths, it is not possible to produce vias with a diameter less than λ/10. The vias produced therefore disturb the waves passing through the substrate by diffraction.

In the second embodiment, the interface between two successive rings constitutes a jump in the local relative permittivity, i.e. an index jump. This interface therefore disrupts the direction of propagation of the waves by refraction.

In both cases, the reflected wave no longer allows us to precisely establish constructive interference in front of the reflecting plane.

These various problems therefore alter the properties of the antenna, in particular its radiation pattern.

The aim of this invention is to solve these problems.

For this purpose, the invention relates to a wide-band frequency wire antenna comprising: a radiating element, the radiating element comprising at least one metal wire shaped around an axis of the antenna, in a transverse radiating plane; a reflecting plane, the reflecting plane being transverse to the axis, the radiating plane being located at a predetermined height (h0) above the reflecting plane; and a substrate, the substrate being interposed between the radiating element and the reflecting plane, and having a constant thickness, characterized in that a local relative electrical permittivity and/or a local relative electrical permeability of the substrate is a function of the radius, i.e. of the distance from the axis, and of a height, i.e. of a distance from the reflecting plane, the local relative electrical permittivity being, at constant height, increasing as a function of the radius, and, at constant radius, increasing as a function of the height at least for a portion of the substrate in the vicinity of the reflecting plane.

• la permittivité électrique relative locale et/ou la perméabilité électrique relative locale est, à rayon constant, décroissante en fonction de la hauteur au moins pour une portion du substrat au voisinage de l'élément rayonnant.
• la permittivité électrique relative locale et/ou la perméabilité électrique relative locale est, à rayon constant, une fonction cosinus de la hauteur.
• la permittivité électrique relative locale et/ou la perméabilité électrique relative locale est une fonction continue du rayon et de la hauteur.
• le substrat résulte de la combinaison d'au moins un premier matériau présentant une première permittivité électrique relative et/ou une première perméabilité électrique relative, avec un second matériau présentant une seconde permittivité électrique relative différente de la première et/ou une seconde perméabilité électrique relative différente de la première, une concentration relative des premier et second matériaux étant fonction du rayon et de la hauteur.
• la combinaison des premier et second matériaux est réalisée par la mise en oeuvre d'une technologie de fabrication additive, notamment d'impression tridimensionnelle.
• le premier matériau présente une pluralité de premiers interstices, certains desdits premiers interstices étant remplis par le second matériau et/ou le second matériau présente une pluralité de seconds interstices, certains desdits seconds interstices étant remplis par le premier matériau.
• les premiers interstices et/ou les seconds interstices ont une dimension caractéristique qui dépend du rayon et/ou de la hauteur.
• les premiers interstices et/ou les seconds interstices ont une forme parallélépipédique ou sphérique, la plus grande dimension d'un interstice étant de préférence inférieure à λ/10.

According to particular embodiments, the antenna comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

• the local relative electrical permittivity and/or the local relative electrical permeability is, at constant radius, decreasing as a function of the height at least for a portion of the substrate in the vicinity of the radiating element.
• the local relative electric permittivity and/or the local relative electric permeability is, at constant radius, a cosine function of the height.
• the local relative electric permittivity and/or local relative electric permeability is a continuous function of radius and height.
• the substrate results from the combination of at least a first material having a first relative electrical permittivity and/or a first relative electrical permeability, with a second material having a second relative electrical permittivity different from the first and/or a second relative electrical permeability different from the first, a relative concentration of the first and second materials being a function of the radius and the height.
• the combination of the first and second materials is achieved by implementing additive manufacturing technology, in particular three-dimensional printing.
• the first material has a plurality of first interstices, some of said first interstices being filled by the second material and/or the second material has a plurality of second interstices, some of said second interstices being filled by the first material.
• the first interstices and/or the second interstices have a characteristic dimension which depends on the radius and/or the height.
• the first interstices and/or the second interstices have a parallelepiped or spherical shape, the largest dimension of an interstice preferably being less than λ/10.

• La figure 1 est une vue en perspective d'une antenne selon l'invention ;
• La figure 2 est une demi-section selon un plan axial de l'antenne de la figure 1 ;
• La figure 3 est un graphe représentant la permittivité électrique relative locale dans le substrat de l'antenne de la figure 1 en fonction du rayon r (distance à l'axe A) et la hauteur h (distance au plan réflecteur) ;
• La figure 4 illustre une structure possible du substrat de l'antenne de la figure 1 permettant d'obtenir la distribution de permittivité électrique relative locale représentée sur la figure 3 ; et,
• La figure 5 est un graphe représentant le gain en fonction de la fréquence d'une antenne selon l'état de la technique et d'une antenne selon l'invention.
• Les figures représentent un mode de réalisation préférentiel de l'antenne selon l'invention.

The invention and its advantages will be better understood upon reading the detailed description which follows of a particular embodiment, given solely as a non-limiting example, this description being made with reference to the appended drawings in which:

• There figure 1 is a perspective view of an antenna according to the invention;
• There figure 2 is a half-section along an axial plane of the antenna of the figure 1 ;
• There figure 3 is a graph representing the local relative electrical permittivity in the antenna substrate of the figure 1 as a function of the radius r (distance to the axis A) and the height h (distance to the reflecting plane);
• There figure 4 illustrates a possible structure of the antenna substrate of the figure 1 allowing to obtain the local relative electrical permittivity distribution represented on the figure 3 ; And,
• There figure 5 is a graph representing the gain as a function of the frequency of an antenna according to the state of the art and of an antenna according to the invention.
• The figures represent a preferred embodiment of the antenna according to the invention.

As shown in the figures 1 And 2 , the wide frequency band wire antenna 2 comprises, stacked along an axis A, a reflector plane 8, a substrate 6 and a radiating element 4.

An origin O is chosen at the intersection of the axis A and the reflecting plane 8.

The coordinate along the A axis is called height h. It is therefore the distance to the reflecting plane 8.

A direction D is chosen extending radially with respect to the axis A in the reflecting plane 8. The coordinate according to the direction D is called radius r. It is therefore the distance to the axis A.

The radiating element 4 is arranged in a radiating plane S, located at a height h ₀ from the reflector plane 8.

The radiating element 4 is for example produced by etching a metal layer carried by a support film 5.

The radiating element 4 comprises, for example, first and second metal wires 10 and 12 which are respectively shaped according to a spiral, in particular an Archimedean spiral, around the axis A.

The reflecting plane 8 is for example a disk with axis A and radius r ₀ . It is made of a metallic material. Its function is to reflect any incident wave whatever its frequency.

The substrate 6 has the general external shape of a disk with axis A of radius r ₀ and constant thickness, equal to the height h ₀ .

The substrate 6 is in contact, by a lower surface 14, with the reflecting plane 8. The substrate 6 is in contact, by an upper surface 15, with the radiating element 4, or more precisely with the support film 5 of the radiating element 4.

A feed device (not shown in the figures) for the radiating element 4 is positioned below the reflector plane 8. The reflector plane 8 and the substrate 6 are advantageously provided with a passage (not shown), along the axis A, for the passage of the feed lines for the radiating element 4.

The higher the operating frequency F, the closer the active zone Z of antenna 2 gets to axis A. It is therefore the peripheral part of antenna 2 which radiates at low operating frequencies and the central part of antenna 2 which radiates at high operating frequencies.

According to the invention, the substrate 6 has a local relative electrical permittivity ε _r at the point P(r,h) which is a function of both the radius r and the height h. It can therefore be written: ε _r (r,h).

There figure 3 represents a possible example of this function. On the figure 3 , iso-permittivity curves have been plotted and the corresponding value of the local relative electrical permittivity ε _r indicated.

The dependence of the permittivity ε _r on h, for a given radius r, is such that for h close to 0, that is to say for points P(r, h) of the substrate in the immediate vicinity of the reflecting plane 8, the permittivity is minimal, preferably equal to unity.

Thus, the material of the substrate 6 in contact with the reflecting plane 8 has a low permittivity such as to avoid the generation of creeping waves.

Advantageously, the dependence of the permittivity on h, for a given radius r, is such that for h close to h ₀ , that is to say for the points P(r, h) of the substrate 6 in the immediate vicinity of the radiating plane 4, the permittivity is minimal, preferably equal to unity.

Thus, the material of the substrate 6 in contact with the radiating element 4 has a low permittivity such as to avoid coupling between two consecutive strands of the radiating element 4.

Moreover, independently of one and/or the other of these behaviors at the lower and upper limits of the substrate 6, the dependence of the local relative permittivity ε _r (r,h) is advantageously continuous in h and in r.

Thus, the substrate material does not disturb the propagation of waves passing through the substrate.

It should be noted that in the state of the art, the relative electrical permittivity considered is an effective permittivity, obtained by integration over the height h, at a given radius r.

THE figure 3 represents, in gray level, an example of a substrate whose permittivity ε _r at a point P(r, h) depends on the radius r and the height h of this point.

The local relative electric permittivity combines the three improvements identified above, namely a value close to unity on the lower surface 14, a value close to unity on the upper surface 15, and continuity at all points.

The local permittivity, for a given radius r, has a first minimum for a zero height, then increases with the height, to reach a maximum (for example in the middle of the substrate (h ₀ /2), then decreases with the height h, to reach a second minimum for the height h ₀ .

où y est un paramètre de valeur constante et prédéfinie, et n est une variable pouvant être un entier ou une fonction dépendant de r et/ou hPreferably, the dependence on h and r of the local relative electric permittivity is of the general form: $${\mathrm{\epsilon }}_{\mathrm{r}}\left(\mathrm{r}\mathrm{,h}\right)=\frac{\left[{\left(\frac{\mathrm{\pi }}{2}\frac{\mathrm{<figure-callout\; id="0"\; label="r\; h"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">r</figure-callout>}}{h_{}}\right)}^2-{\mathrm{\epsilon }}_{\min }\right]\ast \frac{\left(\mathrm{n}+1\right)}{{\mathrm{h}}_0^{\mathrm{n}}}\ast {\mathrm{h}}^{\mathrm{n}}}{\mathrm{y}}+{\mathrm{\epsilon }}_{\min }$$

where y is a constant and predefined value parameter, and n is a variable which can be an integer or a function depending on r and/or h

In the embodiment of the figure 3 , the permittivity takes the particular form: $${\mathrm{\epsilon }}_{\mathrm{r}}\left(\mathrm{r}\mathrm{,h}\right)=\mathrm{HAS}\left(r\right)\cos \left(\frac{h-\frac{{\mathrm{<figure-callout\; id="0"\; label="h"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">h</figure-callout>}}_0}{2}}{\frac{{\mathrm{<figure-callout\; id="0"\; label="h"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">h</figure-callout>}}_0}{2}}\right)+{\mathrm{\epsilon }}_{\min }$$

In this example, the local relative electric permittivity ε _r is a cosine function of the height h, at a given radius r.

The value of the minimum of this function is e _min , which is preferably 1.

The value of the maximum of this function for a given height h depends on the radius r.

As in the state of the art, the effective permittivity at a given radius r, i.e. the integral according to the variable h of the local relative electrical permittivity ε _r (r,h) between 0 and h ₀ , is a function of the radius r adapted to allow the desired constructive interference, the principle on which this antenna technology is based.

As illustrated by the figure 4 , for the production of the substrate 6, an additive manufacturing process is preferably used, for example three-dimensional printing.

The material constituting the substrate 6 results from the combination of at least two materials, respectively a first material, having a first low relative permittivity, and a second material, having a second high relative permittivity.

The relative concentration of the first and second materials at a point P(r,h) is a function of the coordinates h and r.

For example and preferably, the first material is deposited so as to have a plurality of first interstices, some of said first interstices being filled by the second material and/or the second material has a plurality of second interstices, some of said second interstices being filled by the first material.

For example, three-dimensional printing makes it possible to structure the substrate into cells.

To produce the cell 21, the first material is deposited to form the walls 32 of the cell while providing a gap 31, which is left empty.

To produce the cell 22, the first material is deposited to form the walls 34 of the cell, while leaving a gap 33, the latter then being filled with the second material.

To produce the cell 23, the second material is deposited to form the walls 36 of the cell while leaving a gap 35, the latter then being filled with the first material.

To make the cell 24, the second material is deposited to form the walls 37 of the cell, without leaving any gaps. The cell is full.

The thickness of the walls (and therefore the size of the gaps) is adjusted for each cell so as to obtain the value of the local relative electrical permittivity sought, taking into account the properties of the materials used.

Advantageously, the first interstices and/or the second interstices have a characteristic dimension which depends on the distance from the axis and/or the distance from the radiating plane and/or the reflecting plane.

The first interstices and/or the second interstices have a rectangular parallelepiped shape (as a first approximation). Alternatively, they have a spherical shape.

The largest dimension of a gap is less than λ/8, preferably less than λ/10, more preferably less than λ/15.

The structure of the substrate has, due to this alveolar structure, good mechanical resistance.

There figure 5 is a graph representing the gain (in Decibel dB) as a function of the operating frequency (in Hertz Hz) of an antenna according to the state of the art and of an antenna according to the invention. The gain is here evaluated along the axis of the antenna.

It is noted, in particular for the first half of the frequency spectrum, that the gain of the antenna according to the invention is much more stable in frequency with gain values often higher than those of an antenna according to the state of the art.

Alternatively, instead of characterizing the antenna by a local relative electrical permittivity function of r and h, it could be characterized by a local relative electrical permeability function of r and h.

Claims (9)

1. A wideband wire antenna (2) comprising:

   - a radiating element (4), the radiating element comprising at least one metal wire (10, 12) shaped around an axis (A) of the antenna, in a transverse radiating plane (S);

   - a reflecting plane (8), the reflecting plane being transverse to the axis (A), the radiating plane being located at a predetermined height (h₀) above the reflecting plane (8); and,

   - a substrate (6), the substrate being interposed between the radiating element (4) and the reflecting plane (8), and having a constant thickness,

   characterized in that a local relative electrical permittivity and/or a local relative electrical permeability of the substrate (6) is a function of the radius (r), i.e. the distance to the axis (A), and a height (h), i.e. a distance to the reflecting plane (8), the local relative electrical permittivity and/or a local relative electrical permeability being, at constant height, increasing as a function of the radius, and, at constant radius, increasing as a function of the height at least for a portion of the substrate (6) in the vicinity of the reflecting plane (8).
2. The antenna according to claim 1, wherein the local relative electrical permittivity and/or the local relative electrical permeability is, at constant radius (r), decreasing with height at least for a portion of the substrate (6) in the vicinity of the radiating element (4).
3. The antenna according to claim 2, wherein the local relative electrical permittivity and/or the local relative electrical permeability is, at constant radius (r), a cosine function of the height (h).
4. The antenna according to any one of the preceding claims, wherein the local relative electrical permittivity and/or the local relative electrical permeability is a continuous function of the radius (r) and the height (h).
5. The antenna according to any of the preceding claims, wherein the substrate (6) results from the combination of at least a first material having a first relative electrical permittivity and/or a first relative electrical permeability, with a second material having a second relative electrical permittivity different from the first and/or a second relative electrical permeability different from the first, a relative concentration of the first and second materials being a function of the radius (r) and height (h).
6. The antenna according to claim 5, wherein the combination of the first and second materials is achieved by using an additive manufacturing technology, in particular three-dimensional printing.
7. The antenna according to claim 5 or claim 6, wherein the first material has a plurality of first interstices, some of said first interstices (31, 33) being filled by the second material and/or the second material has a plurality of second interstices (35), some of said second interstices being filled by the first material.
8. The antenna according to claim 4, wherein the first interstices and/or the second interstices have a characteristic dimension which depends on the radius (r) and/or on the height (h).
9. The antenna according to claim 7 or claim 8, wherein the first interstices (31, 33) and/or the second interstices (35, 37) have a parallelepipedal or spherical shape, the largest dimension of an interstice preferably being less than λ/10.

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