GB2272332A - Antenna devices and microwave resonators - Google Patents

Abstract

A device forming an artificial capacitive surface admittance is applied to the production of bidimensional cavities, resonators and antennas. The device comprises two planar assemblies positioned facing one another and separated by a planar dielectric layer (21). Each planar assembly comprises electrically conductive patterns (22) which are electrically insulated from one another, said patterns being staggered with respect to each other in such a way that the device behaves like a dielectric layer having a high dielectric constant. The device may be used to tune a cavity or a dielectric resonator. <IMAGE>

Description

2272332 Device forming an artificial capacitive surface admittance and

application to the production of bidimensional cavities, resonators and antennas.

DESCRIPTION

The present invention relates to a device forming an artificial capacitive surface admittance, which is also called an artificial capacitive surface impedance (an admittance Y being the opposite to an impedance Z).

This admittance is fixed in certain embodiments and regulatable in others.

The range of electromagnetic waves to which the invention more particularly relates is that of radio waves and microwaves.

The invention more particularly applies to the production of various devices using electromagnetic properties associated with capacitive surface admittances and is more particularly applicable to the production of waveguides, antennas and tunable microwave resonators.

Consideration will now be given to two known examples of a natural surface admittance.

The first example is that of a resistive plane. Such a plane is obtained by depositing on a thin, planar, electrically insulating support, a very thin layer of a metal or a semiconductor, or a conductive polymer, so as to obtain a clearly defined, homogeneous value of the surface resistance Rs for the thus formed layer. For an electromagnetic wave arriving under a normal incidence, said layer constitutes a resistive impedance equal to Rs, provided that the influence of the support is B 11414.3 PV made negligible, which can be easily brought about.

The resistive plane obtained is diagrammatically shown in fig. 1A, where it is possible to see the conductive layer 2 formed on the insulating support 4. The resistive surface impedance formed is in particular used for producing an anti-reflecting screen, known as a Salisbury screen. A Salisbury screen is a device aiming at cancelling out the reflection coefficient of a metal surface.

Fig. 1B diagrammatically shows such a device. The metal surface is represented by the metal plane 6. The latter is a perfect reflector for a microwave.

is The Salisbury screen is formed by placing the resistive plane 2 formed on the substrate 4 upstream of the metal plane 6 at a distance Lo/4 therefrom, so as to be on the path of the incident electromagnetic wave of wavelength Lo.

Moreover, the surface resistance of the resistive plane 2 is equal to the impedance of the vacuum, which is 377 ohms (square root of the ratio of the magnetic permeability of the vacuum Mo to the permittivity of the vaccum Eo).

It is found that the incident electromagnetic wave is completely absorbed by the resistive layer 2 and consequently the module of the reflection coefficient, which was 1 for the metal plane 6 alone, is now 0 for the structure shown in fig. 1B.

The second example of a natural surface impedance is constituted by what is called a dielectric sheet. In this case it is a reactive impedance and more specifically a capacitive impedance. This dielectric sheet is constituted by a single, thin, planar, B 11414.3 PV 2 dielectric material plate 8 (fig. 2A), whose thickness is designated h and whose dielectric constant is designated E.

This plate 8 can be looked upon as an impedance located on a plane and therefore as a surface impedance or admittance, to the extent that its thickness h proves the following condition:

h < <L with L=Lo/E 0.5 (1) in which Lo is the wavelength in the vacuum or in air of an incident electromagnetic wave and L is the wavelength in the dielectric material of the plate 8 of said incident wave at the considered frequency f.

The value of the corresponding surface admittance Y is then:

Y = j.w.E.Eo.h in which w--2.pi.f 2 3 =_, pi being the number equalling approximately 3.14.

It can be seen that the aforementioned condition (1) implies that the modulus of the admittance Y is low, unless E is very large.

An interesting borderline case is that where E tends towards infinity, whereas h tends towards 0, so that the product E.h remains constant and finite. This is an ideal capacitive surface admittance because, in the extreme case, h is zero.

B 11414.3 PV A known application of a thin dielectric plate such as the plate 8 is the planar guide.

It is possible to demonstrate and experimentally prove that a planar dielectric plate of dielectric constant E, thickness h, which is electrically isolated or insulated in vacuum or air, or even placed within another dielectric material with a dielectric constant El below E, is able to guide an electromagnetic wave. Such an electromagnetic wave guided by the plate is propagated in the plane of said plate in the form of modes.

A mode is a particular distribution of the electric and magnetic fields in the plane, which is perpendicular to the wave propagation direction, said distribution being retained during said propagation: the shape or profile of the mode then being retained during propagation.

There is a double infinity of modes, namely transverse electric modes because their electric field is both parallel to the plate plane and perpendicular to the wave propagation direction, and transverse magnetic modes because their magnetic field is parallel to the plate plane and perpendicular to the wave propagation direction. Among all these modes, only the fundamental transverse electric mode TEo is covered directly by the present invention.

Thus, when the dielectric constant E of the plate 8 tends towards infinity and the thickness h of said plate simultaneously tends towards 0 in such a way that the product Ech remains finite, i.e. when the dielectric plate is effectively reduced to the aforementioned ideal surface admittance, said fundamental mode TEo still exists.

B 11414.3 PV 7 It is then possible to speak of a mode guided by a surface admittance. This mode is characterized by an electric field profile Ex, whose shape is shown in fig. 2B.

A trihedral Oxyz (fig. 2A) is defined, in which 0 is the centre of the planar plate 8, the axis y is perpendicular to said plate and the axes x and z are parallel to the plate plane. Thus, in fig. 2B it is possible to see the variations of Ex as a function of y.

Fig. 2B also shows the variations of the components Hy and Hz of the magnetic field corresponding to mode TEo, as a function of y. In fig. 2B, the guidance of said mode TEo leads to the electric field Ex being at a maximum on the plane of the plate and decreasing exponentially on moving away from said plate.

It is possible to define a confinement width of the mode TEo or rather a confinement thickness of said mode, more simply referred to hereinafter as "thickness of the mode", as the thickness of the region in which the electric field Ex maintains a high value compared with its maximum Emax (obtained for y=O), namely a value substantially equal to or higher than e-lxEmax, in which e is the number of approximately 2.718.

These guidance properties of mode TEo by a thin plate made from a dielectric material with a high dielectric constant can be utilized for producing radio or microwave resonators.

Such a resonator can be called a "surface admittance bidimensional cavity". Fig. 3 diagrammatically illustrates such a cavity.

B 11414.3 PV In the example shown in fig. 3 use is made of a thin, planar dielectric plate 10, whose thickness is h and which is e.g. shaped like a rectangle of length Lg and width W. The cavity of fig. 3 is defined by four metal walls 12, which are perpen- dicular to the plate 10, bear on the edges of said plate and extend on either side of the plane of the plate 10 in such a way that the latter is equidistant of the ends 14,16 of the walls.

The mode TEo which is guided by the plate 10 and whose profile extends on either side of the plane of the plate 10 is reflected on the metal walls 12 in exactly the same way as with a planar wave, but provided that the height Ht of said walls 12 significantly exceeds half the thickness of the mode (whose definition was given hereinbefore) and is at least ten times the half- thickness of the mode, i.e. at least ten times half the thickness of the mode. The term "height Ht of the wall" is understood to mean the distance between the thin plate 10 and one of the ends 14,16 of the walls, said distance being counted perpendicular to the plate 10, so that the distance between the two ends 14 and 16, counted perpendicular to said plate, is 2xHt.

The phenomenon of guiding an electromagnetic wave by a very thin dielectric plate with a high dielectric constant, in the same way as the resonances associated with said phenomenon in bidimensional cavities are known or are derived in obvious manner from known phenomena and are even used in practice, particularly for the measurement of dielectric constants.

Reference can be made in this connection more particularly to fig. 2 of the document:

B 11414.3 PV (1) Article of G. Kent, "A dielectrometer for the measurement of substrate permittivity% Microwave Journal, vol. 34, No. 12, December 1991, pp. 72 to 82.

The construction of a capacitive surface admittance by natural means, i.e. as stated hereinbefore, by using a dielectric material plate suffers from two disadvantages.

Firstly in order to bring about an optimum approach to the conditions according to which h is zero, E (dielectric constant of the material) infinite and hxE finite, it is necessary to use a material with a high dielectric constant of approximately 100, such a material being very infrequently encountered in the range of ultra-high and radio frequencies.

The value of the complex admittance, for a given pulsation or ripple w, is fixed by the thickness and the nature of the dielectric material. Thus, it is impossible to produce a regulatable admittance in this way.

The present invention is directed at a device forming a capacitive surface admittance, i.e. which is equivalent from the radio standpoint to a thin plate made from a dielectric material having a high dielectric constant, said device avoiding the first of the two aforementioned disadvantages and, in a special embodiment, the second of said disadvantages.

The present invention relates to a device forming a capacitive surface admittance, which incorporates two planar assemblies positioned facing one another and separated by a planar dielecttric layer, each planar assembly incorporating planar, electrically conductive patterns, which are electrically isolated from one another, characterized in that said assemblies are reciprocately displaced parallel to the dielectric layer in B 11414.3 PV such a way that the device behaves like a dielectric layer having a high dielectric constant of approximately 50 or higher.

In a special embodiment of the invention, the assemblies or arrays of patterns respectively form identical networks, which are periodic or cyclic in each of the two directions of their planes and which are reciprocately displaced by a translation of a half-period or half-cycle in each direction. The patterns can be metallic or superconductive.

According to a first embodiment of the device according to the invention, the dielectric layer separating the two assemblies is a dielectric plate on the two faces of which are respectively formed the two assemblies.

According to a second embodiment, the two assemblies are respectively formed on two electrically insulating supports having a low dielectric constant below 50 and the dielectric layer separating said two assemblies is a vacuum or air layer.

The present invention also relates to a tunable bidimensional cavity, incorporating a device forming a capacitive surface admittance and an electrically conductive wall surrounding said device, said cavity being characterized in that the device is according to the second embodiment referred to hereinbefore, in that the two supports of said device are plates positioned perpendicular to the cavity wall and in that said cavity is provided with means for varying the spacing between said two plates in order to tune the cavity.

According to an embodiment of said cavity, the patterns are superconductive and only occupy the centre of the plates, so as to be remote from the wall, so that said cavity has a high overvoltage.

B 11414.3 PV The present invention also relates to a resonator, characterized in that it comprises the device according to the second aforementioned embodiment and in that the two supports of said device are cylindrical, coaxial, dielectric bars, each bar having a planar face perpendicular to the axis of the bar and which carries the corresponding conductive pattern assembly, the tuning frequency of the resonator being modified by varying the spacing between the two bars.

The present invention also relates to an antenna, characterized in that it comprises the device according to the invention and means for the electromagnetic excitation of said device.

According to a first special embodiment, the antenna also comprises an electrically conductive wall surrounding the device, the two planar assemblies of said device being perpendicular to said wall, and the height of said wall is less than or substantially equal to the half-thickness of the mode guided by the surface admittance formed by the device.

According to a second special embodiment, the antenna also comprises an electrically conductive wall surrounding the dev- ice, the two planar assemblies of said device being perpendicular to the wall, and the wall has an opening extending from one end to the other of said wall, parallel to a direction perpendicular to the planar assemblies.

According to a third embodiment, the antenna comprises the device having two supports according to the invention, said two supports being coaxial, cylindrical bars, each bar having a planar face perpendicular to the axis of said bar and which carries the assembly of the conductive patterns corresponding thereto and the height of at least one of the two bars is appro- ximately the half-thickness of the mode guided by the surface.

B 11414.3 PV - 10 admittance formed by the device.

The invention is described in greater detail hereinafter relative to nonlimitative embodiments and with reference to the attached drawings, wherein show:

Fig. 1A, already described, a diagrammatic sectional view of a resistive plane.

Fig. 1B, already described, a diagrammatic view of a Salisbury screen.

Fig. 2A, already described, a diagrammatic sectional view of 10 a dielectric plate.

Fig. 2B, already described, variations as a function of the distance from the plate of fig. 2A of electric and magnetic components of the fundamental transverse electric mode guided by the plate of fig. 2A.

Fig. 3, already described, a diagrammatic view of a bidimension al cavity having a natural capacitive surface admittance.

Fig. 4 diagrammatically two assemblies of conductive arrays forming part of a device according to the invention.

Fig. 5 a sectional view of said device.

Fig. 6 a diagrammatic sectional view of another device according to the invention.

Fig. 7 a diagrammatic view of a bidimensional cavity having a tunable natural surface admittance.

B 11414.3 PV Fig. 8 variations of the frequency of the fundamental mode of a tunable cavity according to the invention as a function of the spacing between the plates forming part of the cavity, for different values of the length of said cavity.

Fig. 9A a diagrammatic perspective view of a tunable cavity according to the invention.

Fig. 9B a diagrammatic, longitudinal sectional view of the cavity of fig. 9A.

Fig. 10 a diagrammatic view of another device according to 10 the invention having superconductive patterns.

Fig. 11 a diagrammatic, longitudinal sectional view of a tunable dielectric resonator according to the invention.

Fig. 12 a perspective diagrammatic view of an antenna with an artificial surface admittance according to the invention.

Figs. 4 and 5 diagrammatically show a device according to the invention forming an artificial, capacitive surface admittance. This device comprises two planar assemblies or arrays 18 and 20 positioned facing one another and parallel to one another, as can be seen in fig. 5. These two assemblies are separated 20 by a vacuum or air layer 21.

Each of the two assemblies 18 and 20 comprises electrically conductive patterns 22 (e.g. metallic patterns), which are planar and electrically isolated from one another. Moreover, the two assemblies 18 and 20 of conductive patterns 22 are respectively formed on two planar, electrically insulating or isolating plates 24 and 26, whose dielectric constant is below 50.

B 11414.3 PV In an indicative and non-limitative manner, the conductive patterns 22 are made from copper and the plates 24 and 26 are made from glass.

h is the distance between the conductive patterns formed on the plate 24 and the conductive patterns formed on the plate 26.

Fig. 4 shows the conductive patterns formed on the plate 24 (part A of fig. 4) and those formed on plate 26 (part B of fig. 4) as if the device according to the invention was "open".

For each of the plates 24 and 26, the corresponding patterns are obtained by forming a thin metal layer on said plate and by forming, through said thin metal layer, trenches 28 exposing the dielectric material of the plate and separating the conductive patterns from one another.

Moreover, the assemblies 18 and 20 of conductive patterns 22 respectively form identical networks, which are periodic or cyclic in each of the two directions of their planes and which are reciprocately displaced, parallel to the layer 21, by a half-period or half-cycle translation along each direction.

In the embodiment of figs. 4 and 5, each of the networks is a periodic network having square meshes of patterns 22, which are themselves square. The side of each square having a value p greatly exceeding h (p being approximately 10 h or more). The width t of the isolating trenches 28 is well below p (said 25 width not exceeding p/10).

Bearing in mind the displacement of the networks in translation, the facing square patterns have parallel edges, but each of the nodes of the grating formed by the isolating trenches 28 B 11414.3 PV on one of the plates faces the centre of a square pattern of the other plate.

In order to understand the operation of the device shown in figs. 4 and 5, it is necessary to assume that an alternating electric field is applied parallel to the planes of the plates, e.g. with the aid of two elongated electrodes 30,32 visible in part A of fig. 4. The electrodes 30 and 32 are parallel to rows of conductive patterns 22 and the length and spacing of said electrodes are large compared with the spacing p of 10 the networks.

By means of an appropriate generator 34, an a.c. voltage is applied between the electrodes 30 and 32 so as to obtain the alternating electric field.

Under these conditions, the electric current flowing between the measuring electrodes 30 and 32 under the influence of the voltage applied thereto, alternately passes from one assembly of conductive patterns to the other, each row of conductive patterns constituting the electrodes of capacitors, which are connected in series and whose inter-electrode spacing is h.

The current observed, whose distribution is in reality periodic and bidimensional, can be treated like a homogeneous surface current on assuming a scale large compared with the spacing p of the networks and which is therefore even larger compared with h. Thus, there is the equivalent of a surface admittance, 25 which is capacitive and calculated in the following way.

If the linear measuring electrodes 30 and 32 have a unit length and if their distance is equal to said length, there is in series n capacitors of capacitance Cl with:

B 11414.3 PV n = 1/(p/2) = 2/p Cl = Eo x (p/2)/h = Eo x p/(2h).

The sought admittance (opposite to impedance) is therefore that of a capacitance Cs, which is such that:

i.e.:

Cs = Cl/n = Eo x p 2 /(4h).

Therefore the device of figs. 4 and 5 is equivalent to a homogeneous dielectric layer of thickness h and dielectric constant Eeq such that:

j.w.Eo.Eeq.h = j.w.Eo.p 2 /(4h) Eeq = (p/2h) 2.

It is therefore possible to easily produce high equivalent dielectric constants, e.g. with p=10 mm and h=0.5 mm, Eeq equals 100.

Moreover, the value of Eeq and hEeq and consequently the surface admittance value for a given pulsation or ripple w can easily be adjusted by changing the thickness h.

Another device according to the invention is diagrammatically shown in section in fig. 6. This device comprises a planar plate 34 made from a dielectric material having a dielectric constant E.

The aforementioned assemblies 18 and 20 of conductive patterns 22 are, in the case of fig. 6, respectively formed on the two opposite faces of the plate 34.

B 11414.3 PV P In this case, the device is equivalent to a homogeneous dielectric layer of thickness h (thickness of the plate 34) and dielectric constant Eeq such that:

2 Eeq = E.(p/2h).

In a purely indicative and non-limitative manner for p=10 mm, h=0.5 mm and E=10 (which is a current value), we obtain Eeq=1000 which is a very high value.

However, in the special embodiment shown in fig. 6, the quantities E, p and h have given values and Eeq, which therefore also has a given value, is not adjustable.

Other devices according to the invention can be envisaged. The conductive patterns can have shapes different from those of a square and in particular anisotropic shapes. Instead of being metallic, said patterns can be superconductive (they can in particular be made from a superconductive material with a high critical temperature). The relative translations between the networks of conductive patterns can be different from that indicated hereinbefore.

An essential point of the invention is to produce the equivalent of a dielectric layer, which has a high dielectric constant and which is preferably regulatable by geometrical parameters such as the thickness of the device or the relative translation of the networks of conductive patterns thereof.

The considerations which have been put forward are definitively based on the likening of the assembly of two networks of conductive patterns to a continuous bidimensional medium. This is correct, provided that it is only necessary from the electromagnetic standpoint to consider scales which are high compared B 11414.3 PV with the spacings p of the networks and therefore compared with the thickness h. In practice, the scale is given by the wavelength at the operating frequency or, in the case of resonators using a fundamental mode, by the length and width of 5 said resonators.

Different applications of the device according to the invention will now be described.

A first application of said device is the bidimensional cavity with tunable surface admittance. A description has already been given hereinbefore of a bidimensional cavity with natural surface admittance, i.e. a cavity using a true dielectric plate. The disadvantages of such a cavity appear as soon as it is wished to make said cavity tunable.

Initially it would even seem impossible to make such a cavity tunable, because all the geometrical parameters are fixed by the dimensions of the dielectric plate used. Thus, if the dielectric constant of the plate is very high and if the thickness of said plate is very low, i.e. on approaching an ideal surface admittance, it is then no longer necessary for the reflecting metal walls (cf. fig. 3) to be perfectly continuous at the point where they meet the plane of the dielectric plate.

However, it is possible to add a supplementary two-part wall 12a (fig. 7), so as to provide a passage for the dielectric plate 10 and thus make it possible to reversibly vary, e.g. the cavity length Lg (length of the plate present in the cavity) and therefore make said cavity tunable (in the example of fig. 7 the wall 12a used as the reflector is connected to other walls 12 by sliding contacts 36 and said wall 12a is rendered mobile by means of a U-shaped piece 37, which holds the two parts of the reflector and guides the latter in translation, B 11414.3 PV which permits the variation of Lg).

However, the system shown in fig. 7 suffers from a disadvantage. Thus, in the cavity, the frequency fo of the fundamental mode is the solution of an equation of form:

fo x Lg = F(fo) in which F is a slowly variable function of fo. Thus, the frequency fo is approximately inversely proportional to Lg.

It is therefore necessary to have a significant variation of the length Lg in order to significantly vary the tuning frequ- ency of the cavity.

For example, for a cavity produced with a 1 mm thick dielectric plate with a dielectric constant of 100, the frequency of the fundamental mode is 490 MHz, when the length Lg is adjusted to 240 mm. To double this frequency, said length Lg must be brought to 80 mm, which involves a 160 mm displacement of the mobile wall 12a.

In general terms, all resonators tuned by length variation, no matter what their type, suffer from this disadvantage of requiring large displacements of the mobile member thereof in order to obtain large frequency variations.

To this end, reference can be made to the following document:

(2) Technique des mesures en micro-ondes (hyperfrequences), realise sous la direction de Carol G. Montgomery, vol. 1, pp. 340 to 350, Editions Chiron B 11414.3 PV which describes cavities having a resonant frequency adjustable by the mechanical displacement of a piston.

A bidimensional cavity with artificial surface admittance according to the invention makes it possible to obviate this disadv5 antage.

This bidimensional cavity according to the invention does not use a dielectric plate as in fig. 7, but a device according to the invention forming an artificial surface admittance, e.g. of the type of the device described relative to figs.

4 and 5.

In this case, the two dimensions of the cavity, Lg and W, remain fixed and it is the distance h between the two arrays of conductive patterns which is modified for varying the resonant frequency of the cavity.

This leads to limited displacements of the mobile part, i.e. of one of the two arrays of conductive patterns with respect to the other in the present case, adequate for producing large variations of the tuning frequency.

This is illustrated by the curves of fig. 8, which represent the variations of the reduced tuning frequency Fr as a function of the reduced thickness hr for different reduced lengths Lgr. The reduced frequency Fr is such that:

Fr = f 10/Fo in which f10 is the frequency of the fundamental mode of the cavity (designated 10), f10 being independent of the cavity width W and Fo is a reference frequency such that:

B 11414.3 PV Fo = c/(2Lg) in which Lg is the cavity length and c the speed of light in vacuum.

Fo is also the first resonant frequency of a Fabry Pe/rot reson5 ator of length Lg.

The two other parameters hr and Lgr are such that:

hr = h/p and Lgr = Lg/p in which p represents the spacing of the networks of conductive patterns and h represents the distance between the networks.

This ratio Lg/p must be looked upon as fixed a priori by the validity condition of the surface admittance calculation. Thus, the resonator length Lg must contain a large number of conductive patterns and Lgr Z 5 is appropriate.

Fig. 8 shows that the tuning frequency becomes very sensitive to the thickness when hr and h tend towards 0 and correlatively when Fr becomes well below 1. In a purely indicative and nonlimitative manner consideration is given to a cavity which resonates at 490 MHz, the value considered hereinbefore.

In order to be in the range of high tuning frequency variations, the example is taken of Fr = 0.4, which leads to Fo=1225 MHz.

Therefore the resonator length Lg is equal to 122 mm. By choos ing Lgr equal to 5, we obtain a spacing p of 24.4 mm and due to the curvecorresponding to Fr equal to 0.4, we obtain hr= 0.0068 and h=0.166 mm.

To double the tuning frequency, it is necessary to double Fr, B 11414.3 PV so that Fr=0.8 and the corresponding curve of fig. 8 leads to hr=0.0412, i.e. h=1.005 mm.

In this case, the necessary displacement of the mobile member is only 0. 839 mm, which is well below the above-obtained value 5 of 160 mm for a cavity tuned by the variation of its length Lg.

A cavity tunable by surface admittance variation according to the invention is diagrammatically shown in perspective in fig. 9A and partially and in longitudinal section in fig. 9B.

This bidimensional cavity is defined by conductive walls 38 forming the reflecting walls of said cavity.

In the represented embodiment, said cavity seen in cross-section is shaped like a rectangle of length Lg and width W, but other shapes are possible, the important point being that the cavity, in cross-section, has fixed dimensions.

The device 40, of the type shown in figs. 4 and 5, is placed between the walls 38, perpendicular to said walls and equidistant of the upper and lower ends thereof. In order to bring this about the walls 38 are transversely cut at mid-height, 20 which leads to two half-assemblies of walls.

The insulating plate 24 of the device 40 is fixed to the lower end of the upper half-assembly, whilst the insulating plate 26 of the device 40 is fixed to the upper end of the lower half-assembly, as shown in fig. 9B.

An electric contact is established between the conductive patterns 22 on the edge of the insulating plate 24 and the walls 38 defining the cavity, whilst the conductive patterns 22 B 11414.3 PV of the insulating plate 26 are electrically insulated from said walls 38.

In addition, a metal bellows 42 is provided for reestablishing the electrical continuity of the walls 38 defining the cavity, as can be seen in figs. 9A and 9B.

Coupling antennas 44 (fig. 9A) and 46 (fig. 9B) are positioned facing one another on two opposite walls 38 and in the vicinity of the device 40 and are used for exciting and detecting the resonant mode.

Moreover, the upper and lower ends of the walls have outer ledges 50,48 between which are fixed piezoelectric tubes 52, whose length is varied by appropriate, not shown means, which make it possible to vary the spacing h between the networks of conductive patterns 22 respectively associated with the plates 24 and 26 in order to vary the cavity tuning frequency.

Thus, a tunable cavity is obtained using the resonance of a mode guided by a surface admittance between the reflecting walls defining said cavity.

It is also possible to produce a bidimensional cavity having artificial surface admittance according to the invention and which is not tunable, by placing a device according to fig. 6 in a wall having conductive walls with a rectangular crosssection, which is open at its two ends, the insulating plate of the device being perpendicular to the tube axis and being fixed at mid-height thereof.

The conductive patterns placed on one side of the insulating plate are also electrically connected to the conductive walls, whilst the conductive patterns located on the other side of B 11414.3 PV the insulating plate are electrically insulated from said walls.

There are also two coupling antennas facing one another on opposite walls of the tube, in the vicinity of the insulating plate of the device.

It is also possible to produce a tunable cavity according to the invention having a high overvoltage. To bring this about use is made of the structure of figs. 9A and 9B, except that in the device 40 the patterns 22 are then superconductive and only occupy the centre of the insulating plates 24,26, so as to be remote from the conductive walls of the cavity, as can be seen in fig. 10 (where the walls are not shown).

Fig. 11 shows diagrammatically and in perspective a tunable dielectric resonator according to the invention. It is a surface admittance resonator having two bars 54,56 made from a dielectric material. In fig. 11 they are identical, coaxial, cylindrical bars, whose common axis is X and both have a circular cross-section of diameter D and height a.

The two bars 54,56 respectively have planar end faces perpendicular to the axis X and positioned facing one another. In addition, these two end faces respectively carry networks 58,60 of reciprocately electrically insulated conductive patterns, said networks being displaced from one another as explained relative to figs. 4 and 5.

The distance h between the two networks, which is substantially equal to the distance.between the two bars, determines the tuning frequency of the resonator of fig. 11.

B 11414.3 PV By means of the networks 58 and 60 an artificial surface admittance is obtained. For producing the bars 54 and 56 use is made of a dielectric material having a high dielectric constant of e.g. 15 to 30.

Means M, symbolized by an arrow in fig. 11, are provided for varying the spacing between the bars 54 and 56, so as to vary the tuning frequency of the resonator obtained, which is also comparable with the bidimensional cavity described relative to figs. 9A and 9B, except that in fig. 11 there is no reflecting metal wall.

However, in the resonator of fig. 11, due to reflections on the side faces of the cylinders, the resonance modes are still linked with the establishment of a standing wave system confined by surface admittance. These lateral surfaces are virtually as effective as metal mirrors, bearing in mind the high value of the dielectric constant of the material from which bars 54 and 56 are made.

The good operation of the resonator of fig. 11 requires the height a of each bar to exceed the half-thickness of the mode guided by the surface admittance and is e.g. ten times higher than said half-thickness of the guided mode.

It is pointed out that the electromagnetic energy is supplied to the resonator of fig. 11 by conventional means such as a coupling antenna or loop.

A description will now be given of various special embodiments of an antenna having a fixed or adjustable artificial surface admittance in accordance with the invention.

B 11414.3 PV It is a radio or microwave antenna, which can be likened to the artificial surface admittance bidimensional. cavity described relative to figs. 9A and 9B. However, the metal walls forming reflecting walls are, in the case of said antenna, dimensioned in a completely different way, because unlike in the case of the cavity, the aim is to obtain a good electromagnetic radiation of said antenna. Different antenna embodiments are envisageable.

In a first special embodiment (not shown), use is made of an artificial surface admittance of the type according to figs. 4/5 or fig. 6 and whereof the electrically insulating plate or plates are circular and said surface admittance is partly closed by the metal walls extending over the same height on either side of the surface admittance, said height being less than or approximately equal to the half-thickness of the mode guided by the surface admittance.

In order to radiate electromagnetic energy, the antenna is provided with appropriate, known exciting means (coupling antenna or loop).

In a second special embodiment, a certain length of the edge of the insulating plate or plates used for producing the artificial surface admittance does not have a reflecting wall. An example of an antenna corresponding to this second embodiment is diagrammatically shown in fig. 12.

The antenna of fig. 12 comprises an artificial surface admittance 62 according to fig. 6, which has a semicircular dielectric plate 64, on the two faces of which are respectively formed networks of conductive patterns 65,66, which are reciprocately displaced as described relative to figs. 4 and 5.

B 11414.3 PV A semicylindrical metal wall 68 bears against the circular edge of the plate 64 and the conductive patterns 66 on one of the faces of the plate 64 and in the vicinity of the circular edge of the plate are electrically connected to said metal wall 68, whilst the conductive patterns 65 on the other face of the plate 64 are electrically insulated from said wall 68. Moreover, the plate 64 is at the same distance d from the ends 69 and 70 of the metal wall 68. A coupling antenna or loop 72 is also provided in the vicinity of one of the faces of the insulating plate 64 and is used for supplying or energizing the antenna. Thus, the antenna according to fig. 12 has a fixed artificial surface admittance.

It is also possible to produce an antenna of the same type, but which has a variable artificial surface admittance by using in place of a device according to fig. 6, a device according to figs. 4 and 5. In this case, the metal wall 68 is cut transversely equidistantly of the two ends.

The two plates of the device which is then used are respectively fixed to the free edges of the two half-walls resulting from said cutting action and the electrical continuity between the half-walls is reestablished by means of a metal bellows.

The conductive patterns in the vicinity of the curved edge of one of the plates are electrically connected to the corresponding half-wall, whilst the patterns located on the other plate are electrically insulated from the corresponding halfwall.

Means are also provided for varying the spacing between the two insulating plates in such a way as to vary the thus formed artificial surface admittance, which makes it possible to tune the antenna.

B 11414.3 PV Another embodiment of an antenna according to the invention (not shown) is in accordance with the dielectric resonator diagrammatically shown in fig. 11, except that the height a of each of the cylindrical bars or only one of them is then chosen to be approximately the same as the half- thickness of the mode guided by the surface admittance of said resonator. In order to radiate the electromagnetic energy, the antenna is then supplied with appropriate, known supply means (coupling antenna or loop).

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Claims (13)

CLAIMS (GB)

1. Device forming a capacitive surface admittance, said device incorporating two planar assemblies (18,20;58,60;65,66) placed facing one another and separated from one another by a planar 5 dielectric layer (21, 34,64), each planar assembly having planar, electrically conductive patterns (22), which are electrically insulated from one another, characterized in that the assemblies are reciprocately displaced, parallel to the dielectric layer, in such a way that the device behaves like a dielectric layer having a high dielectric constant.

2. Device according to claim 1, characterized in that the assemblies of patterns (22) respectively form identical networks, which are periodic in accordance with each of the two directions of their planes and which are reciprocately displaced by a translation of a half-cycle in each direction.

3. Device according to either of the claims 1 and 2, characterized in that the patterns (22) are metallic or superconductive.

4. Device according to any one of the claims 1 to 3, characterized in that the dielectric layer separating the two assemblies is a dielectric plate (34,64), on the two faces of which are respectively formed these two assemblies (18,20;65,66).

5. Device according to any one of the claims 1 to 3, characterized in that the two assemblies (18,20) are respectively formed on two electrically insulating supports (24,26;54,56) having a low dielectric constant and in that the dielectric layer separating said two assemblies is a vacuum or air layer (21).

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6. Tunable bidimensional cavity incorporating a device (40) forming a capacitive surface admittance and an electrically conductive wall (38) surrounding said device, characterized in that the device is in accordance with claim 5, in that the two supports of said device are plates (24,26) positioned perpendicular to the cavity wall and in that said cavity is provided with means (42,52) for varying the spacing between the two plates for tuning the cavity.

7. Tunable bidimensional cavity according to claim 6, charact- erized in that the patterns are superconductive and only occupy the centre of the plates, so as to be remote from the wall, said cavity thus having a high overvoltage.

8. Resonator, characterized in that it comprises the device according to claim 5 and in that the two supports of said device are coaxial, cylindrical dielectric bars (54,56), each bar having a planar face perpendicular to the axis (X) of said bar and carrying the assembly of conductive patterns (58,60) corresponding thereto, the tuning frequency of the resonator being modified by varying the spacing between the two bars.

9. Antenna, characterized in that it comprises the device (62) according to any one of the claims 1 to 5 and means (72) for the electromagnetic excitation of said device.

10. Antenna according to claim 9, characterized in that it also comprises an electrically conductive wall surrounding said device, in that the two planar assemblies of said device are perpendicular to said wall and in that the height of said wall is less than or substantially equal to the half-thickness of the mode guided by the surface admittance formed by the device.

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11. Antenna according to claim 9, characterized in that it also comprises an electrically conductive wall (69) surrounding the device (62), in that the two planar assemblies (64,66) of said device are perpendicular to the wall (68) and in that said wall has an opening extending from one end to the other of said wall parallel to a direction perpendicular to the planar assemblies (65,66).

12. Antenna according to claim 9, characterized in that the device is in accordance with claim 5, in that the two supports of said device are coaxial, cylindrical bars, each bar having a planar face perpendicular to the axis of said bar and carrying the assembly of conductive patterns corresponding thereto and in that the height of at least one of the bars is approximaely the half-thickness of the mode guided by the surface admittance formed by the device.

13. Device forming a capacative surface admittance substantially as hereinbefore described with reference to Figures 4 to 12.

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