EP0489934B1 - Flat antenna - Google Patents

Abstract

A flat antenna has a multilayer structure consisting of a screening layer (1) of an electroconductive material, a layer (2) with a strip-type power supply and a radiation layer (4) consisting of a plate with slot-type radiators (5, 6) electro-magnetically connected to the corresponding strips (3) of the power supply circuit, all the layers being mutually separated by means of dielectric spacers (7, 8). The antenna contains a grid (9) of three-dimensional cells (10) and carries a layer (11) partially transparent to the frequency band of the received waves. The surface of the grid (9) is made of an electroconductive material and the grid (9) is located on the plate with the radiation layer so as to form, by means of each of its cells (10), a three-dimensional resonator in which is located at least one slot-type radiator (5 or 6). The length and the width of each cell (10) exceed the average wave length (μ), and its height differs from half the average wave length (μ) by a value of approximately 0.02-0.07 of the wave length (μ). The antenna comprises also an adapter (14) connected to the screening layer (1) and to the strips (3) of the power supply.

Description

Technical field

The present invention relates to antenna technology and relates in particular to a surface antenna.

Underlying state of the art

An antenna with a high amplification factor is required to receive signals from the satellites in the 12 GHz range. Conventional mirror antennas meet this requirement. However, the mirror antennas are quite extensive, their operating data deteriorate under the influence of rain, snow and wind. Because of the large sail area of the mirror antenna, which causes high wind loads, the mast or any other attachment for antenna construction must be quite stiff and therefore quite extensive and heavy. The exterior view of the mirror antenna does not match that of residential and public buildings. These factors made the development of surface antennas necessary.

The area antennas have a lower mass, the sail area depends to a lesser extent on the weather conditions. The aesthetic correspondence of the area antennas with the exterior view of the residential and public buildings allows the former to be arranged on the walls of the buildings. Since the area antennas make it possible to implement the signal reception from a satellite through window glass, they can be installed directly in apartments.

A flat planar antenna is known which consists of a shielding layer in the form of a plate made of conductive material and of a radiation layer arranged above the former, which contains printed cavity emitters and a feed circuit made of conductive material, and an insulating insert between these layers (Marata Takao Ohmaru Kenji "A flat panel antenna with two-layer structure for satellite broadcasting reception ", NHK Lab. Note - 1989 - N 374, p. 1-12). The printed cavity radiator in square form has a galvanic contact with strips of the supply circuit. In order to achieve secondary diffraction maxima in the To eliminate the directional diagram, the emitters are at a distance of 0.7 to 0.9 λ, where λ is the wavelength.

• unmittelbare Strahlung der Leiter der Speiseschaltung,
• Strahlung infolge einer Wellenbeugung an T-Verzweigungen, Impedanztransformatoren und Krümmern einer Mikrostreifenleitung,
• Streuung der Oberflächenwelle, die sich längs der Grenze Isoliersubstrat-Luft fortpflanzt, an Schichtinhomogenitäten und Antennenkanten,
• Wärmeverluste im Dielektrikum der Schicht und der Einlage der Mikrostreifenleitung,
• Wärmeverluste in den Streifen und der Abschirmung der Mikrostreifenleitung bedingt sind.

Such a flat planar antenna has high losses due to

• direct radiation from the supply circuit conductors,
• Radiation due to wave diffraction at T-branches, impedance transformers and bends of a microstrip line,
• Scattering of the surface wave that propagates along the insulating substrate-air boundary, at layer inhomogeneities and antenna edges,
• Heat losses in the dielectric of the layer and the insert of the microstrip line,
• Heat losses in the strips and the shielding of the microstrip line are caused.

An antenna based on a cavity resonator is known, which contains a housing with an insulating filler, to which a waveguide is connected. Uniformly distributed rectangular coupling openings are provided on one of the housing walls and are arranged in relation to a wave field excited within the housing in such a way that half-waves of the same polarity (DE, A, 3530647) arise in these coupling openings.

To adapt the impedance of the waveguide to the housing, it protrudes a lot into the housing. In addition, there can be a wave-type filter between the waveguide and the housing, which has the shape of a side has flat hollow body which merges directly into the housing. However, the known antenna does not allow a predetermined operating frequency range to be realized, to ensure a sufficiently small side lobe level, and has large dimensions in the direction orthogonal to the radiating antenna opening.

A surface antenna for receiving waves in the UHF range is known from a geostationary broadcasting satellite (US, A, 4851855) which contains a system of coplanar printed surface radiators which are located between two layers of synthetic resin. One of the surfaces of the radiator system is an antenna opening. The antenna also contains a flat supply circuit, which is formed by strips of conductive material and is arranged between the two layers of synthetic resin, and a plan screen underneath. The system of radiators and the feed circuit, the feed circuit and the screen are separated from one another by intermediate layers.

The intermediate layers are executed in the antenna mentioned in the form of a frame made of metal, synthetic resin or wood. The antenna feed network represents a suspended symmetrical stripline.

By using the stripline, veluste can be avoided in the known antenna, which are caused by direct radiation from the conductors of the supply circuit, the radiation due to wave diffraction at T-branches, impedance transformers and from microstrip bends. Heat losses in the stripline dielectric are also reduced.

However, the known antenna also has high heat losses in the inner conductor and the shielding of the stripline due to a large extent of the strip, a large number of binary power dividers, impedance transformers, and line bends.

A surface antenna in the form of a multilayer structure is known, which is characterized by a shielding layer conductive material, a layer with a strip feed circuit and a radiation layer in the form of a plate with slot radiators is formed, which are electromagnetically coupled to the respective strips of the feed circuit. The layers mentioned are separated from one another by intermediate layers of insulating material. A transition is connected to the shielding layer and the strips of the feed circuit. A galvanic coupling of the conductors of the layers with the feed circuit and the slot radiators is missing (Hirofumi Ishizaki "Sguare Antennas Edge Jinto BS Antenna Market" - JEI, 1990, Vol. 37, N 8, series N 432, pp. 63 to 64).

The antenna works as follows in transmission mode.

The signal from the transmitter is given to the input of the transition and further fed to the slot radiators via the feed circuit. The slots are excited by the field of an electromagnetic wave that propagates along the stripline.

The known antenna also has high heat losses in the stripline and the shielding thereof due to a large extension of the feed line, a large number of binary power dividers, impedance transformers, line bends, and a large drop in power along the line, which cannot be reduced.

Mainly in the 12 GHz range, wave losses along the stripline with a good dielectric as filler for the line contribute to heat losses in the conductors - in the strip and the shielding of the line.

The line losses increase with the deterioration of the adaptation in the feed network, while the adaptation essentially depends on the number of slot radiators, because with the increase in the number of these in the antenna grid, the number of binary power dividers, impedance transformers, line bends is increased. So For example, a 2 ^M = 256 radiator grid has a number M = 9 binary power divisions in an antenna with a linear polarization and M + 1 = 10 dividers on the way from the antenna input to the radiator with a circular polarization. In connection with the high density of the radiators in the grille, the elbows are rectangular.

A large number of the elements of the feed line in the grid, sharp kinks in the strip line deteriorate the line adaptation and consequently cause an increase in antenna losses.

Disclosure of the invention

The invention has for its object to provide a surface antenna with such a design that allows a high directional factor in the receive wave range with a reduction in losses in the feed circuit of the antenna by reducing the number of slot radiators by using resonators with a to ensure partially permeable surface.

The object is achieved in that the surface antenna in the form of a multilayer structure, the shielding layer made of conductive material separated from one another by insulating material intermediate layers, layer with a strip feed circuit and radiation layer in the form of a plate with slot radiators which are electromagnetically coupled to the respective strips of the feed circuit are formed and contains a junction connected to the shielding layer and the strips of the supply circuit, according to the invention additionally contains a grating with cavity cells and a layer arranged above and partially permeable to the reception wave range, the grating surface being made of conductive material and the grating on one Radiation layer plate forming a cavity through each of its cells is arranged, in which at least one slot radiator is arranged, the width and length of each cell exceeding the mean wavelength and the height of which deviates from half the mean wavelength by a size of approximately 0.02 to 0.07 of the wavelength.

The introduction of the resonator grating into the surface antenna allows the number of slot radiators to be significantly reduced while maintaining the directional factor of the surface antenna by transforming the field of the radiator in the field of the natural oscillation of the resonator, thereby eliminating the possibility of secondary diffraction maxima appearing in the directional diagram. The reduction in the number of radiators makes it possible to simplify the supply circuit and to keep the losses in it lower.

It is imperative that the partially permeable layer is made in the form of a plate made of conductive material with coupling openings, the grating height having to be below half a medium wavelength.

The use of the plate made of conductive material with the coupling openings as a partially permeable layer allows this layer to be produced by stamping.

It is also expedient for the partially permeable layer to be in the form of an insulating film with electrically insulated metal plates attached to it on both sides, the grating height having to be above half a medium wavelength.

The use of the insulating foil with the metal plates allows the semi-permeable layer to be produced using a printing technique.

It is also expedient that the surface antenna is manufactured in such a way that the width and length of each cell is ≧ 4 λ, where λ is the wavelength, while the number of slot radiators in each cell is two and they are parallel to a wall of the Cell at intervals of this lie, which are 1/4 and 3/4 of the length of the wall orthogonal to the slots.

The use of two slot radiators arranged in the resonator in the manner mentioned makes it possible to expand the operating frequency range of the antenna, since such an arrangement excludes the excitation of the next interference oscillation in the resonator and a corresponding distortion of the directional diagram.

It is also expedient that the surface antenna is designed in such a way that in each grid cell there are additionally two slot radiators which are parallel to one another and orthogonal to the main slot radiators at intervals which are 1/4 or 3/4 of the length of the Slits are orthogonal wall, the lengths of the strips corresponding to each pair of the orthogonally arranged radiators should differ from one another by 1/4 λ.

The use of four slot radiators in the resonator, which are arranged and fed in the manner mentioned, enables the wave emission or reception with a circular polarization of the field in the operating frequency range.

Brief description of the drawings

• Fig. 1 eine dreidimensionale Ansicht der Hauptteile einer auseinandergenommenen erfindungsgemässen Flächenantenne;
• Fig. 2 eine erfindungsgemässe Ausführungsform einer teildurchlässigen Schicht;
• Fig. 3 eine Anordnung von erfindungsgemässen Schlitzstrahlern auf einer Strahlungsschichtplatte;
• Fig. 4 eine grafische Darstellung eines zweidimensionalen Verteilunsgesetzes für eine tangente Komponente der Stärke eines elektrischen Feldes anhand der Messangaben bei einem Abstand 4/5 λ von der Antennenapertur;
• Fig. 5 ein experimentelles Richtdiagramm der ersten Ausführungsform der Antenne in der Ebene eines Vektors E;
• Fig. 6 ein experimentelles Richtdiagramm der ersten Ausführungsform der Antenne in der Ebene eines Vektors H;
• Fig. 7 eine grafische Darstellung der Abhängigkeit des Verstärkungsfaktors der ersten Ausführungsform der Antenne von der Frequenz;
• Fig. 8 ein experimentelles Richtdiagramm der zweiten Ausführungsform der Antenne in der Ebene des Vektors E;
• Fig. 9 ein experimentelles Richtdiagramm der zweiten Ausführungsform der Antenne in der Ebene des Vektors H;
• Fig. 10 eine grafische Darstellung der Frequenzabhängigkeit des Antennenverstärkungsfaktors in der zweiten Ausführungsform.

The present invention is explained in more detail below on the basis of the specific exemplary embodiments thereof with reference to the accompanying drawings. It shows:

• 1 shows a three-dimensional view of the main parts of a disassembled surface antenna according to the invention;
• 2 shows an embodiment according to the invention of a partially permeable layer;
• 3 shows an arrangement of slot radiators according to the invention on a radiation layer plate;
• 4 shows a graphical representation of a two-dimensional distribution law for a tangent component of the strength of an electric field on the basis of the measurement data at a distance of 4/5 λ from the antenna aperture;
• 5 shows an experimental directional diagram of the first embodiment of the antenna in the plane of a vector E;
• 6 shows an experimental directional diagram of the first embodiment of the antenna in the plane of a vector H;
• 7 shows a graphical representation of the dependence of the gain factor of the first embodiment of the antenna on the frequency;
• 8 shows an experimental directional diagram of the second embodiment of the antenna in the plane of the vector E;
• 9 shows an experimental directional diagram of the second embodiment of the antenna in the plane of the vector H;
• 10 is a graphical representation of the frequency dependence of the antenna gain factor in the second embodiment.

Best embodiment of the invention

The antenna contains a shielding layer 1 (Fig. 1), a layer 2 with strips 3 of a supply circuit made of conductive material, a radiation layer 4, which is a conductive plate with a system of slot radiators 5, 6, intermediate layers 7 and 8, a grid 9 with Cavity cells 10, a partially permeable layer 11 with coupling openings 12, a cladding 13, if necessary a coaxial strip or a strip-waveguide transition 14.

The shielding layer 1 is made of aluminum, copper, silver or other material with high electrical conductivity. To protect the conductive shielding layer 1 from corrosion, its surface can be covered with a 5 to 20 μm thick dielectric layer.

Layer 2 is produced in the form of a three-plate construction, which ensures corrosion protection for the conductors of the supply circuit.

The layer 4 is also produced in the form of a three-plate construction, the upper side of the conductive plate with the system of slot radiators 5, 6 being covered with a plastic film after the plate has been soldered or glued to the grid 9.

The intermediate layers 7, 8 are made of foam polystyrene or another dielectric with a low dielectric loss factor.

The grid 9 and the partially permeable layer 11 are made of a highly conductive material such as aluminum, copper, silver or a material similar in terms of electrical conductivity. The grid 9 is either made of dielectric with a metal coating applied thereon or of a highly conductive material.

The layer 11 is produced in the form of a metal plate which is perforated by round or square openings 12. In the other embodiment, the layer 11 (FIG. 2) is produced in the form of an insulating film with electrically insulated metal plates 15 attached to its two sides.

After assembly of the antenna, the conductive plate of layer 4, the grating 9 and layer 11 are galvanically contacted with one another over the entire line of contact. The galvanic coupling is achieved either by soldering or gluing using a conductive adhesive.

The conductive plate of layer 4 together with the cells 10 of the grating 9 and the layer 11 forms nxm cavity resonators with a partially permeable surface, where n and m represent the numbers of the resonators in the direction of the axes x and y of a right-angled coordinate system in the plane of the antenna opening. The length and the width of the cells 10 are selected in the order of magnitude of a few wavelengths as a function of the predetermined operating frequency range, for example of 4λ x 4λ. The height of the grating 9 is chosen from the requirement that the sum of the height of the grating 9 and the half thickness of the layer 11 deviate from the half wavelength by a size which is proportional to the square root of the relative frequency range, for example by a size of 0.02 to 0.07 of the wavelength. The choice of the number nxm of the resonators is realized on the condition of securing a predetermined antenna amplification factor.

In transmission mode, the surface antenna according to the invention works as follows.

The signal from the transmitter (not shown in FIG.) Is applied to the input of the coaxial strip transition 14 (FIG. 1) and is subsequently fed to the slot radiators 5, 6 via the strips 3 of the feed circuit.

The slots are excited by the field of the electromagnetic wave that propagates along the stripline. Furthermore, the cavity resonators are excited by the slots, whereupon the electromagnetic energy is emitted through the layer 11.

The field structure in the resonator with a partially permeable wall strives asymptotically towards the field structure in the closed resonator with increasing quality.

$${\text{E}}_{\text{y}}\text{= cos}\frac{\text{πx}}{\text{l}}\text{· sin}\frac{\text{πz}}{\text{a}}\text{· cos ωt}$$

beschrieben wird, worin

ω

die Kreisfrequenz der Schwingungen im Resonator,

l

die Seitenlänge des Resonators,

a

die Resonatorhöhe

bedeuten.The slots of the radiators 5, 6 (Fig. 1) excite a fundamental vibration in the resonator, which is approximately due to the relationships $${\text{E}}_{\text{x}}\text{= cos}\frac{\text{πy}}{\text{l}}\text{· Sin}\frac{\text{πz}}{\text{a}}\text{Sin ωt}$$

$${\text{E}}_{\text{y}}\text{= cos}\frac{\text{πx}}{\text{l}}\text{· Sin}\frac{\text{πz}}{\text{a}}\text{· Cos ωt}$$

is described in which

ω

the angular frequency of the vibrations in the resonator,

l

the side length of the resonator,

a

the resonator height

mean.

The amplitudes of the higher vibrations are small compared to the amplitude of the fundamental vibration due to the filter properties of the resonator.

   i(γ) - Einheitsvektor in Richtung der Achse x(y), erzeugt.As a result of the passage of the wave through the layer 11, a field distribution according to the law becomes in the antenna opening $$\overline{\text{E}}\text{= i. cos}\frac{\text{πy}}{\text{l}}\text{Sin ωt + J cos}\frac{\text{πx}}{\text{l}}\text{· Cos ωt,}$$

i ( γ ) - unit vector in the direction of the axis x (y).

In the present field distribution in the opening, the antenna forms a directional diagram with a radiation maximum in the normal direction to the opening, while the radiation field has a circular polarization.

The additional introduction of the grating 9 and the layer 11 into the antenna makes it possible to arrange the slot radiators 5, 6 in the grating at much greater distances. The analysis of the results of the numerical investigations using a strict solution to the electrodynamic task of excitation of a resonator antenna shows that when working in the 5% frequency band, the slots of the radiators 5, 6 on the conductive plate of the layer 4 from one another in the plane H at a distance of 4 λ. Based on the results of the experimental investigations, it was found that the distance between the two slots in the plane E can be approximately twice the wavelength. The experimental pattern with a linear polarization of the radiation field, which enables a 5% frequency band with the side length of the cell 10 of 4 λ x 4 λ and with n = m = 1, has two slits in the plane of the vector E lie.

It must be emphasized that despite the fact that the slots on the layer 4 plate with a 2 λ are arranged, the antenna grating according to the invention nevertheless has no secondary diffraction maxima. The thing is that the additional introduction of the grating 9 and the layer 11 converts the antenna in the form of a grating of discrete radiators into an aperture antenna, in the opening of which the field strength is constant in the direction of one coordinate and in the direction of the other within the limits of each resonator after the cosine law changes. The field distribution mentioned in the antenna opening ensures suppression of secondary interference maxima, which would occur at larger distances in the grating.

• die Länge der Leiter der Speiseschaltung wird kleiner;
• die Anzahl der sekundären Leistungsteiler in der Speiseschaltung nimmt ab, und demzufolge verringert sich die Anzahl der Impedanztransformatoren und der Speiseleitungskrümmer;
• im Zusammenhang mit einer kleineren Dichte der Schaltungselemente werden die Leistungsteiler nicht in Form von rechtwinkligen Winkelstücken, sondern in Form von allmählichen Leitungskrümmern an den Verzweigungsstellen ausgeführt;
• die Streifenleitung wird mit breiten Streifen 3 ausgeführt.

Thanks to the larger distances between the slot radiators 5, 6 in the grid, the feed circuit for the antenna is simplified:

• the length of the conductors of the supply circuit becomes smaller;
• the number of secondary power dividers in the feed circuit decreases, and consequently the number of impedance transformers and feeder manifolds decreases;
• in connection with a lower density of the circuit elements, the power dividers are not designed in the form of right-angled elbows, but in the form of gradual elbows at the junction points;
• the stripline is carried out with wide strips 3.

When using the wider strips 3 in the strip line, the value of the longitudinal attenuation of the wave in the line decreases. The reduction in the number of circuit elements, the use of the gradual transitions make it possible to achieve a better adaptation in the circuit, which reduces the line losses which are due to multiple wave reflections from the inhomogeneities of the circuit. The reduction the line length in the supply circuit reduces the line losses.

In addition, the choice of the wide strips 3 for the feed line reduces the demands on the manufacturing accuracy for the line.

As a result of the use of the resonators, the antenna according to the invention has a higher frequency selectivity compared to the known flat antenna grids than the radiation elements of which slots, microstrip lines with bends, dipoles, microstrip emitters, horn radiators are used. In connection with this, the interference with signal reception from the satellite for a direct television broadcast outside the working frequency band is reduced.

The proposed antenna offers an advantage in comparison to the mirror antennas currently used for systems for direct television broadcasting in terms of a constructive and aesthetic correspondence with the interior of the apartments, with the exterior view of the residential and public buildings, has a much smaller size in Wave beam direction.

The following are examples which confirm the possibility of realizing the present invention.

In the first embodiment of the surface antenna, which corresponds to the number 1 × 1 of the cells 10 in the grid 9, the plate of the layer 4 has two rows of the two radiators 5 and 6 in each row of orthogonally arranged slots, as shown in FIG. 3 is on. The internal dimensions of the cell 10 of the grid 9 are 95x95 mm the same. The slots are located in points with the coordinates A (-47.5; 0); B (47.5, 0); C (0; 47.5); D (0; -47.5). The wall height of the grid 9 is 11.7 mm.

The layer 11 is 1 mm thick by punching Copper plate with the formation of square openings 12 measuring 7.5x7.5 mm. The hole spacing in the direction of the two coordinates is 11.5 mm.

The supply circuit is based on a symmetrical strip line. The width of the strip 3 is 3.8 mm, the thickness 0.018 mm. The height of the stripline is 3 mm. The intermediate layers 7 and 8 are made of foam polystyrene.

According to the data from the measurements with the aid of a waveguide, the dielectric constant of the foam polystyrene is 1.13 and its dielectric loss factor is 5 · 10⁻⁵. The feed circuit is constructed in such a way that the magnetic fluxes in the slot radiators 5, 6 of each row are in phase and are 90 ° out of phase with the flows of the orthogonal row. The signal reaches the antenna through the stripe-coaxial transition 14.

In Fig. 4 there are graphs showing the measured distribution law for a tangent component of the strength of the electric field E illustrate in the antenna aperture. The measurements are carried out at a distance of 4/5 λ from the antenna aperture on a frequency of 11.7 GHz.

The experimental directional diagrams in the plane of the vector E and in the plane of the vector H are shown in Figs. 5 and 6, respectively.

The dependency of the antenna gain factor K on the frequency F is given according to the experimentally determined measurement data.

In the second embodiment of the surface antenna, the aluminum grating 9 has 4x4 cells 10. The internal dimensions of the cells 10 are equal to 95x95 mm. The wall height of the grid 9 is 11.7 mm. Two systems of the orthogonally arranged slot radiators 5 and 6 are accommodated on the plate of layer 4. Are in every system the slots are arranged in four rows to eight parallel slots of the radiators 5 or 6 in the row.

The layer 11 is made of an aluminum sheet 1 mm thick by perforating it to form square openings 12 of dimensions 7.5x7.5 mm. The grid 9, the layer 11 and the conductive plate of the layer 4 together form sixteen cavity resonators with a partially permeable surface. The arrangement of the slots on the surface of each resonator is identical to the arrangement of the slots in the first antenna. The strip line of the feed circuit has the same dimensions in cross section as the line of the first antenna. The signal reaches the antenna through a strip-waveguide transition 14. The antenna cladding 13 is made of foam polystyrene with a density of 0.6 g / cm³. The external dimensions of the antenna (without taking into account the dimensions of the coaxial strip transition) are 400x400x24 mm. The antenna weight is equal to 2.7 kg.

The experimental directional diagrams of the second antenna in the plane of the vectors E and H are shown in Figs. 8 and 9, respectively.

The dependency of the gain factor K of the second antenna on the frequency F is specified on the basis of the measurement data in FIG. 10.

At the maximum of the directional diagram, the ellipticity factor is no worse than 1 dB in the frequency range from 11.7 to 12.2 GHz.

Industrial application

The invention can be successfully used in systems for satellite and ground connection, in satellite television systems, in particular as an antenna for direct signal reception in satellite television in the 12 GHz range.

Claims (5)

1. Flat top antenna in the form of a multilayered structure in the case of which antenna the layers are separated relative to one another by intermediate layers (7, 8) of insulating material and the antenna is formed from a screening layer (1) made of conductive material, from a layer (2) having strips (3) of a strip feed circuit and from a radiating layer (4) in the form of a plate with slot radiators (5, 6) which are coupled electromagnetically to the respective strips (3) of the feed circuit, and which contains a passage (14) connected to the screening layer (1) and to the strips (3) of the feed circuit, characterised in that it additionally contains a grid (9) with cavity cells (10) and a layer (11) which is disposed thereon and is partially permeable for the operating frequency range, the surfaces of the grid (9) being made of conductive material and the grid (9) on the plate being galvanically contacted by the radiating layer (4), forming a cavity resonator through each of its cells (10), in which at least one slot radiator (5, 6) is disposed, the width and the length of each cell (10) exceeding the average wavelength (λ) and its height differing from half the average wavelength (λ) by a value of approximately 0.02 to 0.07 of the wavelength (λ).
2. Flat top antenna according to Claim 1, characterised in that the partially permeable layer (11) is produced in the form of a plate of conductive material with coupling openings (12), the height of the grid (9) being less than half the average wavelength (λ).
3. Flat top antenna according to Claim 1, characterised in that the partially permeable layer (11) is in the form of an insulating foil with electrically insulated metal plates (15) mounted on both sides thereof, the height of the grid (9) being more than half the average wavelength (λ).
4. Flat top antenna according to Claim 1, characterised in that the width and the length of each cell (10) is ≧ Δλ, λ being the wavelength; in that the number of the slot radiators (5) in each cell (10) is two; and in that the slot radiators (5) lie parallel to a wall of the cell (10) at spacings therefrom which are in each case 1/4 and 3/4 of the length of the wall which is orthogonal relative to the slots of the radiator (5).
5. Flat top antenna according to Claim 4, characterised in that in each cell (10) of the grid (9) there are additionally two slot radiators (6) which are parallel to one another and orthogonal relative to the main slot radiators (5) at spacings which are 1/4 and 3/4 respectively of the length of the wall which is orthogonal relative to the slots of the additional radiator (6), the lengths of the strips (3) corresponding to each pair of the orthogonally disposed slot radiators (5, 6) differing from one another by 1/4 of the wavelength (λ).

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