Radar cross section reconfigurable antenna based on magnetically reconfigurable frequency selective absorber

Abstract

The embodiment of the present invention discloses a helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber, which relates to the field of radar stealth and can effectively take into account both the out-of-band stealth performance and the in-band radar scattering size of the antenna. The present invention includes: a broadband helical antenna radiator (1) is connected to a metal floor (6) to form a common metal ground relationship. The broadband helical antenna radiator (1) is perpendicular to the magnetically reconfigurable frequency selective absorber and is located in the middle thereof. The overall structural design of the antenna can achieve the switching of the overall radar cross section of the antenna by changing the on-off state of the PIN diode in the switchable band-stop frequency selective structural layer (2), thereby achieving the purpose of stealth state switching. The overall structural design realizes the low scattering and reconfigurable radar cross section performance of the antenna, while meeting the requirements of good impedance matching and antenna radiation performance.

Description

Technical Field

[0001] The invention relates to the field of radar stealth, and in particular to a helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber. Background Art

[0002] The key technical aspect of stealth technology research is how to successfully minimize the radar cross section. With the advancement of stealth technology, scholars from several countries have proposed many strategies to reduce the radar cross section. Electromagnetic absorption materials, target shape design, passive cancellation technology, and active cancellation technology are basic strategies. These methods are mainly used for platforms without antennas. However, in actual engineering applications, the antenna system, as an important device for communicating with the outside world, will show strong electromagnetic scattering at certain specific angles, becoming an important radar cross section source for the system. Therefore, how to reduce the radar cross section of the antenna becomes a key part of achieving the stealth of the target system. As a core component of communication, the main function of the antenna is to effectively transmit and receive electromagnetic waves. The antenna can easily scatter electromagnetic waves that are easily detected.

[0003] With the development of science and technology, how to effectively reduce the scattering of antennas has become an urgent problem to be solved in engineering practice. Loading absorbing materials or distributed resistors on the antenna to convert the opposite electromagnetic wave energy into heat energy is an early method to reduce the radar cross section of the antenna. This method is more effective in reducing the radar cross section of the antenna, but it has a huge impact on the radiation characteristics of the antenna. Shape design technology is the most direct way to reduce the radar cross section of the antenna while ensuring the normal operation of the antenna.

[0004] Therefore, how to effectively balance the out-of-band stealth performance and in-band radar scattering size of the antenna has become an issue that needs to be studied. Summary of the invention

[0005] The embodiments of the present invention provide a helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber, which can effectively take into account both the out-of-band stealth performance and the in-band radar scattering size of the antenna.

[0006] To achieve the above object, the embodiments of the present invention adopt the following technical solutions:

[0007] A broadband helical antenna radiator 1, a switchable band-stop frequency selection structure layer 2, a dielectric substrate 3, an air spacer layer 4, a broadband magnetic absorbing material layer 5 and a metal floor 6; wherein, in order from top to bottom, the switchable band-stop frequency selection structure layer 2, the dielectric substrate 3, the air spacer layer 4, the broadband magnetic absorbing material layer 5 and the metal floor 6 constitute the magnetic reconfigurable frequency selective absorber; the broadband helical antenna radiator 1 is perpendicular to the magnetic reconfigurable frequency selective absorber and is located in the middle of the magnetic reconfigurable frequency selective absorber, wherein a hole is opened at the center of the switchable band-stop frequency selection structure layer 2, a part of the dielectric substrate 3 is exposed from the hole, and the broadband helical antenna radiator 1 passes through the hole and is connected to the dielectric substrate 3; a PI N diode is installed in the switchable band-stop frequency selection structure layer 2.

[0008] The diameter of the broadband helical antenna radiator 1 is D=8mm, and the inner diameter of the coil is r=0.63mm. The distance between two adjacent coils of the broadband helical antenna radiator 1 is h=12mm, and the helical angle of the helical antenna is about 15°; the broadband helical antenna radiator 1 is wound around the switchable band-stop frequency selection structure layer 2 in a clockwise direction. The end of the last coil at the bottom of the broadband helical antenna radiator 1 is connected to one end of a vertical part, and the other end of the vertical part passes through the hole and is connected to the dielectric substrate 3; one end of the impedance matching line is connected to the bottom end of the vertical part, and the other end is connected to the broadband helical antenna radiator 1, wherein the highest point of the vertical part is the zero point of the z-axis, and the z-axis coordinate of the connection point of the impedance matching line and the vertical part is Zmin=-2.0mm, and the z-axis coordinate of the connection point of the impedance matching line and the broadband helical antenna radiator 1 is Zmax=3mm. The vertical part continues to extend downward from the coordinate Zmin for 3.2mm and passes through the dielectric substrate 3.

[0009] The thickness of the switchable band-stop frequency selection structural layer 2 is t1=0.25mm, and the material is F4B substrate; a metal pattern is printed on the upper surface of the switchable band-stop frequency selection structural layer 2; wherein the metal pattern is: two concentric metal square rings, the side length of the inner ring is I1=1.0mm, and the side length of the outer ring is I2=2.0mm.

[0010] The outer ring of the metal square ring serves as an outer loop, and two vertical metal arms extending from the upper surface of the switchable band-stop frequency selection structure layer 2 and two horizontal metal arms extending from the lower surface are respectively connected to the side of the outer loop through respective pin diodes (PIN diodes) and resistors; wherein, the width of each vertical metal arm and each horizontal metal arm is w=0.5mm, and the length is also I3=8.5mm. After installation, the pin diodes and resistors are connected to the access end of the outer loop as metal arms, and the total length of the access end of the outer loop is l4=2.5mm.

[0011] The thickness of the dielectric substrate 3 is t2=2.0 mm, and the thickness of the air spacer layer 4 is ta=6.0 mm; the thickness of the broadband magnetic absorbing material layer 5 and the metal floor 6 are the same, both tab=3.2 mm, and the broadband magnetic absorbing material layer 5 is made of a magnetic material FGM-125 having magnetic loss characteristics, and the metal floor 6 is made of a metal material, and the metal material includes copper or aluminum.

[0012] The air spacer layer 4 is filled with polymethacrylimide (PMI) foam having a dielectric constant εr=1.05 and a loss tangent tanδ=0.005.

[0013] In the embodiment of the present invention, the radar cross-section reconfigurable helical antenna based on the magnetic reconfigurable frequency selective absorber is provided, and the broadband helical antenna radiator 1 is connected to the metal floor 6 to form a common metal ground relationship. The broadband helical antenna radiator 1 is perpendicular to the magnetic reconfigurable frequency selective absorber and is located in the middle thereof. The overall structural design of the antenna can realize the switching of the overall radar cross section of the antenna by changing the on-off state of the PIN diode in the switchable band-stop frequency selective structural layer 2, thereby achieving the purpose of switching the stealth state. The overall structural design realizes the low scattering and radar cross-section reconfigurable performance of the antenna, while meeting the requirements of good impedance matching and antenna radiation performance. Since the reflective frequency selective absorber is used to replace the metal grounding of the antenna, the radar cross-section switching of the antenna is realized, while the gain, axial ratio and other characteristics of the original helical antenna are maintained, and the reduction of the in-band and out-of-band radar cross-section is realized, so that the antenna has the ability of stealth and communication switching. The advantage of this embodiment is that the reflective frequency selective absorber is used to replace the metal grounding of the antenna, and the switching electromagnetic characteristics from in-band reflection and out-of-band absorption response to broadband absorption response can be realized, so that the antenna has the ability to switch the stealth state, thereby taking into account the out-of-band stealth performance of the antenna. By designing an antenna composed of a switchable resistance unit and a magnetic dispersion element, the scattering gain is fixed due to the uniform reflection characteristics of the antenna unit, and the radiation pattern of the antenna can be changed by controlling the bias voltage, so a reconfigurable low-scattering antenna with normal radiation performance can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments provided by the embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0015] Figure 1a , Figure 1bThe geometric structure of the magnetically reconfigurable frequency selective absorber provided by the embodiment of the present invention;

[0016] Figure 2 A top view of a switchable band-stop frequency selective structural layer of a magnetic reconfigurable frequency selective absorber provided by an embodiment of the present invention;

[0017] Figure 3 A bottom view of a switchable band-stop frequency selective structural layer of a magnetic reconfigurable frequency selective absorber provided by an embodiment of the present invention;

[0018] Figure 4 The geometric structure and impedance matching line design of the broadband helical antenna radiator of the helical antenna with reconfigurable radar cross section based on the magnetic reconfigurable frequency selective absorber provided in the embodiment of the present invention;

[0019] Figure 5 The axial ratio of the radiator of the broadband helical antenna of the helical antenna with reconfigurable radar cross section based on the magnetically reconfigurable frequency selective absorber provided in the embodiment of the present invention;

[0020] Figure 6 The E-plane radiation pattern of the helical antenna with reconfigurable radar cross section based on the magnetic reconfigurable frequency selective absorber provided in an embodiment of the present invention;

[0021] Figure 7 The H-plane radiation pattern of the helical antenna with reconfigurable radar cross section based on the magnetic reconfigurable frequency selective absorber provided in the embodiment of the present invention;

[0022] Figure 8 A 3D gain pattern of a helical antenna in an "off" state with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber provided in an embodiment of the present invention;

[0023] Fig. 9 A 3D gain pattern of the “on” state of a helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber provided in an embodiment of the present invention;

[0024] Fig.10 A normalized antenna pattern of the "off" state of the helical antenna with reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber provided in an embodiment of the present invention at 6.0 GHz;

[0025] Fig.11 The normalized antenna pattern of the “on” state of the helical antenna with reconfigurable radar cross section based on the magnetically reconfigurable frequency selective absorber provided in the embodiment of the present invention at 6.0 GHz;

[0026] Fig.12A single-station radar cross-section diagram of the simulation and measurement of a radar cross-section reconfigurable helical antenna based on a magnetically reconfigurable frequency selective absorber provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention will be described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention. It can be understood by those skilled in the art that, unless specifically stated, the singular forms "one", "one", "said" and "the" used herein may also include plural forms. It should be further understood that the term "including" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or their groups. It should be understood that when we say that an element is "connected" or "coupled" to another element, it can be directly connected or coupled to other elements, or there may also be intermediate elements. In addition, the "connection" or "coupling" used here may include wireless connection or coupling. The term "and / or" used herein includes any unit and all combinations of one or more associated listed items. It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as generally understood by those of ordinary skill in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0028] The embodiment of the present invention provides a helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber, such as Figure 1a , 1b As shown, including:

[0029] A broadband helical antenna radiator 1, a switchable band-stop frequency selective structure layer 2, a dielectric substrate 3, an air spacing layer 4, a broadband magnetic absorbing material layer 5 and a metal floor 6;

[0030] Among them, in a hierarchical order from top to bottom, the switchable band-stop frequency selective structure layer 2, the dielectric substrate 3, the air spacing layer 4, the broadband magnetic absorbing material layer 5 and the metal floor 6 constitute the magnetically reconfigurable frequency selective absorber, thereby forming a relationship in which the spiral antenna and the magnetically reconfigurable frequency selective absorber share a metal ground.

[0031] The broadband helical antenna radiator 1 is perpendicular to the magnetically reconfigurable frequency selective absorber and is located in the middle of the magnetically reconfigurable frequency selective absorber, wherein a hole is opened at the center of the switchable band-stop frequency selective structural layer 2, a part of the dielectric substrate 3 is exposed from the hole, and the broadband helical antenna radiator 1 passes through the hole and is connected to the dielectric substrate 3;

[0032] A PIN diode is installed in the switchable band-stop frequency selection structure layer 2. The overall structure design of the antenna can switch the overall RCS of the antenna by changing the on-off state of the PIN diode in the switchable band-stop frequency selection structure layer 2, thereby achieving the purpose of switching the stealth state.

[0033] The diameter of the broadband helical antenna radiator 1 is D=8mm, and the inner diameter of the coil is r=0.63mm. The distance between two adjacent coils of the broadband helical antenna radiator 1 is h=12mm, and the helical angle of the helical antenna is about 15°. The broadband helical antenna radiator 1 is wound around the switchable band-stop frequency selective structure layer 2 in a clockwise direction.

[0034] The end of the last coil at the bottom of the broadband helical antenna radiator 1 is connected to one end of a vertical part, and the other end of the vertical part passes through the hole and is connected to the dielectric substrate 3; one end of the impedance matching line is connected to the bottom end of the vertical part, and the other end is connected to the broadband helical antenna radiator 1, wherein the highest point of the vertical part is the zero point of the z-axis, and the z-axis coordinate of the connection point between the impedance matching line and the vertical part is Zmin=-2.0mm, and the z-axis coordinate of the connection point between the impedance matching line and the broadband helical antenna radiator 1 is Zmax=3mm. The specific shape of the broadband helical antenna radiator 1, viewed from one end of the vertical part connected to the antenna, is first parallel to the PEC board and then extends upward to the other end at a speed similar to an exponential rate of change.

[0035] The vertical portion continues to extend downward by 3.2 mm from the coordinate Zmin and passes through the dielectric substrate 3 .

[0036] In this embodiment, the thickness of the switchable band-stop frequency selection structural layer 2 is t1=0.25mm, and the material is F4B substrate; a metal pattern is printed on the upper surface of the switchable band-stop frequency selection structural layer 2; wherein the metal pattern is: two concentric metal square rings, the side length of the inner ring is I1=1.0mm, and the side length of the outer ring is l2=2.0mm.

[0037] The outer ring of the metal square ring serves as an outer loop, and two vertical metal arms extending from the upper surface of the switchable band-stop frequency selection structure layer 2 and two horizontal metal arms extending from the lower surface are respectively connected to the side of the outer loop through respective pin diodes (PIN diodes) and resistors; wherein, the width of each vertical metal arm and each horizontal metal arm is w=0.5mm, and the length is also I3=8.5mm. After installation, the pin diodes and resistors are connected to the access end of the outer loop as metal arms, and the total length of the access end of the outer loop is l4=2.5mm.

[0038] Among them, the top switchable band-stop frequency selection structure layer 2 in its unit structure is an active frequency selection surface, which can switch from in-band reflection and out-of-band transmission to full transmission. It is realized by printing metal patterns on both sides of the F4B substrate with a thickness of t1=0.25mm. Its metal structure consists of two metal square rings with the same center and side lengths of I1=1.0mm and l2=2.0mm respectively. The two vertical metal arms on the upper surface and the two horizontal metal arms on the lower surface are connected to the four sides of the external loop through pin diodes and resistors. The width of the vertical metal arm and the horizontal metal arm is w=0.5mm, and the length is I3=8.5mm. The total length of the pin diode and the resistor is l4=2.5mm. The two resistors are used to introduce ohmic loss, thereby widening the absorption band. For the magnetically reconfigurable frequency selective absorber 2-6, the top switchable band-stop frequency selection structure layer 2 in its unit structure, the packaged PIN diode used for switching is SMP1320-079LF. The unit structure is Figure 1a and Figure 1b As shown, the magnetic reconfigurable frequency selective absorber is composed of many unit structures. Among them, 2 to 6 constitute a unit structure, for example: Figure 1a and Figure 1b Every four points connected in a quadrilateral form a unit structure, and many unit structures form the parts other than the broadband helical antenna radiator.

[0039] In the preferred solution of this embodiment, the thickness of the dielectric substrate 3 is t2=2.0 mm, the thickness of the air spacer layer 4 is ta=6.0 mm; the thickness of the broadband magnetic absorbing material layer 5 and the metal floor 6 are the same, both tab=3.2 mm.

[0040] The broadband magnetic absorbing material layer 5 is made of a magnetic material FGM-125 having magnetic loss characteristics, and the metal floor 6 is made of a metal material, including copper or aluminum.

[0041] The air spacer layer 4 is filled with polymethacrylimide (PMI) foam having a dielectric constant εr=1.05 and a loss tangent tanδ=0.005.

[0042] For example, refer to Figure 1a , Figure 1b , Figure 2 and Figure 3 Described are the geometric structure of the magnetically reconfigurable frequency selective absorber of the radar cross section reconfigurable helical antenna proposed by the present invention, and the top view and bottom view of the switchable band-stop frequency selective structure layer. Figure 1a and Figure 1b As shown, the magnetic reconfigurable frequency selective absorber structure designed by the present invention comprises, from bottom to top, a switchable band-stop frequency selective structure layer 2, a dielectric substrate 3, an air spacing layer 4, a broadband magnetic absorbing material layer 5, and a metal floor 6. Figure 4 The invention discloses a broadband helical antenna radiator with a geometric structure and impedance matching line design. The bottom of the broadband helical antenna radiator is connected to a metal floor. The rest of this part of the magnetically reconfigurable frequency selective absorber is hollowed out except for the metal floor. The entire magnetically reconfigurable frequency selective absorber is composed of 2 to 6 units arranged and combined. The axis of the antenna is such as Figure 6 As shown, the E and H surface radiation patterns of the antenna refer to Figure 6-7 , 3D gain pattern and normalized antenna pattern reference in the "off" state of the antenna Figure 8-10 , 3D gain pattern and normalized antenna pattern reference in the "off" state of the antenna Figure 9-11 , antenna simulation and measured single-station RCS as Fig.12 As shown, the measured results are basically consistent with the simulation results.

[0043] In this embodiment, the designed magnetic reconfigurable frequency selective absorber is composed of 2-6 layers, and the broadband helical antenna radiator 1 is connected to the metal floor 6 to form a common metal ground relationship. The broadband helical antenna radiator 1 is perpendicular to the magnetic reconfigurable frequency selective absorber and is located in the middle thereof. The overall structural design of the antenna can achieve the switching of the overall radar cross section of the antenna by changing the on-off state of the PIN diode in the switchable band-stop frequency selective structural layer 2, thereby achieving the purpose of switching the stealth state. The overall structural design realizes the low scattering and reconfigurable radar cross section performance of the antenna, while meeting the requirements of good impedance matching and antenna radiation performance.

[0044] By using a reflective frequency selective absorber to replace the metal grounding of the antenna, the radar cross section switching of the antenna is realized, while the gain, axial ratio and other characteristics of the original helical antenna are maintained, and the reduction of the in-band and out-of-band radar cross sections is achieved, so that the antenna has the ability of stealth and communication switching. The advantage of this embodiment is that by using a reflective frequency selective absorber to replace the metal grounding of the antenna, the switching electromagnetic characteristics from in-band reflection and out-of-band absorption response to broadband absorption response can be realized, so that the antenna has the ability to switch the stealth state. By designing an antenna composed of a switchable resistor unit and a magnetic dispersion element, due to the uniform reflection characteristics of the antenna unit, the scattering gain is fixed, and the radiation pattern of the antenna can be changed by controlling the bias voltage, so that a reconfigurable low-scattering antenna with normal radiation performance can be realized. In practical applications, the main working frequency band is 5-6GHz, which realizes the reduction of the in-band and out-of-band radar cross sections, and maintains the gain, axial ratio and other characteristics of the original helical antenna.

[0045] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The above is only a specific implementation method of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or replacements that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A helical antenna with a reconfigurable radar cross section based on a magnetically reconfigurable frequency selective absorber, characterized in that: include: A broadband helical antenna radiator (1), a switchable band-stop frequency selection structure layer (2), a dielectric substrate (3), an air spacing layer (4), a broadband magnetic wave absorbing material layer (5) and a metal floor (6); Wherein, in a hierarchical order from top to bottom, a switchable band-stop frequency selective structural layer (2), a dielectric substrate (3), an air spacing layer (4), a broadband magnetic absorbing material layer (5) and a metal floor (6) constitute the magnetic reconfigurable frequency selective absorber; The broadband helical antenna radiator (1) is perpendicular to the magnetically reconfigurable frequency selective absorber and is located in the middle of the magnetically reconfigurable frequency selective absorber, wherein a hole is opened at the center of the switchable band-stop frequency selective structural layer (2), a portion of the dielectric substrate (3) is exposed from the hole, and the broadband helical antenna radiator (1) passes through the hole and is connected to the dielectric substrate (3); A PIN diode is installed in the switchable band-stop frequency selection structure layer (2); The end of the last coil at the bottom of the broadband helical antenna radiator (1) is connected to one end of a vertical portion, and the other end of the vertical portion passes through the hole and is connected to the dielectric substrate (3); One end of the impedance matching line is connected to the bottom end of the vertical part, and the other end is connected to the broadband helical antenna radiator (1), wherein the highest point of the vertical part is taken as the z-axis zero point, and the z-axis coordinate of the connection point between the impedance matching line and the vertical part is Zmin=-2.0mm, and the z-axis coordinate of the connection point between the impedance matching line and the broadband helical antenna radiator (1) is Zmax=3mm; The vertical portion continues to extend downward by 3.2 mm from the coordinate Zmin and passes through the dielectric substrate (3).

2. The radar cross section reconfigurable helical antenna based on a magnetically reconfigurable frequency selective absorber according to claim 1, characterized in that: The diameter of the broadband helical antenna radiator (1) is D = 8 mm, and the inner diameter of the coil is r = 0.63 mm; The distance between two adjacent coils of the broadband helical antenna radiator (1) is h=12 mm, and the helical angle of the helical antenna is 15°; The broadband helical antenna radiator (1) is wound around the switchable band-stop frequency selection structure layer (2) in a clockwise direction.

3. The radar cross-section reconfigurable helical antenna based on a magnetically reconfigurable frequency selective absorber according to claim 1, characterized in that: The switchable band-stop frequency selective structure layer (2) has a thickness of t1=0.25 mm and is made of F4B substrate; A metal pattern is printed on the upper surface of the switchable band-stop frequency selection structure layer (2); Wherein, the metal pattern is: two concentric metal square rings, the side length of the inner ring is l1=1.0mm, and the side length of the outer ring is l2=2.0mm.

4. The radar cross-section reconfigurable helical antenna based on a magnetically reconfigurable frequency selective absorber according to claim 3, characterized in that: The outer ring of the metal square ring serves as an outer loop, and two vertical metal arms extending from the upper surface of the switchable band-stop frequency selection structure layer (2) and two horizontal metal arms extending from the lower surface are connected to the side of the outer loop through respective pin diodes and resistors; Among them, the width of each vertical metal arm and each horizontal metal arm is w=0.5mm, and the length is l3=8.5mm. After the pin diode and resistor are installed, they are connected to the access end of the external loop as metal arms, and the total length of the access end of the external loop is l4=2.5mm.

5. The radar cross-section reconfigurable helical antenna based on a magnetically reconfigurable frequency selective absorber according to claim 1, characterized in that: The air spacer layer (4) is filled with polymethacrylimide (PMI) foam having a dielectric constant εr = 1.05 and a loss tangent tanδ = 0.005.

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