Patent Application
20140009351
The invention relates to a horn that radiates a radioelectric wave coming from an input waveguide, comprising a grating placed over the aperture of the horn. It is notably applicable to the field of reflector antennas. The invention also relates to a satellite antenna equipped with this horn.
Conventionally, an antenna for emitting and receiving an electromagnetic wave may be produced by associating a waveguide with a radiating element that may, for example, take the form of a horn. A horn-shaped waveguide, more simply called a horn, has a rectangular transverse (i.e. perpendicular to the propagation direction of the wave) cross section that gradually increases towards the aperture. Such a waveguide makes it possible to promote propagation, along its longitudinal axis, of an electromagnetic wave polarised along an axis orthogonal to the longitudinal axis of the horn. The electric field of the electromagnetic wave may be decomposed into a component parallel to the shortest sides of the aperture, and into a component parallel to the longest sides of the aperture. The first component is called the principal or co-polarisation component. The other component is called the cross-polarisation component. In the context of certain applications, it is desirable to reduce as much as possible the amplitude of the cross-polarisation component. One solution consists in placing a grating over the aperture of the horn. A grating is generally made of a metal, for example aluminium. It is formed from a set of plates placed parallel to the longest sides of the aperture of the waveguide. The grating allows the co-polarisation component to pass through and filters the cross-polarisation component of an electromagnetic wave. For a relatively directional horn, for example with a gain higher than 25 dBi, equipped with a grating, it is possible to obtain a cross-polarisation component the amplitude of which is about 40 to 45 dB smaller than the amplitude of the co-polarisation component. However, the effectiveness of the filtering very clearly or even completely decreases when the horn is less directional. This is notably the case for the test horns used in microwave anechoic chambers. Also, the filtering is effective only over a narrow frequency band. With the increasing demand for better antenna performance, it is becoming useful to develop horns providing an attenuation of the cross-polarisation component of at least 40 dB relative to the co-polarisation component, and this over widened frequency bands, for example by about 40% to 50%.
One aim of the invention is notably to provide a horn having improved properties with respect to the filtering of the cross-polarisation component of the electric field of an electromagnetic wave, both in terms of amplitude of the cross-polarisation component and in terms of bandwidth. For this purpose, one subject of the invention is a waveguide comprising a horn-shaped segment, an entrance, an aperture, and a grating placed next to the aperture, at least one linearly polarised electromagnetic wave being able to propagate between the entrance and the aperture along a first axis, the grating comprising a frame encircling a set of plates extending longitudinally and continuously from a first short side of the frame to a second short side of the frame, so as to form a linear polarising filter attenuating the cross-polarisation component of the electric field of the electromagnetic wave, said cross-polarisation component being orthogonal to a second axis orthogonal to the first axis. The plates comprise corrugations dimensioned and positioned so as to reinforce the attenuation of said cross-polarisation component.
Notably one advantage of the invention is that it can be tailored to any type of horn, notably pyramidal or trifurcated horns. These horns are relatively light, and relatively simple to design and manufacture. Relative to a corrugated horn, a pyramidal or trifurcated horn weighs about half as much. Also, the invention has the advantage of improving the standing wave ratio and the gain of the horn.
The invention may be used in the test equipment of radiofrequency anechoic chambers so as thus to allow more precise and more reliable measurement results to be obtained on the levels of cross-polarisation and of the orientation of the principal polarisation of the devices tested. With better cross-polarisation levels, and by virtue of its simplicity of manufacture and its favourable weight, it will also be possible to use the invention for satellite antenna applications.
The corrugations for example consist in rectangular slits cut in the side opposite the entrance of the waveguide.
Advantageously, the corrugations have dimensions that vary depending on their position in the direction in which the plates extend longitudinally between the first and second short sides of the frame, as a function of the frequency of the electric field of the electromagnetic wave having locally the largest amplitude at the respective corrugations. The filtering may thus be optimised over a wide frequency band.
The depth of the slits is, for example, substantially equal to a quarter of the wavelength corresponding to the frequency of the electric field having locally the largest amplitude at the respective slits, and being essentially oriented along the second axis. The depth of the slits is, in another example, substantially equal to a quarter of the wavelength corresponding to a frequency of an operating frequency band of the waveguide, the electromagnetic wave emitted in said operating frequency band having an electric field essentially oriented along the second axis. Moreover, the higher the frequency, the smaller the width of the slits. Again with the aim of optimising filtering over a wide frequency band, the interval between two adjacent corrugations in the direction in which the plates extend longitudinally is substantially equal to a quarter of the wavelength corresponding to the frequency of the electric field of the electromagnetic wave having locally the largest amplitude at the respective slits. The interval between two adjacent corrugations in the direction in which the plates extend longitudinally is, as a variant, substantially equal to a quarter of the wavelength corresponding to a frequency of an operating frequency band of the waveguide, the electromagnetic wave emitted in said operating frequency band having an electric field essentially oriented along the second axis.
Advantageously, with the aim of optimising the attenuation, the frame comprises corrugations. Advantageously, with the aim of optimising the attenuation, the frame comprises corrugations extending right through the thickness of at least one side of the frame in a direction perpendicular to the first axis.
Advantageously, the frame comprises corrugations extending right through the thickness of at least one side of the frame along the second axis and/or corrugations extending right through the thickness of at least one side of the frame along a third axis orthogonal to the first axis and to the second axis.
According to one particular embodiment, the corrugations are aligned in sets along the second axis, the corrugations of a given set having identical dimensions.
Again according to one particular embodiment, the grating is placed at a non-zero distance away from the aperture of the waveguide along the first axis.
In order to enhance the filtering of the cross-polarisation component, the waveguide may comprise at least one additional grating, the gratings being spaced apart pairwise along the first axis by a distance ranging from the wavelength corresponding substantially to a central frequency of an operating frequency band of the waveguide, to one eighth of this wavelength. One or more of the additional gratings may be placed parallel to the grating placed next to the aperture. Moreover, one or more of the additional gratings may each comprise corrugations. Each additional grating may be substantially identical to the grating placed next to the aperture.
According to one particular embodiment, the grating comprises a frame that substantially follows the perimeter of the waveguide aperture, the frame comprising protruding parts extending in a plane orthogonal to the first axis. The protruding parts for example form a sawtooth profile. The protruding parts may extend towards the interior and/or towards the exterior of the frame.
Advantageously, the plates extend longitudinally in a direction substantially parallel to a third axis orthogonal to the second axis and orthogonal to the first axis.
Advantageously, so as to obtain a better attenuation, the plates extend longitudinally in a direction making, with a third axis orthogonal to the second axis and orthogonal to the first axis, an angle between 0.05° and 5° about the first axis.
Advantageously, the waveguide is designed to operate in an operating frequency band, the plates having a height along the z-axis substantially equal to half a wavelength corresponding to a frequency in the operating frequency band of the waveguide.
Another subject of the invention is a satellite antenna comprising a waveguide such as described above.
The last subject of the invention is a method for testing a radio-frequency device in which a waveguide such as described above is used.
The invention will be better understood and other advantages will become apparent on reading the following description, given with regard to the appended drawings in which:
In the rest of the description, the central frequency of the operating frequency band of an antenna will be denoted f0, the speed of light in the propagation medium considered will be denoted C0, and the wavelength corresponding to the frequency f0 will be denoted λ0 (where λ0=C0/f0). f0 is the central frequency of the electric fields of the electromagnetic waves emitted in the operating band of the antenna. These electric fields are, even before they arrive at the grating, essentially oriented along the y-axis.
In the embodiment of
The expression “two short sides of the frame” is understood to mean the two shortest sides of the frame and the expression “two long sites of the frame” is understood to mean the two longest sides of the frame.
The plates 151-153 are placed parallel to the x-z plane in the embodiments of the figures. They are placed so as to allow an electromagnetic wave the electric field of which is polarised along the y-axis to pass and to filter any other electromagnetic wave the electric field of which is not polarised along the y-axis. The grating 15 thus forms a linear y-axis polarising filter. The term “filtering” is understood to mean the attenuation of the amplitude of the electric field. The grating 15 notably attenuates what is called the cross-polarisation component of the electric field of an electromagnetic wave, i.e. the component oriented along the x-axis. The grating in particular attenuates the cross-polarisation components of the electric fields of the electromagnetic waves the respective frequencies of which are contained in the operating frequency band of the waveguide. The geometric properties of the grating 15 are essentially defined depending on the operating frequency band of the antenna. The geometric properties having the most significant impact on the electromagnetic properties of the grating are the height of the grating 15 and the interval between adjacent plates, and likewise between the external plates 151 and 153 and the internal edge of the frame 150. Advantageously, the height of the grating 15 along the z-axis is substantially equal to half the wavelength λ0 (λ0/2). The interval between two adjacent plates, and between the external plates 151 and 153 and the internal edge of the frame 150 is advantageously substantially equal to a quarter of the wavelength λ0 (λ0/4). Other geometric properties have a secondary influence on the electromagnetic properties of the horn 10. This may notably be the position of the grating 15 relative to the aperture 14. Advantageously, the grating 15 is placed a substantially zero distance away from the x-y plane of the aperture 14. The thickness of the frame 150 along the x- and y-axes and that of the plates 151-153 along the y-axis have little influence on the performance of the grating 15. The thickness of the plates 151-153 depends directly on the size of the aperture 14 of the horn along the y-axis, on the number of plates, and on the interval between the plates. From an electromagnetic point of view, the thickness of the plates 151-153 may be very small. However, the plates 151-153 must be sufficiently thick to be manufacturable and strong. By way of example, the plates may be substantially equal to 1 mm in thickness. The thickness of the frame 150 is essentially chosen in order to allow it to withstand the mechanical stresses undergone by the horn 10. In particular, since the plates 151-153 are relatively thin, the frame 150 must be sufficiently thick to prevent the plates 151-153 from twisting. For a horn intended to be used in an antenna operating in the Ku frequency band, i.e. the frequency band between 10.00 GHz and 15.00 GHz, the frame 150 is for example between 2 and 10 mm in thickness.
The geometric properties of the grating 21 are defined identically to those of the grating 15 of
The attenuation of electromagnetic waves the electric field of which is not polarised along the y-axis, in particular the attenuation of cross-polarisation components, is enhanced by the geometric properties of the corrugations 22, namely their dimensions and their positions. These geometric properties of the corrugations 22 are defined depending on the operating frequency band of the antenna. The geometric properties having the most significant impact on the electromagnetic properties of the grating are the depth of the corrugations and the interval between adjacent corrugations along the x-axis. The depth of a corrugation 22 is defined as the distance along the z-axis between, on the one hand, the external surface of the frame 210 or the plates 211-213 and, on the other hand, the bottom of the slit 22 considered. The depth of the corrugations is advantageously dimensioned to form a “quarter-wave trap”. In other words, it is substantially equal to a quarter of the wavelength λ0 (λ0/4). However, in order to preserve optimal filtering over the entire width of the frequency band, it is possible to consider a number of particular frequencies in the frequency band. Specifically, low-frequency signals have a tendency to disperse more towards the edges of the grating than to the centre, whereas higher frequency signals are more directional and therefore concentrate more at the centre of the grating. This property may be used in order to dedicate various portions of the grating to filtering particular separate frequencies. In the example of
The following frequencies are considered: f0=12.5 GHz, f1=14.75 GHz, f2=14.25 GHz, f3=12.75 GHz and f4=11.7 GHz. Each frequency f1 to f4 is associated with a set of corrugations 221, 222A-222B, 223A-223B or 224A-224B. These frequencies allow the depths h1 to h4 of the corrugations of the sets 221 to 224, respectively, to be defined. With C0=3.108 m/s, the wavelengths associated with the frequencies f0 to f4 are respectively λ0=24 mm, λ1=20.34 mm, λ2=21.05 mm, λ3=23.53 mm and λ4=25.64 mm.
For the various zones of the grating 21 located between the corrugations, the following frequencies are considered: f12=14.5 GHz, f23=13.75 GHz, f34=f0=12.5 GHz and f40=10.3 GHz. They allow the intervals between adjacent corrugations to be defined. The wavelengths associated with these frequencies are λ12=20.69 mm, λ23=21.82 mm, λ34=24.00 mm, and λ40=29.13 mm, respectively. For these frequencies, the dimensions of the grating 21 are for example the following:
H=12 mm, set to λ0/2;
d=8.25 mm;
ec=7.0 mm;
el=1.0 mm;
h1=5.08 mm; h2=5.26 mm; h3=5.88 mm; h4=6.41 mm;
d12=5.17 mm; d23=5.46 mm; d34=6.00 mm; d40=7.28 mm;
e1=0.75 mm; e2=1.0 mm; e3=1.25 mm; e4=1.5 mm.
In other words, the dimensions and/or the intervals between the respective slits are defined by the wavelength corresponding to the frequency of the electric field having locally the largest amplitude at the grating 21, and in particular at the respective slits 22.
As may be seen in
As a variant (not shown) or in addition to the corrugations in the long sides, the short sides 250a and 250b of the frame comprise corrugations extending right through their respective thicknesses in a direction perpendicular to the z-axis. The corrugations of each short side extend right through the thickness of the short side perpendicularly to the longitudinal direction of the short side. In this way they emerge onto the top and bottom of the side. In the case where the short sides extend longitudinally along the y-axis, the corrugations extend right through their respective thicknesses in the x-direction. The corrugations formed in a side take, for example, the form of a channel extending longitudinally in a direction perpendicular to the longitudinal direction of said side (y-direction) and have a rectangular cross section in the y-z plane.
Advantageously, at least one of the sides of the frame comprises corrugations extending right through its thickness.
In the graphs of
In the graph of
The corrugated grating according to the invention also has the advantage of improving the standing wave ratio by about 1 to 5 dB and the gain of the horn by a few tenths of a decibel. It makes it possible to obtain cross-polarisation component maximum amplitudes 40 dB below the co-polarisation component maximum amplitudes with pyramidal horns.
In the example of
Moreover, a horn according to the invention may comprise a number of gratings in addition to the grating 21 placed next to the aperture 14 of the horn 20. These additional gratings also contain corrugations in their plates and/or in the edges of their frame. The gratings are, for example, regularly spaced apart from each other (pairwise) by a distance ranging between the wavelength λ0 and one eighth of this wavelength. The additional gratings may optionally be identical to the grating 21.
In the embodiment of the figures, the plates extend longitudinally in the direction substantially parallel to the x-axis. The position and dimensions of the corrugations are defined in and/or relative to this axis. In other words, the longitudinal direction of the plates forms an angle smaller than 0.05° with the x-axis, about the z-axis.
As an advantageous variant, the plates extend longitudinally in a direction forming, with the x-axis, about the z-axis, an angle at least equal to 0.05° and between 0.05 and 5°. In this case, the position (for example the interval between the corrugations) and the dimensions of the corrugations (for example their width) are defined in and/or relative to the longitudinal direction of the plates. In certain practical cases, this embodiment advantageously enables a better attenuation of cross-polarisation components extending along the x-axis.
In these two embodiments, given that the angle formed between the longitudinal direction of the plates and the x-axis is at most equal to 5°, the plates are assumed and said overall to extend longitudinally along the x-axis.
The plates form overall rectangular parallelepipeds having a side extending along the z-direction
Embodiments in which the depth of the corrugations, the interval between the corrugations, or the height of the plate were equal to a fraction (a quarter or half) of the wavelength of the central frequency were described above. As a variant, these dimensions and positions are equal to a fraction (a quarter or half) of the wavelength of a frequency contained in the operating frequency band of the waveguide.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12 01240 | Apr 2012 | FR | national |
