ANTENNA APPARATUS WITH REFLECTOR OR LENS CONSISTING OF A FREQUENCY SCANNED GRATING

Abstract: The present invention relates to an antenna apparatus for high frequency applications, essentially composed of feed antenna (15) and reflector (16) or lens (19) consisting of a frequency scanned grating which reflects or transmits the incident electromagnetic field into a diffracted field consisting of the first order diffracted grating-lobe, where the shape of curvature of the reflector or the lens is not directly determined by desired radiation characteristics but can be chosen to obtain other benefits. The invention is characterized in that the frequency scanned grating has a quasi-periodic lattice geometry, where the quasi-periodic lattice geometry essentially determines the radiation pattern of the reflected or transmitted diffracted field from the grating and thereby is essentially determinant for the radiation pattern and the radiation properties of the antenna apparatus.

Description: ANTENNA APPARATUS WITH REFLECTOR OR LENS CONSISTING OF A FREQUENCY SCANNED GRATING The present invention relates to an antenna apparatus for high frequency applications, essentially composed of feed antenna and reflector or lens consisting of a frequency scanned grating (blazed grating) which reflects or transmits the incident electromagnetic field into a diffracted field composed of the first order diffracted grating-lobe. In the field of microwaves and millimeterwaves the antenna is a very important system component. Depending on the application different demands are put on the performance of the antenna. For communication links and for direct broad¬ cast satellite (DBS) systems etc., antennas with large antenna gain are requested also having a simple construction that are inexpensive to manufacture. In these areas, the ordinary parabolic reflector antenna is commonly used. However, there is a great interest in designing planar antennas, such as printed circuit array antennas, lens or reflector antennas consiting of Fresnel plates, etc., which are less voluminous and which can be attached directly on a wall or a roof. A difficulty for the planar antennas is to find simple constructions having a good antenna efficiency. Periodic grating structures are of increasingly importance in modern antenna design and have found applications such as frequency selective surfaces, metallic radomes, polarizers, etc.. Periodic gratings can also be designed to give fre¬ quency scanning properties. The principle considered is to select a grating periodicity such that the first order diffracted wave (first order diffracted grating-lobe) is propagating when the grating is illuminated by an electro- magnetic field. This diffracted wave is then to serve as the frequency scanned beam since its direction of propagation is frequency dependent. The concept requires that the grating transfers the power of the incident field to the propagating diffracted wave. Grating structures with this property are commonly referred to as blazed gratings or frequency scanned gratings. The present invention provides an antenna system, essen¬ tially composed of feed antenna and reflector or lens con¬ sisting of a frequency scanned grating, where the shape of curvature of the reflector or the lens is not directly determined by the desired radiation characteristics but can be chosen to obtain other benefits. For instance, a planar reflector or lens can be chosen, giving a construction with a high antenna efficiency that is simple and inexpensive to manufacture. The present invention also provideds an antenna apparatus suitable for applications where antennas with a frequency controllable radiation direction are desired. The antenna according to the invention is characterised in that the frequency scanned grating, which reflects or trans- mitts the incident electromagnetic field into a diffracted field composed of the first order diffracted grating-lobe, has a quasi-periodic lattice geometry. The quasi-periodic lattice geometry essentially determines the radiation pat¬ tern of the reflected or transmitted diffracted field from the grating and thereby essentially determines and shapes the radiation pattern and the radiation properties of the antenna apparatus. The invention will now be described further with reference to the accompanying drawings, in which: Fig. la shows the cross-sectional view of a frequency- scanned reflection grating consisting of electrical¬ ly conducting elements. Fig. lb shows the top view of the same frequency-scanned reflection grating as in Fig la. Fig. 2 shows examples of different alternative shapes of the electrically conducting elements. Fig. 3a shows the cross-sectional view of a frequency- scanned reflection grating consisting of apertures in a electrically conducting surface. Fig. 3b shows the top view of the same frequency-scanned reflection grating as in Fig 3a. Fig. 4a shows the cross-sectional view of a frequency- scanned reflection grating with coordinat system and dimensions given in order to describe the lattice geometry. Fig. 4b shows the top view of the same frequency-scanned reflection grating as in Fig 4a. Fig. 5a shows the side view of an embodiment with a feed antenna and a planar reflector consisting of a frequency-scanned reflection grating. Fig. 5b shows the top view of the same embodiment as in Fig 5a. Fig. 6 shows two coordinate systems. Fig. 7a shows the cross-sectional view of a frequency- scanned transmission grating consisting of electri- cally conducting elements in three layers. Fig. 7b shows the top view of the same frequency-scanned transmission grating as in Fig 7a. Fig. 8a shows the side view of a construction example with a feed antenna and a planar lens consisting of a frequency-scanned transmission grating. Fig. 8b shows the top view of the same construction example as in Fig 8a. The invention is based on so called frequency scanned gra- tings or blazed gratings. This type of grating has the property of reflecting or transmitting an incident electro¬ magnetic field into the first order diffracted grating lobe. Fig 1 shows an example of a frequency scanned reflection grating consisting of electrically conducting elements 1 etched in a periodic lattice geometry on a dielectric sub¬ strate 2 and placed over an electrically conducting ground- plane 3 with the support of a dielectric spacer 4. The periodic element 1 consists in this case of single dipoles. In practical applications a grating consists of hundreds or thousands of elements placed in a periodic lattice geometry. Examples of other types of elements that can be used instead of the single dipoles are shown in Fig 2, and are crossed dipoles 8, rings 9, tripoles 10, squares 11, Jerusalem crosses 12, etc.. Fig 3 shows another example of a frequency-scanned reflec¬ tion grating where the periodic lattice of electrically conducting elements is replaced by an electrically conduc¬ ting plane 13 perforated by apertures 14 in a periodic lattice geometry. Here the periodic aperture element 14 is crossed slots. Other types of aperture elements are possible and can have shapes similar to those shown in Fig 2. For a frequency-scanned reflection grating the periodicity of the grating (the lattice periodicity) is normaly chosen such that when illuminated by an electromagnetic wave 5 a reflected scattered field is obtained where, besides the reflected fundamental wave 6, also the first order diffrac¬ ted wave 7 (first order diffracted grating-lobe) propagates. The reflected fundamental wave has a direction of reflection decided only by the illumination angles of the incident wave 5 with respect to the grating. Whereas for the diffracted wave 7 the direction of reflection depends on the illumina¬ tion angles, the lattice geometry of the grating, and the frequency of the incident electromagnetic field. A frequency-scanned grating can be theoretically analyzed by assuming a plane and infinite periodic grating illuminated by a plane electromagnetic wave. These assumptions can be made if the eventual radious of curvature of the grating and the distance the between grating and the source of the electromagnetic field are large compared to the wavelength and the grating periodicity. For a periodic reflection grating with a lattice geometry as defined in Fig 4 the fol¬ lowing relation between the illumination angles (Θ,Φ) of the incident wave 5 and the reflection angles ( B_ 1 , Φ_ 1 ) of the first order diffracted grating-lobe 7 is obtained: sin(c ) sinθ-1,cosΦ-1, = sinθ cosΦ - D_-s„i.„,o„ -α „, )x λ cos(α 2 ) sinθ _.sinΦ _. = sinθ sinΦ - -1 -1 D-sinto -GU ) where λ is the wavelength and D l r D 2 , a l r a 2 , describes the periodicity, see Fig 4. The angle θ is defined as the angle between the propagation direction of the incident wave 5 and the z-axis, and the angle Φ as the angle between the plane of incidence and the x-axis. In the same manner θ_ is defin¬ ed as the angle between the propagation direction of the first order reflected diffracted wave 7 and the z-axis, and Φ_ ! as the angle between the plane of reflection for the diffracted wave 7 and the x-axis. It follows from the above equations that for an incident wave 5 with a wavelength and illumination angles (Θ,Φ) the direction of reflection ( Θ_ 1 , Φ_ 1 ) for the diffracted wave 7 is determined by the lattice periodicity , that is D x , D 2 , a l r 2 . It is the first order reflected diffracted wave (grating- lobe) 7 that is to serve as the frequency scanned beam by designing the grating structure such that essantially all the power of the incident field is scattered into this diffracted wave. Methods and solutions for how gratings with this property can be designed are described in; F.S. Johans¬ son, "Periodic arrays of metallic elements as frequency scanning surfaces", Procedings Fifth Intern. Conf. on An¬ tennas & Propagation, York, UK, pp 71-74, March 1987, and in F.S. Johansson; "Frequency-scanned gratings consisting of photo-etched arrays", IEEE Trans. Antennas and Propagation, Vol. AP-37, no.8, pp 996-1002, August 1989. From these dokuments it is clear that power conversions to the first order reflected diffracted wave can be achieved with a conversion loss less then 1%. The present invention makes use of the fact that for a fre¬ quency scanned reflection grating the reflection direction of the first order diffracted wave depends on the grating periodicity. By letting the grating not being strictly periodic but having a quasi-periodic lattice geometry, it is possible to control and shape the radiation pattern of the reflected diffracted field. With a reflector consisting of a frequency-scanned reflection grating, efficiently con¬ verting the incident field to the first order diffracted field, it will be the diffracted field that essentially determines the radiation pattern of the complete antenna system. Hence the radiation pattern of the antenna system will be determined by the quasi-periodic lattice geometry of the grating-reflector. Here quasi-periodic lattice geometry means that the periodicity varies smoothly along the reflec- tor surface. Fig 5 shows an example of an antenna according to the inven¬ tion, consisting a feed horn 15 illuminating a planar re¬ flector 16 composed of a frequency-scanned reflection gra- ting. In this example the frequency scanned reflection grating consists of electrically conducting dipoles etched in a quasi-periodic lattice geometry. The frequency-scanned reflection grating can for instance be realized as shown in Fig 1. In this example the quasi-periodic lattice geometry is chosen such that when the feed horn 15 illuminates the reflector 16 with an electromagnetic field 5 at a fixed frequency, a reflected diffracted field 7 is obtained which predominately radiates in one common direction. That is, the quasi-periodic lattice geometry is chosen such that a point focus for the first order diffracte field is obtained at the position of the feed horn. With a grating-reflector that has an efficient power conversion to the diffracted field the reflector antenna system will basically have an antenna gain comparable with the traditional parabolic reflector antenna. Since the reflection direction of the diffracted field also depends on the frequency, it follows that the radiation direction of the antenna system is frequency dependent. Hence the antenna can be used as a frequency-scanned an¬ tenna. The quasi-periodic lattice geometry of the frequency scanned reflection grating in Fig 5, has been determined by the condition that each electromagnetic ray 5 from the feed 15 illuminating the reflector 16 is to be reflected into dif¬ fracted rays 7 that are parallel with each other. It is then assumed that locally on the reflector the reflection proper- ties can be approximated by the corresponding case of a periodic infinite grating illuminated by a plane wave. That is, the above equations are assumed to hold locally on the reflector. This assumption is justified if there is a smooth periodicity variation and the distance between the reflector 16 and the feed 15 is large compared to the periodicity and the wavelength. Referring to the coordinate systems defined in Fig 4 and Fig 6 the condition of parallel diffracted rays means that the relation between the local illumination angles (Θ,Φ) and the local lattice geometry (D 1 ,D 2 ,α 1 ,α 2 ) is obtained by letting θ_ x = and Φ_ x = 180° in the above equa¬ tions. Y is then the desired radiation direction with re¬ spect to the normal of the surface, (in this example chosen to 30°). In the above example the lattice geometry was chosen in order to obtain maximum antenna gain. However, it should be appreciated that the quasi-periodic lattice geometry can as well be selected in order to obtaine other features, such as specially shaped radiation patterns. In accordance with the invention also lens antenna systems can be designed by the use of frequency scanned transmission gratings. Fig 7 shows an example of a frequency scanned transmission grating consisting of three layers of conduc¬ ting elements 1 placed in a periodic lattice geometry and separated by dielectric substrates 2. Here the periodic element 1 consists of dipoles. The principle operation of a frequency scanned transmission grating is to select a lattice periodicity such that when illuminated by an electromagnetic wave 5 a scattered field is obtained where, besides the fundamental reflected and transmitted waves 6,17, also the first order diffracted waves (first order diffracted grating-lobes) 7,18 are pro¬ pagating. In comparision to the reflection grating the difference is that we now also have a transmitted field composed of the transmitted fundamental wave 17 and the first order transmitted diffracted wave 18. In the case of a frequency scanned transmission grating it is normally the first order transmitted diffracted wave 18 that is to serve as the frequency scanned beam. Hence the grating structure is to be designed such that essentially all the power of the incident field is diffracted in to this wave. Methods and solutions for how gratings with this property can be designed are described in; F.S. Johansson, "Frequency-scanned gratings consisting of photo-etched arrays", IEEE Trans. Antennas and Propagation, Vol. AP-37, no.8, pp 996-1002, August 1989. Fig 8 shows an example of a lens antenna according to the invention. The antenna consists of a feed horn 15 and a lens 19 composed of a frequency scanned transmission grating with electrically conducting dipoles placed in a quasi-periodic lattice geometry. The frequency scanned transmission grating can for instance be a three-layer grating structure of the type shown in Fig 7. The quasi-periodic lattice geometry of the lens in Fig 8 is determined by the condition of having a diffracted field radiating essentially in one and the same direction when the feed horn illuminates the lens by an electromagnetic field. Also the lens antenna will have frequency scanning properties. In the examples above the antennas have been described in transmit mode. However, due to reciprocity it is obvious that the antennas can operate as receivers as well. The invention is not limited to the presented exemplary em- bodiments above, but can be varied in several ways within the scope of the following appended claims. It is of course possible to also let the dimensions of the element to vary along the surface of the quasi-periodic gratings in order to improve the conversion efficiency to the diffracted field. It is also possible to use frequency scanned gratings con¬ sisting of a conducting surface corrugated in a quasi-perio¬ dic lattice geometry or a frequency scanned grating consis¬ ting of dielectric sheets with a density and/or shape that varies in a quasi-periodic lattice geometry.

Claims: CLAIMS 1. Antenna apparatus for high frequency applications, essen¬ tially composed of feed antenna (15) and reflector (16) or lens (19) consisting of a frequency scanned grating (blazed grating) which reflects or transmitts the incident elec¬ tromagnetic field into a diffracted field consisting of the first order diffracted grating-lobe, c h a r a c te r i z e d t h e r e i n , that the frequency scanned grating has a quasi-periodic lattice geometry, where the quasi-periodic lattice geometry essentially determines the radiation pattern of the reflec¬ ted or transmitted diffracted field from the grating and thereby is essentially determinant for the radiation pattern and the radiation properties of the antenna apparatus. 2. Antenna apparatus in accordance with claim 1, c h a r a c t e r i z e d t h e r e i n, that the frequency scanned grating consists of one or seve- ral surfaces with electrically conducting elements (1,8-12) placed in a quasi-periodic lattice geometry and/or one or several electrically conducting surfaces (13) perforated with apertures (14) in a quasi-periodic lattice geometry. 3. Antenna apparatus in accordance with claim 2, c h a r a c t e r i z e d t h e r e n, that the frequency scanned grating is build up of one or several dielectric substrates (2,4), where the surfaces with the electrically conducting elements (1) and/or the electri- cally conducting surfaces (13) perforated with apertures (14) are etched or printed on some of the dielectric layers. 4. Antenna apparatus in accordance with claim 1, 2, or 3, c h a r a c t e r i z e d t h e r e i n, that the frequency scanned grating also includes an elec¬ trically conducting surface (3). 5. Antenna apparatus in accordance with claim 1, 2, 3, or 4, c h a r a c t e r i z e d t h e r e i n, that the frequency scanned grating is a planar structure.

Inventors: JOHANSSON STEFAN, JOHANSSON, STEFAN
Applicants: JOHANSSON STEFAN, JOHANSSON, STEFAN
Application year: 1990
Main classification: H01Q19
IPC codes: H01Q3, H01Q15, H01Q19