EP0562355A2 - Antenna for radar surveillance - Google Patents

Abstract

The antenna is designed as a reflector antenna having a double-curved reflector and a double-exciter supply, the reflector surface being especially shaped in two planes in order to produce sum and difference characteristics, and in consequence making possible a special direction-finding behaviour. The illumination region, which is limited in a rectangular manner, requires a beam cross-section which is like a trapezoid in the sum characteristic. The special direction-finding behaviour is defined in that azimuth direction finding is possible within this illumination region using the amplitude or phase monopulse method. An antenna which is constructed according to the invention can advantageously be used in the case of a traffic-control radar, in movement signalling devices or, completely generally, for surveillance of an area. <IMAGE>

Description

The invention relates to an antenna according to the preamble of claim 1.

In order to control the flow of traffic at intersections of multi-lane carriageways, a traffic control radar is proposed (European patent application 92 100 151.7), which provides information about position, speed and possibly also the type of vehicles available. The position of the vehicle is sufficiently defined by the distance and the assignment of the vehicle to a lane. The distance and speed of the vehicle can be determined from the echo signals in a known manner from the transit time and Doppler effect. A rough classification of the vehicles might also be possible using certain recognition patterns. The assignment to a lane can be determined by an azimuthal angular bearing, e.g. according to the amplitude or phase monopulse method. For this purpose, however, the antenna must have special DF properties in monopulse mode.

Previously known monopulse antennas, used in particular for the target tracking radar, consist of a centrally fed parabolic antenna. The resulting radiation lobe is rotationally symmetrical in cross section and bundled in longitudinal section according to the aperture width (C.Rint: "Handbook for High Frequency and Electrical Technicians", Volume 4, 1980, 10th Edition, page 705). In no way are such monopulse antennas particularly adapted to the requirements of road illumination.

Typical radar antennas fed with a double exciter with a fan beam and cosec² vertical characteristics in the beam cross section are also known, e.g. from the essay by G.V. Trentini, W. Jatsch: "Omnidirectional radar antennas for monopulse direction finding", in the journal "Frequency" 21 (1970), 5, pages 144-149. In this case, the longitudinal beam cut is not optimal, but is reasonably well adapted to the area to be illuminated. The beam cross section, however, does not meet the requirements.

The shaping of the beam cross section of reflector antennas is known in connection with the illumination of earth areas by satellite antennas (e.g. NTG technical reports volume 52, 1975, satellite radio systems, W. Rebhan: "Possibilities and assessment criteria for the illumination of earth areas by satellite on-board antennas", page 178 -186). An attempt is made to keep the field strength within the illumination area at a constant value, although level fluctuations are quite permissible. In relation to the problem at hand, one would only optimize the sum characteristic, but not the difference characteristic or the discriminator curve. The adoption and application of these methods would therefore generally not lead to the required direction finding of the antenna.

The object of the invention is to provide an antenna for radar monitoring of an at least approximately rectangularly delimited coverage area, in particular for a road traffic control radar, which has suitable bearing properties.

This object is achieved in a generic antenna by the features specified in the characterizing part of patent claim 1.

Appropriate developments of the invention are specified in the subclaims.

• Fig. 1 einen Vertikalschnitt einer zur Verkehrsregelung eingesetzten Radarantenne nach der Erfindung,
• Fig. 2 eine Ansicht von oben auf die Fahrbahnfläche und die mit der Antenne nach Fig.1 auszuleuchtende Fahrbahnfläche,
• Fig. 3 ein Ausleuchtgebiet in Antennenkoordinaten,
• Fig. 4 Summen- und Differenzcharakteristika,
• Fig. 5 Diskriminatorkurven,
• Fig. 6 eine geometrische Darstellung zur Erklärung der Erzeugung der Reflektorfläche aus dem vertikal verlaufenden Mittelschnitt des Reflektors,
• Fig. 7 und 8 den Reflektormittelschnitt bei konvergentem bzw. divergentem Strahlengang,
• Fig. 9, 10 und 11 den konstruktiven Aufbau eines der Verkehrsregelung dienenden Radargerätes mit einer Antenne nach der Erfindung in einer Ansicht von oben, von der Seite bzw. von vorne,
• Fig. 12 Reflexionszonen im Reflektor bei verschiedenen Einfallwinkeln.

The invention is explained below with reference to twelve figures. Show it

• 1 is a vertical section of a radar antenna used for traffic control according to the invention,
• 2 shows a view from above of the road surface and the road surface to be illuminated with the antenna according to FIG. 1,
• 3 shows a coverage area in antenna coordinates,
• 4 sum and difference characteristics,
• 5 discriminator curves,
• 6 is a geometrical representation for explaining the generation of the reflector surface from the vertically running central section of the reflector,
• 7 and 8 the reflector center section with convergent or divergent beam path,
• 9, 10 and 11 the structural design of a traffic control radar device with an antenna according to the invention in a view from above, from the side or from the front,
• Fig. 12 reflection zones in the reflector at different angles of incidence.

First, in connection with FIGS. 1 and 2, the requirements for the radiation properties of a radar antenna 1 used for traffic detection are dealt with. As a rule, the radar antenna 1 will be at a relatively low height of approx. 4 m above the center, possibly also at the edge of one Lane 2 are located. The detection area 3 begins at approx. 10 m and ends at approx. 200 m, so that the corners A, B, C, D of the surface of the road to be illuminated appear from the antenna 1 at very different angles.

The antenna 1 must therefore have a total radiation characteristic whose beam width varies greatly depending on the elevation and whose vertical section is similar to the typical cosecans curve of radar antennas. A common radar antenna, such as that used for flight surveillance, would not be suitable for several reasons: firstly, the azimuth bearing in the present problem is caused by a monopulse and not by mechanical rotation of the reflector; secondly, the fan-shaped beam cross section is not adapted to the coverage area, and thirdly, there is one The cosec² characteristic in the vertical section is not optimal, since it requires distant objects with a constant reflection cross section. As FIGS. 1 and 2 show, the radar antenna 1 is located in the near field of the scattering objects to be considered. It has been shown that the situation shown in FIGS. 1 and 2 results in better selectability of vehicles 4, 5 with a less strongly decreasing radiation characteristic.

und $\text{v = sin ϑ. sin φ}$

ab (ϑ = Polarwinkel in der xz-Ebene im antennenfesten Koordinatensystem x,y,z; φ = Azimutwinkel in der xy-Ebene im antennenfesten Koordinatensystem x,y,z), so würde sich bei idealer Ausleuchtung der in Fig.3 gezeigte trapezähnliche Strahlquerschnitt A,B,C,D ergeben, wobei die Linien u = const. (z.B. u = u₁ und u = u₂) gleichzeitig Niveaulinien einer cosec²-Charakteristik im Vertikalschnitt darstellen. Die eingezeichneten, in sich geschlossenen Isoradiolinien entsprechen dagegen einer realistischen, der Ausleuchtfläche bereits recht gut angepaßten Strahlungsverteilung. Sie zu erreichen ist das erste Ziel der Reflektorformung. Zur Peilung nach dem Monopulsprinzip ist auch noch eine Differenzcharakteristik erforderlich. Der Quotient aus Differenz- und Summensignal, nämlich die Diskriminatorkurve, soll innerhalb des Ausleuchtbereichs einen monotonen und linearen Verlauf haben, um eine eindeutige und gleichbleibend genaue Zuordnung des Peilsignals zum Peilwinkel sicherzustellen. Dies ist das zweite Ziel der Reflektorformung.If the surface to be illuminated is formed in the antenna system fixed coordinate system x, y, z of FIGS. 1 and 2 with the normalized coordinates $\text{u = sin ϑ. cos φ}$

and $\text{v = sin ϑ. sin φ}$

ab (ϑ = polar angle in the xz plane in the antenna-fixed coordinate system x, y, z; φ = azimuth angle in the xy plane in the antenna-fixed coordinate system x, y, z), the ideal illumination would make the trapezoid-like one shown in Fig. 3 Beam cross section A, B, C, D result, with the lines u = const. (eg u = u₁ and u = u₂) simultaneously represent level lines of a cosec² characteristic in vertical section. The drawn, closed isoradiol lines, on the other hand, correspond to a realistic, the illuminated area already quite well adapted radiation distribution. Reaching them is the first goal of reflector shaping. A differential characteristic is also required for direction finding according to the monopulse principle. The quotient of the difference and sum signal, namely the discriminator curve, should have a monotonous and linear course within the illumination area in order to ensure a clear and consistently accurate assignment of the direction-finding signal to the direction-finding angle. This is the second goal of reflector shaping.

Figures 4 and 5 give two examples with the help of two diagram sections u = u₁ and u = u₂ for the desired courses of the sum, difference characteristics (Fig.4) and discriminator curves (Fig.5). In general, the quotient Q from the difference and sum amplitude is a complex quantity, it being possible to evaluate the phase or amplitude information in order to obtain the bearing signal. The required change in the beam width of an aperture radiator can in principle be achieved by changing the aperture width or the aperture assignment. In the case of reflector antennas, the aperture width is preferably kept constant in order to limit the overexposure of the primary field. The setting of the amplitude and phase of the aperture assignment is, however, only possible to a limited extent with a single reflector antenna, and above all not independently of one another. As a rule, with increasing broadening of the radiation lobe, the originally smooth course of the radiation characteristic becomes increasingly wavy, because the widening actually only comes about by superimposing several strongly bundled individual lobes. Thus, the illumination of a surface is possible without further ado; However, it is not a matter of course that within this footprint the discriminator curve also enables a clear bearing. The sum and difference characteristic is in the usual way by a double exciter feeding system generated in combination with a hybrid. For the consideration of the sum characteristic, both exciters can be replaced approximately with an exciter with equivalent aperture size. The problem therefore consists in shaping the contour of the reflector for a given primary radiator and given height and width of the reflector edge in such a way that the above-mentioned properties of the radiation characteristics are achieved in the sum and difference modes.

Die Ergänzung zur Fläche erfolgt bei der in allen Azimutschnitten stark bündelnden Fächerkeule durch differentielle Streifen fiktiver Paraboloide P, deren Brennweiten F vom Radiusvektor r (ψ) des Mittelschnitts abhängen, vergleiche dazu die Gleichungen (3) bis (6). Das "Blatt" paralleler Strahlen, das unter einem bestimmten Winkel ϑ auf den Mittelschnitt M einfällt, wird durch die Teilfläche eines Rotationsparaboloids P, dessen Achse parallel zu den einfallenden Strahlen liegt, in einem gemeinsamen Brennpunkt reflektiert. Die Brennweite F ist im Falle der Fächerkeule nur eine Funktion des Primärfeldwinkels ψ. Um nur die geforderte Verbreiterung der Strahlungskeule im Azimut zu erreichen, werden diese erzeugenden Paraboloide elevationsabhängig deformiert. Im einfachsten Fall kann man die Brennweite F so verändern, wie dies durch eine Erregerdefokussierung bewirkt würde, vgl. dazu Gleichung (7). Wesentliche Parameter sind hierbei: Der Faktor der Brennweitenvergrößerung (M_F), der Anfangs- und Endwert des Elevationswinkelbereichs (ϑ_FO, ϑ₁), in dem die Brennweite verändert wird, und die hier parabolisch gewählte Übergangsfunktion Δ (ϑ), die die kontinuierliche Veränderung der Brennweite abhängig vom Elevationswinkel ϑ beschreibt, ferner auch die Vorgabe der Gewinnfunktion ${\text{G (ϑ) = cosec}}^{\text{n}}\text{(ϑ)}$

mit vorzugsweise n < 2. Die Brennweite F wird somit eine Funktion sowohl des Primärals auch des Sekundärwinkels. Bezogen auf das in Fig. 6 dargestellte Koordinatensystem lauten die Definitionsgleichungen der Reflektorfläche im Parameterform:

Im folgenden wird eine vorteilhafte Realisierungsform einer Antenne nach der Erfindung beschrieben. Je nach Zuordnung der Randwinkel des Primär- bzw. Sekundärfelds ergeben sich zwei rechnerisch gleichwertige Lösungen für den Mittelschnitt mit einem konvergenten oder divergenten Strahlengang.The following explains how to calculate the reflector area of a radar antenna according to the invention. 6 shows the relevant geometry. In the classic procedure, which is described, for example, in the book by S.Silver: "Microwave Antenna Theory and Design" McGraw-Hill, New York, 1949, pages 502 to 509, the reflector shape of a typical radar antenna is carried out in two steps: the determination of the mid-section M (the "backbone") and the addition to the surface (the "ribs"). The middle section M is determined as an iterative solution of a differential equation assuming a constant reflector width so that the required cosec ⁿ radiation distribution in vertical section G (ϑ) is achieved with a given primary field I (ψ). The design equations in integral form are given in equations (1), (2), the coordinate system according to FIG. 6 being required.

The area is supplemented in the case of the fan club, which is strongly bundling in all azimuth sections, by means of differential strips of fictitious paraboloids P, the focal lengths F of which depend on the radius vector r (ψ) of the central section, compare equations (3) to (6). The "sheet" of parallel rays that is incident on the central section M at a certain angle ϑ is reflected by the partial surface of a paraboloid of revolution P, the axis of which is parallel to the incident rays, at a common focal point. The focal length F is only a function of the primary field angle ψ in the case of the fan club. In order to only achieve the required broadening of the radiation lobe in the azimuth, these generating paraboloids are deformed depending on the elevation. In the simplest case, the focal length F can be changed as it would be caused by exciter defocusing, cf. see equation (7). The main parameters here are: the factor of the focal length enlargement (M _F ), the start and end value of the elevation angle range (ϑ _FO , )₁) in which the focal length is changed, and the transition function Δ (ϑ) chosen here parabolically, which is the continuous change describes the focal length as a function of the elevation angle ϑ, and also specifies the profit function ${\text{G (ϑ) = cosec}}^{\text{n}}\text{(ϑ)}$

with preferably n <2. The focal length F thus becomes a function of both the primary and the secondary angle. Based on the coordinate system shown in Fig. 6, the definition equations of the reflector surface are in parameter form:

An advantageous embodiment of an antenna according to the invention is described below. Depending on the assignment of the contact angles of the primary or secondary field, there are two computationally equivalent solutions for the middle cut with a convergent or divergent beam path.

Fig. 7 shows the convergent solution, in which a caustic occurs on the excitation side of the reflector in the beam path of the reflector. 8 shows the divergent solution for the middle section, in which a caustic occurs in the beam path of the reflector on the side of the reflector facing away from the exciter side. The convergent solution (Fig. 7) enables a more compact antenna design and is therefore preferred.

The optimization of the reflector surface can be done purely mathematically in an iterative process, which includes the geometrical-optical contour synthesis and the physical-optical radiation analysis. The parameter set for the contour synthesis can be varied systematically until the desired radiation properties arise.

9 to 11 show a top view, side view and front view of the structure of a radar device for traffic detection with a reflector antenna according to the invention with special monopulse direction finding properties. The reflector 6 of the antenna has a rectangular border and is fed diagonally from above by two horns 7 and 8 abutting one another on the longitudinal side. The main beam direction of the exciters formed by the horn radiators 7 and 8 includes an acute angle with the z-axis of the antenna coordinate system. To protect against the effects of the weather, a housing 9 lined with absorber material with a radome 10 in the form of an essentially cylindrical, is downward inclined surface provided. Various housings 11, 12, 13 of the radar device are located in the housing 9 behind the reflector 6, for example for the power supply, for processing or the RF preamplification 9 directly on the waveguide feeds 14, 15 for the two horn radiators 7, 8. An interesting information about the geometrical-optical approximation method used here is shown in FIG. 12 using the example of mathematically determined reflector zones that contribute to the radiation in vertical section in certain directions ϑ. These zones must be visible as parts of the differential strips already described. For ϑ = 0 ^o (radiation maximum), the entire width of the lower half of the reflector 6 is used, while with increasing angle of elevation (ϑ = 11 ^o , ϑ = 22 ^o ) the reflection zone moves upwards and decreases according to the desired broadening. For ϑ = 22 ^{o there} are already isolated reflection zones, which indicate the approximate nature of the model described.

The synthetic method described here is only an approximation in the sense of geometric optics - this already applies to a normal fan club. There are exact methods of geometric optics in the literature, which usually cannot do without analysis with the help of physical optics either. Compared to this, this complex and complex differential-geometric considerations based on the classic solution provides immediate synthesis methods of continuous reflector deformation with astonishingly good results. It is even possible to use this method to create asymmetrical beam cross-sections, as required, for example, by illuminating the road from a corner.

It should also be noted that the antenna according to the invention not only for radar monitoring in connection with a road traffic regulation, but in general for monitoring an area, i.e. also for the purpose of motion detection or for illuminating an area, such as a section of a street.

Claims (10)

Antenna designed for an azimuthal angular bearing for radar monitoring of an at least approximately rectangular, limited illumination area, in particular for a road traffic regulation radar,
characterized in that the antenna is designed as a reflector antenna with a double curved reflector and a double excitation supply,
that the reflector surface for generating sum and difference characteristics is specially shaped in two planes, so that a special bearing behavior is made possible,
that the beam cross section is trapezoidal in the sum characteristic, so that the at least approximately rectangularly delimited area can be illuminated, and that the special bearing behavior is defined by the fact that azimuthal bearing is clearly possible within this illumination area with the aid of the amplitude or phase monopulse method. Antenna according to claim 1,
characterized in that the reflector surface is shaped in the vertical central section so that ideally a cosec ⁿ characteristic with preferably n <2 is generated, and that the reflector surface is supplemented by differential strips of specially curved surfaces, preferably, for example, deformed rotational paraboloids is. Antenna according to claim 2,
characterized in that in the case of differential stripes of deformed paraboloids of revolution, the focal length is changed as a function of elevation, and thus defocusing of the two exciters, or as viewed from the exciters, an equivalent deformation of the paraboloid surface is achieved. Antenna according to claim 3,
characterized in that the change in focal length is described by a suitable transition function. Antenna according to one of the preceding claims,
characterized in that the reflector has a rectangular border. Antenna according to one of the preceding claims,
characterized in that an asymmetrical configuration of the differential strips producing the reflector surface is provided to generate a radiation characteristic which is asymmetrical with respect to the vertical plane. Antenna according to one of the preceding claims,
characterized by a convergent design, in which a caustic occurs on the excitation side of the reflector in the beam path of the reflector. Antenna according to one of claims 1 to 6,
characterized by a divergent design, in which a caustic occurs in the beam path of the reflector on the side of the reflector facing away from the exciter side. Antenna according to one of the preceding claims,
characterized by a central arrangement of the two pathogens, in which the pathogens are located within the beam path in front of the reflector. Antenna according to one of claims 1 to 8,
characterized by a so-called offset arrangement of the two exciters, in which the exciters are outside the beam path from the reflector.

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