EP0822610B1 - Reflector antenna - Google Patents

Description

The invention relates to a reflector antenna according to the Cassegrainprinzip, in which on the surface of a reflector, an absorbent in the form of an annular surface is applied.

DE 26 10 506 C2 describes a reflector antenna, in which on the surface of a reflector, an absorbent is applied in an annular surface. Preferably, the absorbent is placed on the edge of the subreflector of a Cassegrain antenna. The absorption medium is selected such that the electromagnetic radiation is attenuated in a narrow frequency band with up to 20 dB, while in the other frequency ranges the attenuation reaches approximately 0 dB. Thus, this antenna can be operated in several frequency bands with almost constant gain and sufficient opening angle.

In contrast, however, in reflector antennas, which are used as sensors, the electromagnetic radiation to be reflected only in the range of the operating frequency, while in the other frequency ranges for the purpose of camouflage a broadband attenuation is desired.

It is therefore an object of the invention to design a parabolic antenna with subreflector according to the Cassegrain principle, which reflects the electromagnetic radiation only in the range of the operating frequency, while in the other frequency ranges, a high attenuation is achieved.

This object is achieved according to the invention in a reflector antenna according to the Cassegrain principle of claim 1.

Further advantageous embodiments of the reflector antenna are described in the subclaims.

The construction of the main reflector according to the invention has various advantages. Due to the partial construction of the main reflector by means of a frequency-selective radar-absorbing construction, it is possible to dispense with the use of a frequency-selective radome designed for the operating frequency of the antenna, thus eliminating the resulting radome reflections in the frequency range of a radar observing the antenna. If the entire main reflector according to the prior art is produced in a frequency-selective radar-absorbing construction, the result is reflection losses greater than 1 dB due to the interaction between the radar-absorbing construction and the frequency-selective layer in the operating frequency range of the antenna. These losses reduce the antenna gain and thus also considerably limit the range of the antenna. By means of the described invention, only a limited area of the parabolic surface is realized in such a construction, so that significantly lower reflection losses are achieved without significant absorption losses in the frequency range of the antenna observing radars, which reduces according to the area ratio of the frequency-selective radar-absorbing circular area to the total parabolic area of the main reflector become. If z. B. realized 20% of the main reflector in a frequency-selective radar-absorbing design, the reflection losses decrease to about 0.2 dB.

Fig. 1

eine Seitenansicht (Schnitt) einer Cassegrain-Antenne,

Fig. 2

die Draufsicht zu Fig. 1,

Fig. 3

den Verlauf eines einfallenden Strahles im Fall einer 3-fach Reflexion.

An embodiment of the invention is shown schematically simplified in the drawing and will be explained in the following description. Show it

Fig. 1

a side view (section) of a Cassegrain antenna,

Fig. 2

the plan view of Fig. 1,

Fig. 3

the course of an incident beam in the case of a 3-fold reflection.

FIG. 1 shows a section through a Cassegrain antenna which consists of a main reflector 1 and a subreflector 2. The feed horn has not been reproduced for the sake of simplicity. Both reflectors have metallically reflecting surfaces 8. In the main reflector, a combination of an absorbent 3 and a frequency-selective structure 4 is embedded. The frequency-selective structure 4 is in this case on the absorbent 3, that all not the resonance frequency of the structure 4 corresponding frequencies are attenuated in the absorbent 3. In the exemplary embodiment according to FIGS. 1 and 2, the absorbent 3 has an approximately annular boundary which results from the geometrical location of all points of incidence of rays on the main reflector which are n-fold between the subreflector and main reflector (n = 3, 5, 7, ...) are reflected.

The present antenna design is based on the finding that in the irradiation of a parabolic antenna by means of an external radiation source in addition to the 1-fold reflections that are generated especially on the Subreflektorhalterungen, the subreflector itself and inhomogeneities or edges of the main reflector, n-fold Reflections (n = 3, 5, 7, ...) arise between the sub and the main reflector. These multiple reflections, in particular the 3-fold and 5-fold reflections, contain the highest backscattered energy components over a relatively large aspect angle range and within broad frequency bands.

So far, it has been customary to cover the antenna with a frequency-selective radome, wherein the frequency of the incident external radiation must be far enough away from the operating frequency of their own system, since the radome must be made transparent to the operating frequency. However, this method is useful only in cases where the device-specific requirements allow a configuration of the radome, which allows a reflection of the incident radiation in a direction other than the direction of incidence by means of a corresponding shaping of the radome. However, simple single reflections on the radome surface can not be avoided.

Due to the construction according to the invention of the main reflector, which has an annular subarea of a frequency-selective and radar-absorbing construction, it is possible to dispense with the use of a frequency-selective radome.

In this case, the frequency-selective radar-absorbing construction is designed in such a way that almost lossless metallic reflection takes place in the range of the operating frequency (eg 35 GHz), whereas a radar absorbing effect is achieved in the frequency range of incidental external radiation (eg 2 to 20 GHz) , The broadband Radarabsorption can be realized by a lossy sandwich construction or by a monolithic absorber 3. The frequency-selective structure applied to the absorption layer 3 can be formed with the aid of differently shaped individual reflector elements: for this purpose metallic Jerusalem crosses or cross dipoles have become known in addition to metallic circular rings.

In FIG. 3, the course of a 3-fold reflection is shown in simplified form. The incident beam 5 strikes the main reflector 1, which is not covered here with an absorbent. The reflected from the main reflector 1 beam hits the subreflector 2, where it is then reflected again at the main reflector 1 and then thrown back as a reflected beam 6 in about the same direction as the incident beam 5. Similarly, the other n-fold reflections will also expire. The location of all points of impact of the reflected rays on the main reflector 1 characterizes the region and the boundary of the absorbent 3. In a symmetric Cassegrainsystem of Fig. 1 and 2, the absorbent 3 forms an annular surface whose geometry can be derived as follows. The outer radius of the annular surface is a function of the focus foc of the main reflector 1, the geometry parameters a, b and r (= radius) of the subreflector 2, as well as the distance s of the vertices of sub and main reflector: $${\mathrm{R}}_{\mathrm{Max}}=\mathrm{f}\left(\mathrm{foc}.\mathrm{a}.\mathrm{b}.\mathrm{r}\right)$$

The inner radius of the circular ring corresponds to the radius r of the subreflector. The main reflector is described here by a paraboloid of revolution whose center cross-section satisfies the formula: $${\mathrm{f}}_{\mathrm{main\; reflector}}\left(\mathrm{x}\right)=\frac{{\mathrm{x}}^2}{4\mathrm{foc}}$$

The subreflector is a rotational hyperholoid whose center cross section corresponds to the formula: $${\mathrm{f}}_{\mathrm{subreflector}}\left(\mathrm{x}\right)=\mathrm{a}\sqrt{1+\frac{{\mathrm{x}}^2}{\mathrm{b}}},$$

For determining the outer radius of the annular surface, both formulas are related in a coordinate system, so that a new function for the subreflector results from this: $${\mathrm{f}}_{\mathrm{subreflector}}\left(\mathrm{x}\right)=\mathrm{a}\sqrt{1+\frac{{\mathrm{x}}^2}{\mathrm{b}}}+\mathrm{s}-\mathrm{a},$$

Here s describes the distance of the vertices of main and subreflector. The relevant condition for a 3-fold reflection results from the fact that the incident wavefront from the main to the sub-reflector is reflected such that the angle of incidence corresponds to that of the surface normal of the subreflector. The point of intersection of the incident beam on the main reflector then gives the maximum radius of that circular area on the main reflector, for which just a triple reflection can take place when the point of intersection of the reflected beam at the subreflector lies at its outer boundary. By the slope of the normal of the subreflector at the point x = r of the reflected beam from the subreflector is given by $${\mathrm{f}}_{\mathrm{reflects}}\left(\mathrm{x}\right)=-\left(\mathrm{x}+\mathrm{r}\right){\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}^2\frac{\sqrt{1+{\left(\frac{\mathrm{r}}{\mathrm{b}}\right)}^2}}{\mathrm{a}\mathrm{x}}+{\mathrm{f}}_{\mathrm{subreflector}}\left(-\mathrm{r}\right),$$

mit der Steigung der Funktion f_Subreflektor(x) gemäß $${\mathrm{f}}^{\prime }\left(\mathrm{x}\right)=\frac{\mathrm{d}}{\mathrm{d}\mathrm{x}}{\mathrm{f}}_{\mathrm{Subreflektor}}\left(\mathrm{x}\right)=\frac{\mathrm{a}\mathrm{x}}{{\mathrm{b}}^2\sqrt{1+{\left(\frac{\mathrm{x}}{\mathrm{b}}\right)}^2}}$$

_Reflected with the intersection of the functions f (x) and f _{main reflector} (x), there is now an expression for the maximum radius of the circular ring surface: $${\mathrm{R}}_{\mathrm{Max}}=\left|\frac{-2\mathrm{foc}}{{\mathrm{f}}^{\text{'}}\left(-\mathrm{r}\right)}\sqrt{{\left(\frac{-2\mathrm{foc}}{{\mathrm{f}}^{\text{'}}\left(-\mathrm{r}\right)}\right)}^2-\frac{4\mathrm{foc}\mathrm{r}}{{\mathrm{f}}^{\text{'}}\left(-\mathrm{r}\right)}+4\mathrm{foc}\times {\mathrm{f}}_{\mathrm{subreflector}}\left(-\mathrm{r}\right)}\right|,$$

with the slope of the function f _subreflector (x) according to $${\mathrm{f}}^{\text{'}}\left(\mathrm{x}\right)=\frac{\mathrm{d}}{\mathrm{d}\mathrm{x}}{\mathrm{f}}_{\mathrm{subreflector}}\left(\mathrm{x}\right)=\frac{\mathrm{a}\mathrm{x}}{{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}^2\sqrt{1+{\left(\frac{\mathrm{x}}{\mathrm{b}}\right)}^2}}$$

This results in the following dimensions for a calculated example: Focus: foc = 142.40 mm Radius of the main reflector: r _{main ref.} = 150.00 mm Geometry parameters of the subreflector: a = 42.40 mm b = 52.63 mm r = 30.00 mm Vertex distance s = 120.00 mm Circular ring of the absorber R _max = 76.34 mm R _min = 30.00 mm

Claims (5)

1. A reflector antenna according to the Cassegrain principle comprising a main reflector (1) and a sub-reflector (2), in which an absorption means (3) in the form of an annular surface is applied on the surface of the main reflector (1), wherein

   - the absorption means (3) is arranged on the main reflector (3);

   - a frequency-selective structure (4) is applied to the absorption means (3);

   characterised in that the outer radius (Rmax) of the annular surface of the absorption means (3) is given by the maximum radius of that circular surface on the main reflector (1) for which precisely 3-fold reflection of the incident electromagnetic waves can just take place.
2. A reflector antenna according to Claim 1, characterised in that the inner radius of the annular surface of the absorption means (3) corresponds to the parallel projection of the outer radius of the sub-reflector (2) onto the main reflector (1).
3. A reflector antenna according to Claim 1 or 2, characterised in that the absorption means (3) is produced by means of a sandwich structure which contains lossy materials.
4. A reflector antenna according to Claim 1 or 2, characterised in that the absorption means (3) consists of a monolithic absorber.
5. A reflector antenna according to at least one of Claims 1 to 4, characterised in that the frequency-selective structure (4) consists of metal elements such as annuli, Jerusalem crosses or crossed dipoles.

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