DE3337941A1 - Passive radar reflectors - Google Patents

Abstract

Thin, preferably circular sheets of metallised polymeric material having a diameter equal to 0.1 to 1 wavelength of the radiation to be reflected are eminently suitable as passive radar reflectors for the millimieter wavelength range.

Description

Passive radar reflectors

The invention relates to metallized flakes that are used as passive Radar reflectors, so-called chaff, are used to detect unwanted signals (returns) to a radar system. These are essentially Rectangular strips of aluminum foil, some of which are on paper, glass fibers, polyamide threads and the like are concealed.

While conventional radars for air surveillance in a frequency range of around 1010 Hz, i.e. working in the centimeter wave range, is to be expected that future radar systems because of the improved resolution and the lower Antenna dimensions will prefer much shorter wavelengths.

The millimeter waves in the range are of particular interest here the so-called atomospheric window, i.e. in the frequency range from 30 - 300 GHz, where the propagation of the radar waves as a result of absorption by water vapor and Oxygen is less obstructed.

For millimeter waves, however, chaffs of around 0.5 to 5 mm are now required Length required.

It has therefore already been proposed (cf. DE-A 3 246 289) for Disturbance of such radar systems Radar chaffs made of metallized carbon fibers to be used with a diameter of 7.5 - 8 Wm.

With these short fiber dipoles, the radar backscatter is through the ohmic losses in the thin metal layer on the fiber of approx. 5 - 10 Wm strength already severely affected, which is the use of highly conductive metals such as copper or silver requires.

In addition, thin dipoles give a strong dependence on backscattering on the direction of polyrization of the radar radiation. Furthermore there is especially at closer packing the risk of bending the dipoles, resulting in a frequency-wise Detuning and thus contributes to a reduction in radar backscatter. These Bending occurs especially when a short cut from the filament yarn or Monofilament is to be produced.

Even a nickel-plated filament yarn can be made from organic high polymers, e.g. polyacrylonitriles, aramids, polyester or fiberglass and carbon filament etc., do not cut properly, especially when exact lengths are required. deformation The chaff can also cause cracking in the metal layer and thus deterioration contribute to reflection.

It has now been found that these disadvantages can be largely avoided can, if you chaff in the form of thin metallized, preferably circular or more polygonal Leaflets made of polymeric materials with a diameter corresponding to 0.1 - 1.0 wavelength of the radiation to be reflected is used.

Circular and square leaflets with a diameter are preferred or an edge length from a quarter to a half radar wavelength.

It must be seen as extremely surprising that surprising with these chaffs compared to fiber dipoles of half a wavelength intense radar echoes could be achieved. In addition, there was a material Less dependence of the backscatter on the radar frequency than when using fiber dipoles of equal length.

According to the invention, metallized flakes are depending on Frequency range in diameters from 1 to 10, preferably 1.5 to 8 mm and in thicknesses from 2 to 20 µm made of thermoplastic or thermoset high polymers, for example Polycarbonate, polyester, polyamide, polypropylene, phenolic resin, polyhydantoin or metallized inorganic materials such as thin glass, etc. are used.

Metallized paper and art paper based on polyethylene are preferred.

Electroless, wet chemical processes are particularly suitable for metallization, in which the substrates have previously been treated with palladium sol (e.g. according to DE-A 2 743 768) or organometallic compounds (e.g. according to DE-A 3 025 307) have been activated. The metallic coating should preferably a layer thickness of 0.05-0.5 ijm.

Along with copper, silver, and aluminum, nickel is especially good for that Suitable for metallization.

The nickel film applied to the substrates is characterized by its high Oxidation resistance and abrasion resistance of other metals such as copper and silver substantially superior. In the case of thicknesses of approx. 0.1 μm, in the millimeter range Reflection losses of less than 1 db achieved.

The particularly preferred circular shape of the chaff results in not just one higher mechanical stability against bending and cracking, but also a less impairment of backscatter in the event of deformation. It also offers the leaflet shape aerodynamic advantages. Show compared to fiber dipoles Circular leaflets in free fall have a much stronger tendency to expand reflecting cloud and longer dwell time in the air, e.g. by whirling up in the rising air.

In addition, the use of the new chaff is not the same as the one mentioned at the beginning Area restricted. There are also numerous possible applications in the rescue sector, e.g. to localize accident victims on water and land who are at the scene of the accident by simply scattering the chaff.

Example Using an office punch, TYVEK (registered trademark of Du Pont) circular leaves with a diameter of 5 mm punched out. These are placed in a hydrochloric acid bath (pH 1) at room temperature colloidal palladium solution according to DE-A 1 197 720 immersed. After lingering under With a slight movement of goods between 10 seconds and 2 minutes, the goods are removed and rinsed with water at room temperature. Then you give it in an approx. 5% strength Sodium hydroxide solution at room temperature. With a slight movement of goods, the goods are between Treated about 30 seconds and 2 minutes, then removed and then with water rinsed for about 30 seconds at room temperature. Then you wear the goods e.g. in an alkaline nickel bath in a solution of 0.2 mol / l nickel-II chloride, 0.9 Mol / l ammonium hydroxide, 0.2 mol / l sodium hypophosphite, in which there is so much ammonia initiates that the pH at 20 "C is 8.9.

After approx. 3 minutes the nickel layer has reached a thickness of 0.15 µm. The material is removed from the bath and thoroughly washed with water until it has a neutral reaction rinsed from room temperature and dried.

In addition to the advantageous properties mentioned above, it stands out the metallized product by a low bulk volume, whereby it metallized Bead is superior. Of course, it is also possible to start with the complete Metallize sheets of paper and then shred them.

Claims (5)

Claims 1.) Passive radar reflectors for the millimeter wave range from 30 - 300 GHz, characterized in that reflectors in the form of thin leaflets made of metallized polymeric materials with a diameter corresponding to 0.1 up to 1 wavelength of the radiation to be reflected can be used. 2. Passive radar reflectors according to claim 1, characterized in that that the leaflets are rectangular or - preferably - circular. 3. Passive radar reflectors according to claim 1, characterized in that that the diameter of the leaflets 1.5 - 15 mm and their thickness between 2 - 20 µm amounts to. 4. Passive radar reflectors according to claim 1 + 2, characterized in that that for metallization an electroless wet chemical applied metal layer of 0t05 to 0.5 m thickness is used. 5. Passive radar reflectors according to claim 1, characterized in that that the leaves are nickel-plated.