Corner reflector

Corner reflectors are mostly used as a radar reflector retro-reflectors for microwave.

Corner reflectors produce a particularly strong echo signal (radar echo) and thus a safe target mark ( target) on the radar screen for objects that have otherwise only a very small effective reflection surface for radar. They consist in the basic element consists of two or three at an angle of exactly 90 ° to each other electrically conductive surfaces. For this reason element different forms of corner reflectors are constructed. Corner reflectors reflect radio waves and microwave radiation in exactly the direction from which the radiation source takes place, without having to be aligned perpendicular to it like a mirror (see retroreflection ).

• 2.1 Active radar reflectors
• 2.2 tube reflectors
• 2.3 Luneburg lenses

Operation

Corner reflectors have geometric dimensions (circumference) in the range of possible integer multiple of the wavelength of the frequency zurückzustrahlenden. If these dimensions in the range of the wavelength of the radiation are, then the effective reflecting surface is essentially determined by the resonance behavior of the reflector. The corner reflector receives the incoming radiation and acts as a passive or comparable parasitic antenna element as in a Yagi antenna. The corner reflector then pass to high frequency voltages, which in turn is the cause of the corner reflector as the reflector. The corner reflector has when it is in resonance, even then a much larger effective reflecting surface than its geometric size, it can be expected. The larger the dimensions of the corner reflector in relation to the wavelength, the less is the influence of the resonance ( see Mie scattering ). From about ten times the wavelength, the influence of the resonance is negligibly small. From these dimensions retroreflection is purely optical laws before. Out of the corner reflector is thus a retro-reflector: Incoming electromagnetic waves are exactly reflected by double or triple reflection in the direction from which they come. Thus even small objects with low reflection surface acquire a strong reflection back towards the source of radiation, they provide a much stronger radar echo from other reflective body and appear much brighter on the radar screen.

The double reflection on the electrically conductive plates is done synchronized in phase, because the distances of the individual phases are equal to ( a b c = a ' b ' c '). At any angle of incidence of the corner reflector so as a standing at right angles to the direction of incidence plate acts. Phasenkongruenz this is given only in the direction from the source of radiation. The effective area corresponds approximately to the optical projection of the corner reflector to the level of the plate. When approaching the wavelength of the source radiation to the geometric dimensions of the corner reflector is similar to a resonant behavior caused as the Mie scattering. A triple reflection is mathematically considered as two double reflections.

Corner reflectors with two faces

Any reflection of electromagnetic waves is lossy. A twofold reflection energy has advantages over a three -reflection. Among other reasons corner reflectors are often constructed from two surfaces and it is advantageous to equip smaller corner reflectors with a layer of stabilization and alternatively use a corner cube with three surfaces, which, in order to obtain the same effective reflective surface, it must be slightly larger, so a larger wind load has. Another advantage is that in contrast to a reflector angle with the three faces at an angle of two reflector surfaces at an illumination participates directly in front of the entire area of reflection. Disadvantage is that they only reflect energy back exactly to the origin when the lower edges of the surfaces lying on a horizontal plane. For the theoretical calculation of the effective reflecting surface of a corner reflector with two square faces, the formula is:

In the application of the formulas should be noted that if the corner reflector being less than ten times the wavelength of the interrogating radar, by local resonances of the practical value of the effective reflection surface of the representation of both upwardly and downwardly from the theoretically calculated value of up to four times can. Since the frequencies of commonly used navigation radars are known ( for example from 9.3 to 10.5 GHz), have become established in practice standard sizes as an approximate multiple of the wavelengths used, which lie with their geometric size just so that a positive deviation from the theoretical calculated value comes into play. The larger the angle of the reflector, the less is the influence of the resonance. From ten times the wavelength of this influence is negligible.

In applications in which electromagnetic waves must be reflected within a plane ( for example in the nautical: only parallel to the water surface ), corner reflectors are only two surfaces which are disposed at an angle of 90 ° to each other, are used. Since these corner reflectors are not by wind, currents and waves always perpendicular to the water surface, they are mounted in a floating eg within a sphere made ​​of insulating material in an oil bath. A label at the base weight ensures independent of the pitch and roll of the boat inside the ball for a stable vertical position. This externally spherical angle reflector can thus be " set " like a pennant at the mast. Corner reflectors are as far as possible attached to smaller boats at the top of a mast to reduce coverage losses caused by the curvature of the earth.

Corner reflectors with only two surfaces are used on some military airfields. Here they are parallel in two rows to the start and runway and provide the on-board radar a good orientation. The plane in which retro-reflected, here is perpendicular to the earth's surface parallel to the direction of take-off and landing strip. The surfaces of the reflector are mounted so as to be seen in the picture above principle. If the airplane is in the correct landing approach, this angle reflectors are visible to the RADAR. For other directions, these reflectors are nearly ineffective to make the airfield for RADAR investigation of serious find.

Corner reflectors with three faces

Corner reflectors with three reflective surfaces at an angle of 90 ° to each other are used where retroreflection in three-dimensional space is necessary. This triangular reflectors function as the optical analogue of the triple mirror. The maximum effective reflecting surface of the symmetric triangular corner reflector shown in the picture occurs in the direction of the axis of symmetry, it is calculated by:

The magnitude and phase of the reflected energy to remain independent of the angle of incidence in the frequencies used by the radar relatively constant up to the area in which the reflected wave is in the order of magnitude of the dimensions of the reflector surfaces. The individual surfaces of the corner reflector should therefore be large compared to the wavelength. The larger a corner reflector is, the more energy is reflected.

If eight pieces are joined together by this calculated angle reflector that is reflected in every direction, then the observing radar four equal angle reflectors are always facing more or less effective. The result of the formula is so then to be multiplied by a factor. This type of corner reflectors has a very complicated diagram of the backscatter, in which the particular size of the effective reflective surface depending on the direction of the incident radar radiation can be inserted.

Advantage of the corner reflector with three faces is its lageunabhängigere function (it must also be vertically tilted slightly ) and the greater mechanical stability. This design is therefore used when the angle can change in two directions. Drawback is that it must be made ​​larger than a dihedral corner reflector at the same effective reflecting surface, as the tip ends of the corner reflector not participate in the reflection in the direction of origin. To these ends is dependent on position may reflect only twice, so that this energy is channeled in a different direction. Of the total area of ​​an equilateral hexagon is effective in the direction of the axis of symmetry. Some manufacturers therefore refrain from these corners, so that the angle reflector receives a polygonal shape like in this picture. Often types of circular surfaces are used. They offer a good compromise between radar cross section and wind load. They are often housed in a plastic ball to protect against ice formation and reduce the wind load on.

The highest return reflection with respect to the side length have shapes of three squares ( cubes with three missing pages ), it takes in the direction of the missing cube corner:

With

For those in the inland mostly in the I / J- band navigation radar operating at a wavelength of about 3 to 4 cm, an angle reflector made of square aluminum plates is sufficient with an edge length of 20 cm as a radar target.

A corner reflector, which radiates back around in all directions, is drawing from 12 isosceles triangles. In the practical construction (see instructions ) is assumed equal to three large square plates, one of which is cut in half diagonally. Then cuts with a section thickness equal to the sheet thickness are inserted into the sheets and pushed into each of the four parts. With an edge length of the square sheets of 0.3 m has such a corner reflector in the manner customary for the sport and commercial shipping X-band (ca. 9.4 GHz ) an effective reflecting surface of about 8 m, which is about as much as a simple sailing boat. Such a reflector provides a radar devices even at great distances not be overlooked target characters on the screen. To avoid risk of injury, the corners can be cylindrically ground without this significantly degraded the radar cross section.

Corner reflectors with four faces

This type is most commonly used in buoys in shipping. There are, depending on location but only two or three areas effectively. In the horizontal position, an angle reflector is given with only two reflective vertical surfaces. The horizontal surfaces not only provide mechanical stability, but are effective at a tilt of the buoy. As long as the buoy in calm water is vertical, only the two vertical surfaces are effective. Only when the buoy is tilted by wind, current or swell, then the observing radar is presented with a three -surface angle reflector. Is the forward inclination, the base plate as a third effective surface, the inclination to the rear, the upper plate.

In a calm sea and exact vertical position of the corner reflector, the reflecting surface of the corner reflector having two faces, at an inclined position, the reflective surface is reduced to the value of a corner cube with three surfaces.

Special shapes

Active radar reflectors

In the recreational boating and rescue workers also transponder and search and rescue radar transponders are used. Receiving a radar pulse the electronics, a strong radar pulse is returned. This appears on the radar as a major "Echo " as the ship itself Additionally, the active radar reflectors on a light or an audible warning that a radar pulse has been received, and so have to go to other vessels in the vicinity. A big advantage over passive reflectors is the dramatically increased range, since with transponders according to the radar equation, the received power is only proportional to 1 / r ² decreases proportionally and not 1/r4. Transponders can also return coded signals which enable automatic identification.

Tube reflectors

Under the name " radar reflectors for sailing ships" tubular products are sold, in which several smaller angle reflectors are included, which are measured by their dimensions so that they hit exactly the wavelength of the navigation radar. 10.5 GHz, the frequency corresponding to a wavelength of about 3 cm. Through the now occurring resonance frequency of this reflector, this has an approximately four times larger effective reflective surface, than would be expected according to quasi-optic principles. With minimal variations in the lengths of the reflectors present in the resonance tube for different frequencies in the J- band can be achieved, so that the sum of a band-pass behavior for this band is achieved. These reflectors are a compromise between the largest possible radar cross section and the least possible wind resistance. They are used on sailboats. Their disadvantage is that they are effective only in a vertical position. Therefore, it is advantageous to attach it only at the upper end or to loosen the bottom line so that they can hang vertically, even in an inclined position of the boat. Possibly the use of two of these radar reflectors ( one starboard and one port ) shall be considered, for example, if the sail prevents a vertical hanging down. According to the manufacturer, they have an effective reflecting surface of at least 2 m and a diameter of 3 cm and ten individual reflectors.

Rigid mounting as in this picture is to be avoided. Even when the boat picks up speed, the bow lifts out of the water and the boat takes up an oblique position. The corner reflector loses so quickly effectiveness. With a deviation of 5 ° from the vertical position, it can already be ineffective.

Luneburg lenses

Luneburg lenses can be manufactured as all-round radar reflector, then wear them instead of the rear mirror coating only a narrow horizontal belt made of a conductive layer. You must always depend exactly perpendicular when used as a radar reflector on boats, so that the horizontal on both sides of the belt entering the ball radio waves are precisely focused onto the reflective metal strips on the back. Luneburg lenses have the advantage that their rear beam pattern is completely constant and no minimums or has gaps, as is the case with corner reflectors as a radar reflector. According to this principle, the Tri -Lens Radar Reflectors are made, for example.

Installation of radar reflectors

Thus, a radar reflector its theoretically possible effectiveness can also reach certain conditions for the preparation of the radar reflector must be considered:

• Mounting as high as possible on a sailing ship, for example, at the top of the mast or the top spreader. This ensures that the sailing ship will still reflects an adequate radar return if essential parts such as the hull or the rigging already lie behind the radar horizon.
• Fixed firmly by the radar reflector, for example, so screwed that it can not slip or rotate. This avoids that the radar reflector to be non-stationary, so-called pumping, echoes leads as "flying" for example, in an attachment to traps, the backstay or the uphaul be the case. When pumping echo is indicated on the radar screen of the receiver, the reflected signal alternately and not shown. However, raster scan radar systems for sports and commercial shipping suppress under circumstances such pumping echoes, so that the sailing ship is not perceived.
• Proper alignment of the radar reflector.

For the correct orientation of the radar reflector must be considered whether it is an object with a in all directions almost the same reflecting surface, such as in a motor boat, or whether certain directions already have a sufficiently good reflecting surface, such as z. B. in a sailing ship. When a sailing ship the rig or the standing rigging already leads to lateral impingement of radar beams to sufficient reflections whereas this insufficiently ahead and astern of the case. In this case, a radar reflector should preferably reflect upon ahead and astern, that is, have a particularly large effective reflecting surface.

An octahedral radar reflector with eight triangular corner reflectors, the so-called cornern, can be mounted in three different positions in principle:

• Four of creation, in which a tip is aligned to the top, just a bit down. However, this position has a poor reflection characteristics in the horizontal direction, since effectively contribute only four corner reflectors to the rear radiation.
• The six generation, in which a corner reflector straight up and an angle of the reflector is aligned downwards, so that this position is also referred to as " rain tuck position ". This position has a horizontally in all directions almost equally good and balanced reflection characteristics. It is suitable especially for motor vessels.
• The boat position, a special form of six generation, in which a corner reflector just forward and a corner reflector is aligned to the rear. This position has a very good reflection ahead and astern, and less so that it is suitable to the sides, especially for sailing ships.

Expedient

If no radar reflector on board and you will be surprised at its cruise through dense fog, can be bound to the mast as a makeshift any large metal part. For example, if the largest pot from the galley is hoisted on the mast, then increases the visible radar cross section of a small boat, but results due to the "flying" fixing to be pumped echoes. Alternatively, a metal foil or a metal-coated film ( film rescue, etc.) may be used.

Application

Corner reflectors as targets for radar devices are used:

• As a calibration standard for a free-space calibration of antennas ( for example, in air traffic control to the direction of check of a precision approach radar)
• As navigation aids for marking shipping lanes in difficult water ( on floating navigational aids, on bridges )
• To mark the start and runway at airports
• Radar reflectors on vessels to their better visibility through surveillance radar of other ships and on land
• To simulate a large missile using so-called decoys
• On a weather balloon to radar tracking and measurement of wind speeds at high altitudes.