However i have another doubt hope any science students or professionals can help me in the article quoted by Mr.Gambit on wiki there is a sentence which says this is difficult don't know how to infer it :
A radar reflector can be made from a Luneburg lens by metallizing parts of its surface. Radiation from a distant radar transmitter is focussed onto the underside of the metallization on the opposite side of the lens; here it is reflected, and focussed back onto the radar station. A difficulty with this scheme is that metallized regions block the entry or exit of radiation on that part of the lens, but the non-metallized regions result in a blind-spot on the opposite side.
Please help me
I will attempt...
First...You need to understand what is a 'lens'...
Lens (optics) - Wikipedia, the free encyclopedia
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam.
In layman language, a 'lens' is simply a device that focus an EM signal, visual spectrum or not, into a desirable direction. Simplistically, if you put a large bowl in front of your mouth and speak, the audio wave would be largely focused and redirected, or echo-ed, in the direction that the bowl is facing, back at you or at an angle. You have just created a simple audio 'lens'. The materials and constructions of the lens should be appropriate to the properties of whatever type of wave you are working upon, of course.
Second...An object, or a body, can be an 'emitter' or 'transmitter' of EM radiation in two ways: Either the body is
ACTIVELY broadcasting EM radiation. Or when the body is reflecting an EM wave that came from an external source. A music radio station with its tall tower antenna is a good analogy. The antenna tower is an active transmitter of EM radiation that contain contents: music. But if an aircraft's radar EM wave happened to impact this antenna tower, the aircraft will see a radar return, or echo, from the tower itself.
Voila...You have a body that is an emitter of EM radiation in two ways. The advantage is using a radar is that you would know, based on the echo, the general direction of the tower, whereas with a simple radio receiver, all you would know is that there is a music radio station around somewhere.
So when we combine the two concepts, we have a directional reflector of any EM wave: A Luneburg Lens.
A Luneburg Lens is essentially an antenna for either transmit or receive purpose or both. But because it is a 'lens', or a focusing device, its operations, transmit and receive, will be directional, meaning at whatever direction it is looking at. When a Luneburg Lens is used as a reflector, even though it is essentially an antenna, it will not be an active transmitter but merely a passive, or responsive, transmitter, meaning it will be an 'emitter' only when an EM wave hit it.
Now we get to the difficult to understand part...Check out the source below...
http://www.ussailing.org/safety/images/radLensref.jpg
Imagine that band is a metalized region on the ball. When an EM wave hit this ball, whose internal construction is a Luneburg Lens, the metal will block a portion of the EM wave from entering the inside of the ball.
Not all but only a portion of the wave.
The rest of the wave will be focused and redirected like a normal Luneburg Lens operation but whatever that is coming out of the ball will have a portion of that wave blocked from leaving the ball, as indicated by this sentence...
Radiation from a distant radar transmitter is focussed onto the underside of the metallization on the opposite side of the lens; here it is reflected, and focussed back onto the radar station.
Remember the ball example. Not all of the ball is metalized, meaning covered but only a narrow band, a metal strip, that is on the ball.
A difficulty with this scheme is that metallized regions block the entry or exit of radiation on that part of the lens, but the non-metallized regions result in a blind-spot on the opposite side.
So when the EM wave impact this ball with a metal strip on its surface, part of the wave is blocked from entering the ball. Inside the ball, the Luneburg Lens operation does its job but when the wave exit the ball, part of this wave is blocked from leaving the ball.
The wiki source gave a somewhat misleading description of this 'blind spot' implying that it is a negative aspect of creating a Luneburg Lens reflector with a defect in design. Not at all. For coastal marine safety reasons, the coast radar often does not care if this Luneburg Lens is reflecting or not. By having that metal band on the ball, the EM wave that exit the internal Luneburg Lens is still visible for other radar stations in the area, like on other ships and would like to know if there are other vessels around to avoid collisions. The coastal marine radar transmitter is simply providing a much appreciated service -- broadcasting EM waves. Other ships with their radars turned on but not actively transmitting will receive echoes as created by this Luneburg Lens ball. No need to have so many transmitters around.
In air combat training, which includes missile defense, sometimes a Luneburg Lens reflector is a much better training aid than omnidirectional reflectors. Remember that a Luneburg Lens is a focusing device, hence generally the EM wave created by the device is directional.
An air target equipped with an omnidirectional radar reflector would be to train radar crews on basic radar detection of this airborne body. However, a Luneburg Lens reflector, which is a directional reflector, thereby not always visible, is a much more difficult training aid airborne adversary. A Luneburg Lens reflector forces radar operators, from ground to air, to be more attentive to spikes or anomalies in their areas of responsibilities. Spikes and anomalies are not constant. They could be of nature such as flocks of birds or a genuine military threat. A Luneburg Lens reflector forces radar crews to study tactics instead of simple radar detection. How does a bomber move, electronically speaking, compared to the smaller and more agile fighter?
In weapons testing and development, facing a Luneburg Lens opponent, I may be forced to redesign my missile to detect less constant targets, such as a slow moving truck against uneven terrain. A Luneburg Lens would simulate that non-constant radar echo -- the truck. But if I redesign my missile to be overly sensitive, my missile could end up chasing flocks of birds or swarms of insect or even trees swaying in the wind. A Luneburg Lens reflector would give me an electronic reference since the device itself could be constructed to emit an echo whose properties are like that of a metal body -- truck -- but who is traveling in uneven terrain so this echo is non-constant. This type of testing would allow me to fine tune my missile's radar to even the briefest echo of any man-made objects.
