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Comment: Re:NOT Invisibility Cloak: RADAR Cloak (Score 1) 197

by HuskyDog (#26538739) Attached to: A Step Toward an Invisibility Cloak

The key issue is how little of the radio waves are reflected.

Indeed, I couldn't agree more, and this is a property which rarely seems to be mentioned in meta-material discussions. Supposing that such a material passed 95% of the energy undisturbed and only reflected 5%. I think that this would rightly be regarded as an excellent technical achievement, and after all some glass isn't that good, but it would seem to be of limited military value. As we know from the radar equation's R^4 term, this will only reduce detection range by a little more than 50%, and you can easily buy commercial radar absorbing materials which will do much better than this.

Of course, your absorbing/transmitting solution also has to meet many other criteria. It needs to work over a wide range of frequencies, polarisations and angles; be rugged enough to survive on the outside of a military vehicle without needing maintenance every five minutes; not be so heavy that your aeroplane can't take off and most of all, be affordable (I appreciate that military affordability is not the same as normal affordability, but never the less budgets are much tighter these days). Of course, progress is probably being made in all these area's but my feeling is that there is still a long way to go.

My other big problem with meta-materials is seeing how they can possibly be applied in real life. As I see it, they are basically being used to make a sort of cunning lens which diffracts the wavefront around the object to be concealed. Sadly, the geometry of the situation seems to result in devices which have a high degree of symmetry (for example, cylinders). I can imagine how this might be extended to a sphere (great if we need stealthy cannon balls) or maybe even a prolate spheroid (which perhaps we could fashion into some sort of missile), but to deal with the complex shape of an aircraft in such a way that the different parts of the wavefront passing around different parts of the wings and fuselage all add up correctly in phase over a useful range of angles seems to me to be an extremely difficult problem.

Still, I am not going to sit here and say that it can't be done. The people working on these problems are no doubt very brainy and may come up with a practical solution, but presumably the paint and shape solution is also being advanced and it is my feeling that this older simpler technology is going to stay ahead for a considerable period of time.

Comment: Re:NOT Invisibility Cloak: RADAR Cloak (Score 4, Insightful) 197

by HuskyDog (#26511145) Attached to: A Step Toward an Invisibility Cloak

Sigh, here we go again! Radars and optical vision do not work in remotely the same way. Creating invisibility in the two different realms is a completely different problem.

In most vision situations there are two critical factors which don't occur in the great majority of radars. The first is illumination of the target from angles other than the viewing angle (OK, there are bistatic radars, but they are not common) and the other is a background which is illuminated. Try to think about this for just a few moments. Why can't we all make ourselves invisible just by wearing matt black clothing? Well, obviously because we will stand out against the background unless we happen to be standing in front of black wall or wandering around in a coal mine. The whole point of the fictional 'invisibility cloak' is that it works in all circumstances. We can already be invisible in certain carefully controlled environments, that after all is what camouflage is all about.

But, a radar is rather like wandering about in the above mentioned coal mine, or perhaps a dark forest with a miner's lamp fixed to your head. The background is basically black and the illumination comes from the viewing direction. In this scenario, someone dress entirely in black would be effectively invisible. And that is the key point to grasp. In the world or radar we can achieve invisibility simply by making sufficiently 'black' 'paint'. The weird ability of these meta-materials to allow the illumination to pass through the target un-disturbed is of no benefit. Since we don't have a receiver on the other side of the target to detect this energy it isn't relevant. Now, sure, we can all dream up complex bistatic radars which rely on the obscuration of the signal to detect the target, but I remain to be convinced that such a thing can be made sufficiently versatile to be useful.

Can I stress that I am not suggesting the these meta-materials don't have an application in the world of radar. They seem to me to be particularly useful where one wants to remove a fixed object which obscures the view of your radar. For example, consider a radar on a ship. It may well find that in some directions its view is obscured by other parts of the superstructure. If the could cover these other bits of the ship with meta-materials such that the radar pulses could pass 'through' and back again undisturbed, then our radar's field of view would be increased. Such an application would work perfectly well with even the relatively narrow band materials presented previously.

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