GAO: The DSN is actually falling apart |
GAO: The DSN is actually falling apart |
Guest_BruceMoomaw_* |
May 24 2006, 11:55 PM
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Guest_Richard Trigaux_* |
May 26 2006, 05:46 PM
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A suggest RNeuhaus, there exist new antenna technologies which could do the same job than a large dish at a lesser cost. But it is physically impossible to reduce the size of the antennas, as this size is necessary to:
-gather signal power -obtain a directionnal effect. (Both characteristics go together). -dipole arrays. As indicated, they are simple receivers (dipoles or other microwave devices) mounted in an array. Each dipole has a phase shifter commanded by a computer in such a way that when we combine all their outputs, we receive the signal from only a specified direction. Such arrays are used for military radars in aircraft noses, saving the wheight of the orientation mechanism. At a pinch, the dipoles can be set on a non-planar surface, provided that the computer knows their position. The cost of such an antenna will rise as the square of their diametre, as each dipole has its own electronics. I don't know if they are cheaper than conventionnal dishes, probably not as there are no attemps to build any. Anyway it is simple to built phase shifters for a fraction of the wavelength, but a large dish would require electronic phase shifters for many times the wavelength, so I don't know if this is possible. Flat striplines antennas use printed-circuits to build dipoles (or slots) and achieve their coupling. They still need to be oriented and rigidified like a dish, so like this we don't gain much. But, combined with the previous technology, we could use only one phase shifter per rank of dipoles. Such phase shifters, much less numerous, could be mechanical, and thus cheaper and with as much shift as desired. Such an antenna could simply lie flat on the ground (or better on water), needing only to be oriented into azimuth, the phase shifters doing the job of tuning the site. This is, as far as I know, the only way to save the building of a large mechanical structure, which makes all the cost of a large dish. New materials with designed electromagnetic properties. This is very new in antenna technologies: materials which are regular arrays of filled conductors and gaps. Their geometries can be designed for, as an example, set a given propagation speed th electromagnetic waves travelling into them. So we could design lenses made of such materials. For a space application, the lens could be made of wires, rolled into a deflated baloon. Once into space, the baloon is inflated, putting all the wires in place. Then all the thing is rigidified, for instance with some polymer which hardens with UV. The ballon soon loses its gas, but no matter. The receiver is a but further, at the focal point of the lens. The advantage of this is that it don't require a large sized construction into space, a thing we don't know to do. All those previous methods have the obvious drawback that they have a limited bandwidth. Only a true parabole could have a large bandwidth. inflated parabolic dish . This idea is not new, and it is appealing: just with inflating a baloon in space makes a dish. But obstacles are many: -inaccurate building -the baloon soon loses its gas. To overcome this I would suggest some use of electrostatic forces, repelling the two sides of the baloon, and keeping it "inflated" without gas. On the reflecting side, there would be many electrodes, each commanded independently, like into the adaptative optic mirrors. So they could maintain a correct shape, accurate enough, and focalise waves on the other side of the baloon where the receivers would stand. Into space this would work neatly. |
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