For this reason, at the end of the seventies, the large antenna networks were conceived: the European VLBI network and the American VLBA one. Simultaneously observing a source with all the network elements, a “radio image” can be built, such an image showing the same details that would have been obtained using a filled-aperture antenna having the dimensions of the whole network!
The radiotelescope networks solved the problem about how to increase the resolving power, allowing a very detailed study of the most distant radiogalaxies. However, the virtual instrument sensitivity is still depending on the geometric mean of all the single antenna areas. As a consequence, the whole network is very directive but not sensitive.
To obtain a fully effective radiotelescope is then necessary to widen its collecting area; that’s why a new radiotelescope concept is nowadays spreading: SKA (Square Kilometre Array), a telescope having a total surface of 1.000.000 square metres.
SKA will be built thanks to the efforts of many international Institutions and Research Institutes. The further possible step will be to build a network, following the present VLBI example, composed by several elements, each having a surface of a square kilometre. Such a network will be characterised by both a high resolving power and a high sensitivity. The SKA project is now being discussed, and different technical solutions are being proposed: parabolic, flat, spherical or parabolic-cylinder antennas may be used to compose the array.