Any description of an antenna is likely to include the words wire, cylinder, dish or other term describing a metal object. In some cases, however, a permanent antenna could be a liability. In military communications systems, a large antenna can be detected by enemy radar. It would be convenient to have an antenna that disappeared when the RF disappeared. The ability to electronically control the shape of the antenna may also be useful.
Scientists and engineers have been working on antennas using plasma. Plasma isn't a solid, liquid or gas. Sir William Cooke, an English physicist, identified plasma as a fourth state of matter. Plasma antennas use plasma as the guiding medium for electromagnetic radiation.
A vertical column of plasma can be used like a monopole. A lens made of plasma can be used to deflect or focus a beam and the lens can be modified simply by changing the shape of the "lens."
If you are interested in these unusual antennas, Plasma Research Laboratory web site contains links to various plasma resources. The Plasma Research Laboratory is part of the Research School of Physical Sciences and Engineering at the Australian National University has link. Plasma antennas are demonstrated using diagrams and Java applets at http://wwwrsphysse.anu.edu.au/~ggb112/.
