Now you see it . . .
By Mark Schrope in San Francisco YOU’RE deep in enemy territory and need to radio your base to call in an air strike-but the long-range radio antenna you erect gives away your position to enemy radar, and you’re sunk. Help may soon be at hand, however, in the shape of a new antenna that can be made invisible to radar in a fraction of a second. The antenna, developed by Gerard Borg and his colleagues at the Australian National University (ANU) in Canberra, replaces the metal in a standard aerial with a plasma confined inside a gadget similar to a large fluorescent strip light. Borg revealed the technology at a mid-November plasma physics meeting of the American Physical Society in Seattle. The plasma is formed by a radio wave which is shot into a shock-resistant dielectric tube containing a noble gas by a single metal electrode at the base of the antenna. This wave moves up the antenna, stripping electrons from the gas molecules as it goes, ionising them to form a plasma. The free electrons allow the plasma to act like a metal, which has many free electrons drifting inside it. And just like a metal antenna, those electrons can be forced to oscillate to create the electromagnetic waves that transmit a radio signal. When the antenna is in receive mode, incoming radio waves cause the electrons to oscillate in the same fashion. The most striking aspect is that when the electrode at the base of the plasma antenna is turned off, the gas inside instantly becomes neutral, so the antenna is as invisible to radar as air. The idea of using plasma as an antenna has been around for more than a decade, but military secrecy has made it difficult to assess the work of other groups, Borg says. An American team is thought to be developing a plasma antenna that passes a radio frequency current between two metal electrodes. But Borg says ANU’s single-electrode design will be easier to hide from radar. “They’ve done some very nice work,” says Mark Rader, a physicist on the US project at the University of Tennessee in Knoxville, commenting that the ANU device has proved some of the advantages that were predicted for plasma antennas, particularly that they are powerful enough to replace whole arrays of standard antennas. “They’ve been able to prove a couple of pet theories I’ve had, but haven’t had the resources to try,