A conventional metal antenna has a defined size and shape. These dimensions directly influence the frequencies it can emit or receive. Changing them during operation generally requires mechanical parts or several antennas. Researchers have tested another solution: temporarily transforming air into plasma instead of using metal.
The prototype plasma-filament antenna produced by a laser. The light beam passes through a metal ring that injects the radio signal without direct contact.
Credit: Prya Darshni / NC State University.
The principle begins with a sufficiently intense laser pulse. Along its path, it strips electrons from air molecules and creates a plasma filament. This very thin channel then contains charged particles capable of interacting with a radio signal. Visually, the device appears as a narrow beam of light.
One problem remained: how can an electrical signal be sent into an antenna that is not solid? Direct metal contact would disrupt the filament. The researchers therefore placed a metal ring around the laser’s path. This ring produces an electromagnetic field that transfers radio energy to the plasma without touching it.
During the experiment, the filament emitted a signal at 30 MHz. A receiver placed about 5 cm away measured a signal 2.5 times stronger when the plasma was present than with the laser blocked. This difference confirms that the filament itself was indeed radiating radio waves.
The main advantage lies in the possibility of modifying the antenna without moving a metal structure. By changing the laser’s parameters, the researchers can alter the filament’s length. An antenna’s length partly determines its operating frequencies. Directing the beam could also make it possible to quickly change the antenna’s direction.
Such a property could be useful when an antenna needs to scan multiple frequencies or change orientation. The researchers specifically mention aircraft and certain radar systems. An antenna produced on demand could eliminate the need to deploy long mechanical structures.
The device remains an experimental demonstration, however. The researchers have shown emission at 30 MHz, but not yet the reception of a signal by this antenna. They believe the same principle should work in the opposite direction, which will need to be verified experimentally.
The team is now working to improve performance and explore other frequencies. It is also considering use in low Earth orbit, where the residual atmosphere could provide enough material to create the plasma. The range, energy efficiency, and system behavior under these conditions remain to be measured.