When most people picture an antenna, they think of a slim metal rod, whether the thin, extendable rod on a portable FM radio or the towering structures used for radio and television broadcasting. In reality, antennas come in all shapes and sizes, from round parabolic dishes to conductive patterns printed onto circuit boards. However, whatever shape they take, they are typically solid structures. In work that would delight any Star Wars fan, scientists have now created an antenna from a plasma beam.
Researchers at North Carolina State University have demonstrated a laser-generated plasma antenna that can transmit radio waves at 30 MHz. The proof-of-concept technology could allow engineers to create antennas of different lengths and orientations simply by adjusting a laser, eliminating the need for bulky mechanical antenna deployment systems.
“The plasma beam antenna looks like a lightsaber and is tunable, meaning we should be able to transmit across a broad range of frequencies,” says Prya Darshni, a PhD student at NC State and the study's corresponding author. She added that while the team has only demonstrated transmission, they believe the antenna should also be able to receive radio signals.
So, why bother creating plasma beam antennas? One limitation of conventional antennas is that their physical dimensions, particularly length, determine the radio frequencies at which they operate efficiently. An antenna designed for one frequency might perform poorly at another, substantially different frequency. Of course, we already have technologies that address this problem.
Software-defined radios (SDRs), for example, can switch between frequencies without requiring hardware swaps. However, the antenna itself must still be capable of operating efficiently at those frequencies. Wideband, multiband, and electronically tunable antennas enable multifrequency operation, but each involves different design trade-offs.
The challenge becomes even more pressing in applications such as space exploration, where satellites may require relatively long antennas that must be folded for launch and deployed once in orbit. Changing their physical dimensions or orientation can require additional mechanical systems, adding weight and complexity. Now, imagine replacing that metal antenna with a laser-generated column of conductive gas. Need a different antenna length? Adjust the laser. Need to change its orientation? Point the laser in another direction. These are the possibilities the researchers are exploring.
To build the device, the team used a laser with carefully controlled power and beam diameter to ionize a narrow column of air. The concentrated laser energy strips electrons from gas molecules, producing a thin shaft of plasma, known as a plasma filament, containing free electrons and positively charged ions. Because these charged particles can respond to electromagnetic fields, the filament can behave somewhat like a conventional metal antenna.
In English, the laser creates the antenna, while the plasma it produces serves as the conductive structure that transmits radio waves. The laser beam itself isn't carrying the radio signal.
Now, plasma antennas aren't entirely new. Researchers have previously demonstrated antennas made from ionized gases, including laser-generated plasma columns that required additional high-voltage discharges. The NC State team has now demonstrated a different approach: a laser-generated filament and a contactless mechanism to feed radio-frequency signals into it, which brings us to the second innovation.
We have a plasma antenna, but how do we connect it to the rest of the radio equipment? Conventional antennas are typically connected to transmitters through conductive metal feeds. However, attaching a metal wire directly to the delicate plasma filament would distort its structure. The researchers solved this problem by developing a contactless antenna feed consisting of a conductive metal ring that acts as a capacitor.
The laser passes through the ring, creating a plasma filament that extends through its center. A radio-frequency generator then supplies the ring with an electrical signal, producing an alternating electric field that interacts with the plasma without physically touching it. This field causes the plasma's charged particles to respond to the radio-frequency signal, allowing the filament to radiate electromagnetic waves.
To test whether this setup worked, the researchers supplied a 30-MHz signal to the metal ring and placed a receiving antenna about 5 cm (2 inches) away. They first measured the received signal with the laser blocked, so the capacitive feed operated without a plasma antenna. They then repeated the measurement with the laser producing the plasma filament, keeping the radio-frequency input the same.
With the plasma filament present, the received signal strength increased by a factor of 2.5, representing a 150% improvement over the setup without it. Both antennas were positioned with matching linear polarization, and the measurements were conducted in the near field. The results provided experimental evidence that the laser-generated plasma filament functioned as a transmitting antenna.
Potential applications of the technology extend beyond ordinary radio communications. In satellites and spacecraft, engineers could theoretically generate antennas of different lengths without carrying and deploying equivalent metal structures. Adjusting the laser's direction could also change the plasma antenna's orientation, potentially offering greater flexibility for radar systems and directional communications.
“By controlling the parameters of the laser, you can control the characteristics of the plasma filament – including its length,” says Darshni. “This is valuable for applications where you need an antenna that can sweep all frequencies. There are also applications where it is important to be able to control the angle of the antenna, in order to target the direction of radar sweeps or to improve the strength of a signal you want to pick up,” she continues. “The technique we’ve demonstrated here would allow users to control the angle of the plasma filament antenna via beam steering – simply shifting the direction of the laser.”
However, a considerable distance remains between this laboratory demonstration and a practical antenna system. The researchers have demonstrated transmission at only one frequency, 30 MHz, with the receiving antenna just 5 cm away. They have yet to demonstrate the proposed frequency tuning, beam steering, radio reception, or long-distance communications.
There are also questions about how much laser power would be required to maintain a useful plasma filament, how efficiently it could transmit radio signals, and how reliably it would operate in different environments. For now, the achievement is largely a proof of concept. “This is the first step, but it is a big step – it is the first time anyone has ever demonstrated that plasma-filament antennas can work,” says Darshni. “Now that we’ve shown it is possible, we can begin improving its performance.”
The study was published in the IEEE Journal of Microwaves.
Source: North Carolina State University
