Science1 publisherNot yet confirmed elsewhere2 min readPublished
NC State researchers transmit 30 MHz radio from a filament of laser-ionized air
NC State researchers turned a laser-made plasma filament in air into an antenna that transmitted radio at 30 MHz. They propose tuning its length with the laser so one antenna covers many bands without deployment hardware, beyond what the single-frequency test shows.
The Scientist · Science desk
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What happened
- The signal reaches the plasma through a metal ring that acts as a capacitor, with the laser fired through it so the ring's field couples to the filament without contact.
- Darshni said the team has not shown the filament can receive radio signals, though she sees no reason it would not.
- Darshni said shifting the laser's direction would let users steer the antenna's angle, for aiming radar sweeps or strengthening a received signal.
- Arthur Dogariu of Texas A&M University and Princeton University co-authored the paper and helped with the measurements.
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Why it matters
- capability If laser settings can set the filament's length across bands, a transmitter could change frequency by changing the beam, with no deployment hardware to extend or retract.
- constraint Radar and two-way links need an antenna that also listens, and for this one that rests on Darshni's expectation until a receive test is published.
- cost On a satellite the antenna's mass and power move into the laser and its supply, so any payload benefit depends on laser figures the team has yet to report.
At 30 MHz a radio wave is about 10 metres long [8], so a conventional half-wave dipole for that frequency runs to roughly 5 metres [9]. Antenna length sets the frequencies an antenna can transmit and receive. Changing that length to sweep a band is hard in applications such as space exploration, according to NC State's account of the work [10]. The team's proposal is to set the length with a laser. "By controlling the parameters of the laser, you can control the characteristics of the plasma filament," said Prya Darshni, an NC State Ph.D. student and the paper's corresponding author [11][7].
The filament is a thin shaft of air ionized by a laser beam of a chosen power and diameter [2]. To make it radiate, a radio-frequency generator feeds the ring capacitor around it, and the resulting field sets the filament transmitting at the signal's frequency [4].
The published result is transmission at 30 MHz [1]. Darshni said the antenna "is tunable, meaning we should be able to transmit across a broad range of frequencies" [12]. Her sentence is a forecast, and the paper's title names one frequency [1]. The release does not report the filament's length, the power it radiated, its efficiency, or the laser power needed to make it.
I think the contactless feed is the firmer result for now. It was built and operated [3]. The frequency sweep, reception and beam steering are all described by the team in the conditional [12][5][15]. Paul Franzon, a co-author and the Cirrus Logic distinguished professor of electrical and computer engineering at NC State, described the goal as "a customized antenna without complex mechanical deployment mechanisms" [16][7].
Satellites are the application the team points to first, because payload and the ability to scan a wide range of frequencies both matter there [17]. "In low Earth orbit, there is sufficient air to form a plasma," Franzon said [18]. The filament in this work was formed by ionizing air [2]. Forming and driving one at orbital densities would be a separate experiment.
Darshni described the result as "the first time anyone has ever demonstrated that plasma-filament antennas can work" [13]. That priority claim comes from the team itself. "Now that we've shown it is possible, we can begin improving its performance," she said [14].
What to watch
- A measurement of the filament antenna transmitting at frequencies other than 30 MHz, which would test the tunability claim directly.
- Published figures for radiated power, efficiency and the laser power each filament needs.
- A filament formed and driven at the low air densities of low Earth orbit, the condition Franzon's satellite case depends on.