In the ever-evolving world of wireless communication, the demand for adaptive, high-performance, and interference-resistant antenna systems continues to grow. Indeed, the electromagnetic spectrum is becoming increasingly crowded, and future communication systems should be able to quickly switch to a different frequency or even a different band. Traditional metal antennas have been the backbone of communication systems for decades, but their physical limitations have driven researchers to explore alternative technologies. Plasma antennas – using ionized gas instead of solid metal conductors – offer a highly reconfigurable, lightweight, and low-noise solution for next-generation communication, radar, and electronic warfare applications.
What Are Plasma Antennas?
A plasma antenna operates on the same fundamental principles as a conventional metallic antenna—using an electrical conductor for transmitting and receiving electromagnetic waves. However, instead of relying on a solid metal structure to conduct electrical currents, a plasma antenna uses ionized gas, which can be turned on and off, tuned, and reconfigured dynamically.
The plasma in these antennas is typically generated using electrical discharges, such as radio-frequency (RF) or microwave excitation, in a gas-filled tube or chamber. A fluorescent light tube is perhaps the simplest example. When ionized, the gas inside the tube forms plasma, i.e. becomes conductive, and can support electromagnetic wave propagation, allowing it to function as an antenna. When the plasma is turned off, the antenna effectively “disappears,” leaving no physical conductive structure behind.
J. Hettinger obtained the first US patent on plasma antenna (called then an “aerial conductor for wireless signaling”) in 1919, but then the plasma antenna technology experienced a hiatus for about 80 years. In the last several decades, a number of research groups around the world, starting with the work of Haleakala Inc. in the US and a group at the Australian National University, have explored plasma antennas, obtaining some encouraging results. A number of scientific and engineering questions, however, remain to be answered.
Advantages of Plasma Antennas
1. Reconfigurability and Frequency Agility
One of the most significant advantages of plasma antennas is their ability to be dynamically reconfigured. Unlike traditional antennas, which are fixed in size and shape, plasma antennas can change their operating frequency, radiation pattern, and polarization by adjusting the plasma density and excitation parameters. This makes them ideal for software-defined radio (SDR) applications and military systems that require rapid frequency hopping to avoid jamming.
2. Low Radar Cross Section (RCS)
Since plasma antennas can be turned off when not in use, they offer a low radar cross section (RCS), making them virtually undetectable to enemy radar systems. This property is particularly valuable for applications where minimizing an object’s visibility to radar is critical. Additionally, since plasma tubes do not conduct electricity when turned off, plasma antennas have low insertion loss.
3. Compact Reconfigurable Arrays
Since plasma antennas do not require bulky metal structures, they can be designed to be more compact and lightweight. The miniaturization is helped by the modern technology of microplasmas, or microcavity plasmas, with sub-millimeter sizes of plasma cavities. The small plasma tubes, or cavities, can be arranged in a pixelated array where each element, or pixel, can be individually controlled (i.e. be turned on/off and have different power and phase of the signal sent to it). Thus, pixelated plasma arrangements can naturally form phased arrays. Also, turning on or off selected plasma pixels, any pattern can be quickly ‘drawn’ in the array aperture, similar to drawing any picture in the pixelated screen. This way, multiple different antennas (each for a certain frequency or band) can be effectively combined within the same aperture, with rapid switching from one antenna to the other, removing the need to have many different physical antennas each with its own aperture. This makes plasma antennas particularly suitable for aerospace applications, including satellite communications and UAV-based radar systems, where weight and space constraints are major concerns.
4. Reconfigurability at High Power
Semiconductor technology offers a solution to tunability and reconfigurability via so-called varactors (variable capacitors). However, semiconductors are easily damaged by high power and the associated heat. In contrast, plasma antennas and other plasma-based systems are robust with respect to high power and heat; plasmas can naturally dissipate excess energy and avoid permanent damage, making them resilient in high-power environments.
Key Challenges and Recent Developments
Despite their advantages, plasma antennas are still in the early stages of practical deployment. Several challenges must be addressed before they can replace or supplement traditional antenna technologies in commercial and military applications.
1. Gain and Efficiency
Gain is the key parameter of any antenna, and to make plasma antennas attractive for applications, their gain should be comparable to (or at least not much worse than) that of metallic antennas. It is somewhat surprising that in the numerous plasma antenna studies no analysis or measurement of this key parameter was conducted until very recently. To the best of my knowledge, our group at Purdue University was the first to address this issue.
The gain of an antenna is the product of its directivity and efficiency. The directivity is determined by the antenna geometry, and here plasma antennas are as good as metallic ones. The efficiency is the ratio of radiated power to the total power (radiated plus dissipated in the antenna). Since weakly ionized plasmas are poor conductors, i.e. the electrical conductivity of weakly ionized plasmas is generally orders of magnitude lower than that of metals, one would expect that the losses in plasma antennas would be much higher and the efficiency – much lower than those in metallic antennas. This would certainly be a concern.
However, the experiments with plasma monopole antenna (made of a fluorescent light tube) conducted at Purdue several years ago have shown that when enough power was supplied to the antenna, i.e. when the electron density in the plasma was sufficiently high, the gain at the resonant frequency of several hundred MHz matched that of the reference metallic antenna of the same length. That was a pleasant surprise that bodes well for the applications.
The key factor behind this encouraging finding was that the plasma antenna had a much larger effective cross section than the metallic wire had. The diameter of the plasma tube was about 1 cm whereas the metallic wire had a diameter of about a millimeter. Moreover, the skin depth, i.e. the effective depth of penetration of the electromagnetic field, was greater than a centimeter for the plasma but on the order of a micrometer for the metal. The resulting electrical resistance of the plasma antenna, despite the low conductivity, turned out to be only about an order of magnitude lower than that of the reference metallic antenna.
Additionally, the small electrical resistance, on the order of a few Ohm, of the metallic antenna created a mismatch with the standard 50-Ohm transmission line in the antenna feed, whereas the higher resistance of the plasma antenna was nearly matched to the transmission line, thus bringing the measured gain of the two antennas close to each other.
Again, this simple but fundamental result is quite encouraging for applications.
2. Noise
The so-called white broadband noise is present in all electronic systems. Such noise cannot be filtered out. For a receiver, noise is especially important: since the system has to distinguish weak signals on the background of noise, and the signal-to-noise ratio must be high enough, thus noise suppression is critical. In many systems, including plasma antennas, the Johnson-Nyquist thermal noise is the dominant one. The Johnson-Nyquist noise intensity is proportional to the absolute temperature of the charge carriers. Therefore, to reduce the noise, the charge carrier temperature must be at least not higher than the room temperature, and for a high-sensitivity RF sensors, cryogenic temperatures are used. In this regard, the use of plasma antennas poses a problem: in weakly ionized plasmas, the gas as a whole is relatively cold, but the steady-state temperature of electrons (the charge carriers) is very high, on the order of 10,000-50,000 K, which corresponds to an unacceptably high level of noise.
Recently, the studies conducted at Purdue University by our group have addressed this critical issue. The study of plasma driven by repetitive short pulses demonstrated that with proper selection of the parameters and conditions (such as the gas pressure and its composition), the electron thermalization, i.e. the drop in electron temperature after the ionizing pulse, is much faster than the decay of electron density. Thus, there is a significant time interval in the pause between the ionizing pulses during which the electrons are thermalized with the room temperature while the electron density is still sufficiently high for a good electrical conductivity.
Direct measurements of thermal noise as a function of time were also conducted. These measurements have confirmed that the noise level does follow the evolution of electron temperature, dropping to an acceptable level between the pulses.
Based on these encouraging findings, a new concept can be proposed. Plasma antennas can be used in a transceiver (a combination transmitter/receiver in a single package) the following way. Plasma is energized by a repetitively-pulsed RF source at a frequency which needs to be transmitted. The transmission then occurs during the pulses. The reception is properly synchronized and time-gated so that it occurs during the time intervals between the pulses when the plasma conductivity (and thus the antenna efficiency) is high enough but the electron temperature and the noise level are low.
As an added benefit, the repetitively-pulsed, low duty cycle mode of operation reduces the power consumption and alleviates the associated heating of the antenna structure.
This new concept provides specific guidance for practical applications.