The classical definition of a charge is limited to assigning opposite polarities to it, which is used to design electric current circuits whose charges are sustained by an energy from a potential (voltage) difference for example. However, recent research and reviews of physical plasma behavior revealed that the energy acquired by the charges can be studied closely in space and time domains using other common attributions such as charge (dimming) or (brightness) [1].
Physical plasma can serve as a conductive medium for transmitting or receiving electromagnetic (EM) waves, offering an alternative to metal antennas. This concept is developed due to recent advances in a telecommunication concept that is known as Gaseous Plasma Antennas (GPAs). GPAs exploit plasma’s unique properties as a dispersive medium, whose EM behavior depends on wave frequency and plasma density, both of which can be controlled via many variables. This enables real — time reconfiguration of antenna parameters such as shape, frequency, and radiation patterns, allowing rapid beam steering and reduced interference by activating only the necessary elements [2].
In addition, high-speed imaging in displays can also be controlled through the integration of in-pixel storage and light-emitting components for brightness adjustment too. This represents an innovative advancement for the flat panel display technology that may enable True Random Number Generation (TRNG) features. This capability is achieved by capturing rapid ambient events and noise for example and storing them within the display’s pixels. Further, AI algorithms can be employed to analyze and adjust data, including power consumption, heat, and light emission, collected from each display panel set [3].
Elements of light sources that may operate as plasma electric discharges for GPAs, such as the Cold Cathode Fluorescent Lamp (CCFL), were used before also as backlighting elements that are sealed under pressure and kept under a low pressure for use in the technology of Liquid Crystal Displays (LCDs) as an example. In this configuration, such backlighting elements can enable the display to operate as a switchable and tunable antenna in addition to its role as an illuminating element to the display screen [4] [5].
1. Analysis
Electromagnetic behavior of the plasma is highly dependent on the frequency of the EM wave and the plasma’s electron density. The relationship between plasma density and frequency is critical: for an EM wave to propagate through a plasma, its frequency ( f ) must be greater than the plasma’s characteristic frequency, known as the plasma frequency ( fp ). The plasma frequency is directly proportional to the square root of the electron density (ne).
If the frequency of the EM wave is lower than the plasma frequency (f < fp), the wave is reflected, similar to how a metal surface reflects EM waves. This is the principle behind the reflection of radio waves by the ionosphere, a naturally occurring plasma layer in Earth’s upper atmosphere. Conversely, if the EM wave frequency is higher than the plasma frequency (f > fp), the wave can propagate through the plasma. By controlling the electron density of the plasma, which can be done via electrical input or pressure change for example, the plasma frequency can be dynamically adjusted, allowing for real-time control over whether the plasma transmits or reflects a given frequency. This dynamic control over the plasma’s properties enables rapid reconfiguration of antenna parameters such as shape, frequency, and radiation patterns. This allows for rapid beam steering and reduced interference by activating only the necessary elements of the antenna. Another distinctive feature of GPAs is its ability to deactivate the plasma by simply turning off the electrical input, which effectively makes the antenna invisible to radar as an example.
2. Discussion
The typical peak electron densities in the F2 layer of the ionosphere range from approximately 10 ⁵ to 10 ⁶ cm⁻³. Consequently, to achieve reflection for communication applications for example of even the lowest frequency microwave (~1 GHz), the electron density would need to be increased by approximately four to five orders of magnitude beyond the typical peak densities observed in the natural ionosphere.
For example, in the case of a Cold Cathode Fluorescent Lamp (CCFL) plasma source, operating within the electron density range of 10 ¹¹ to 10 ¹² cm⁻³, the corresponding plasma frequency range is calculated to be approximately 2.84 to 8.97 GHz. So, when a radar system probes a plasma with these electron densities, the interaction with the plasma varies across different frequency bands. Below 2.84 GHz (L-band and lower S-band) radar waves with frequencies below the plasma frequency are subject to reflection by the plasma, leading to a cutoff phenomenon and between 2.84 GHz and 8.97 GHz (upper S-band to C-band) radar waves within this frequency range experience significant interaction with the plasma, potentially leading to strong absorption due to resonance effects as the radar frequency approaches the local plasma frequency. Extending into the lower portion of the X-band (8–12 GHz), and above 8.97 GHz (X-band and higher), radar waves with frequencies above the plasma frequency are generally able to propagate through the plasma, although they may experience some attenuation or phase shift depending on the collision frequency within the plasma.
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Generally, and in the case of longitudinal waves propagation for example, such as acoustic waves, particles oscillate parallel to the wave’s propagation direction. In atmospheric air, sound propagates through compressions and rarefactions, with air molecules oscillating around their equilibrium positions. The speed of sound in the atmosphere is influenced by factors such as the atmosphere temperature and the molecular composition. At 0°C, the speed of sound in dry air is approximately 331 m/s, increasing with temperature due to higher molecular kinetic energy. The composition of atmosphere affects sound propagation. Nitrogen (N₂) and oxygen (O₂) are the primary constituents, determining the baseline properties for sound transmission. The speed of sound in these gases is approximately 354.4 m/s for nitrogen and 330 m/s for oxygen at 27°C. Other components, such as argon (Ar), carbon dioxide (CO₂), and water vapor (H₂O), have more subtle effects. Argon, being a monatomic gas, has fewer internal energy states and thus contributes less to sound attenuation compared to diatomic gases like N₂ and O₂. CO₂, a triatomic molecule, has specific vibrational modes that can absorb sound energy more effectively, especially at certain frequencies and humidity levels. Water vapor, being lighter than N₂ and O₂, decreases the average molecular weight of air, leading to a slight increase in the speed of sound in humid conditions.
As for transverse waves, they involve oscillations perpendicular to the direction of wave propagation. In solids and liquids, these waves can propagate due to the medium’s rigidity. However, atmospheric air, being a gas, lacks the necessary rigidity to support transverse mechanical waves. In this case, the molecules in gases are not bound in a fixed structure and can move freely, preventing the establishment of the restoring forces required for transverse wave propagation. Consequently, transverse mechanical waves do not propagate through the bulk of atmospheric air. Despite this, electromagnetic waves, such as light and radio waves, are inherently transverse and do not require a medium to propagate. These waves can travel through the vacuum of space and are only slightly affected by the medium they pass through.
In atmospheric air, the speed of electromagnetic waves is slightly less than in a vacuum due to the medium’s refractive index, but this effect is generally minor for visible light and more relevant for specific wavelengths, such as infrared, which can be absorbed by gases like CO₂ and H₂O vapor. Additionally, waves at the surface of water, known as surface waves, exhibit both longitudinal and transverse characteristics but occur at the interface of two media, not within a bulk gas.
Now, at an elevation of 100 km and above, the region of the Earth’s atmosphere known as the ionosphere is reached, where physical plasma is present due to the ionization of neutral gases by solar radiation. But electron density in the earth ionosphere is not constant; it is highly dynamic and varies significantly with time of day where densities are much higher during the day due to solar radiation, and lower at night as electrons recombine, seasonal changes, where the angle of the Sun’s incidence affects the amount of solar radiation, leading to seasonal variations and solar activity.
Periods of high solar activity (e.g., as tracked by counting sunspot number, EUV output) result in increased solar radiation and, consequently, higher electron densities throughout the ionosphere. Therefore, while typical values can be provided, the exact electron density at any given moment at these altitudes is highly dependent on these variable factors. All of these ambient conditions are to be considered when operating plasma antennae whether on earth or in space.
3. Conclusion
This short review explores Gaseous Plasma Antennas (GPAs) that offer a flexible alternative to traditional metal antennas by using ionized gases as a conductive medium. A key benefit is their ability to dynamically adjust characteristics like shape and frequency by electrically controlling plasma density. This allows for real-time beam steering and reduced interference and makes the antenna radar more invisible GPAs also facilitate improved integration of high and low-frequency systems, potentially offering benefits over metallic counterparts. Interestingly, the electron densities required for GPAs align with those found in Cold Cathode Fluorescent Lamps (CCFLs), suggesting that this display backlighting technology could potentially be repurposed to create tunable antennas within displays, thereby extending their device utility and further using advanced techniques for more efficient power consumption. In addition, interaction between radar waves and plasma is frequency dependent, so for a typical GPA electron density of 10 ¹¹ to 10 ¹² cm⁻³, radar waves below ≈ 2.84 GHz are reflected, those between ≈ 2.84 GHz and ≈ 8.97 GHz are significantly absorbed, and those above ≈ 8.97 GHz can generally propagate through. This contrasts with mechanical waves such as acoustic sound waves, which rely on a physical medium for propagation, or electromagnetic waves that propagate readily through physical plasma. While the natural ionosphere itself contains plasma, its variable and generally lower density make engineered plasma sources essential for practical GPA applications.
References
1. Hwu, K. I., and Y. H. Chen. “A novel dimming technique for cold cathode fluorescent lamp.” IEEE Transactions on Industry Applications 46, no. 6 (2010): 2196–2201.
2. Magarotto, Mirko, Fatemeh Sadeghikia, Luca Schenato, Davide Rocco, Marco Santagiustina, Andrea Galtarossa, Ali Karami Horestani, and Antonio-Daniele Capobianco. “Plasma antennas: A comprehensive review.” IEEE Access (2024). DOI: 10.1109/ACCESS.2024.3411142.
3. Hala, A. M. “Mixed modes pixels display technology: is there a room for physical plasma inside modern day display screens?”. J Sen Net Data Comm 4(3), 01–04.
4. Lim, S. K. “LCD Backlights and light sources.” In Proceedings of ASID, vol. 6, pp. 8–12. 2006.
5. Surti, Mustafa, Chintan Panchal, and Bhaumik Vaidya. “Comparative study on low power display technique based on LCD systems with back-light as LED.”
