Conceptually, the idea of plasma antenna is older than the transistor. In the early part of 20th century, due to extensive research in nuclear science from which the idea of plasma originated, it was theoretically estimated that the plasma medium can be exploited to make antennae having better capabilities and properties than metal antennae. Following this, the first patent for such an antenna was issued to J. Hettinger in 1919.
But now the question arises why the term ‘plasma antenna’ triggers a sense of unfamiliarity amongst us. The answer to this question is that although the concept of plasma antenna emerged very early but, due to non-availability of proper technology, this concept was practically realised only after 1995. And at that time also plasma antenna models were built around big and heavy discharge tubes that limited their flexibility.
It was in the year 2000 when a new branch in this field emerged in which this concept was realised utilising solid-state semiconductors. Although this concept is not mature enough, a lot of research has taken place and is still going on. Moreover, the California Institute of Technology has been successful in realising and applying this concept to create compact and dynamically-configured antennae.
In the case of metal antennae, taking a rough estimate, they can be used up to 4GHz of frequency range; beyond that, we need to make use of some hybrid or microstrip antennae. Now, as per the latest research, plasma antennae can be configured to operate up to 20GHz, which is far better than the range possible with traditional metal antennae.
Talking about research, most of the researches carried on this subject are by defence organisations. Commercial organisations are also contributing to this but their limited resources has been an issue.
Besides, since solid-state semiconductor realisation is very limited, discharge tubes are used for prototyping. The discharge tubes have their own disadvantages that limit their practical usefulness in commercial applications.
Understanding the plasma
Now, before talking of a plasma antenna, we should understand the term ‘plasma.’ There are five states of matter known to this date, namely, solid, liquid, gas, plasma and supercooled solid as shown in Fig. 1. Plasma is the fourth state of matter. The story of plasma starts with gases. A substance is said to be a gas if its boiling point is below room temperature under atmospheric pressure. More specifically, the intermolecular forces of attraction existing amongst the molecules are almost negligible. So that means higher the boiling point, higher the intermolecular forces of attraction.
Talking of electrical property of gases, we can say they are generally insulators. Now what happens when we supply thermal energy to the gases is that the heat absorbed is used to cut off the intermolecular forces. By applying more heat energy to the gases, we can convert them into a plasma state. Overall, this process is known as ionisation, i.e., the conversion of atoms to ions and electrons. For plasma to exist, ionisation is necessary. The term plasma density is a synonym to electron density.
Definition. Plasma can be defined as a set of quasi-neutral particles with free electric charge carriers which behave collectively. In this definition, two terms are very important:
(i) quasi-neutral. Meaning that there is the same quantity of positive and negative particles so, as a whole, it behaves as a fluid without net charge.
(ii) collectively. Meaning that plasma as a whole is capable of carrying out processes that generate electric and magnetic fields to which it can react. This is one of the most important properties that lead to some unparallel characteristics.
So basically, plasma is produced “when enough atoms are ionised to significantly affect the electrical properties of a gas under normal conditions.” It is not any alien form but only a state of ionisation, a state of matter. There are eleven elements that exist as gas under normal conditions. Out of these, group 18 elements are of prime importance (if we are analysing discharge tubes) as they are inert. Plasma is much visible matter in the universe, being about 99 per cent of all the matter. Besides astronomical plasma, we can distinguish two main groups of plasma from laboratory point of view:
(i) Thermal plasma. Here electrons and ions are at thermal equilibrium. But for this thermal equilibrium to exist, very high temperatures are required, specifically in the range of 4000K-20,000K. But this equilibrium makes this kind of plasma to be unfit for the antenna system application (welding, plasma torches, sintering and etching).
(ii) Non-thermal plasma. Here ions and neutral particles are at lower temperature as compared to electrons, or we can say that the electrons are somewhat ‘hotter’ than ions. This fits for the antenna system application.
Fluorescent lamps and neon signs are examples of plasma states. In case of lamps, the extra electrons added at the electrode ionises the gas with the help of mercury atoms and the result is ionised state of plasma.
The most common way of generating a low-temperature plasma is by applying electric field to a neutral gas. The neutral gas always contains a few electrons and ions. When an electric field is applied to it, these free carriers are accelerated and they collide with atoms and molecules. This is similar to an avalanche breakdown, that is, a large number of charge carriers are formed at the loss of old carriers, and so there is a balance in carrier generation. In this way, steady-state plasma is developed.
Consider DC discharge system for a rough idea. If we subject the gas to high pressure, there would be a high-collision rate and this could produce thermal plasma. But DC discharge system requires non-thermal plasma, so we will pressurise the gas at low pressure to produce a low-collision rate. A high electric field is applied to the electrode plates and the phenomena stated above takes place. Following this pattern, if we apply more voltage, we will get more plasma density. But if we apply electric current in microsecond pulses, we can obtain high plasma density at the same input power. This can be advantageous as the voltage requirements of the system are determined by the nature of gas, electrode material, current density and length of the tube.
Earlier, the DC discharge systems were very bulky but now engineers have been able to reduce their size. but the ionisation instrument is still bulky and hence it limits its universal application.
Interaction of plasma with EM waves
As we are interested in designing antenna systems, the interaction and behaviour of plasma with EM waves must be investigated for better understanding. As per some theses, various relations are set up (qualitatively) to study their interaction. Plasma contains quasi-neutral particles, which means they are highly conductive. For analysing interaction with EM waves, four important parameters are conductivity, electrical permittivity, magnetic permittivity and propagation constant.
Qualitatively, the interaction of plasma with EM waves can be formulated as:
1. Plasma with high-collision frequency behaves as a lossy medium. This is due to the reason that with increase in plasma pressure and electron density, the rate of collision increases, so considerable amount of energy will be lost. (This is the reason why thermal plasma is unfit to be used in antennae.)
2. If W > Wp. EM wave frequency is greater than plasma frequency (an inherent property of plasma), so wave propagates in plasma and the plasma has dielectric properties which are electrically controllable.
3. If W < Wp. The propagation constant is imaginary. The wave is vanishing with the plasma medium. The wave can be absorbed or reflected depending on the collision frequency.
4. Effect of magnetic field. The plasma can be shaped into specific geometry to match the needs of intended application. This is done by applying magnetic field which will make a cyclic motion of the charged particle. This is also an important property as it frees the traditional image of antennae in our mind, although geometries are limited.
The plasma antenna
antenna is a matching section between two output terminals of a transmitter and space, or between space and two input terminals of a receiver. Radiation is simply the transfer of energy through a medium. So a plasma antenna is a type of transmission and reception device that makes use of plasma medium rather than metal components.
The electron behaviour of plasma antenna is completely different from that of metal antenna. In the plasma antenna, the functioning concept is altogether different. It is due to ‘electrons in free space’ rather than ‘electrons moving freely.’ The design allows for extremely short pulses that are very important to many forms of digital communication and radars.
One fundamental distinguishing feature of a plasma antenna is that the gas ionising process can manipulate resistance. When de-ionised, it has infinite resistance and hence it does not react with RF. when ionised, it will have some resistance due to which it will react with EM waves.
Methods of producing radiations
There are two methods of producing radiation:
1. m-radiation method. In this method, the radiation is produced by current oscillations on the surface of a metal and by disturbing current on the interface between plasma and medium. (Example is surface wave-driven plasma antenna.)
2. d-radiation method. In this method, the excitation is applied to the interface which disturbs current between plasma and medium and radiation takes place. It is just like the radiation in dielectric antennae.
Plasma antennae are of two types:
1. Gas chamber. In this type, we use the DC discharge system with a very high-voltage source applied to cathode and anode, and then the signal is superimposed on it (a plasma column). This is a primitive implementation of plasma antenna.
2. Solid-state semiconductor type. The concept of this type of plasma antenna is that the charge carriers in metal and semiconductor behave similar to those in gas plasma. The medium properties will vary as per constructions. However, the interaction of EM waves with charge carriers will have very similar properties to that of quasi-neutral particles.
The semiconductor having enough free carriers to interact with EM waves is called semiconductor plasma with very high electron density that can be obtained by heating, current injection or by optical excitation.
Features and applications
Salient features and applications of some of the plasma antennae are:
1. Reflectors. As stated earlier, if EM wave frequency is smaller than plasma frequency, the wave will be reflected or absorbed. This feature has been exploited in the design of radar-absorbing materials for stealth applications.
2. Windowing. It is a term coined for RF signals being transmitted through plasma tubes which are off or low enough in plasma density. This feature has been exploited in side-lobe reduction and broadband jamming equipment.
3. Stacking. Plasma antennae can be stacked into one another for different frequencies. For instance, the inner antenna is optimised for high frequency and outer antenna is optimised for low frequency, so both of them will work independent of each other. Also, the plasma antennae are more susceptible to frequencies not detected by metal ones.
4. In war. Plasma antennae do not melt, have heat and fire resistance and, as ohmic losses are very less, they have wider range of power-handling capability. They can be easily prepared for use in wars.
5. Car anti-collision radar system. Perhaps this is the most awaited commercial application of any antenna system. Plasma antennae solve this problem. The optically-excited semiconductor antenna array combined with adequate logic control in California Institute of Technology resulted in such a system. However, this is still in its early phase and prototypes have been developed. Just think of its application in low-visibility areas!
6. Mobile industry. If the plasma silicon antennae are used in towers, the towers would be able to transmit denser beams and provide more phone support than traditional antennae, though this could make humans and animals more prone to radiation-related problems. But due to eradication of network interference, this would directly reduce the spectrum cost. HF CDMA plasma antennae can have low probability of interception, which is an important parameter of CDMA communication.
The author is a fourth-year B.Tech student at Vidya College of Engineering, Meerut