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Plasma Antenna
ABSTRACT
The plasma antennas offer a new solution to new requirements that are imposed on antenna systems with the advancing communication technology and increasing demand on wider frequency bands. In this the plasma antennas are investigated for the radar and communication applications. The interaction of gas and semiconductor plasma with electromagnetic waves is inspected theoretically, and several experiments on the interaction of microwaves with gas plasma are conducted. Results of these experiments show that a relatively simple setup can produce plasma dense enough to interact with microwaves of frequency about 8 GHz. The previous studies of other institutes on plasma antennas are surveyed, emphasizing the results important for the use in radar and communication applications. Finally, semiconductor plasma is introduced, and an

antenna system utilizing the semiconductor plasma generated by optical excitation is proposed.

Keywords: Plasma antenna, plasma frequency, DC plasma.

Plasma Antenna

1. INTRODUCTION
On earth we live upon an island of "ordinary" matter. The different states of matter generally found on earth are solid, liquid, and gas. Sir William Crookes, an English physicist identified a fourth state of matter, now called plasma, in 1879. Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that whichis not visible. Important to ASI's technology, plasmas are conductive assemblies ofcharged and neutral particles and fields that exhibit collective effects. Plasmas carry electrical currents and generate magnetic fields. When the Plasma Antenna Research Laboratory at ANU investigated the feasibility of plasma antennas as low radar crosssection radiating elements, Redcentre established a network between DSTO ANU researchers, CEA Technologies, Cantec Australasia and Neolite Neon for further development and future commercialization of this technology. The plasma antenna R & D project has proceeded over the last year at the Australian National University in response to a DSTO (Defence Science and Technology Organisation) contract to develop a new antenna solution that minimizes antenna detectability by radar. Since then, an investigation of the wider technical issues ofexisting antenna systems has revealed areas where plasma antennas might be useful. Theproject attracts the interest of the industrial groups involved in such diverse areas as fluorescent lighting, telecommunications and radar. Plasma antennas have a number of potential advantages for antenna design. When a plasma element is not energized, it is difficult to detect by radar. Even when it is energized, it is transparent to the transmissions above the plasma frequency, which falls in the microwave region. Plasma elements can be energized and deenergizedin seconds, which prevents signal degradation. When a particular plasma element is notenergized, its radiation does not affect nearby elements. HF CDMA Plasma antennas willhave low probability of intercept( LP) and low probability of detection( LPD ) in HFcommunications.

Plasma Antenna

2. PLASMA ANTENNA TECHNOLOGY

Since the discovery of radio frequency ("RF") transmission, antenna design has been an integral part of virtually every communication and radar application. Technology has advanced to provide unique antenna designs for applications ranging from generalbroadcast of radio frequency signals for public use to complex weapon systems. In itsmost common form, an antenna represents a conducting metal surface that is sized toemit radiation at one or more selected frequencies. Antennas must be efficient so themaximum amount of signal strength is expended in the propogated wave and wastedin antenna reflection.

Plasma Antenna

Plasma antenna technology employs ionized gas enclosed in a tube (or other enclosure) as the conducting element of an antenna. This is a fundamental change from traditional antenna design that generally employs solid metal wires as the conducting element. Ionized gas is an efficient conducting element with a number of important advantages. Since the gas is ionized only for the time of transmission or reception, "ringing" and associated effects of solid wire antenna design are eliminated. The design allows for extremely short pulses, important to many forms of digital communication and radars. The design further provides the opportunity to construct an antenna that can be compact and dynamically reconfigured for frequency, direction, bandwidth, gain and beamwidth. Plasma antenna technology will enable antennas to be designed that are efficient, low in weight and smaller in size than traditional solid wire antennas. When gas is electrically charged, or ionized to a plasma state it becomes conductive, allowing radio frequency (RF) signals to be transmitted or received. We employ ionized gas enclosed in a tube as the conducting element of an antenna. When thegas is not ionized, the antenna element ceases to exist. This is a fundamental change fromtraditional antenna design that generally employs solid metal wires as the conductingelement. We believe our plasma antenna offers numerous advantages including stealth formilitary applications and higher digital performance in commercial applications. We alsobelieve our technology can compete in many metal antenna applications. Our initialefforts have focused on military markets. General Dynamics' Electric Boat Corporationsponsored over $160,000 of development in 2000 accounting for substantially all of ourrevenues. Initial studies have concluded that a plasma antenna's performance is equal to a copper wire antenna in every respect. Plasma antennas can be used for any transmissionand/or modulation technique: continuous wave (CW), phase modulation, impulse, AM,FM, chirp, spread spectrum or other digital techniques. And the plasma antenna can bezenon).

Plasma Antenna

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Plasma Antenna

3.GENERATION OF PLASMA BY DC DISCHARGE

3.1. INTRODUCTION
The first studies on the plasma medium are conducted on the DC discharge systems. A DC discharge system is basically applying a constant electric field on a lowpressure gas via electrodes of various physical properties. While explanation of the DC discharge setup is so simple, the mechanism underlying the discharge and explanation of behavior of the plasma medium in the discharge is quite complex. Our inspection will be limited to those properties that helps us calculate the electrical circuitry parameters required to form a plasma medium with the properties desired for the interaction with the electromagnetic waves. The electrical circuitr parameters we are interested in are the voltage current characteristics and the current required to reach a desired electron number density.

3.2. QUALITATIVE INSPECTION OF DC DISCHARGE The regions in the DC discharge are formed because of the mobility difference between the electrons and ions; electrons move much faster than ions under the effect of applied electric field. Due to this mobility difference, the contribution of ions to current densities and conductivity was neglected in the interaction of plasma with electromagnetic waves. The same mobility difference gives rise to the regions called sheath regions. After the gas is ionized, the electrons in the cathode region are repelled by the negative charge of the cathode, while ions are attracted to the cathode. Since the ions are much heavier than th electrons, the electrons will move away from the cathode very rapidly while ions take a longer time to reach the cathode. Due to this difference in the mobility of electrons and ions, a region with a net positive charge forms in front of the cathode. Similarly the anode repels the ions with its positive charge while attracting theelectrons. Due to the same mobility difference, the electrons are consumed by the anode much faster than the ions can move away from the vicinity of the anode. This effect forms another region with a net positive charge in front of the anode. The two regions in front of the electrodes with a net positive charge are called the sheath regions. Sheath regions are formed even in the situation of existence of conductive walls within an ionized gas, for the electrons will collide more frequently with the wall than ions. This process will charge the conductive wall negatively, repelling all other electrons in the vicinity and leaving a positively charged gas blanket in front of the wall. Effectively the plasma will appear as if it has a positive potential with respect to surrounding areas. Between the sheath regions, there is an ionized region where the overall charge isneutral. This region, having a positive potential as explained above, is called the positive column. In positive column the electric field intensity is very low. The E.J energy is consumed by collision of particles with each other and with the surrounding chamber walls. As the positive column can

Plasma Antenna

extend almost without limit, it is the region of the DC discharge that is to be utilized as a radiator or a reflector I plasma antenna applications.

3.3. SHEATH REGION Experiments with the DC discharges show that most of the voltage drop across a DC discharge occurs in the cathode sheath. Also since the ion flux dominates the current in the sheath region; it is much easier to calculate the current passing through the DC glow by calculating the ion flow in the sheath region. The voltage and currentcalculations in the sheath region may give an approximate value for voltage and current required to drive a DC discharge with the required electron density in the positive column region. Thus characteristics of the DC discharge may be analyzed easily by inspecting the sheath regions. When only a fraction of molecules in a gas is ionized, the resulting plasma is described as cold plasma. In cold plasma, the particle collisions can be neglected. The plasma that we are interested in is cold and ions move much slower than the electrons. To simplify the calculations and formulations is assumed that there is negligible collision between particles. Also in the cathode sheath region, there is negligible ionization, recombination and collision of particles

4. EXPERIMENTS ON THE INTERACTION OF PLASMA AND ELECTROMAGNETIC WAVES

4.1. INTRODUCTION Theoretical study of the interaction of plasma medium with electromagnetic waves has been introduced in the previous sections. The possibility of plasma antennaconcept has been verified in this manner. However, to be of use in practical applications, feasibility of plasma systems must be demonstrated. Such a demonstration requires experimentation on basic wave-plasma interaction. The experimental setup basically consists of a plasma generating apparatus, antennas of appropriate frequency range and microwave measurement devices. The plasma in our experimental setup is subjected to microwaves, and various measurements are conducted to show the interaction between plasma medium and electromagnetic waves is realizable.

4.2. PLASMA GENERATING APPARATUS

The plasma generating apparatus can take many shapes and can include various components, depending on the parameters of plasma that are specific to application. In our experimental setup, DC discharge plasma at 1 Torr (1 mm Hg) pressure is generated as the interacting medium. Such plasma is much simpler to generate compared to other methods, the required voltages to ignite and sustain the discharge are quite low compared to other pressure ranges [2-7, 13]. The plasma generation apparatus consists of an air-tight tube, a vacuum pump, a vacuum gauge to measure

Plasma Antenna

the pressure, two electrodes on each end of the tube, and DC discharge circuitry. The vacuum pump must be able to reach pressures on the order of mTorr scale, especiall if the gas used in the plasma generation is different from air.

4.2.1. SELECTION OF ELECTRODES AND GASES Brass electrodes and air is used in the experimental setup initially. The brass cathode, due to continuous ion bombardment, suffered from a phenomenon called sputtering. Sputtering is the coating of inner walls of the plasma chamber with the cathode material. Although sputtering is a technique widely used in material processing, metallic coating of the plasma tube is an undesired feature in plasma antenna applications. The metallic coating not only degrades the DC discharge by initiatin break-downs at very low voltage levels, but also interferes with the microwave in an undesired manner.Sputtering requires ion energies above a threshold and the heating of the cathode by the continuing ion bombardment. Brass, being a very soft metal, quickly erodes under
Plasma Antenna

the ion bombardment. The resulting metallic coating can only be wiped off the chamber walls by strong acids such as HCl with 15% volume density. Instead of brass, steel or aluminum can be used. Staining of steel degrades the discharge by deteriorating the discharge homogeneity. Thus, aluminum was used in the experimental setup as electrode. Different gases have different voltage requirements to reach a specific current value. Each gas has a different ignition voltage depending on the gas pressure, and mixtures of gases may have completely different discharge characteristics. For the sake of commercial availability and low voltage requirement, Argon plasma I preferred. Air at 1 Torr pressure requires a voltage about 3000V to generate a current density of 250A/m2 through the plasma, while only 2000V was enough with Argon. 4.2.2. DC DISCHARGE CIRCUITRY

Due to the high voltage and current levels required for the generation of plasma, DC generators with the required voltage and current capability were not commercially available. Thus a DC supply had to be built from a transformer, a bridge diode and capacitor banks. The voltagecurrent characteristics of the plasma discharge require adjustable resistor banks as well. The combined DC discharge circuitry is given in Fig. 4.2 Current passing through the discharge is adjusted by Variac in the transformer input and resistor block connected in series to the discharge tube. The discharge circuit. Bridge diodes and capacitor banks form the DC power supply. The circuit is fed by a high voltage transformer, which is adjusted by a Variac

4.2.2.1. VOLTAGE-CURRENT CHARACTERISTICS OF DISCHARGE PLASMA

The DC discharge plasma mainly has 4 stages, each of which having different voltage-current characteristics. 1. Ignition: The gas, when not ionized, acts as an insulator between the electrodes. As the electric field intensity increases the electrons are ripped off the atoms, resulting in the ionization of the gas and a sudden increase in the conductivity of the gas. Without limiting resistors connected in series to the discharge tube, the current level increases very rapidly, resulting in either damage in the circuitry, or ceasing of the operation due to drained capacitor banks. 2. Normal Glow: After the ignition, if the current levels are controlledappropriately, the discharge may enter the normal glow stage. At this stage, the current level changes very much even with the slight changes in discharge voltage. This state is not suitable for plasma antenna operation, for the electron density of the plasma is prone to change very much with small fluctuations in the discharge voltage. 3. Abnormal Glow: After a certain current level, slope of the voltage-current characteristic curve increases. Voltage must be increased considerably to increase the current passing through the

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plasma. This stage is the most suitable for our continuous DC discharge plasma, for the electron density in the plasma is not affected very much from the fluctuations in the discharge voltage. 4. Break-Down: Conductivity of the plasma increases very suddenly and irreversibly after some electric field intensity specific to the setup. Once a discharge enters breakdown, the only way to establish normal operation is to extinguish and reignite the plasma. This mode of operation must be avoided, for the sudden and uncontrolled increase in current levels may damage the discharge circuitry. Current limiting resistor bank is a method of preventing breakdowns, and may provide circuit protection in case of breakdowns by discharging the capacitors in a controlled manner. 4.2.2.3. CAPACITOR BANKS The current requirement of the DC discharge is calculated in section 3.3.1. Considering the previous experiments [2-7], the voltage requirement of the system for maximum current level was deduced to be around 2000V. Using the commercially available 100F-400V capacitors, a capacitor bank with 3200V maximum voltage capability is assembled. Capacitor bank of the DC power supply has a capacitance of 150F, maximum voltage of around 3000VDC. 4.2.3. STRIATION EFFECT To be able to conduct microwave reflection experiments, plasma with sheet geometry is more suitable compared to plasma with cylindrical geometry. Magnetic fields and rod shaped electrodes are used to obtain plasma sheet in [2-7]. To obtain a simpler geometry and due to the available setup size, two glass plates are inserted into cylindrical chamber on each side of rod shaped electrodes. The glass plates prevented the plasma from diffusing into the cylindrical chamber, limiting the plasma in a sheet shaped geometry of 1cm thickness. The decreased chamber cross section resulted in a phenomenon called striation Effect. The voltage required to obtain the desired current density increased significantly due to the increased rate of particle collision with the chamber walls. Therefore plasma electron densities required for microwave reflection could not be reached. The initial plasma tube with glass plates, striations in the plasma are visible in this figure. The striation phenomenon limits the pressure range that can be used for a specific chamber size. Striations can be observed in long chambers with narrow cross sections and low plasmapressures. Thus, chamber dimensions must be chosen accordingly.

4.3. MICROWAVE EXPERIMENTS ON DC DISCHARGE PLASMA

The most significant parameter that governs the interaction of plasma medium and electromagnetic waves is the plasma frequency. The relation between the plasma frequency and wave frequency determines the behavior of plasma medium. The aim of microwave experiments on the plasma medium is to demonstrate the effect of plasma frequency.

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For this aim, two experiments have been conducted. The first experiment was aimed to show that plasma becomes opaque for the microwave frequencies below the plasma frequency, while the objective of second experiment was to demonstrate the microwave reflection from a plasma column. In literature it has been found out experimentally that while microwave cut-off can be obtained by plasma with frequency equal to that of microwave, reflection quality equivalent to a metallic sheet required a plasma frequency of at least 2 times higher than wave frequency.

4.3.1. MICROWAVE CUT-OFF EXPERIMENTS

The setup for microwave cut-off experiments consisted of two horn antennas of appropriate frequency range facing each other, with the cylindrical discharge tube in between them. Transmission loss S21 between the two horns is measured using a sweep signal generator and a network analyzer The calculated plasma frequency for a current density of 250A/m2 is about 9 GHz. The results of the experiments show very significant transmission loss at low frequencies, especially up to 4 GHz. Attenuation can be observed up to 8 GHz witless transmission loss for higher frequencies due to non-uniform plasma electro density. The plasma electron density has a radial gradient, with electron density decreasing towards the chamber walls. The decreasing plasma electron density in the radial direction results in smaller plasma region with enough electron density to cutoff the higher frequencies.

5. UNIQUE CHARACTERISTICS OF A PLASMA ANTENNA

One fundamental distinguishing feature of a plasma antenna is that the gas ionizing process can manipulate resistance. When deionized, the gas has infinite resistance and does not interact with RF radiation. When deionized the gas antenna will not backscatter radar waves (providing stealth) and will not absorb high-power microwave radiation (reducing the effect of electronic warfare countermeasures).A second fundamental distinguishing feature is that after sending a pulse the plasma antenna can be deionized, eliminating the ringing associated with traditional metal elements. Ringing and the associated noise of a metal antenna can severely limit capabilities in high frequency short pulse transmissions. In these applications, metal antennas are often accompanied by sophisticated computer signal processing. By reducing ringing and noise, we believe our plasma antenna provides increased accuracy and reduces computer signal processing requirements. These advantages are important in cutting edge applications for impulse radar and high-speed digital communications. Based on the results of development to date, plasma antenna technology has the following additional attributes: No antenna ringing provides an improved signal to noise ratio and reduces multipath signal distortion. Reduced radar cross section provides stealth due to the non-metallic elements.

Changes in the ion density can result in instantaneous changes in bandwidth over wide dynamic ranges.

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After the gas is ionized, the plasma antenna has virtually no noise floor. While in operation, a plasma antenna with a low ionization level can be decoupled from an adjacent high-frequency transmitter. A circular scan can be performed electronically with no moving parts at a higher speed than traditional mechanical antenna structures.

It has been mathematically illustrated that by selecting the gases and changing ion density that the electrical aperture (or apparent footprint) of a plasma antenna can be made to perform on par with a metal counterpart having a larger physical size.

Our plasma antenna can transmit and receive from the same aperture provided the frequencies are widely separated. Plasma resonance, impedance and electron charge density are all dynamically reconfigurable. Ionized gas antenna elements can be constructed and configured into an array that is dynamically reconfigurable for frequency, beamwidth, power, gain, polarization and directionality - on the fly.

A single dynamic antenna structure can use time multiplexing so that many RF subsystems can share one antenna resource reducing the number and size of antenna structures.

6. SPONSORED WORK
To date, plasma antenna technology has been studied and characterized by ASI Technology Corporation revealing several favorable attributes in connection with antennaapplications. The work was carried out in part through two ONR sponsored contracts. NCCOSC RDTE Division, San Diego, awarded contract N66001-97-M-1153 1 May 1997. The major objective of the program was to determine the noise levels associated with the use of gas plasma as a conductor for a transmitting and receiving antenna. Bothlaboratory and field-test measurements were conducted. The second contract N00014-98-C-0045 was a 6-month SBIR awarded by ONR on November 15, 1997. The majorobjective of this effort was to characterize the GP antenna for conductivity, ionizationbreakdowns, upper frequency limits, excitation and relaxation times, ignition mechanisms, temperatures and thermionic noise emissions and compare these results to areference folded copper wire monopole. The measured radiation patterns of the plasmaantenna compared very well with copper wire antennas. ASI Technology Corporation is under contract with General Dynamics Electric Boat Division and in conjunction with the Plasma Physics Laboratory at the University of Tennessee, an

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inflatable plasma antenna is being developed. This antenna is designed tooperate at 2.4 Ghz and would be mounted on the mast of an attack submarine. In additiona prototype plasma waveguide and plasma reflector has been designed and demonstratedto General Dynamics. The gas plasma antenna conducts electron current like a metal and hence can be made into an antenna but with distinct advantages.

7. The following technological concepts are important to plasma antennas:

1. Higher power - Increased power can be achieved in the plasma antenna than in the corresponding metal antenna because of lower Ohmic losses. Plasmas have a much widerrange of power capability than metals as evident from low powered plasma in fluorescentbulbs to extremely high-powered plasmas in the Princeton University experimental fusionreactors. In this range, a high-powered plasma antenna is still low powered plasma. Sinceplasmas do not melt, the plasma antennas can provide heat and fire resistance. The higherachievable power and directivity of the plasma antenna can enhance target discriminationand track ballistic missiles at the S and X band. 2. Enhanced bandwidth - By the use of electrodes or lasers the plasma density can be controlled. The theoretical calculations on the controlled variation of plasma density I space and time suggest that greater bandwidth of the plasma antenna can be achieved than the corresponding metal antenna of the same geometry. This enhanced bandwidth can improve discrimination. 3. EMI/ECI - The plasma antenna is transparent to incoming electromagnetic signals inthe low density or turned off mode. This eliminates or diminishes EMI/ECI thereby producing stealth. Several plasma antennas can have their electron densities adjusted sothat they can operate in close proximity and one antenna can operate invisible toothers.In this physical arrangement mutual side lobe and back lobe clutter is highly reduced andhence jamming and clutter is reduced. 4. Higher efficiency and gain - Radiation efficiency in the plasma antenna is higher due to lower Ohmic losses in the plasma. Standing wave efficiency is higher because phase conjugate matching with the antenna feeds can be achieved by adjusting the plasmadensity and can be maintained during reconfiguration. Estimates indicate a 20dbimprovement in antenna efficiency. 5. Reconfiguration and multi-functionality - The plasma antenna can be reconfigured onthe fly by controlled variation of the plasma density in space and time with far moreversatility than any arrangement of metal antennas. This reduces the number of requiredelements reducing size and weight of shipboard antennas. One option is to constructcontrolled density plasma blankets around plasma antennas thereby creating windows(low-density sections of the blanket) for main lobe
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transmission or reception and closing windows (high-density regions in the plasma blanket). blanket). The plasma windowing effectenhances directivity and gain in a single plasma antenna element so that an array will have less elements than a corresponding metal antenna array. Closing plasma windows where back lobes and side lobes exist eliminates them and reduces jamming and clutter. This sidelobe reduction below 40db enhances directivity and discrimination. In addition,by changing plasma densities, a single antenna can operate at one bandwidth (e.g. communication) while suppressing another bandwidth (e.g. radar). 6. Lower noise - The plasma antenna has a lower collision rate among its charge carriers than a metal antenna and calculations show that this means less noise. 7. Perfect reflector - When the plasma density is high the plasma becomes a loss-less perfect reflector. Hence there exist the possibilities of a wide range of lightweight plasma reflector antennas.

8. MARKET APPLICATIONS OF PLASMA TECHNOLOGY

Plasma antennas offer distinct advantages and can compete with most metal antenna applications. The plasma antenna's advantages over conventional metal elementsare most obvious in military applications where stealth and electronic warfare are primaryconcerns. Other important military factors are weight, size and the ability to reconfigure.

Potential military applications include:

Shipboard /submarine antenna replacements. Unmanned air vehicle sensor antennas. ("identification friend or foe") land-based vehicle antennas. Stealth aircraft antenna replacements. Broad band jamming equipment including for spread-spectrum emitters. ECM (electronic counter-measure) antennas. Phased array element replacements. EMI/ECI mitigation Detection and tracking of ballistic missiles Side and back lobe reduction Military antenna installations can be quite sophisticated and just the antenna portion of a communications or radar installation on a ship or submarine can cost in the millions of dollars. Plasma antenna technology has commercial applications in telemetry, broad-band communications, ground penetrating radar, navigation, weather radar, wind shear detection and collision avoidance, high-speed data (for example Internet) communication spread spectrum communication, and cellular radiation protection.

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9. ADVANTAGES:
The advantage of a plasma antenna is that it can appear and disappear in a few millions of a second. This means that when the antenna is not required, it can be made to disappear, leaving behind the gas filled column that has little effect on the electromagnetic fields in the proximity of the tube. The same will be true for fiber glass consideration. and plastic tubes, which are also under

The other advantage of plasma antenna is that even when they are ionized and in use at the lower end of the radio spectrum, say HF communications, they are still near transparent to fields at microwave frequencies. The same effect is observed with the use of ionosphere, which is plasma. Every night amateur radio operators bounce their signals off the ionosphere to achieve long distance communications, whilst microwave satellite communication signals pass throughthe ionosphere.

10. CONCLUSION
As part of a blue skies research program, DSTO has teamed up with the ANUs Plasma Research Laboratory to investigate the possibility of using plasmas like those generated in fluorescent ceiling lights, for antennas. The research may one day have far reaching applications from robust military antennas through to greatly improved external television aerials. Antennas constructed of metal can be big and bulky, and are normally fixed in place. The fact that metal structures cannot be easily moved when not in use limits some aspects of antenna array design. It can also pose problems when there is a requirement to locate many antennas in a confined area. Weapons System Division has been studying the concept of using plasma columns for antennas, and has begun working in collaboration with ANU plasma physicists Professor Jeffrey Harris and Dr. Gerard Borg. Work by the team has already led to a provisional patent and has generated much scientific interest as it is so novel. It offers a paradigm shift in the way we look at antennas and is already providing the opportunity to create many new and original antenna designs. Plasma is an ionized gas and can be formed by subjecting a gas to strong electric or magnetic fields. The yellow lights in streets are a good example of plasmas though a better example is the fluorescent tubes commonly used for lighting in homes. The type of plasma antenna under investigation is constructed using a hollow glass column which is filled with an inert gas. This can be ionized by the application of a strong RF field at

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the base of the column. Once energized, the plasma column can be made to exhibit many of the same characteristics of a metal whip antenna of the type mounted on most cars. The metal whips that may be considered for a plasma replacement are anywhere from a few centimetres to several metres long. There are many potential advantages of plasma antennas, and DSTO and ANU are now investigating the commercialization of the technology. Plasma antenna technology offers the possibility of building completely novel antenna arrays, as well as radiation pattern control and lobe steering mechanisms that have not been possible before. To date, the research has produced many novel antennas using standard fluorescent tubes and these have been characterized and compare favorably with their metal equivalents. For example, a 160 MHz communications link was demonstrated using plasma antennas for both base and mobile stations. Current research is working towards a robust plasma antenna for field demonstration to Defence Force personnel.

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11. REFERENCES:
1. noel.martin@dsto.defence.gov.au 2. Science festival Stories Based on a Presentation by Dr. Martin at the 1999 Science Festival 3. http://rsphysse.anu.edu.au

Plasma Antenna