低温等离子体天线技术

Cool Plasma Antenna Technology


   
天线是所有射频通讯系统的重要部分。射频信号的收发从一般的广播到各种复杂的武器系统,其共同点都是使用一定尺寸的金属导体在所选用的频率上辐射或接收电磁波。由于金属天线一旦设计好,在结构上难以改变,在实际使用中受到环境、匹配等因数的影响其阻抗匹配等参数不容易达到比较理想的状态,即降低了金属天线的效率。
    等离子天线技术不采用传统的金属设计,而采用气体等离子体天线技术。这种崭新的天线设计技术在军事和民用上具有很大的发展潜力。相对于传统天线,等离子天线具有无可比拟的优点。是一种全新的概念,在诸多军事和民用领域都有良好的应用前景。由于等离子天线使用被电离的气体作为电磁能量传导介质。通过控制等离子体的形态和强度等参数可以对天线带宽、频率、增益和方向性等特性进行动态的重构。用等离子天线技术所设计的天线与传统的天线相比,其效率将更高、重量更轻、体积更小、尺寸更短、带宽更宽。由于等离子天线是气体形态,在外型和流体力学方面更具隐蔽性。西方很多国家都开展了等离子天线军事方面的研究,其重要科研及应用价值不言而喻。
    这种新型天线设计概念与传统的金属天线相比具有许多独特的优点,主要包括:①隐形性。当除去电离状态后,等离子体天线将不会产生后向散射雷达波,也不会吸收可降低电子对抗效能的高功率微波辐射。②适应于多种信号。等离子体天线具有可动态重构的特性,如带宽、频率、增益和指向性。③便于远程部署。等离子体天线可以比常规天线设计更轻、体积更小。④效率更高。等离子体天线很好地降低冲击激励效应,从而提高了短脉冲雷达的性能。这些独特的优点将使等离子体天线技术具有广阔的应用前景,如用于海军水面舰与潜艇雷达天线、隐形飞机雷达天线和弹道导弹防御雷达天线等。


What is Plasma?

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 which is not visible. Important to ASI's technology, plasmas are conductive assemblies of charged and neutral particles and fields that exhibit collective effects. Plasmas carry electrical currents and generate magnetic fields.

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 general broadcast of radio frequency signals for public use to complex weapon systems. In its most common form, an antenna represents a conducting metal surface that is sized to emit radiation at one or more selected frequencies. Antennas must be efficient so the maximum amount of signal strength is expended in the propogated wave and not wasted in antenna reflection.

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 the gas is not ionized, the antenna element ceases to exist. This is a fundamental change from traditional antenna design that generally employs solid metal wires as the conducting element. We believe our plasma antenna offers numerous advantages including stealth for military applications and higher digital performance in commercial applications. We also believe our technology can compete in many metal antenna applications. Our initial efforts have focused on military markets. General Dynamics' Electric Boat Corporation sponsored over $160,000 of development in 2000 accounting for substantially all of our revenues.

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 transmission and/or modulation technique: continuous wave (CW), phase modulation, impulse, AM, FM, chirp, spread spectrum or other digital techniques. And the plasma antenna can be used over a large frequency range up to 20GHz and employ a wide variety of gases (for example neon, argon, helium, krypton, mercury vapor and zenon). The same is true as to its value as a receive antenna.

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 elements are most obvious in military applications where stealth and electronic warfare are primary concerns. 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.
· IFF ("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 backlobe 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.

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.
· 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.
Sponsored Work

To date, plasma antenna technology has been studied and characterized by ASI Technology Corporation revealing several favorable attributes in connection with antenna applications. 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. Both laboratory 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 major objective of this effort was to characterize the GP antenna for conductivity, ionization breakdowns, upper frequency limits, excitation and relaxation times, ignition mechanisms, temperatures and thermionic noise emissions and compare these results to a reference folded copper wire monopole. The measured radiation patterns of the plasma antenna 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 inflatable plasma antenna is being developed. This antenna is designed to operate at 2.4 Ghz and would be mounted on the mast of an attack submarine. In addition a prototype plasma waveguide and plasma reflector has been designed and demonstrated to General Dynamics.

The following discussion illustrates why there is military and government support for plasma antenna concepts. The gas plasma antenna conducts electron current like a metal and hence can be made into an antenna but with distinct advantages. 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 wider range of power capability than metals as evident from low powered plasma in fluorescent bulbs to extremely high-powered plasmas in the Princeton University experimental fusion reactors. In this range, a high-powered plasma antenna is still low powered plasma. Since plasmas do not melt, the plasma antennas can provide heat and fire resistance. The higher achievable power and directivity of the plasma antenna can enhance target discrimination and 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 in 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 in the low density or turned off mode. This eliminates or diminishes EMI/ECI thereby producing stealth. Several plasma antennas can have their electron densities adjusted so that they can operate in close proximity and one antenna can operate invisible to others. In this physical arrangement mutual side lobe and back lobe clutter is highly reduced and hence 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 plasma density and can be maintained during reconfiguration. Estimates indicate a 20db improvement in antenna efficiency.

5. Reconfiguration and mutifunctionality - The plasma antenna can be reconfigured on the fly by controlled variation of the plasma density in space and time with far more versatility than any arrangement of metal antennas. This reduces the number of required elements reducing size and weight of shipboard antennas. One option is to construct controlled density plasma blankets around plasma antennas thereby creating windows (low-density sections of the blanket) for main lobe transmission or reception and closing windows (high-density regions in the plasma blanket). The plasma windowing effect enhances 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.