Multiple Tube Plasma Antenna

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.

(1) Field of the Invention

The present invention relates generally to communications antennas, and more particularly to a plasma antenna for High Frequency (HF) communications.

(2) Description of the Prior Art

Current communication methods for underwater environments include the use of mast mounted antennas, towed buoys, and towed submersed arrays. While each of these methods has merits, each presents problems for use in an underwater environment. The mast of current underwater vehicles performs numerous sensing and optical functions. Mast mounted antenna systems occupy valuable space on the mast which could be used for other purposes. For both towed buoys and towed submersed arrays, speed must be decreased to operate the equipment.

Plasma antennas are of interest for communications with underwater vessels since the frequency, pattern and magnitude of the radiated signals are proportional to the rate at which the ions and electrons are displaced. The displacement and hence the radiated signal can be controlled by a number of factors including plasma density, tube geometry, gas type, current distribution, applied magnetic field and applied current. This allows the antenna to be physically small, in comparison with traditional antennas. Studies have been performed for characterizing electromagnetic wave propagation in plasmas. Therefore, the basic concepts, albeit for significantly different applications, have been investigated. These efforts have included a Corona Mode antenna that utilizes the corona discharges of a long wire to radiate ELF signals, a propane plasma antenna, and studies of electromagnetic propagation in plasmas. Other research has focused on characterizing the electromagnetic waves that exist in plasmas. In addition, U.S. Pat. No. 3,914,766 to Moore discloses a pulsating plasma antenna which has a cylindrical plasma column and a pair of field exciter members parallel to the column. The location and shape of the exciters, combined with the cylindrical configuration and natural resonant frequency of the plasma column, enhance the natural resonant frequency of the plasma column, enhance the energy transfer and stabilize the motion of the plasma so as to prevent unwanted oscillations and unwanted plasma waves from destroying the plasma confinement. However, as configured, the Moore antenna lacks the capability of being electronically steered and dynamically reconfigured. Such steering and reconfiguration would allow the antenna to be more efficient and operate in a wider band of frequencies. U.S. Pat. No. 5,594,456 to Noris et al. discloses an antenna device for transmitting a short pulse duration signal of predetermined radio frequency that includes a gas filled tube, a voltage source for developing an electrically conductive path along a length of the tube which corresponds to a resonant wavelength multiple of the predetermined radio frequency and a signal transmission source coupled to the tube which supplies the radio frequency signal. The antenna transmits the short pulse duration signal in a manner that eliminates a trailing antenna resonance signal. However, as with the Moore antenna, the band of frequencies at which the antenna operates is limited since the tube length is a function of the radiated signal.

Accordingly, it is an object of the present invention to provide a wideband, physically compact antenna capable of transmitting signals in a range from High Frequency (HF) to Super High Frequency (SHF).

Another object of the present invention is to provide an antenna that is electronically steerable and dynamically reconfigurable.

Still another object of the present invention is to provide an antenna which can be mounted within the mast structure of a submarine.

A further object of the present invention is to provide an antenna which can be generally formed into various shapes in order to conform to the structure to which it is attached.

Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.

In accordance with the present invention, an antenna is provided which utilizes ionized gas, or plasma, to propagate electromagnetic signals in the HF band. The gas is ionized using either lasers or electric potentials and the resulting plasma is confined within two or more coaxial tubes contained within a pressure vessel. Both the tubes and pressure vessel are non-metallic. External magnetic fields, temperature, or electric potentials are used to change the shape and directivity of the plasma to effect the gain and directivity of the antenna. Instrumentation measures the density of the plasma providing a means to measure incoming signals as well as to regulate the radiation frequency. The plasma antenna overcomes the frequency limitations of conventional antennas since the ion/electron movement within the plasma can be controlled by other than electromagnetic forces. This allows the plasma antenna to respond, i.e., radiate, signals at frequencies which do not require the frequency of the radiated signal to be a fractional part of an electromagnetic wavelength.

A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:

FIG. 1 is a representation of a plasma antenna of the present invention having a coaxial cylindrical tube design;

FIG. 2 shows a computed radiation pattern of a plasma antenna having a simple tube design;

FIG. 3 shows a computed radiation pattern of a plasma antenna of the present invention;

FIG. 4 is a schematic representation of an antenna system having a plasma antenna of the present invention; and

FIG. 5 is a cross section of an alternate embodiment of a plasma antenna of the present invention having a multi-chambered outer tube.

Referring now to FIG. 1, there is shown a representation of a plasma antenna 10 according to the present invention. Plasma antenna 10 is a coaxial design having an inner plasma tube 12 to radiate the intended signal and an outer plasma tube 14 which can be used as a dynamically reconfigurable reflector. Plasma tubes 12 and 14 are constructed using well known electron/plasma tube construction techniques and may be fabricated from a variety of inert glasses also well known in the art. Voltage driven electrodes 16 are used to modulate the plasma density of inner and outer tubes 12 and 14, thus creating plasma waves which in turn are converted to electromagnetic waves for radiating a signal. The conversion process is well known in the art and is described in various works, e.g., "Radiative Properties of a Plasma Moving Across a Magnetic Field 1. Theoretical Analysis," Roussel-Dupre', R. and Miller, R., Phys. Fluids B 5(4), April 1993. The electrodes vary the plasma density by increasing the number of ions and free electrons in the tube. The potential difference excites the electrons and allows them to move into an energy state that is sufficient to break free of the parent molecule (or atom), thus producing a free electron and ionized gas.

In addition to passing signals, plasma antennas are capable of reflecting electromagnetic signals. The reflection properties of plasma are also well known and described in the art, e.g., Principles of Plasma Physics, Krall, N., McGraw-Hill, 1973. The reflection/transmission property of plasma is relevant to the design of an antenna since the phenomena can be used as a reflector to re-direct a radar signal radiated by a driving antenna. Such a reflector is discussed in "Navy Research Lab Tests Plasma Antenna," Nordwall, B., Aviation Week and Space Technology, Jun. 10, 1996. Here, the plasma sheet could be steered electronically resulting in a fast, multi-functional, antenna reflector. Another potential application of the reflective/transmission properties of the plasma is the reduction of the radar cross section of an antenna. The plasma's transmission properties will reduce the radar cross section as long as the plasma antenna is operating at a frequency below that of search radars. For a number of vessels, the antenna mounting structure typically would reflect more radar signals than the actual antenna element such that the radar cross section reduction may not be significant. However, significant reductions may be obtained for submerged vessels where the antenna is mounted on top of a mast extending above the water surface, or is mounted as a conformal antenna on a radar transparent, or stealth, sail area of the submerged vessel. A reduced radar cross section is probably of greater importance for surface ships since these antennas tend to be relatively large and may contribute significantly to the radar cross section of the ship. Controlling the plasma density within the outer tube allows the outer tube to be used as a reflector to direct the radiation pattern of the inner tube and to reduce the radar cross section of the antenna.

FIG. 2 illustrates the computed radiation pattern for a 3.lambda., plasma line antenna, where .lambda. is the wavelength of interest. The current distribution for the radiation pattern consists of a cosine on a pedestal, i.e., there is an offset of the cosine wave near the electrodes. The resulting beam width of the plasma line is approximately 18 degrees and has distinct side lobes. FIG. 3 illustrates the computed radiation pattern for a 3.lambda. line antenna using the outer tube as a reflector that has an efficiency of 85%. As can be seen, the outer tube reflector will significantly reduce the back lobe and concentrates the energy towards the front of the antenna.

Referring now to FIG. 4, there is shown a schematic representation of a plasma antenna system 18 utilizing plasma antenna 10 of FIG. 1. Ionizer module 20 is responsible for creating and maintaining the ion concentration within plasma antenna 10. In the preferred embodiment of FIG. 1, energized electrodes 16 of FIG. 1 are used to ionize the plasma. However, ionization can be achieved by several methods including electric potential difference, photoionization by the use of lasers, RF heating, discharge and magnetic squeezing under magnetic confinement. The electrode method is the desired approach since it provides the greatest amount of flexibility, in terms of variability and controllability, and is the easiest and most efficient method to implement. Depending on the application, ionizer module 20 may have a number of power supplies 22 ranging from one to the number of electrodes 16. It is anticipated that a single supply would be sufficient for most applications. Ionizer module 20 also contains a series of attenuator networks 24 that allows each electrode 16 to receive a different voltage level. In the transmission mode, signal generator or transmission module 26 converts the transmission signals into a format suitable for plasma antenna 10 by modulating plasma frequencies as is well known in the art, such as by alternating the magnetic field in the antenna tube through a series of electromagnets or wire coils. In the receiving mode, receiver 28 measures signals arriving at plasma antenna 10, again using methods well known in the art, such as a sensing wire, or cathode follower, within the plasma. The operation of ionizer module 20, signal generator 26 receiver 28 are controlled and monitored by controller module 30. In a preferred embodiment, controller module 30 has a microprocessor for performing the control and monitoring functions, including controlling the attenuator paths and power supply levels.

Though the plasma antenna of the present invention has been shown as cylindrical, it will be understood that the tubes may have any cross section, e.g., square or rectangular. Such shapes will effect the radiation and reflectivity patterns for the antenna. While such tubes may be more difficult to fabricate in comparison to cylindrical tubes, the planar surfaces may allow finer control of plasma density. Referring now to FIG. 5, a cross section of an alternate embodiment of the antenna of FIG. 1 is shown. Outer tube 12 is divided into six separate chambers 12a though 12f and each chamber is provided with electrodes, 16a through 16f, respectively, to independently control the plasma densities within the chambers 12a-f. Such a configuration allows more precise control of the plasma density within outer tube 12 and thus greater control of the radiation pattern of antenna 10.

Thus, it will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.