Experimental and Theoretical Results with Plasma Antennas Igor Alexeff, Fellow, IEEE, Ted Anderson, Sriram Parameswaran, Eric P
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166 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 2, APRIL 2006 Experimental and Theoretical Results With Plasma Antennas Igor Alexeff, Fellow, IEEE, Ted Anderson, Sriram Parameswaran, Eric P. Pradeep, Student Member, IEEE, Jyothi Hulloli, and Prashant Hulloli Invited Paper Abstract—This report is a summary of an extensive research pro- passes through the same reflector. The idea is that a gram on plasma antennas. We have found that plasma antennas are plasma antenna can be so configured that a high-fre- just as effective as metal antennas. In addition, they can transmit, quency, electronic-warfare signal can pass through the receive, and reflect lower frequency signals while being transparent to higher frequency signals. When de-energized, they electrically antenna without appreciable interaction, while the an- disappear. Plasma noise does not appear to be a problem. tenna is transmitting and receiving signals at a lower frequency. Index Terms—Active antennas, antennas, plasma antennas, plasma devices. 6. Mechanical Robustness: We have developed two kinds of robust plasma antennas. In one design, the glass tubes comprising the plasma antenna are encapsulated in a di- I. INTRODUCTORY SUMMARY electric block. In a second design, the plasma antennas are E have had the following experimental demonstrations composed of flexible plastic tubes. We have found that the Wof plasma antennas. Most of these demonstrations are plasma does not damage the plastic tubes over periods of documented on videotape, and are available on request. several hours if the plastic tubes are kept cool. Heat, not 1. Transmission and Reception: We have demonstrated plasma, causes damage to plastic. Mechanical Reconfigurability transmission and reception of operating plasma antennas 7. : We have been able me- over a wide frequency range (500 MHz–20 GHz). The chanically to manipulate the operating plasma antenna surprising results were that the efficiencies are compa- composed of flexible plastic tubes. In particular, we have rable to a copper wire antenna of the same configuration, designed a plasma antenna that may be compressed and and the noise level seemed comparable with a wire an- stowed when not being used. Plasma Waveguides tenna. The noise measurements will be repeated with a 8. : We have demonstrated a coaxial precision noise meter. plasma waveguide. The advantage of such a waveguide is 2. Stealth: When de-energized, the plasma antenna reverts that it reverts to dielectric tubes when de-energized, and to a dielectric tube which has a small radar scattering cross does not have large RADAR cross section. Noise Reduction section. 9. : We have found that plasma-generated 3. Reconfigurability: At 3 GHz, we have demonstrated a noise is in general not a problem. However, to further parabolic plasma reflector. When energized, it reflects the improve the system, we have discovered several new radio signal. When de-energized, the radio signal passes methods of noise reduction. freely through it. 4. Shielding: The plasma reflector, when placed over a re- II. REVIEW OF PREVIOUS RESULTS ceiving horn and energized, prevents an unwanted 3-GHz The first phase of the plasma antenna project started with the signal from entering. When the antenna is de-energized, idea of a coaxial plasma closing switch, shown in Fig. 1. the signal passes through freely In this switch, the outer conductor was a metal shell, and the 5. Protection from electronic warfare: We have demon- inner conductor was a plasma discharge tube. When the tube strated that with a plasma reflector operating and re- was not energized, the outer shell comprised a metal wave- flecting a signal at 3 GHz, a signal at 20 GHz freely guide beyond cutoff, and no radiation was transmitted. When the plasma discharge tube was energized, the apparatus became Manuscript received August 31, 2005; revised January 24, 2006. a coaxial waveguide, and transmission of radio signals was ex- I. Alexeff, E. P. Pradeep, and J. Hulloli are with the University of Tennessee, Knoxville, TN 37996 USA. cellent. The work was done by W. L. Kang, as a thesis project, T. Anderson is with Haleakala Research and Development, Inc., Brookfield, and was presented at a scientific meeting. MA 01506 USA. The second phase of the research started when researchers S. Parameswaran is with Williams-Sonoma Inc., Memphis, TN 38118 USA. P. Hulloli is with Dell, Inc., West Chester, OH 45069 USA. at the Patriot Scientific Corporation, Carlsbad, CA, read of our Digital Object Identifier 10.1109/TPS.2006.872180 work, and called me in as a consultant. They had an ongoing 0093-3813/$20.00 © 2006 IEEE ALEXEFF et al.: EXPERIMENTAL AND THEORETICAL RESULTS WITH PLASMA ANTENNAS 167 Fig. 1. Coaxial plasma on switch. Fig. 3. Early plasma reflector. Fig. 4. Plasma antenna. Fig. 2. Early plasma antenna. well as the parabolic reflector shown in Fig. 3. With this appa- plasma antenna project, in which they wanted to use the dis- ratus, we demonstrated stealth, reconfigurability, and protection appearing feature of the plasma antenna to prevent ringing on from electronic warfare. signal turnoff. Their problem was poor plasma antenna trans- The fourth phase of research was done at the Malibu Re- mission and reception. A version of the first plasma antenna search Corporation, an antenna design facility in Camarillo, CA. is shown in Fig. 2. My investigation showed that under their We felt that precision measurements were required in a proper conditions of operation, the plasma antenna’s resistance was a facility. In Fig. 4, we show a plasma antenna installed in an megohm, and so did not match the 300 resistance of space. electrical anechoic chamber. Also shown is a metal antenna de- The solution was to pulse the plasma antenna to higher currents, signed to be an identical twin to the plasma antenna. The mi- as the plasma discharge has a resistance that decreases with in- crowaves are generated by a line antenna, focused in one di- creasing current. Under the proper conditions, we found that mension by the metal pillbox, and focused in the second dimen- the plasma antenna transmitted and absorbed radiation virtually sion by either the plasma antenna or a metal twin. The results identically to a metal antenna. In addition, the plasma-generated were remarkably successful, as shown in Fig. 5. First, when the noise appeared to be rather low. plasma antenna was on, the transmission efficiency was virtu- The third phase of research started at the University of Ten- ally identical to the metal antenna. Second, the radiation pattern nessee, Knoxville. The work was transferred to the ASI Tech- was also quite similar to the metal antenna. Third, the noise was nology Corporation, Henderson, NV. At the University of Ten- not particularly worse for the plasma antenna over the metal an- nessee, we constructed the plasma antenna shown in Fig. 2, as tenna. However, when the plasma antenna was de-energized, the 168 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 2, APRIL 2006 Fig. 7. Plasma waveguide. the mast of a ship, yet become transparent to radiation when de-energized. Fig. 5. Radiation pattern. Radar signals would pass through a de-energized waveguide rather than be reflected. In fact, these waveguides could pass in front of operating antennas and be virtually invisible when off. A third plasma antenna application is reconfigurability. The effects of a reconfigurable plasma filter are shown in Fig. 8. In one oscilloscope trace, we observe several spectral lines emitted from an oscillator driven to a nonlinear limit. In the second oscilloscope trace, several of the higher-frequency lines have been removed by the energizing of a plasma interference filter placed between the transmitter and receiver. IV. RECENT RESULTS We have made remarkable discovery in the operation of plasma antennas (patents pending). In the past, our plasma tubes were ionized by direct current (dc). However, if the tubes are ionized by extremely short bursts of dc, we find the Fig. 6. Embedded plasma antenna. following remarkable improvements. The plasma is produced in an extremely short time—2 s. However, the plasma persists reflected signal dropped by over 20 dB! In other words, the re- for a much longer time—1/100 second. This is the reason why flected signal dropped by over a factor of 100. fluorescent lamps can operate on 60 or 50 Hz electric power. For stealth projects, the first metal reflector could be incased Consequently, if the pulsing rate is increased to 1 kHz, the inside the body of a structure. However, this project is really a tubes are operating at essentially constant density. There are proof-of-principle, rather than a deployable system. three benefits to this new mode of operation. First, the exciting current is on for only 2 s, while it is off for III. OTHER PLASMA ANTENNA PROJECTS 1 ms. Consequently, the discharge current is only on for 0.2% of One of the criticisms directed at the plasma antenna is that it is the time, so current-driven instabilities are not present for most fragile. As a researcher from another company told us, he built of the time. However, the current-driven instabilities in general a glass plasma antenna, but it was no good, because it broke have proven to be not serious. when he installed it underneath an airplane. To make a robust In general, operating the plasma tubes in the noncurrent-car- plasma antenna, we imbedded one in an epoxy block, as shown rying, afterglow state should produce considerably less noise in Fig. 6. This imbedded antenna transmits and receives quite than in operating in the current-carrying state. The decrease in well, and has survived several years of hard treatment. plasma noise is obvious, but detailed measurements have been A second, antenna related, plasma application is a plasma deferred till later in the program.