Plasma Antennas: Survey of Techniques and the Current State of the Art
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NPS-CRC-03-001 MONTEREY, CALIFORNIA Plasma Antennas: Survey of Techniques and the Current State of the Art by D. C. Jenn September 29, 2003 Approved for public release; distribution is unlimited. Prepared for: SPAWAR PMW 189 San Diego, CA REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instruction, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188) Washington DC 20503. 1. AGENCY USE ONLY (Leave 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED blank) September 29, 2003 Technical Report (July 2003 to September 2003) 4. TITLE AND SUBTITLE: 5. FUNDING NUMBERS Plasma Antennas: Survey of Techniques and Current State of the Art 6. AUTHOR(S) David C. Jenn 7. PERFORMING ORGANIZATION NAME(S) AND 8. PERFORMING ADDRESS(ES) ORGANIZATION REPORT Naval Postgraduate School NUMBER Monterey, CA 93943-5000 NPS-CRC-03-001 9. SPONSORING / MONITORING AGENCY NAME(S) AND 10. SPONSORING / MONITORING ADDRESS(ES) AGENCY REPORT NUMBER SPAWAR PMW 189 11. SUPPLEMENTARY NOTES The views expressed in this thesis are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. 12a. DISTRIBUTION / AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE Approved for public release, distribution unlimited A 13. ABSTRACT Plasma antennas refer to a wide variety of antenna concepts that incorporate some use of an ionized medium. This study summarizes the basic theory behind the operation of plasma antennas based on a survey of patents and technical publications. Methods of exciting and confining plasmas are discussed, and the current state of the art in plasma technology is examined. 14. SUBJECT TERMS 15. NUMBER OF PAGES plasmas, plasma antennas 27 16. PRICE CODE 17. SECURITY 18. SECURITY 19. SECURITY 20. LIMITATION OF CLASSIFICATION OF CLASSIFICATION OF THIS CLASSIFICATION OF ABSTRACT REPORT PAGE ABSTRACT Unclassified Unclassified Unclassified UL NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. 239-18 NAVAL POSTGRADUATE SCHOOL Monterey, California Rear Admiral D. R. Ellison R. Elster Superintendent Provost This report was sponsored and funded by SPAWAR PMW 189. Approved for public release; distribution unlimited This report was prepared by: _________________________ DAVID C. JENN Professor Department of Electrical and Computer Engineering Reviewed by: Released by: __________________________ _____________________________ HERSCHEL LOOMIS LEONARD FERRARI Chairman Associate Provost and Cryptologic Research Center Dean of Research TABLE OF CONTENTS 1.0 Introduction 1 2.0 Fundamental Plasma Theory 2 3.0 Plasma Generation and Containment 7 4.0 Antenna and Transmission Line Applications 10 4.1 Plasma Mirrors (Reflectors) and Lenses 10 4.2 Linear and Loop Antennas With Plasma Enclosures 13 4.2.1 Ionization Using Electrodes 13 4.2.2 Ionization Using an Electromagnetic Field 15 4.3 Linear Antennas and Transmission Lines by Ionizing the Atmosphere 16 4.4 Plasma Radiation 17 5.0 Microwave Devices 19 5.1 Filters and Phase Shifters 19 5.2 Microwave Tubes 20 6.0 Summary 20 7.0 References and Bibliography 21 7.1 References 21 7.2 Bibliography 24 7.2.1 Miscellaneous Journal Papers 24 7.2.2 Additional Patents 24 Initial Distribution List 27 (THIS PAGE INTENTIONALLY LEFT BLANK) 1.0 Introduction The term plasma antenna has been applied to a wide variety of antenna concepts that incorporate some use of an ionized medium. In the vast majority of approaches, the plasma, or ionized volume, simply replaces a solid conductor. A highly ionized plasma is essentially a good conductor, and therefore plasma filaments can serve as transmission line elements for guiding waves, or antenna surfaces for radiation. The concept is not new. A patent entitled “Aerial Conductor for Wireless Signaling and Other Purposes” was awarded to J. Hettinger in 1919 (Figure 1). Figure 1: Diagram from J. Hettinger’s 1919 patent [From 1]. 1 The advantages of such an approach are numerous. For example, the length of an ionized filament can be changed rapidly, thereby “re-tuning” the antenna to a new frequency1. The antenna can be “turned off” to make it electrically invisible for the purpose of reducing its scattering signature and eliminating its coupling and interference with other nearby antennas. On the other hand, the use of plasma adds complexity to the antenna design. Equipment for establishing and maintaining the ionization must be provided. There is a glow to the plasma that increases its visible signature, and plasma decay generates noise. The ionized volume can take a variety of forms. It can be established in air at atmospheric pressure by using lasers, high power microwave beams, or ultraviolet rays. A plasma might also be generated from a gas filled tube containing a noble gas like neon or argon. Methods that use a tube require less energy to excite and maintain the plasma state, because the gas is pure and the presence of the tube prevents dissipation. The use of a tube requires that it be protected from the environment, which increases the antenna weight and volume, and makes the antenna less durable. This report describes the basic underlying plasma theory, examines methods of exciting and confining plasmas, and summarizes antenna concepts that incorporate plasmas. 2.0 Fundamental Plasma Theory A plasma can be generated from neutral molecules that are separated into negative electrons and positive ions by an ionization process (e.g., laser heating or spark discharge). The positive ions and neutral particles are much heavier than the electrons, and therefore the electrons can be considered as moving through a continuous stationary fluid of ions and neutrals with some viscous friction. Furthermore, the propagation characteristics of electromagnetic (EM) waves in a uniform ionized medium can be inferred from the equation of motion of a single “typical” electron. Such a medium is called a “cold plasma.” This model would be rigorous if the ionized medium was comprised entirely of electrons that do not interact with the background particles (neutrals and ions) and posses thermal speeds that are negligible with respect to the phase velocity of the EM wave. 1 In recent years, antennas with the ability to change their radiation characteristics by modifying their physical or electrical configuration have been called “re-configurable antennas.” 2 In the absence of a magnetic field, the important parameters for a cold plasma are the 3 3 electron density Ne electrons/m and the collision frequency n /m . The complex relative dielectric constant of the plasma is given by [2-5] 2 2 X w p er = er¢ - jer¢¢ = n =1- =1- (1) (1- jZ) w(w - jn ) 2 N e2 æw ö e ç p ÷ where n = e r is the index of refraction, w p = is the plasma frequency, X = ç ÷ , meo è w ø n Z = , and w w = 2p f radians/sec, angular frequency m = 9.0´10-31 kg, electron mass -19 e = 1.59´10 C, electron charge -12 eo = 8.85´10 F/m, permittivity of free space Assuming a time harmonic wave with an e jw t time dependence, a x-polarized electromagnetic plane wave propagating in the +z direction has the form r -g z E(z) = xˆEoe where g is the conventionally defined propagation constant. The real and imaginary parts of the propagation constant are the attenuation and phase constants, respectively, g º a + jb = jko mr e r (2) -12 where ko = w moeo , mo = 4p ´10 H/m is the permeability of free space, and for the plasmas considered here mr =1. 3 For the special case of negligible collisions, n » 0 , the corresponding propagation constant is 2 w p g = jko 1- = jko 1- X (3) w 2 There are three special cases of interest: - jb z 1. w >w p : g is imaginary and e is a propagating wave -az 2. w < w p : g is real and e is an evanescent wave 3. w =w p : g = 0 and this value of w is called the critical frequency, wc which defines the boundary between propagation and attenuation of the EM wave. The intrinsic impedance of the plasma medium is m h = o (4) eo (e ¢- je¢¢) Figure 2 shows the magnitude of the reflection coefficient at an infinite plane boundary between plasma and free space, which is given by the formula h -h G = o (5) h +ho The impedance of free space is ho = 377 ohms. From the figure it is evident that at frequencies below the plasma frequency, the plasma is a good reflector. 4 0 -10 -20 -30 -40 (|R|) 0 1 -50 20*log -60 -70 -80 -90 -100 0 1 2 3 10 10 10 10 Frequency, MHz Figure 2: Reflection coefficient for a plane wave normally incident on a sharp plasma/air 12 3 boundary ( Ne =´110 /m , n = 0 , dashed line is the plasma frequency, fp = 8.9 MHz). EM waves below the plasma frequency (w < w p ) are attenuated at a rate determined by the attenuation constant -az E(z) ~ e = exp(- zko X -1). (6) The loss in decibels per meter (dB/m) is 20log 10 {exp(- ko X -1)}. (7) Loss is plotted in Figure 3 for several electron densities. This shows that plasma can be a good absorber once the EM wave enters the plasma medium, a feature that has been exploited in the design of plasma radar absorbing material (RAM) for stealth applications [6].