| Plasma is composed of positive and negative charged particles or ions, different types of charged particles are independent of each other and the sum of positive charges is equal to that of negative charges, so in general it is neutral. When the frequency of electromagnetic(EM) wave is much lower than that of plasma, plasma will exhibit the conductive properties similar to metal, and then the plasma column radiating EM energy can be used to replace the metal antenna. When the source removed, the plasma antenna becomes a dielectric tube filled with inert gas, and it is easy to avoid enemy radar detection, which is one of the most important characteristics of plasma antenna. However,in some time, the plasma antenna is working when it is scanned by enemy radar EM waves in certain frequency bands, the can scattering EM waves will be caused by the plasma column, so scattering is a parameter of plasma antenna which should be paid attention to. At the same time, EM waves in certain frequency bands can be absorbed and scattered by plasma, thus metal column target covered with plasma is a stealth one. The application of the plasma column in EM radiation and scattering problems are studied in the paper.At present, much work concerning the plasma antenna has been reported, but there is little knowledge of depth analysis on radiation mechanism of plasma antenna. So the substantial problem for the radiation of a plasma column antenna is investigated. Due to physics characteristics of plasma changes with the change of signal frequency, it is found that there are two operation modes(guided and leaky modes) which could be used as radiation mode exist in different signal frequency bands for a given plasma frequency. The cutoff conditions of the two modes are presented by formula derivation and theoretical analysis for the plasma column, respectively. A plasma column works as guided mode and radiates EM power as a metal antenna in the low frequency, and it may operate as leaky mode and be used as a plasma column leaky wave antenna in the high frequency. The far field of leaky mode is derived from Schelkunoff’s equivalence principle. Radiation patterns of two modes are given, and indicate that the pattern could be reconfigured conveniently by controlling plasma density due to the nature of plasma. Impedance is an important step in the design of plasma column antenna. In this paper, the internal impedance is analyzed through building a model of a plasma cylinder with EM wave propagating in it, the radiation resistance of plasma antenna are analyzed by the method of Poynting vector. The results indicate that the internal resistance could be reduced and the radiation resistance be increased efficiently by adjusting the plasma density and the collision frequency properly, accordingly the efficiency of the antenna is improved too. Through the establishment of a model for antennas array involving two center-fed parallel plasma columns, and based on the equivalent circuit theory, the mutual impedance between plasma antennas is calculated for the uniform and non-uniform distribution of plasma column, and the data shows that the mutual impedance between two plasma antennas is depend on distance between them and the plasma parameters. A planar plasma sheet antenna fed by microstrip line is analyzed. It is found that the resonance frequency of the antenna increases along with the increase of plasma frequency. Then the planar plasma antenna is conformal to a cylindrical surface, simulation results indicate that the resonance frequency of conformal antenna offsets and increases as the radius of conformal cylinder increases, the radiation diagram of plasma conformal antenna is also given. Based on this,the radiation performances of metallic patch antenna with parasitic coupling plasma sheets are investigated. Using shifted parasitically coupled plasma sheets, the bandwidth of metallic patch antenna can be effectively increased, and plasma frequency also has an impact on it.When designing a plasma antenna, the scattering of plasma column must be considered. In the paper, a finite difference time domain(FDTD) simulation model of plasma antenna bounded by infinite square dielectric column is developed, and the scattering characteristics of the plasma antenna are analyzed with the help of the model, it is found that the radar cross section(RCS) of plasma antenna can be effectively reduced by choosing reasonable dielectric constant of tube and the related parameters of plasma. A model for conducting cylinder coated with an un-magnetized eccentric plasma cylindrical layer is be set up, and its scattering cross section is mainly studied by making use of the coordinate transformation and the superposition of cylindrical wave functions. The results show that RCS is impacted evidently by the distance between the two eccentric axes when the radius of plasma surrounding the conducting cylinder remains unchanged. And with increase of electron density, the backscattering cross section of the target cylinder became smaller. At same time, comparing with the coaxial model, the backscattering cross section of the eccentric model has a smaller value in a wide frequency band. It is very useful for the study of plasma stealth technology. In a word, to reduce RCS within a certain frequency bands and realize target stealth, there must be an overall optimization for plasma parameters and geometric parameters of the eccentric model. |
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