The large radar cross section of wind turbine generator (WTG) blades combined with high tip speeds can produce significant Doppler returns when illuminated by a radar. Normally, an air traffic control radar system will filter out large returns from stationary targets, however the Doppler shifts introduced by the WTG are interpreted as moving aircraft that can confuse radar operators and compromise safety. A possible solution to this problem that we are investigating is to incorporate an active layer into the structure of the WTG blades that can be used to dynamically modulate the RCS of the blade return. The active blade can operate in one of two modes: firstly the blade RCS can be modulated to provide a Doppler return that is outside the detectable range of the radar receiver system so that it is rejected: a second mode of operation is to introduce specific coding on to the Doppler returns so that they may be uniquely identified and rejected. The active layer used in the system consists of a frequency selective surface controlled by semiconductor diodes and is a development of techniques that we have developed for active radar absorbers. Results of experimental work using a 10GHz Doppler radar and scale model WTG with active Doppler imparting blades are presented.
Conventional (i.e. passive) radar absorbers are widely used for modifying the radar cross-section (RCS) of current
military platforms but such absorbers may not have adequate performance to satisfy future requirements. Active absorbers, however, offer the potential to overcome the so-called Rozanov performance limit and to enable additional 'smart' functionality such as monitoring damage, adaptive control of RCS or target appearance, Identification-Friend-or-Foe (IFF) and Absorb-While-Scan (AWS) This paper outlines the concept and basic properties of a novel type of
active radar absorber, the so-called Phase-Switched Screen (PSS). The basic PSS topology is then modified so as to enable it to operate as a smart radar absorber when used together with an external sensor and feedback control loop. The theoretical predictions are confirmed using data measured on transmission-line analogues of the smart PSS
structure.
Composite materials comprising microparticles of the environmentally stable conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), a transition metal/metal salt redox couple in a solid polymer electrolyte matrix have been prepared and characterised. These materials show rapid and reversible changes in their DC and microwave impedances when small DC or AC fields are applied across them from the edges. The composites may be switched from a high impedance state to a low impedance state with the imposition of hte fields for more t han one thousand switching operations with little or no deterioration in performace. When the fields are removed, the initial high impedance state is restored. The extent of the change is very dependent on the choice of redox pair and also on the composition of the polymer electrolyte phase. Copper has been shown to give the largest changes in microwave impedance from 750Ω at 0V to 5Ω at 5V. In this paper, we present a series of waveguide results for composites containing 26wt% PEDOT together with a comparison with other conducting polymer composites, the effect of redox couple on the extent of switching and a proposed mechanism for the switching process.
A brief description of the theory of passive and active absorbers is presented followed by details of an experimental study into a new design of adaptive absorber. The absorber is a single-layer planar structure based upon the topology of a Salisbury screen, but in which the conventional resistive layer is replaced by an active frequency selective surface (FSS) controlled by pin diodes. The resulting structure has superior reflectivity-bandwidth characteristics compared to conventional passive absorbers of corresponding thickness. Experimental results are presented and compared to those obtained from a transmission line model, and show that the reflectivity response of the absorber can be dynamically controlled over the frequency band from 9-13GHz
Composite materials comprising the environmentally stable conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), a transition metal/metal salt redox couple in a solid polymer electrolyte matrix have been prepared and characterised. DC and microwave measurements on these materials have shown rapid and reversible changes in their impedances and microwave transmission. The composites may be switched from a high impedance state to a low impedance state for several hundred switching operations with no deterioration in performance when small DC or low frequency AC fields are applied across coaxial discs of the materials from the edges. When the fields are removed, the initial high impedance state is restored. The extent of the change is very dependent on the choice of redox pair and also on the composition of the polymer electrolyte phase. Copper has been shown to give the largest changes in microwave impedance from 185Ω(0V) to 11Ω (5V) at 300MHz. In this paper, we present a series of results for composites containing 26wt% by mass of PEDOT with several transition metal redox couples. The results of lithium tetrafluoroborate concentration in the polymer electrolyte phase on the microwave transmission loss of the PEDOT composite are discussed. The effect of copper metal concentration on the magnitude of the impedance change is also presented along with a proposed mechanism for the switching process.
Discs and waveguide samples of polymeric mixed conductor nanocomposite materials comprising a conducting polymer and redox active switching agent in a polymer electrolyte have been prepared and studied. These novel materials have been shown to exhibit large, rapid and reversible changes in their microwave impedance when small d.c. electric fields are applied across them from the edges. The results of simultaneous cyclic voltammetry or potential square waves and microwave transmission measurements have shown that the changes are apparantly instantaneous with the application or removal of the applied field. Analysis of the microwave results has shown that the impedance of the materials changes by a factor of up to almost 50 with the imposition or removal of the fields. Nanocomposite materials having either poly(pyrrole) or poly(aniline) as the conducting polymer component and either silver/silver tetrafluoroborate or copper/copper(II) tetrafluoroborate as the redox active components have been investigated. The results of the nanocomposite materials are compared with those of microparticulate composities of similar composition. A new configuration of single layer tunable microwave absorber using only resistive control has been investigated and shown to exhibit wideband, low reflectivity performance combined with reduced thickness. A major advantage of the new topology is the requirement for only a 3:1 change in controllable resistance.
Samples of poly(aniline)-silver-polymer electrolyte particulate composites have been characterized at microwave frequencies when small d.c. electric fields are applied across them in both coaxial line and waveguide measurement test sets. The experimental data shows that the initial conductivity of the materials is dependent on the concentration of sliver metal and suggest that changes in resistance due to chemical switching take place, at least in part, in the manufacture of the composites. When silver is used as the electrodes, the experimental data show that changes in the slope of the cyclic voltammograms coincide with large changes in microwave reflectivity or transmission consistent with increasing conductivity of the composites when fields are applied. The reverse change occurs when the fields are removed. Measurements have shown that the composites are able to switch between the two impedance stats in times of less than one second for well over a million cycles with no apparent depreciation in material properties. Large area films have also been prepared and studied using the 'free space' technique.
Advances in conducting polymer composite materials are raising the prospects of realizing large area dynamically controllable microwave smart surfaces. Candidate structures for these are proposed and their characteristics are analyzed, with due regard to the problems of system integration. The discussion is complemented by modelled and measured performance data.
Discs of polyaniline-silver-polymer electrolyte composites exhibit rapid and reversible changes in their microwave impedance when small electric fields are applied across then in a resonant coaxial line test set. The experimental data show that the initial conductivity of the materials is dependent on the concentration of silver metal and suggests that changes in resistance due to chemical switching take plane, at least in part, in the manufacture of the composites. The experimental data show that changes in the gradient of the cyclic voltammograms coincide with large changes in microwave reflectivity consistent with increasing conductivity of the composite when fields are applied. The reverse change occurs when the fields are removed. Measurements of the switching speed have shown that the composites are able to switch between the different states at in times of less than a second for more than one million switching operations with no depreciation of the material. Large area films have also been studied in the front of waveguide devices and measured in a microwave transmission mode. The results show that large changes in microwave impedance occur with the application of small electric fields (~ 15 V cm-1).
The fabrication of conducting polymer - silver-polymer electrolyte composite materials is described. Discs of these materials mounted in a coaxial test fixture exhibit rapid and reversible changes in their microwave impedance when small electric fields are applied across them. The effect of the concentration of conducting polymer on the cyclic voltammetry and microwave characteristics of the composites is discussed. Comparison of the cyclic voltammetry and microwave results has shown that changes in the gradients of the cyclic voltammograms coincide with large changes in microwave reflectivity. The results are consistent with the conducting polymer being switched from an insulating to a conducting state when the fields are applied. The reverse change occurs when the field is removed. Microwave coaxial line measurements for annular samples are presented and an equivalent network model comprising a parallel resistor and capacitor has been fitted to the measured data. Scanning electron microscopy studies on both the cycled and uncycled composites are presented and the results suggest that during cycling, the silver metal dissolves and is then re- precipitated.
Conventional (i.e. passive) radar absorbing materials (RAM) have been in use now for over 50 years, but it is only with recent advances in conducting polymer composite materials that active RAM has become practicable. Techniques for utilizing these new materials in reconfigurable electromagnetic, or 'smart,' surfaces are reviewed, with due emphasis being given to the problem of system integration. The discussion is complemented by modelled and measured performance data on several smart surface configurations.
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