In this study, a geometric transition radar absorbing material loaded with a binary frequency selective surface (FSS) is presented. The FSS is designed to improve the absorber's reflectivity at low frequencies. A genetic algorithm (GA) was used to optimise the binary FSS pattern and its position within the absorber. The binary FSS loaded absorber was optimised for performance for both single- and dual-polarisation schemes and was subsequently evaluated against a previously published square conductive loop FSS loaded absorber. An image processing technique was utilised to assess the sensitivity of the FSS design to variations of small geometric features.

This paper presents a topology for improving the low frequency performance of pyramidal radar absorbing materials at oblique angles of incidence. The technique uses an embedded frequency selective surface as an impedance matching layer and improves the −40dB reflectivity limit from 6GHz to 4GHz for 102mm pyramidal absorber. The frequency selective surface is based on a resistively loaded dipole FSS and simulations of optimised results of the modified RAM structure are presented.

Three kinds of Schottky diodes based on poly(3,4-ethylenedioxythiophene) nanocomposite were successfully made, respectively, with the metal electrodes such as Al, Zr, and In. Cyclic voltammetry measurement was carried on them. The influence of the work functions of the electrodes, the scan rates etc. on the rectification behavior were thoroughly studied and explained. When the scan rate is too fast above 25mV/s for the nanocomposite with Al electrodes, no electrochemical reaction takes place in the conducting polymer nanocomposite during such a short scan period, and the nanocomposite is not switched on. By comparison with the results of different metals (Al, Zr, In) as the electrode of the YANMIN WANG and BARRY CHAMBERS 110 Schottky barriers at the same voltage and scan rate, the Schottky diode with Zr plate generally has better rectification behavior. Especially, at the Zr electrode, the best rectification performance occurs with the rectification ratio of 1371 at ±2V and the scan rate of 2mV/s, which is pretty high for the conducting polymer/metal Schottky diode. Moreover, when the region of the applied voltage increases, the rectification ratio for Schottky diode at Al electrode increases, but that for Schottky diode at Zr electrode decreases when the voltage range is above ±2V.

A novel type of pyramidal absorber is presented. This absorber utilises a combination of impedance loading and an exponential stepped permittivity profile. The new design offers a significant improvement in the absorber's low frequency performance when compared with existing pyramidal absorbers. Optimisation of the design's parameters, using a PSO algorithm, has been carried out to maximise the absorber's bandwidth at -40dB reflectivity.

This paper describes a new type of pyramidal absorber design for use in anechoic chambers. To improve the absorbers performance at low frequency, it is loaded with impedance in the form of a Frequency Selective Surface (FSS). A technique for optimising the pattern, and therefore impedance, of the FSS is explained. Finally, the reflectivity response of the new design is simulated and compared against those of an unmodified pyramidal absorber and one loaded with a single square loop FSS.

A technique is described for improving the low frequency performance of geometric transition (GT) radar absorbers based on lossy foam pyramids. The technique makes use of the fact that at high frequencies, only the geometric transition region of the absorber is utilized whereas at low frequencies, the whole absorber thickness interacts with the incident wave. Hence the low frequency performance may be improved, without compromising that at high frequencies, by electrically loading the absorber base layer using one or more frequency selective surfaces (FSS) whose elements are typically in the form of single or nested loops. Other advantages of this technique include minimal increases in weight and manufacturing costs. The paper includes comparative predictions of unmodified and loaded GT absorber reflectivity at both normal and oblique incidence and discusses the effect on absorber performance of tolerance variations in the dimensions and location of the loading FSS elements. Finally, free-space reflectivity measurements on unmodified and loaded commercial absorber blocks are made over the frequency range 1-10 GHz and these confirm the validity of the technique.

There is continuing interest in the development of large-area active microwave panels for use as switchable windows, ‘wallpaper’, reflectors or radar-absorbent materials. The density of active devices on such panels may range from a few hundred to tens of thousands per square metre but there is no published information about the reliability of such panels in practice and the effects on their performance of cumulative device failure. A preliminary analysis of such effects in terms of panel radar cross-section and power consumption is provided.

Composite materials containing nanoparticulate PEDOT in a polymer electrolyte matrix containing either a Cu–Cu2+ or Fe2+–Fe3+ redox couples show rapid and reversible decreases of up to 500-fold in their electrical and microwave impedances when small DC or AC electric fields are applied across coaxial line and strip samples from their edges, which are much larger than microparticulate PEDOT composites and make signification progress in this field. The composites show large field-dependent resistances at low applied fields and good electrochemical stability.

A phase-switched screen (PSS) differs from conventional radar absorbers in that an 'ideal' PSS does not absorb incident electromagnetic energy but redistributes it in the frequency domain so that it is undetectable by interrogating radar systems. The PSS achieves this translation in frequency by imposing phase modulation onto the energy scattered from its surface to produce a signal with a low time-average energy spectral density at the illuminating frequency. Previous work has examined the properties of PSS operating under various scenarios including oblique incidence and pulsed illumination. In all of these studies the analysis has been based on a transmission line analogue in which the PSS is assumed to be an infinite planar structure. Although this approach has proved very successful for modelling specular scattering from a PSS, particularly for back-scatter at normal incidence, it does not predict the angular scattering dependence of finite PSS. Hence in this contribution we present some results obtained from a new technique for modelling finite sized PSS using an approach based on time-switched array theory.

This paper describes a new approach to improving the low frequency reflectivity performance of geometric transition radar absorbent materials through the use of impedance loading in the form of one or more FSS layers which are incorporated into the absorber base layer. The discussion includes theoretical predictions and measured data on unmodified and modified commercially available RAM which confirm the validity of the concept.

A two-element time-modulated array system which can be configured to provide active electronic null scanning is presented. The received signal from each element of the array is time switched and combined to provide a phase-modulated output in which the depth of modulation is dependent on the angle of arrival of the received signal. The angular response of the array at the first harmonic of the switching frequency exhibits a deep null which can be scanned in angle to plusmn90deg by controlling the mark-space ratio of the element switching waveform

An active absorber, such as the phase-switched screen (PSS), achieves an apparent reduction in the level of the electromagnetic energy reflected from its surface by using binary phase modulation to redistribute it over a bandwidth which is much wider than that of the receiver [Chambers B. and Tennant A., 2004]. Previous analyses of the PSS have been based on either transmission-line theory [Chambers B. and Tennant A., 2004] or the Fourier series [Chambers B. and Tennant A., 2006]. The former enables the PSS reflectivity bandwidth to be estimated, whereas the latter reveals the frequency spectra of signals reflected from the PSS when different switching strategies are employed. Both these previous analyses assumed ideal switching behaviour (i.e. active layer either perfectly transparent or perfectly reflecting) and ideal binary switching waveforms (i.e. zero rise and fall times). Practical PSSs, however, use active layers consisting of arrays of PIN diode loaded resonant elements [Chambers B. and Tennant A., 2004]. Hence a more general analysis of the PSS based on Finite Difference Time Domain (FDTD) techniques is currently being investigated and initial progress is described below.

A phase-modulated surface (PMS) has been shown to provide an effective method of dynamically controlling the scattering from simple planar or cylindrical targets. Real radar targets however have complex geometry comprised of many scattering centres, and multiple interactions may occur between these. Therefore, the authors explore whether the PMS technique is still effective when applied to more realistic target geometries.

It has been demonstrated that the use of resistive FSS embedded within standard pyramidal RAM has the capability to significantly improve the absorption bandwidth. The authors would like to acknowledge the support of the Nuffield Foundation.

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, but the Doppler shifts introduced by the WTG blades are interpreted as moving aircraft that can confuse radar operators and compromise safety. A possible solution to this problem is to incorporate an active layer into the structure of the WTG blades that can be used to dynamically modulate the radar cross section (RCS) of the blade return. The active blade can operate in one of two modes: first 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 onto 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 developed for active radar absorbers. Results of theoretical and experimental work using a 10 GHz Doppler radar and scale-model WTG are presented.

Experimental results are been presented to show that when a phase-switched screen (PSS) is configured so that alternate rows of the its active surface are modulated with independent square-wave signals, single side-band (SSB)-type frequency modulation can be imposed onto signals scattered from the PSS surface. By adjusting the phase-offset between the two modulating signals, selective suppression of individual upper and lower sidebands can be achieved.

Conventional radar absorbent materials (RAM) fall into two main types based on the Salisbury screen and the Dallenbach layer or their multi-layer counterparts. They operate by absorbing the incident electromagnetic energy and converting it into heat. An alternative approach is to use an active absorber, such as the phase-switched screen (PSS), which achieves an apparent reduction in the level of the electromagnetic energy reflected from its surface by using binary phase modulation to redistribute it over a bandwidth which is much wider than that of the receiver. To date, all the reported PSS implementations have used thin active layers consisting of arrays of PIN diode-loaded resonant elements and hence they may be considered as active versions of the Salisbury screen or Jaumann absorber topologies. In this paper, we consider an alternative route to the PSS, based on a medium with switchable material parameters. This may therefore be regarded as an active version of the Dallenbach absorber

Conventional radar absorbent materials rely on the absorption and conversion into heat of the electromagnetic energy incident upon them. In an alternative approach, the phase-switched screen (PSS) uses a switchable resistive layer to apply binary phase modulation to the reflected signal so that its energy is redistributed into sidebands that lie outside the receiver passband. Here, the characteristics of an alternative type of PSS, whose efficacy depends on periodic changes in the permittivity or permeability of a dielectric layer, are considered. This leads to the concept of the pseudo-multi-layer dielectric PSS, which mimics the performance of a multi-layer absorber but comprises of only a single layer.

Diodes composed of a nanoparticulate composite of poly(3,4-ethylenedioxythiophene) and a Cu-Cu2+ redox couple in a poly(ethylene oxide)-LiBF4 polymer-electrolyte matrix between Ag and Zr electrodes show rectifications in excess of 50000 at applied fields of 4 V These large changes are considered to arise from both rectification at the Zr/ZrO2 composite interface and from the switching of the composite material between two conductivity states by the application of a low potential field. preparation and electrochemical characterisation of these novel active devices are discussed.

Although conventional (i.e., passive) radar absorbers are widely used for modifying the radar cross-section (RCS) of current military platforms, 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, and absorb-while-scan. 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. System implementation issues and the optimum choice of design parameters for a range of operational scenarios are discussed, and theoretical predictions are supported by measured performance data.