B. Chambers

A Two-Element Time-Modulated Array With Direction-Finding Properties

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

A single-layer tuneable microwave absorber using an active FSS

An experimental single-layer active microwave absorber in described. The absorber is a 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. Measured data are presented and show that the reflectivity response of the absorber can be controlled over the frequency band from 9 to 13 GHz.

Influence of switching-waveform characteristics on the performance of a single-layer-phase switched screen

Conventional microwave-absorbing 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) applies phase modulation to the reflected signal so that the energy is redistributed into sidebands with, ideally, none remaining at the original incident carrier frequency f/sub c/. Hence, by adjusting the frequency and shape of the waveform that controls the PSS reflection coefficient, these sidebands may be positioned outside the pass-band of a receiver tuned to f/sub c/. An investigation has been carried out to determine how the choice of control waveform and switching frequency influence the PSS performance.

A smart radar absorber

This paper proposes a configuration for a smart radar absorber which is capable of both self-tuning and absorb while scan operation. The discussion is complemented by modelled and measured performance data.

Control of the harmonic radiation patterns of time-modulated antenna arrays

In this paper we have presented the results of initial research into new switching and sub-arraying schemes for time-modulated antenna arrays. Conventional time-modulated arrays employ switching schemes in which each element of the array has a common start time and an switch-off time which varies according to a prescribed amplitude taper. In this paper we have examined switching strategies in which the element dasiaswitch-onpsila time can be arbitrary. This approach introduces an extra design variable and can be used to modify the sidelobe pattern of the array pattern at harmonic frequencies. In particular we have shown that relatively simple switching schemes can be used to generate a boresight null at harmonic frequencies.

Easing EMC problems in switched mode power converters by random modulation of the PWM carrier frequency

This paper presents a method of reducing the measured amplitude of the high frequency harmonics, contained in standard fixed frequency PWM, by the use of a pseudorandom binary sequence to modulate the PWM carrier frequency, achieving a spread spectrum effect on the EMC spectrum of the system.

Time-Switched Array Analysis of Phase-Switched Screens

In this correspondence, an analysis of phase-switched screens (PSS) based on time-switched array theory is presented. Unlike conventional transmission line analysis, the time-switched array approach allows finite-sized PSS to be analyzed. New modulation schemes for finite-sized PSS based on selective antiphase switching are introduced and analyzed. Modulation sequences in which 50% of the PSS is switched in antiphase are shown to operate differently to conventionally modulated PSS and produce zero backscattered energy at harmonic frequencies. Measurements from an experimental PSS operating at an incident frequency of 10 GHz and time-switched at 5 MHz are presented.

Improvement in the Low Frequency Performance of Geometric Transition Radar Absorbers Using Square Loop Impedance Layers

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.

FDTD modeling of heatsink RF characteristics for EMC mitigation

Due to their size and complex geometry, large heatsinks such as those used in the power electronics industry may enhance the radiated emissions produced by the circuits employing them. Such enhancement of the radio frequency (rf) radiation could cause the equipment to malfunction or to contravene current EMC regulations. In this paper, the electromagnetic resonant effects of heatsinks are examined using the finite-difference time-domain (FDTD) method and recommendations are made concerning the optimum geometry of heatsinks and the placement of components so as to mitigate potential EMC effects.

Towards a `smart window' for microwave applications

Discs of poly(aniline)-silver-polymer electrolyte particulate composites exhibit rapid and reversible changes in their microwave impedance when small electric fields are applied across them in a coaxial line test set in a resonant condition. The effect of composition on the cyclic voltammetry and microwave characteristics of the composites is described. The experimental data show that changes in the gradients of the cyclic voltammograms coincide with large changes in microwave resonant null depth consistent with increasing conductivity of the composite when the fields are applied during electrochemical cycling. The reverse change occurs when the field is removed. 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 suggest that, during cycling, the silver metal dissolves and is then re-precipitated.