A.P. Anderson

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.

The determination of dielectric loss tangent by microwave phase tomography

The possibility of quantitative phase tomography of dielectric objects in matching media is investigated. Computed cross sections of homogeneous and concentric cylinders with complex permittivity are generated from exact analytic expressions for scattered fields. The effect of dielectric loss within the object medium on the validity of the first-order Born approximation is examined. A technique for accurate measurement of weak scattering in the presence of a strong incident field is presented. The subtraction of distortion effects in the reconstructed phase image provides a means for microwave metrology of internal loss distributions within dielectric bodies.

Control of microwave reflectivities of polymer electrolyte-silver-polyaniline composite materials

Abstract Discs of composites of particular poly(aniline hydrochloride) or poly(aniline hydrotetrafluoroborate) and silver in a poly(ethylene oxide)-AgCF 3 SO 3 polymer electrolyte were characterized using microwaves over the frequency range 0.5–3.0 GHz in a coaxial line. The materials demonstrated a rapid and reversible change in their microwave reflectivity when a dc potential of 5 volts was applied across them. A coaxial line test set was used to observe changes of 15–20 dB in microwave reflectivities and displacements of ca 0.25 GHz in the resonance condition. The observations are tentatively discussed in terms of reversible changes in conductivities of the particulate components and the capacitance within ‘microcells’ of polyaniline/polymer electrolyte/silver.

Superdirectivity with Appreciable Bandwidth in Arrays of Radiating Elements FED by Microwave Transistors

The incorporation of transistors into the radiating elements of an array antenna leads to a suppression of the elemental mutual coupling effects in the feed network. Because of this suppressed interaction, it is possible to re-appraise the practical exploitation of Schelkunoffs principle that an arbitrarily high gain can be obtained from a given array size by suitably feeding the individual elements. A four-element end-fire superdirective array with a total length of 3?/8 and a theoretical beam width of 60° has been investigated. The computed radiation patterns of the passive array indicate the extreme sensitivity to changes in frequency consistent with the classical limitations of supergain arrays. An experimental active array is described which permits the superdirective performance to be maintained over an appreciable frequency band.

Microwave Images of Sub-Surface Utilities in an Urban Environment

The principle of identifying concealed object shapes by microwave holography has become established in recent years. An important application would be the imaging of buried cables and pipes if the problem of the high attenuation of ground and the strong surface return could be overcome. It is shown in the present work that a synthetic aperture imaging technique based on polarization discrimination mitigates these problems. The technique has been tested in the real urban environment by producing an image of a gas pipe at depths up to 0.25m below the ground surface. An area of ground is scanned with 0.95GHz radiation, the backscatter is recorded in complex form, and the data is computer-processed to produce focussed images on a 16-level TV display.

Incremental Propagation Procedure for Calculating Scattered Fields from Arbitrary Conductors

A computationally efficient method is described for calculating the scattered fields from conducting bodies of arbitrary shape. The process is non-iterative and incrementally propagates a planar field distribution through the conductor via spectral decomposition of the wave, inserting the necessary boundary conditions as required. Integration is performed numerically using Fast Fourier Transform routines. The technique is compared with two analytic methods for calculating the scattered fields from a perfectly conducting half plane and then from a perfectly conducting cylinder. A generalisation of the method to deal with scattering off bodies composed of any dielectric material is discussed.

A New System for Microwave Holographic Imaging of Buried Services

A new method for improving resolution without increasing operating frequency is proposed. This bistatic version of the holographic technique involves illuminating the buried services from a source antenna placed on, or implanted slightly below the interface. Resolution is improved by scanning in the surface wave region close to the interface. Computer simulations are used to predict the scattered field in sandy soils with 10 and 20% gravimetric moisture content at 300MHz. Some experimental simulations at 500MHz in tap water at 15°C illustrate the improvement in resolution gained by scanning in close proximity to the surface.

Null Steering in Adaptive Arrays using a Genetic Algorithm

Antennas are frequently requiréd to have beam steering, adaptive nulling and direction finding capabilities, and technologies already exist for performing some of these functions. In a new approach, we apply a genetic algorithm to solve the problem of null steering in adaptive arrays. It has been shown that it is possible to steer the array nulls precisely to the required interference directions and to achieve any prescribed null depths. A comparison with the results obtained from analytic solutions, for complex weights, element phases, or element positions, shows some advantage of using the genetic algorithm approach for null steering in linear array patterns. We also show its potential for conformal array design.

Retrieval of the Driving Currents of a Dipole Array from the Radiated Field Magnitude

A technique is described for the retrieval of the driving currents of a dipole array from measured magnitude-only field data and the array configuration. The current values are determined by an iterative algorithm which has been validated by means of simulation with scan-limited data sets. It is intended that the technique be developed to provide a method to diagnose faults on arrays in service.

A Thinned Cylindrical Array for Focused Microwave Hyperthermia

A phased array configuration for microwave induced hyperthermia in cancer treatment is proposed with the objective of heating tumors deep within the body. The computer simulation consists of an elliptic cylinder representing the human thorax partially surrounded by a semi-circular array of radiators and a short linear array of radiators parallel to the main axis of the cylinder. This configuration of only 17 elements yields sufficient energy concentration at the desired location without excessive surface heating. Modification of the array illumination function allows a certain degree of control over the heated volume. The arrangement would be attractive in clinical practice since it neither requires contact nor close proximity to a patient as with radiation treatment in current practice.