Ivor L. Morrow

Effective imaging of buried dielectric objects

Ultrawide-band CW radar have shown good potential for remote imaging of surface laid or shallow buried landmine-like objects. However when sensing fields near to the ground, a number of factors including direct antenna coupling, air/ground coupling, receiver noise floor, and effective dynamic range conspire to degrade or mask the scattered return, resulting in a loss of signal magnitude, range accuracy, and range resolution. The present paper addresses the latter problems using a near-field detection methodology and providing a thorough experimental and systems analysis of the signal-to-clutter issues. The method proves advantageous in sensing weak echoes in the vicinity of the sensor from objects of low dielectric contrast with their environment. To enhance image processing gains, an effective space frequency synthetic aperture technique applicable to the two-media problem is outlined. The algorithm is straightforward and robust enough to be implemented on compact GPR systems operating in real time. The remaining problem of object identification will not be addressed here. To demonstrate the utility of these combined techniques, field experiments, over different frequency bands, were conducted and their results are reported.

Measurement and modeling of a microwave active-patch phased array for wide-angle scanning

An active antenna consisting of an integrated oscillator with a passive radiator has been arrayed and beam-scanned using a new principle for phase shifting. The radiating elements consist of a transistor oscillator whose frequency is controlled by a rectangular microstrip patch antenna. Each oscillator is injection locked to an external source. The phase control of the radiated wave is achieved by varying the bias on the transistor. Individual element performance has been characterized for potential use as an array element and is comprehensively reported. Methods used to achieve configurations with full 360/spl deg/ phase control have been investigated utilizing novel configurations and cascaded oscillator pairs. Close to 360/spl deg/ of radiated phase control from each element has been achieved. Measured results on an experimental four-element S-band array indicate that beam scanning in excess of /spl plusmn/60/spl deg/ can be achieved. Mutual coupling effects on this new form of array are studied both experimentally and theoretically. A van der Pol (1934) model for the weak coupling that is occurring on the array is developed and used to qualitatively predict the phase offsetting on array elements. Reasonable agreement between theory and experiment is obtained and it is observed that good control of the coupling mechanism is essential to array performance within this new form of active integrated phase shifterless array.

ESA measurements using the Hybrid Fibre-Optic Reflection Measurement System

Since the introduction of the Wheeler Cap method to measure the radiation efficiency of an Electrically Small Antenna (ESA), there have been several techniques presented in literature that have either enhanced on the method or provided an alternative technique. This paper presents measurements of the radiation efficiency of an electrically small monopole using the Hybrid Fibre-Optic RF Reflection Measurement System (HRS) integrated into the Wheeler Cap. The HRS isolates the ESA from the measurement system. The ESAs reflection coefficient can then be measured with the isolated ESA inside the Wheeler Cap and in freespace to determine its efficiency. The HRS has also been demonstrated in a far-field measurement range to measure the isolated far-field radiation pattern of the ESA.

Phase control in injection locked microstrip active antennas

This paper describes experimental and theoretical investigations of active antennas using injection locking techniques to achieve phase control. We present a polarisation agile active antenna capable of selectively radiating circular or linearly polarised radiation and discuss oscillator instability considerations and initial results of active mutual coupling analysis that are applicable to this antenna and a recently published active patch scanned array.<<ETX>>

Applications and future prospects for microstrip antennas using heterogeneous and complex 3-D geometry substrates

This paper critically reviews the electromagnetic advantages of altering the dielectric substrate section of the antenna as opposed to the conducting elements. Changing the dielectric has been used to improve the bandwidth, e-ciency and gain of antennas. Heterogeneous substrates have also been employed to lower the efiective permittivity, suppress surface waves for high indexed substrate materials and reduce mutual coupling. In the second half of this paper, 3-D printing has been used to create substrates with reduced material consumption for a lightweight ∞exible wearable antenna. Consumer demand for small devices with wireless connectivity has put increasing pressure on antenna engineers to improve electromagnetic performance in smaller packages. These design constraints are further exacerbated by increasing demand for improved bandwidth, e-ciency and frequency coverage. Including an additional degree of design freedom by manipulating the substrate shape and properties can help address these challenges. These synthetic substrates are very di-cult to manufacture using conventional technologies. Complex 3-D printed geometries can easily be manufactured from computer aided design models that can be exported directly from electromagnetic simulation software. 3-D printing allows the geometry to be varied in all three dimensions, therefore printed cavities of various shapes and sizes can lead to a smooth or discrete change in the efiective permittivity. With the latest advances in additive manufacturing and 3-D printing, the antenna and radiofrequency designer will be able to control the local efiective permittivity and the substrate shape to gain electromagnetic advantages. Section 1 of this paper reviews the advantages of using heterogeneous substrates. Section 2 investigates the minimisation of the substrate volume. Section 3 details how these samples can be manufactured using 3-D printing. Conclusions are drawn in Section 4. 1.1. Control of Surface Waves and Current Modes Microstrip antennas are light, low proflle, conformal, compact structures which are normally fabricated on a homogeneous substrate. They can be regarded as a dielectric fllled parallel plate waveguide radiating at discontinuities (1,2). The size of an antenna can be easily reduced by using a dielectric with a high-valued permittivity but this also increases the energy in the surface wave modes (3). These surface waves decrease the e-ciency of the antenna and also cause interference with the radiation pattern by getting difiracted from the edges of the flnite sized ground plane (4). These detrimental efiects can therefore be reduced by suppression of surface wave modes. Heterogeneous substrates have been utilised, for a circular microstrip antenna, to completely suppress the surface waves caused by TM0 mode, which is the main cause of surface wave radiation for thin substrate microstrip antennas (4). The

Rolled edge ultra-wideband dipole antenna for GPR application

The ever-increasing interest in ground penetrating radar (GPR) for local high resolution interrogation of buried objects or environments has made an impact on the design of ultrawideband (UWB) antennas. Antenna requirements have been stimulated by applications including; utility location, building construction inspection and foreign object detection e.g. archaeological survey or as intended here buried land mine imaging. The strength of signal returns from low dielectric contrast discontinuities tends to be weak, typically less than a few percent of the r.m.s. power transmitted. An obvious solution is to place the receiving antenna in close proximity’ with the air ground interface [I]. More fundamentally, the overall radar system performance places emphasis on two key aspects of antenna design; the efficient transfer of electromagnetic energy (implying low losses) and reducing the level of “clutter” signals originating within the antenna.

Phase Processing Techniques for the Prediction of Induced Current

This paper reviews the currently used minimum phase algorithm approach for the prediction of induced current as a result of the incidence of a time-domain electromagnetic environment such as an electromagnetic pulse. An alternative to this approach is proposed that enables the user to identify a lower bound on the minimum phase prediction. The technique has been applied to over 500 transfer functions with three different high- power electromagnetic environments and the results are presented and discussed within this paper. The paper concludes that the proposed method is a useful technique in providing an alternative method of predicting the induced current.

Fundamental limitations on excitation of a surface wave on a plasma column

Interest in plasma antenna technology has recently been stimulated by applications from a variety of fields. Of particular interest is the case of a plasma column excited below /spl omega//sub /spl rho//, the plasma resonance frequency, which leads to negative permittivity and permits the propagation of a surface wave along the column. We present a condensed analysis of the plasma radiation and power launching constraints on excitation of a surface wave on a plasma column.

A low profile retrodirective frequency selective surface for radar earth observation

This paper presents the design of a low profile retrodirective surface composed of a frequency selective surface (FSS). The principle of operation of re-radiation and scattering from the array surface is outlined. A full-wave analysis of individual FSS unit cells and the assembled reflect array is performed to better understand and design the reflector. A commensurate array of unit cells with progressive phase shift is realised using non-identical dimension dual resonant circular loop FSSs. A prototype reflect array was manufactured and the monostatic backscattered field measured in horizontal and vertical polarisation. Comparative measurements on similar sized flat plate and dihedral reflector surfaces were made that demonstrate the reflect array performance.

Correlation of specific absorption rates in the human head due to multiple independent sources

This paper examines how the SAR in the head is combined when exposed to the field due to multiple sources. The mechanisms when the sources have the same and different frequencies are discussed. FDTD simulation results are included when an anatomically realistic head is excited by various sources including: plane waves, vertically and horizontally orientated dipoles positioned in front and by the side of the head. Results are presented for two sources over the frequency range 0.5 to 4GHz.