Chris Coleman

A Kirchhoff Integral approach to estimating propagation in an environment with nonhomogeneous atmosphere and complex boundaries

For terrestrial radio wave propagation, it has been previously demonstrated that second order effects, such as diffraction, can be estimated through a combination of geometric optics and Kirchhoff style integral relations. In the present paper, it is shown that the approach can also yield accurate estimates when refractive effects, such as atmospheric ducting, are present.

Identification of scintillation signatures on GPS signals originating from plasma structures detected with EISCAT incoherent scatter radar along the same line of sight

Abstract Ionospheric scintillation originates from the scattering of electromagnetic waves through spatial gradients in the plasma density distribution, drifting across a given propagation direction. Ionospheric scintillation represents a disruptive manifestation of adverse space weather conditions through degradation of the reliability and continuity of satellite telecommunication and navigation systems and services (e.g., European Geostationary Navigation Overlay Service, EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionization structures to GPS scintillation. European Incoherent Scatter (EISCAT) measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the EISCAT UHF radar to causally identify plasma structures that give rise to scintillation on the co‐aligned GPS radio link. Large‐scale structures associated with the poleward edge of the ionospheric trough, with auroral arcs in the nightside auroral oval and with particle precipitation at the onset of a substorm were indeed identified as responsible for enhanced phase scintillation at L band. For the first time it was observed that the observed large‐scale structures did not cascade into smaller‐scale structures, leading to enhanced phase scintillation without amplitude scintillation. More measurements and theory are necessary to understand the mechanism responsible for the inhibition of large‐scale to small‐scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionization structures. New insights from this experiment allow a better characterization of the impact that space weather can have on satellite telecommunications and navigation services.

Optimal helical antenna with continuously varying radius using evolutionary optimizers

The size reduction for wire antennas using lumped loading inductors is well known, but introduces significant degradation in efficiency and bandwidth. One of the alternative approaches to solve the problem is the increase of wire length for a fixed height. Various techniques have been investigated to introduce longer wire length, but this paper concentrates on helical windings with continuously varying radii. The continuously varying radii are represented using radial basis functions, which give maximized shape controllability with a minimal number of defining parameters. These parameters that define the shape of the helix are optimized with respect to both bandwidth and efficiency, and matching networks are included in the optimization. The results are obtained from an automated simulation tool (MATNEC) equipped with evolutionary optimizers. Using the proposed strategy, a performance improvement is predicted for an optimized helical winding structure with continuously varying radii.

Optimal positions of loading for a shortened resonant monopole using genetic algorithm

The size reduction for resonant monopoles is an important issue for HF and VHF antennas. Unfortunately, size reduction results in degradation in both efficiency and bandwidth, as is well known from the fundamental limits on small antennas [1]. This paper firstly introduces a MATLAB-controlled NEC2 simulation tool with genetic algorithm optimization, and uses this to find the optimal position of inductors for a shortened monopole with L-section matching networks. Trade-offs between bandwidth and efficiency are investigated and demonstrated in a systematic fashion for a single inductor case. Various multi-inductor loading schemes are also investigated; including central symmetrically distributed and arbitrarily distributed two-inductor loading schemes. The ultimate goal of the research is to find an optimal distribution of inductors on a reduced-size resonant monopole.

Scattering by a plane interface that contains strongly conducting patches

A technique is developed for estimating the scattering effects of a homogeneous half space that contains strongly conducting patches on its surface. The technique directly relates the radiative behavior of the scattered field to that of the field sources acting alone. In particular, the method can be used to relate the performance of an antenna over a perfectly conducting plane to its performance over a finite ground screen. Unlike some previous attempts at this problem, the present approach does not rely upon an impedance boundary condition or upon a physical optics approximation to screen currents.

OTH Skywave MIMO signal model and target detection in presence of multipath

HF Skywave radars exploit the refractive properties of the ionosphere to reach distances beyond the horizon allowing for the surveillance of very large areas. The instability of the ionospheric channel on wide temporal and spatial scales is one of the most important issue to be addressed in the design of such systems, especially when considering a MIMO configuration. In fact, large scale ionospheric disturbances and multipath can affect the propagating signals and degrade the radar performance. The main objective of this paper is to derive a suitable signal model that accounts for ionospheric multipath and fading. ROC curves are then simulated and compared to the case in which multipath is not considered.

Super resolution ISAR imaging via Compressive Sensing

Developing compressed sensing (CS) theory has been applied in radar imaging by exploiting the inherent sparsity of radar signal. In this paper, we develop a super resolution (SR) algorithm for formatting inverse synthetic aperture radar (ISAR) image with limited pulses. Assuming that the target scattering field follows an identical Laplace probability distribution, the approach converts the SR imaging into a sparsity-driven optimization in Bayesian statistics sense. We also show that improved performance is achieved by taking advantage of the meaningful spatial structure of the scattering field. To well discriminate scattering centers from noise, we use the non-identical Laplace distribution with small scale on signal components and large on noise. A local maximum likelihood estimator combining with bandwidth extrapolation technique is developed to estimate the statistical parameters. Experimental results present advantages of the proposal over conventional imaging methods.

Optimized helical monopole antennas for portable VHF communication devices

The helical monopole antenna has been used extensively as compact antenna for portable VHF communication devices. Conventionally, it has a constant pitch and radius along its body, these being chosen to make the antenna resonant at the frequency of interest. By allowing both radius and pitch to vary along the antenna, it is possible to optimize the antenna in terms of efficiency and bandwidth. The current paper investigates the effect of antenna environment upon such optimizations and, in particular, the situation where the VHF antenna is fed against a small radio casing instead of a perfect ground plane. It is shown that the optimization is little affected by such changes to environment and that the optimized designs are robust. The current paper concentrates on optimizations of pitch since helix radius will need to be constrained in the case of a portable radio.