Kelvin J. Nicholson

Improved Near-Field Radar Cross-Section Measurement Technique

This letter presents an improved image-based coherent Doppler tomography (CDT) technique for near-field radar cross-section measurement. The present formulation, developed using the back-projection algorithm, explicitly incorporates the network analyzer calibration in the near-field to far-field transformation. Experimental measurements and computer simulations are presented to validate the improved CDT technique.

The Role of Fibre Orientation on the Electromagnetic Performance of Waveguides Manufactured from Carbon Fibre Reinforced Plastic

Aircraft skins manufactured from carbon fibre reinforced plastic (CFRP) can simultaneously support structural load and act as antennas. This offers the potential for disproportionately large antenna elements and arrays, and thus enhanced aircraft capability. The efficient design of such structures requires that the link between CFRP microstructure and electromagnetic performance be established. This paper presents a method of predicting the electromagnetic attenuation of waveguides manufactured from CFRP. The method considers both the orthotropic, complex conductivity of CFRP, high in the fibre direction and low transverse to it, and the local electric fields in waveguides, which vary with location and frequency. The method was validated experimentally using waveguides manufactured from aerospace grade IM7/977-3 prepreg tape with [0 90]s, [90 0]s and [±45]s ply stacking sequences.

Split-Ring Resonator Loading for the Slotted Waveguide Antenna Stiffened Structure

Slotted waveguide antenna stiffened structure (SWASS) utilizes hat-stiffeners on thin skins or blade stiffeners in sandwich structures as microwave waveguides. By machining slots through the outer skin and into the waveguide, large slotted waveguide antenna arrays may be integrated into a load-bearing structure. However, the slot length is typically resonant with half-wavelength spacing, thereby degrading the load-bearing capacity of the structure. This letter demonstrates a simple method to achieve comparable gain from a slot radiator with subresonant length by means of a single split ring. The adverse structural impact of the individual slot and the slot array may therefore be reduced.

The strong diamagnetic behaviour of unidirectional carbon fiber reinforced polymer laminates

Carbon fibers are finite conductors with a weak diamagnetic response in a static magnetic field. When illuminated with a high-frequency alternating electromagnetic wave such that the skin depth is greater than the fiber diameter, carbon-fiber composites are shown to exhibit a strong dynamic diamagnetic response. The magnetic susceptibility (χm) is controlled by the polarization angle (θ), which is the angle between the incident electric field and conductor direction. A closed form solution for this behaviour was derived using Maxwells equations and an understanding of the induced conductor currents. The equation was verified using simulation and free space “wall” and waveguide measurements on unidirectional IM7/977-3 carbon fiber reinforced polymer laminates. The measured responses ranged from non-magnetic at θ = 90°, χm = 0, up to strongly diamagnetic at θ = 30°, χm = −0.75, over the 8-18 GHz bandwidth. The experimental results are in good agreement with theoretical predictions and computational simulat...

Embroidered Active Microwave Composite Preimpregnated Electronics—Pregtronics

The rapid development of the embroidery of conductive threads into textiles and associated biomedical sensors and on-body antennas has given rise to many healthcare and emergency response applications. In this paper, we investigate the embroidery of conductive threads directly into a grade of composites called “preimpregnated” (pre-preg) materials. These pre-preg materials differ from traditional textiles in that they contain a B-staged epoxy resin that must be baked in an autoclave at temperatures greater than 170 °C, and under pressures upward of 700 kPa to achieve their maximum strength. The experimental characterization of pre-preg-realized transmission line structures embroidered with different types of threads, stitches, and sewing techniques establishes the best fabrication approaches. Unlike embroidery of conductive threads into textiles, the high processing temperatures and pressures of composite materials result in conductivity and dispersion properties similar to those found in copper. We demonstrate how to incorporate passive and active electronic circuit elements directly into this structural material and its subsequent durability during the hardening process. An embroidered pre-preg electronic (pregtronic) ultrawideband amplifier is realized numerically and confirmed experimentally. Because pre-pregs have a relatively long shelf life when stored within a freezer, as well as the capability to be conformably molded to any surface, the study of pregtronics lends itself to the development of storable, conformal electronics for aerospace, automotive, and other structural applications.

Investigations of a Load-Bearing Composite Electrically Small Egyptian Axe Dipole Antenna

An electrically small, metamaterial-inspired Egyptian axe dipole (EAD) antenna has been investigated for use in structural composite materials. The EAD antenna consists of a differentially fed dipole element integrated with a near-field resonant parasitic EAD element. These elements have been adapted to these materials resulting in a system that is impedance matched and radiates efficiently at 307 MHz. Three cases have been identified and investigated to ascertain the performance of the manufacturing techniques and material properties used to build these electrically small antennas, as well as their performance characteristics. Uniquely, an embroidered conductive thread and a new carbon fiber based, nonwoven mat have been investigated for use as the conducting elements. Both cases are compared with a copper variant of the EAD antenna. All three prototypes were tested. Measurements confirm that both the nonwoven mat and the embroidered versions of the EAD antennas perform similarly with maximum realized gains ranging from 1.72 to 1.90 dBi.

Multi-functional composite metamaterial-inspired EEAD antenna for structural applications

An electrically small, load-bearing Egyptian axe dipole (EAD) antenna has been sewn into a low loss, pure quartz glass composite material to investigate its performance. Previous investigations of embroidered Egyptian axe dipole antennas indicated that the dielectric losses of the associated epoxy-based composite, in conjunction with the high effective surface resistance of the conductive textile threads, significantly degrade their performance. Simulations of the EAD antenna using a composite sandwich structure based on an advanced embroidery technique and the much lower loss quartz fabric have shown that a realized gain of 0.9 dBi is possible, a dramatic improvement over previous realizations.

Coaxial Right/Left-Handed Transmission Line for Electronic Beam Steering in the Slotted Waveguide Antenna Stiffened Structure

The slotted waveguide antenna stiffened structure (SWASS) utilizes conventional hat-stiffeners or blade stiffeners in aircraft sandwich structures as microwave waveguides. Slotted waveguide antenna arrays may therefore be integrated into the structure by machining slots through the outer skin. However, the primary mechanical load applied to the structure governs the orientation of these slotted waveguides and so dictates the antenna scan plane. This work extends the SWASS concept by demonstrating a means to achieve electronic phase shifting along the waveguide axis for the purpose of beam steering. This is accomplished by incorporating a varactor loaded coaxial composite right/left-handed transmission line into the SWASS for approximate matched tuning of the dispersion diagram about the design frequency.

Design, manufacture and measurement of a metamaterial with tunable negative-refractive index region

A metamaterial with electronically tunable negative-refractive index region is presented. The proposed structure is loaded with interdigital capacitors fabricated on a barium strontium titanate thin film. When a static electric field is applied to the series connected elements in the metamaterial, the region with negative refractive index is tuned over the WR-137 band. Computer simulation of the unit cell is presented. Experimental measurement of the completed structure in a parallel plate wave guide will be presented at APMC 2009.

Tomographic Characterization of a Multifunctional Composite High-Impedance Surface

The performance of a multifunctional composite high-impedance surface (HIS) has been evaluated using the coherent Doppler tomography (CDT) and finite-impulse response (FIR) filtering techniques. A combination of embroidery and advanced laser manufacturing processes were used to fabricate the conformable multifunctional glass fiber reenforced polymer HIS. The CDT method was utilized because it enabled the generation of a high-resolution tomographic map of the HIS reflectivity. Tomograms generated at high incidence angles (>80° from normal) were used to localize and FIR filter unwanted scattering associated with the ground plane edges and HIS transition regions. The resulting scattered fields from a defect (metallic block positioned in the center of the tomogram) were then used to gain a significantly distinctive insight into the HIS scattering properties. Furthermore, unlike traditional methods for characterizing HISs, the CDT and FIR methods presented herein are applicable to electrically large and conformal HISs.