Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymercomposites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.

A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles

A free-space measurement system operating in the 8.2-40-GHz frequency range is used to measure the reflection and transmission coefficients, S/sub 11/ and S/sub 21/, of planar samples. The complex electric permittivity and the magnetic permeability are calculated from the measured values of S/sub 11/ and S/sub 21/. The measurement system consists of transmit and receive horn lens antennas, a network analyzer, mode transitions, and a computer. Diffraction effects at the edges of the sample are minimized by using spot-focusing lens antennas. Errors due to multiple reflections between antennas via the surface of the sample are corrected by using a free-space TRL (thru, reflect, line) calibration technique. For thin, flexible samples, the sample had to be sandwiched between two half-wavelength (at mid-band) quartz plates to eliminate sagging. Results are reported in the frequency range of 8.6-13.4 GHz for materials such as Teflon, sodium borosilicate glass, and microwave-absorbing materials. >

Free-space measurement of complex permittivity and complex permeability of magnetic materials at microwave frequencies

The reflection properties of multiple electric/magnetic Salisbury screens have been studied. Analytical formulae are developed for the maximally flat design of two- and three-screen planar stackups that have taken into account appropriate values for the spacer materials used to separate the screens. The effect of curvature has also been assessed. It is shown that relatively large reductions in specular reflection are possible over sizeable bandwidths using relatively few Salisbury screens. Furthermore, these reductions are not completely destroyed if the incident wave is not normal to the surface, if the surface is curved or if there are errors in fabrication of the screen. >

Reflection properties of the Salisbury screen

Characterization of M-type barium hexagonal ferrite-based wide band microwave absorber

Available data on microwave absorbing materials finding engineering applications or potentially attractive for use in dielectric, magnetic, and ferroelectric–seignettomagnetic absorbers are summarized.

Microwave Absorbing Materials

The lack of suitable materials having permeabilities (μr) greater than unity at microwave frequencies has previously made it difficult to design thin coatings of broadband radar absorbersat this frequency range. A possible solution is to utilize the hexagonal ferrites that exhibit significant μrvalues (5) over narrow frequency bands and this paper investigates methods of deploying this material to design thin broadband microwave absorbers. Several techniques are evolved involving layering of the materials and staggering of the material electrical parametersand dimensions to achieve a good match ofthe incident waves. The computed results are substantiated by measurements of the manufacturedmaterials; measurement techniques and details of the materials used are given in appendices. It is established that a planar absorbent sheet having a reflection loss > 10 dB over 5–20 GHz is realizable within a practically acceptable thickness.

Techniques for utilization of hexagonal ferrites in radar absorbers. Part 1: Broadband planar coatings

The taper profile of optimized dielectric-rod and horn antennas is synthesized as a series of non-interacting planar radiating apertures. The method is semi-empirical, straightforward to apply, enables the dielectric-rod antenna to be satisfactorily optimized and provides a means of evaluating and optimizing a dielectric-horn antenna with variable wall thickness. The optimum profiles are taken as those which smoothly transform the surface-wave power from the launcher to the radiating aperture. The optimization of the dielectric-rod antenna considerably improves the radiation pattern while computations supported by measurements confirm earlier reports that a dielectric-horn can have a higher gain than a metal horn of similar dimension but side-lobe level is seen to be an important issue. Wide flare-angle horns give ideal E-plane patterns at the expense of a high side-lobe level in the H-plane; for small flare angles the dielectric horn gives similar patterns to the tapered rod antenna and thus preserves rotational symmetry. Calculations throughout are restricted to cylindrical geometry but other geometries and variations on the dielectric-horn principles are described. Useful engineering design data have been compiled for both the dielectric-horn and rod antennas and curves are given which determine near-optimum parameters for gains up to about 20 dB which is seen to be a practical operating limit for these surface wave devices. A unified impression of dielectric antennas emerges with the important conclusion that, when optimized, dielectric-rod and horn antennas are in fact competitive with small metal horns for some applications; furthermore the dielectric-horn antenna, used singly or in arrays, is an ideal device for producing a low side-lobe level in the E-plane.

Engineering approach to the design of tapered dielectric-rod and horn antennas

Every arbitrarily shaped rod is shown to possess a non-radiating guided wave and a new variational solution is derived. A simpler formulation based on the point-matching principle is outlined and likely sources of computational error are scrutinized. In distinguishing between static and wave-like field solutions it is shown that a convergence criterion based on eigenvalues alone can be misleading, point-matching methods being no worse than some other numerical processes in this respect. The validity of the chosen expansion is a critical factor and two new necessary conditions imposed by the boundary geometry are deduced which are of considerable help in selecting an expansion; these being angular periodicity and radial single-valuedness. No sufficient condition for the expansion validity has been given as yet. Relevant mathematical ideas and theories about the point-matching technique are examined and recommendations made on how best to apply the technique. Computations supported by experimental or other evidence are presented including computations of an arbitrarily shaped nonconvex guide and a new equilateral triangular guide. The calculations throughout are restricted mainly to the dominant HEu mode and the present computer program is capable of giving acceptable engineering results for a variety of rod shapes which are of interest.

Point-matched solutions for propagating modes on arbitrarily-shaped dielectric rods

The increasing compactness of modern microwave communication and radar equipment commonly demands that the associated antennas have flat profiles; this paper surveys the various planar antenna design techniques that are available. After summarizing the well-established methods using slotted waveguide arrays, the more recent stripline and cavity backed printed antennas are surveyed; finally the latest techniques using microstrip arrays are introduced and state-of-the-art designs discussed. Outline design and explanatory details are given for each antenna and any special characteristics, advantages and disadvantages are noted; ample references are given throughout the text. Several conclusions are reached about ways of improving the various individual designs but the need to improve the control of both mechanical and electrical tolerances and also more accurately predict the sidelobe behaviour of arrays are seen as important general requirements for further research.

Survey of design techniques for flat profile microwave antennas and arrays

Coatings of hexagonal ferrites on a wire antenna reduce its back-scattering radar cross-sections (r.c.s.) at microwave frequencies without causing significant deterioration of its radiation efficiency at the h.f. and v.h. communication bands. The back-scattering r.c.s. of an infinitely long conducting cylinder coated with a multilayer of hexagonal ferrites is derived and the result modified to take account of the finite length of a monopole antenna. Computed results for layered coatingsare presented to demonstrate the improvement in the microwave absorption bandwidth obtainable for a given coating thickness. It follows that the technique is best applied with all material and geometric parameters having tapered distribution and the ferrimagnetic resonances staggered over the required frequency band. Finally, the theory is illustrated by the measurement of the backscattering losses of a v.h.f. ferrite-coated antenna in the microwave frequency band (for both TM and TE polarizations of the incident waves) and its radiation performance at the v.h.f. range

J.R. James

Papers

Techniques for utilization of hexagonal ferrites in radar absorbers. Part 1: Broadband planar coatings

Engineering approach to the design of tapered dielectric-rod and horn antennas

Point-matched solutions for propagating modes on arbitrarily-shaped dielectric rods

Survey of design techniques for flat profile microwave antennas and arrays

Techniques for utilization of hexagonal ferrites in radar absorbers. Part 2: Reduction of radar cross-section of h.f. and v.h.f. wire antennas