This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substrates, and new metamaterials constructed with ferrite cores wound by metallic wire coils that exhibited unique magnetic properties, with extremely high real and imaginary permeability, which are adjustable or tunable by varying their configurations. Magnetodielectric materials, with matching permeability and permittivity, together with sufficiently low magnetic and dielectric loss tangents, with potential applications in antenna miniaturisation, will be discussed.

Theoretical and experimental studies have been conducted on the effective electromagnetic properties of planar composites at microwave frequencies, with embedded conductive fibers of various volume concentrations. Two types of distribution are considered: random and periodic. Experimental results for the transmission coefficient and effective permittivity are obtained via the free space method. Simulation results are obtained using the finite element method (FEM). Good agreement is found between the measured and computed results, indicating the suitability of the FEM as a theoretical modeling tool for such composites, as compared to other numerical methods, such as the method of moments. It is also found that the type of distribution affects the effective permittivity of the composites: lower microwave permittivity with broader peak response is observed for composites with randomly distributed fibers, in comparison to that with periodically distributed fibers.

Composites with carbon nanotubes (CNTs) as inclusions were prepared and the dielectric properties under bias voltages were studied. It was found that the composites with weight concentrations of CNTs above 6 wt % exhibited tunable dielectric characteristics under an external applied bias voltage with real part decreasing while the imaginary part increases with the increasing applied voltage. The tunable properties may be understood from the percolation threshold point of view. Potential applications of such tunable properties of the CNTs composites include smart materials and structures.

An experimental study on the dipole resonance of long high-conducting fibers embedded in an inhomogeneous composite sheet was conducted. The location of the resonance characterizes the effect of the inhomogeneous environment on the electromagnetic response of the fibers. It is shown that the resonance frequency is determined completely by the thickness and permittivity of the composite sheet, in particular, with the anisotropy of the permittivity. No effect due to inhomogeneity of the environment is observed. This is in disagreement with the scale-dependent effective medium theory (SDEMT) that is conventionally exploited to model the permittivity of fiber-filled composites, because this theory shows that the response of the fibers depends on the inhomogeneity scale of the environment. Therefore, although the SDEMT provides qualitative agreement with the observed behavior of fiber-filled composites, it must be further improved to obtain better quantitative agreement with experimental data. The experimental data obtained can also be useful to the development of microwave dielectrics with complex frequency dispersion behavior, which are necessary for many microwave applications.