For the purpose of studying the effect of using magneto-dielectric nanocomposite materials on the miniaturization of the physical dimension of an active antenna, Borah and Bhattacharyya of the Microwave Research Laboratory in India incorporated nano-sized CoFe2O4 into low-density polyethylene (LDPE). Although details are not disclosed in literature [1], they seem to have incorporated the desired amount of the nano-sized ferrite particles into the LDPE using a co-precipitation process after nano-sized

ferrite had been obtained by a heat treatment of a precipitated nitrate precursor. They reported that the size of a microstrip antenna can be miniaturized if the nano-cobalt ferrite magneto-dielectric composite is used as a substrate for the antenna, without suffering an appreciable increase in losses.Y. J. An and his collaborators of National Defense Academy of Japan made composites of iron (Fe), titanium dioxide (TiO2), and an ethylene propylene diene copolymer (EPDM) with an intention of developing a microwave absorber capable of absorbing power from electromagnetic waves within a specific frequency range [2]. The idea behind this work is as follows. High electric conductivity and high magnetic permeability are two key characteristics of electromagnetic wave absorbers. However, good electric conductors would absorb all the electromagnetic waves from very low to very high frequencies and, therefore, if the desired outcome is the absorption of electromagnetic waves with specific frequencies, a moderate value of electric conductivity is needed. Furthermore, if the frequency of the electromagnetic wave is high, a capacitive current will flow. This means that dielectric substances that exhibit a high permittivity can be a good absorber of electromagnetic waves with high frequencies, such as microwaves. An and his collaborators used iron particles with an average diameter of 4.0 µm and TiO2 particles with an average diameter of 0.98 µm, and they changed the volume ratios of the two particles, keeping the sum of the two volume ratios nearly constant at about 60%. As a result, they found that a composite consisting of Fe (26 vol%), TiO2 (36.5 vol%), and EPDM (37.5 vol%) satisfies the criterion of a reflection loss of –20 dB at 5.8 GHz.Ta-I Yang of University of Maryland, US, and his co-workers produced composites consisting of surfactant-modified iron oxide (Fe3O4) nanoparticles of various sizes incorporated within a styrene-b-ethylene/butylene-b-styrene (SEBS) block copolymer [3]. According to their report, the bandwidth for antenna applications can be improved significantly at an appropriate ratio of dielectric permittivity and magnetic permeability, while keeping the product of the two at a constant value. They first made Fe3O4 nanoparticles with a size of 11 nm, which were then

used as growth seeds to obtain larger particles. By changing the amount of iron-oleate complex, oleic acid, and seeds systematically, they studied their effects on the size of the nanoparticles.