Sergey N. Starostenko

Experimental studies on antenna miniaturisation using magneto-dielectric and dielectric materials

Dielectric or magneto-dielectric materials can be used to miniaturise antennas, but there are many important parameters that must be considered when selecting which material to use. The authors discuss these figures of merit and a rigorous method to compare antennas with different material fillings. A meandered planar inverted-F antenna (PIFA) loaded with magneto-dielectric and dielectric materials is presented as an example antenna for testing. The magneto-dielectric material used is composed of mylar substrate and Fe–SiO2 sheets. Measurement results for the permeability of the material are presented. The radiation mechanism of the meandered PIFA is studied, and the proper position for dielectric and magneto-dielectric filling is discussed and identified. Miniaturisation by commercial dielectric and the presented magneto-dielectric fillings is measured and compared at the same resonance frequency using the radiation quality factor as the figure of merit. It is seen, that the antenna can benefit from the magneto-dielectric filling material in terms of the radiation quality factor only if the placement of the antenna filling is carefully selected.

A broadband method to measure magnetic spectra of thin films

Conventional single-port strip-line techniques to measure permeability of thin films are based on the quasistationary approximation. In this paper, wave phenomena in the measurement cell are taken into account. Particularly, the dielectric response of the sample may be accounted for and removed by the consideration of the geometry of the cell and the sample. An additional measurement with a reference sample with known constitutive parameters allows the directivity error and the frequency response error to be removed from the experimental reflectivity data; accounting for the field inhomogeneity inside the strip cell is also introduced. A proper selection of the reference sample is shown to be critical for the measurement accuracy. The measurement results may also be affected by the demagnetization of the sample. With these corrections introduced, the operating frequency range of the technique is extended up to 8GHz. The measurement results are validated by the data obtained with either standard materials ...

MICROWAVE SCREEN WITH MAGNETICALLY CONTROLLED ATTENUATION

The efiect of magnetic bias on dielectric spectra of composite sheets fllled with Fe or Co-based microwires is studied experimentally and via simulation. The permittivity is measured using a free-space technique within the frequency band from 6 to 12GHz. The bias is applied either parallel or perpendicular to the microwave electric fleld; the bias strength varies from 0 to 2.5kOe. The composites with Fe-based wires reveal a single region of bias dependent permittivity under bias about 800{1000Oe. The composites with Co- based wires reveal two such regions: the high-fleld region is close to that of composites with Fe wires, and the low-fleld region corresponds to the coercive fleld of Co wires (2{3Oe). The high-fleld efiect is related to the dependence of ferromagnetic resonance (FMR) parameters on bias; the low-fleld efiect is related to the rearrangement of the domain structure of Co-based wires. The interference of magnetoimpedance and dipole resonance is analyzed, revealing the efiects ofi wire length, diameter, parameters of magnetic resonance and composite structure. The results are considered in view of application to the problem of controlled microwave attenuation. Simulation shows that the narrower is the FMR spectrum and the higher is the admissible loss of a sheet in a transparent state, the wider is the dynamic range of attenuation control. The attenuation range of a lattice of continuous wires is smaller than that of a screen with identical wire sections, where the magnetoimpedance efiect is amplifled resonantly. At 15GHz frequency the strength of the bias switching opaque sheet with Fe-based wires to the transparent state is about 2000Oe. For 3dB admissible loss, the range of attenuation control about 10dB is feasible in a composite with aligned wire sections. If the aligned sections are distributed regularly, the loss in a transparent state is about 1dB lower.

A laminate of ferromagnetic films with high effective permeability at high frequencies

The paper reports on development of magnetodielectric material with high microwave permeability. The material is a laminate of multi-layer permalloy films deposited onto a thin mylar substrate by magnetron sputtering. The deposited films are arranged into a stack and glued together under pressure to obtain the laminate. With the content of ferromagnetic component in the laminate being 22 % vol., its measured quasistatic permeability is 60. The peak value of imaginary permeability attains 50 and the peak is located near 1 GHz. As compared with the multi-layer films, which the laminate is made of, it exhibits lower magnetic loss tangent at frequencies below the magnetic loss peak and may therefore be useful for many technical applications. Lower low-frequency loss may be attributed to pressing of the glued sample. This rectifies wrinkling appearing due to sputtering of rigid multi-layer film onto flexible mylar substrate and, therefore, makes the magnetic structure of the film more uniform.

Laminates of Thin Ferromagnetic Films for Microwave Applications

Thin ferromagnetic films are known to exhibit the highest possible microwave permeability of known magnetic materials. Magnetic materials with high microwave magnetic performance are useful for many technical applications. However, bulk rather than planar materials are frequently needed for the applications. Bulk materials with high microwave permeability may be produced as laminated structures of thin ferromagnetic films, possibly patterned. The paper presents experimental data on the microwave permeability of such laminated structures based on Fe films. Possible technical applications of the materials under study include thin wideband radar absorbers and miniaturized patch antennas.

Determination of sendust intrinsic permeability from microwave constitutive parameters of composites with sendust spheres and flakes

Intrinsic permeability of sendust alloys is determined from the measured microwave permittivity and permeability of composites filled with either spherical or flaky sendust powders. The permittivity and permeability measurements are performed applying the coaxial reflection-transmission technique in the 0.05 to 18 GHz frequency range. The effects of the filling factor, inclusion shape, and size on composite constitutive parameters are discussed. The permeability of metal inclusion is retrieved from the measured permeability of composites using a generalized Maxwell Garnett equation that accounts for the percolation threshold. The equation parameters are found by fitting the measured dependence of composite permittivity and permeability on frequency and filling. The inclusion dimensions calculated from the found parameters agree with the results of grain-size analyses. The alloy intrinsic permeability is retrieved from inclusion permeability with the account for skinning. The fitted frequency and damping f...

The Study of Structure-Dependent Properties of Thin Magnetic Films at Microwaves by Field-Domain Resonance Technique

The strip-line frequency-domain technique for permeabil- ity measurement is compared to the fleld domain technique. The com- bined setup for microwave measurement of thin fllm permeability with both techniques is proposed. The fleld-domain technique is less afiected by inhomogeneity of measurement strip cell and has signiflcantly higher signal-to-noise ratio, but the obtained parameters are afiected by fllm thickness and may difier from that of the frequency-domain technique. Analysis of the fleld-domain data obtained at a set of frequencies makes it possible to determine the saturation magnetisation, the anisotropy fleld and the damping factor without the knowledge of the amount of substance under study. In case of a simple permeability spectrum the data on metal thickness make it possible to estimate the efiective skin-depth as well. The technique is tested by simulation and is ap- plied to determine permeability of Fe-based fllms vacuum-sputtered on glassceramic and polymer substrates.

Influence of substrate on the high-frequency permeability of thin iron films

The ferromagnetic resonance (FMR) spectra of thin metallic films obtained by magnetron deposition on polymeric and ceramic substrates are investigated in the strip line at frequencies of 0.13–12 GHz via frequency and external magnetic field sweeping. The influence of mechanical stresses on the FMR spectra of films deposited on an elastic (polyethylene rephthalate) substrate is discussed. The magnetostriction contribution to the anisotropy field of a film, as well as the influence of tensile stresses on the quasi-static permeability and FMR frequency, is estimated. It is demonstrated that the microwave properties of a thin metallic film are also specified by the properties of the substrate with such a film. A distinction in the magnetic properties of films with the same composition, which are deposited on different substrates, is explained in terms of the magnetostriction effect.

Metamaterials Fabricated of Amorphous Ferromagnetic Microwires: Negative Microwave Permeability

Microwave permeability of glass-coated ferromagnetic amorphous microwire exhibiting a weak negative magnetostriction has been studied. The diameter of the microwire was about 20 m and the diameter of the metal core was about 12 m. The microwire was wound to comprise a 7/3 washer-shaped composite sample with the volume fraction of magnetic constituent of about 10%. The permeability of the composite sample was measured in a coaxial line in the frequency range from 0.1 to 10 GHz. The composite was found to exhibit a negative permeability within the frequency range from approximately 0.7 to 1.5 GHz, with the permeability being as low as −0.4. Therefore, microwire-based composites, particularly, crossed arrays of microwires may be employed to develop metamaterials for microwave applications. In the composite, the negative microwave permeability is due to the natural ferromagnetic resonance and the negative microwave permittivity is due to the inherent inductance of the wire. Such metamaterials are advantageous in simple design, isotropic in-plane performance, and possible tunability of performance by external magnetic bias. However, for a feasible metamaterial fabricated from microwire arrays, the wires have to exhibit higher magnitude of the ferromagnetic resonance, higher quality factor, and higher resonance frequency.

Control over magnetic spectrum of multilayer magnetic film metamaterial

A RLC electric circuit with magnetic core is studied experimentally and theoretically as a promising design of a metamaterial cell. Laminates made of multilayered ferromagnetic films are used as the magnetic core. The wire coiled around the core allows the frequency dependence of permeability to be adjusted according to needs of a particular task by creating a region of intensive magnetic loss below the ferromagnetic resonance frequency of the bare core. The theoretic analysis is based on the quasi-statics of magnetic fields and electric currents. The intensity of the loss peak is proportional to the value of μ′2/μ″, where μ′ and μ″ are the frequency-dependent components of permeability of the core material. The magnetic spectra of cells with cores made of laminates of NiFe films and FeCo films have been measured. It is shown that the application of the winding allows the magnetic loss peak to be shifted from 1 GHz to 0.3 GHz for NiFe and from 5 GHz to 0.7 GHz for FeCo. The effective imaginary permeabilit...