In this paper, we propose a fast method for measuring the radar cross section of a complex target at non-normal incidences and Fresnel region antenna-to-target distances. The proposed method relies both on the physical optics approach and on averaging the field distribution over the transmitting and receiving antenna apertures. The ratio between the analytical expression of the radar cross section at far-field and Fresnel region results in a field-zone extrapolation factor. The RCS resulting from the scattering parameters measured at Fresnel region distances is then corrected with that field-zone extrapolation factor. The method is suitable to be used in a perturbed, multipath environment by applying the distance averaging technique, coupling subtraction or time gating. Our technique requires a very simple measuring configuration consisting of two horn antennas and a vector network analyzer. The experimental validation of the proposed technique demonstrates reasonable agreement with simulated radar cross section at non-normal incidence.
AbstractOrthoconic antiferroelectric liquid crystals (OAFLC) are promising materials for applications. The saturated value of the tilt angle for such materials is 45°, while for “classical” antiferroelectric liquid crystals (AFLC) it reaches typically 30°. Homogeneously oriented slab of OAFLC, unwound by surface action of cell walls is apparently optically isotropic for the light beam at normal incidence. Such unwound structure of OAFLC one can call isotropic OAFLC (IOAFLC). Optical indicatrix of OAFLC is represented by an oblate sphere (ellipsoid of revolution) with its short axis perpendicular to cell walls. Here the question arises: “Is it possible to create isotropic AFLC (IAFLC)? An effective refractive index of IAFLC should have the same value for the light beam propagating at each direction, hence the optical indicatrix should be a sphere. In the way of theoretical considerations we found that IAFLC can exist for twisted structure of AFLC characterized by the tilt angle as high as 54.3°. Unfortunately, such a big tilt angle is still not achievable in practice. Calculations show that AFLC can be optically isotropic when the helix is not suppressed by cell walls action.
The problem of the scattering of a plane wave by a long, thin, perfectly conducting wire is studied. The scatterer is loaded at its center by a lumped element. The effects of the loading on the scattering of waves are investigated. Numerical results are obtained for the case of normal incidence. The results show that for relatively short wires, the back-scattering cross sections may be modified effectively by central loading, while for longer wires, the modification is rather difficult to achieve. To nullify the back-scattering cross section completely, it is necessary to use active loading if kh > 3.6. A physical explanation is also presented.
Implantation of nitrogen into silicon, followed by high-temperature annealing causes formation of silicon nitride. At high doses, continuous polycrystalline layers are formed, while at lower doses, discrete precipitates are formed. These precipitates show preferred orientation with the silicon matrix. This paper presents the results of characterization of these semi - coherent precipitates using electron diffraction and cross-section TEM.Silicon (110) was implanted with 150 KeV N+ ions at a dose of 1 x 1018 /cm2 at 500° C at near-normal incidence. Annealing was carried out at 1200° C for thirty minutes in a nitrogen atmosphere. The samples were then analyzed by XTEM using a JEOL 2000 FX STEM operated at 200 kV with the electron beam parallel to [112] Si. A continuous polycrystalline layer of silicon nitride was observed at the mean ion range. In addition, a band of precipitates of α-Si3N4 was observed at about 1 μm below the wafer surface, presumably corresponding to the fraction of ions channeled along [110] Si.
SummaryThe theory of a new method for the determination of reflectivity at truly normal incidence is described. A parallel light beam falls on the specimen after passing through a glass cube with semi-silvered diagonal mounted at the centre of an optical goniometer. The various reflected beams are measured by a photomultiplier fixed to the telescope of the goniometer. Experiment has proved the validity of the method and the spectral reflectivity of pyrite in air and in oil has been investigated.
An UWB Physical Optics Approach for Fresnel-Zone RCS Measurements on a Complex Target at Non-Normal Incidence
