Vol 62, No 9 (2017)

A 2D array of radiators with raster frequency scanning focused in the Fresnel zone is presented. The problem of determining the radiator positions for field focusing at a finite distance from the array plane is solved. An approximate matrix model of the array antenna describing its properties as a microwave multiport and allowing determination of the distribution of radiation sources in the array aperture is proposed. The problem of improvement of the array matching in scanning ranges is considered. An approximate model describing radiation from the array is built. This model is used for determining the position and shape of the scanning surface and the field distribution inside the focal spot as well as for the analysis of the dynamics of the field maximum in the frequency scanning sector.

The reflectivity and transmissivity and coherent transmittance of dry homogeneous snow are measured as functions of the thickness of a snow layer at frequencies of 22.2, 37.5, 60, and 94 GHz. The experimental characteristics are approximated using the FIRE, HUT, and Kubelka–Munk models. The experimental frequency dependences of the extinction coefficient and model quantities: absorption, scattering, and flux attenuation coefficients are analyzed. It is shown that the dependence of the model quantities is governed by a power function of frequency in which the exponent for each quantity depends on the structure of snow and the frequency interval in which the remote sensing of the snow cover is performed. It is found that the frequency dependence of the scattering coefficient for coarse-grained snow in the millimeter-wave band is close to linear.

The dispersion and the coupling resistance of spatial harmonics of slow-wave structures formed by a chain of coaxial resonators coupled by arc-shaped waveguides are theoretically analyzed. A 3D vector boundary value problem is solved in the rigorous electromagnetic formulation with the use of projection methods. The 2D problems of excitation of a coaxial resonator are considered and amplitudes of excited waves are determined and used for solution of the 3D problem of the junction of arc-shaped waveguides and a coaxial resonator. The dispersion equation for the considered slow-wave structure is obtained. The results of calculation of the dispersion and the coupling resistance for different numbers and different configurations of coupling apertures are analyzed.

The results of optimization of the geometry of a combined ultrawideband antenna excited by a 1-ns bipolar pulse are presented. Numerical and physical experiments have been performed to optimize the increase in the electric-field strength in the direction of the antenna main beam. It has been shown that the peak field strength can be increased by 6%.

The paper presents an analysis of the errors in the maximum likelihood estimates of the central frequency of narrow band random normal processes from a small number of samples. It is shown that these errors are manifested in the form of large outliers of frequency and caused by the presence of several extrema of the likelihood function. By methods of mathematical modeling, the root-mean-square errors of the estimates of the frequency for different signal-to-noise ratios, correlation times, and sampling periods were obtained and studied. The minimum errors were obtained with sampling intervals of 0.1–0.2 of the period of the central frequency. Methods of reducing large outliers are proposed and studied.

The forced oscillations of the magnetization and elastic displacement in the normally magnetized ferrite plate with magnetoelastic properties are discussed. The process whereby the initial frequency is divided in the ratio defined by the multiplicity of the resonance frequencies of magnetic and elastic subsystems is investigated. The time and amplitude characteristics of the phenomenon are revealed on the basis of the impact excitation model of an elastic system under short-term magnetic actions. Recommendations are given for the practical implementation of excitation-frequency division in the scheme of a magnetostriction transducer.

New promising functional materials—amorphous-crystalline fast-quenched Ti₂NiCu alloys exhibiting the shape memory effect (SME)—are studied. A method for annealing amorphous alloys by electric current pulses is proposed. This method allows one to obtain the needed degree of crystallinity. It is demonstrated that a microcrystalline structure with spherical grains exists in amorphous-crystalline samples. These grains increase in size from 150 nm to 3.2 μm as the degree of annealing increases. The SME is not observed in nontreated samples and is clearly manifested in completely annealed samples. A two-way SME and a trend toward lowering of the martensitic transition temperature are observed in partially annealed samples.

The analytical model of avalanche photodiodes based on the different types of p–n structures is discussed. Formulas for avalanche breakdown voltage V_BD and the exponent in Miller’s relation for dependence between the carrier multiplication coefficient M and the applied voltage V are derived. The obtained results enable us to avoid time-consuming numerical calculations and develop an analytical method for optimizing the structural parameters of avalanche heterophotodiodes (its principles will be reported in Part II).

The results of the development of a focal-plane array (FPA) of the 576 × 6 format with the time delay and integration (TDI) mode are presented. The comparative analysis of different variants of implementation of the TDI mode in ROIC is conducted. The expedience of the upgrade of existing scanning photodetectors of the 288 × 4 format on the basis of the developed 576 × 6 FPA is justified. The result of this upgrade will be simplification of the optical–mechanical scanning unit and improvement of the quality of thermal image.

The 320 × 256 focal plane arrays based on р⁺-B–n-N⁺ tetralayer heterostructures with a wide-gap barrier layer have been investigated. The heterostructures with a narrow-gap n-InGaAs absorbing layer were grown by means of metalorganic vapor phase epitaxy on InP substrates. The band discontinuity between the In_0.53Ga_0.47As absorbing layer and the In_0.52Al_0.48As barrier layer is removed by growing a thin four-component n-AlInGaAs layer with the bandgap gradient variation. Delta-doped layers included into the heterostructures make it possible to lower the barrier in the valence band and eliminate the nonmonotonicity of energy levels. The experimental study of the dark current has been performed. It has been revealed that the average value of the dark current does not exceed 10 fA for the photodiode arrays with a pitch of 30 μm.

Selective metal–AlGaN photodetectors based on the Schottky barrier and operating in UV spectral range have been developed. The selective photodiodes based on Ag–AlGaN Schottky barriers of different composition have been manufactured, which has made it possible to improve the photosensitivity in the UV spectral range and eliminate spurious signals in the long-wavelength part of the UV spectral range. This has made it possible to develop visible-blind photodetectors with the long-wavelength edge of photosensitivity lying at the wavelengths less than 350 nm. The width of the photosensitivity spectrum is within 15–40 nm, depending on the thickness of the Ag layer, which varies from 15 to 150 nm. The proper choice of the composition of the Al_xGa_1–xN solid solution ensures increase in the photoresponse and reduction of the FWHM spectrum width up to 11 nm by matching peaks of the Ag transmission spectrum and the absorption spectrum of the epitaxial layer. The sensitivity is 0.071 A/W. The combination of effects of wideband window and overthe- barrier transfer has made it possible to create the ultraselective UV photodetectors based on Au–AlGaN structures with a half-width of the photosensitivity spectrum of 5–6 nm for the wave range 350—375 nm and a sensitivity of up to 140 mA/W. Based on a structure with the upper Al_xGa_1–xN epitaxial layer (with the AlN content x = 0.1 or x = 0.06), selective photodetectors with the maximum photosensitivity at wavelengths of 355 nm and 362 nm have been developed. Application of an additional less wideband GaN layer has made it possible to independently control the short-wavelength and long-wavelength boundaries of the sensitivity range.