Reflection Of Radio Waves Research Articles

Cement-matrix composites for smart structures

Cement-matrix composites for smart structures are reviewed. Thefunctions include strain sensing, damage sensing, temperature sensing,thermoelectricity, vibration reduction and radio wave reflection. Thefunctions are rendered by the use of admixtures, such as short carbonfibers, short steel fibers and silica fume.

In situ measurement of reflection and diffraction coefficients of UHF radio waves on buildings using a ring array

The lack of reliable estimates of the reflection and diffraction coefficients of radio waves by urban structures is one of the main barriers to deterministic modeling of the UHF channel. In complex environments such as urban areas, the reception of multiple echoes of the transmitted signal generally prevents direct in situ measurements of these coefficients. For the present study, the contributions of the different paths to the measured received power were separated by coupling the channel impulse response sounding with the determination of the direction of arrival (DOA). From experimental data we were able to isolate the field component generated by reflection or diffraction on buildings by both delay and angle of incidence. We deduced the corresponding reflection and diffraction coefficients. The wave DOA was determined using a ring array. The angle of arrival patterns were extracted by two different methods, namely the Wiener inversion method and the beam‐forming method. The coefficients deduced by these two techniques are shown to be very close. These results are also similar to those obtained without the help of DOA separation when delay resolution makes them available. We also show the validity of geometrical rays to represent reflection and diffraction of decimetric waves on the kind of building concerned.

Estimation of the number of frequencies and bandwidth for the surface measurement of soil moisture as a function of depth

The feasibility of obtaining estimates in situ of volumetric soil moisture as a function of depth using measurements of radio wave reflection at the soil surface at several discrete frequencies was demonstrated through computer simulation. By deriving empirical relationships between the number of frequencies, the frequency range, and the number of soil layers for which the soil moisture is estimated, the number of frequencies and measurement bandwidth have been determined. The soil moisture profile was obtained to a depth of 1.5 m by inverting, through function optimization, a simulation of the reflection coefficient from layered dielectric materials. The dielectric properties of soil-water mixtures mere obtained from the models of (1962) and (1980) and the function optimization was done with Powell's direction set method (1992). The number of soil layers in which soil moisture could be resolved was found to be twice the number of frequencies used. The required bandwidth increased with the number of soil layers from 20 MHz for six layers to 140 MHz for 16 layers. Within some assumptions about the nature of the radio wave propagation, the theoretical accuracy of the estimate depended only on the quantization error introduced by having to consider discrete layers of uniform soil with finite thicknesses.

Possible levels of reducing radio wave reflection from an aerodynamic object covered by an absorbing plasma

We estimate the possible levels of reducing backward reflection of radio waves from an aerodynamic object covered by an absorbing plasma created on the object at tropospheric flight altitudes. This effect is analyzed in terms of a model one-dimensional problem of radio wave propagation in inhomogeneous layers of absorbing plasma.

Soil moisture profile estimation from surface measurements at multiple frequencies

It is possible to obtain estimates in situ of volumetric soil moisture content (SMC) as a function of depth using measurements of radio wave reflection at the soil surface at several discrete frequencies. The feasibility of the method was demonstrated through computer simulation by deriving empirical relationships between the number of frequencies, the frequency range, and the number of soil layers for which the SMC is estimated. The SMC profile was obtained to a depth of 1.5 m by inverting, through function minimisation, a simulation of the reflection coefficient from layered dielectric materials. The number of soil layers in which SMC could be resolved was found to be twice the number of frequencies used. The required bandwidth increased with the number of soil layers from 20 MHz for 6 layers to 140 MHz for 16 layers. Within some assumptions about the nature of the radio wave propagation, the theoretical accuracy of the estimate depended only on the quantisation error introduced by having to consider discrete layers of uniform soil with finite thicknesses. The method may have potential for enabling sensitive measurements of SMC profiles at lower cost and lower complexity than methods which use analysis of steps or pulses, such as time-domain reflectometry, and should be well suited for routine use by horticulturists and soil researchers.

Recent advances in radar studies of the polar ionosphere

Early studies of the auroral zone ionosphere were undertaken at Tromso in Northern Norway by Sir Edward Appleton during the Second Polar Year (1932-33). The sounding radars employed were based on the principle of radiowave reflection by the ionospheric layers. During the last 30 years a new generation of radars has been developed, based on the principle of `coherent' and `incoherent' scatter by irregularities in ionospheric plasma. Two of these new systems are described. The EISCAT Svalbard Radar (ESR) is a UHF incoherent scatter radar, which extends the present EISCAT viewing area from the auroral zone well into the polar cap. The second system is the CUTLASS HF coherent radar, which provides a large viewing area including both the auroral zone and polar cap regions. Some of the exciting new results obtained with these radars are discussed in detail.

Radiative transfer in a layer of magnetized plasma with random irregularities

The problem of radio wave reflection from an optically thick plane uniform layer of magnetized plasma is considered in the present work. The plasma electron density irregularities are described by a spatial spectrum of arbitrary form. The small-angle scattering approximation in invariant ray coordinates is proposed as a technique for the analytical investigation of the radiation transfer equation. The approximate solution describing the spatial and angular distribution of radiation reflected from a plasma layer is obtained. The solution obtained is investigated numerically for the case of ionospheric radio wave propagation. Two effects occur as a consequence of multiple scattering: a change in the reflected signal intensity and an anomalous refraction.

Reflection and transmission characteristics of radio waves at a building site due to reinforced concrete slabs

The structure of reinforced concrete (RC) slabs at a building site changes from reinforced iron-bar mesh and concrete to many kinds of concrete walls as construction is completed. As the structure changes, its radio characteristics also change. It is very important to know such characteristics as radio-wave reflection, wave penetration, and wave scattering of concrete in order to use mobile phone and other radio systems at the construction site. This study tries to establish a practically useful scheme to obtain radio characteristics from a simulation experiment using a scale model and electromagnetic analysis. We first analyze the reflection and transmission characteristics, the complex dielectric frequency characteristics of the concrete, and the dependency of these characteristics on the percentage of water included in the concrete. Taking into account the characteristics of the multilayer dielectric model, we carried out an experiment and did calculations for obtaining the characteristics of the basic iron concrete wall, and the validity of the scheme was confirmed. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(8): 68–80, 1998

Reflection and transmission characteristics of radio waves at a building site due to reinforced concrete slabs

Reflection and transmission characteristics of radio waves at a building site due to reinforced concrete slabs

On Seasonal Variation of the 162 kHz Radio Wave Reflection Height: Model Calculations and Their Comparison with Experiments

The seasonal variation of the 162 kHz radio wave reflection height was measured indirectly at Panska Ves using the IPHA method described in Fiser and Matys (1992). The fact that these waves are reflected at the height where the electron concentration is about 3.5 × 10 8 m −3 was used to model this height using a 1-D model of the lower ionosphere electron concentration described by Ondraskova (1993). The comparison of the measurements and model results indicates that: 1. The annual mean of the reflection height derived from measurements is several km higher than that derived from the model; 2. the character of the modelled seasonal variation is different from the real variation shown in Fiser and Lastovicka (1992). The differences are discussed.

Reflection of radiowaves from multilayer protective materials

It is shown that (UHF) radiowave methods for the diagnostics of both thin (surface corrosion) and thick (multilayer composites) materials are highly sensitive. It is also demonstrated that the unambiguous monitoring of the location of defects in multilayer materials can be performed either by using specially selected frequencies for each special type of structures or, more universally, by the solution of inverse multifrequency problems.

Artificial airglow due to modifications of the ionosphere by powerful radio waves

A new model of the artificial airglow due to the ionospheric modification is proposed. The model is based on a recently developed nonlinear theory of stationary striations, which predicts a strong enhancement of the electron temperature inside the striations. According to our model the neutral species can be excited by the hot electrons inside the striations, which spread both upward and downward from the source region located between the radio wave reflection and the upper hybrid resonance point. The discussed model allows us to estimate the local electron temperature inside the striations, and to assess the intensity of red‐ and green‐line emissions due to the excitation of O(1D) and O(1S) levels by the electron impact. These results are checked by comparison with the observations.

Spectral characteristic of the coherent millimeter-wave radar echoes from humans in motion

Results of an experimental investigation of coherent 2-mm wave band signals reflected from a moving human are considered. It is shown that the reflected spectrum is characterized by two maxima located in the vicinity of zero and the mean Doppler frequency. Data are presented on radio wave transmission and reflection factors for some kinds of clothes, as well as statistical characteristics of the reflected spectrum for different directions and magnitudes of the target velocity.

Long-term decrease of mesospheric temperature, 1963–1995, inferred from radiowave reflection heights

Reflection heights of 164 kHz radiowaves in the lower D region at the geographic coordinates 50N, 10E have been continuously monitored by an experimental configuration which has been kept unchanged over more than 30 years, thus providing the longest continouos and homogeneous time series of mesospheric data presently available (apart from NLC observations). A gradual but steady subsidence of the reflection heights since the 1960s has already earlier indicated a long-term change of the structure of the underlying mesosphere. Introducing a minimum of model assumptions, it is now possible to evaluate these data for the summer months (May through August) in terms of neutral air pressure at a fixed altitude of 81.8 km. Our present analysis achieves a clear separation of the long-term trend from a superimposed quasi 11-year solar cycle and provides evidence that during the period from 1963 till 1995 the column-mean temperature between the stratopause and 81.8 km has steadily decreased with a linear rate of about −0.6 K per year, in good agreement with the results derived by other authors from Lidar and rocket soundings.

Reflection of VLF radio waves from the Canadian Rockies

Wideband (2–53 kHz) VLF recordings have been made on an aircraft flight from Honolulu, Hawaii, to St John's, Newfoundland, via Comox on Vancouver Island. Clear periodic variations have been observed in both the amplitude and phase of the VLF signals from both navigation and communications VLF transmitters, and these oscillations reached maximum amplitude in the region above the Coast and Rocky Mountains. We suggest these oscillations were produced by the reflection of the VLF signals from the mountains. The oscillations were seen on signals typical of single‐mode propagation and on those undergoing significant modal interference. We measured reflection coefficients of up to 18% at the Omega navigational frequencies and 43% at the frequency of the NPM transmitter (23.4 kHz). The effect of the reflections at the Omega frequencies was seen more than 3000 km before the mountains, but the effect of reflections at the higher VLF frequencies, although greater in magnitude, was much more localized to the immediate region of the mountains.

Effect of Segmentation of Conductive Coatings on the Radio Wave Reflection from Solar Control Sheet Glass

Solar control glass often becomes an obstacle to television broadcast wave reception inside a room, because coated conductive films can reflect incident electromagnetic waves with frequencies lower than that of the plasma oscillation of the films. Results of calculation of reflection coefficients from theoretical equations showed that the radio wave reflection from solar control glass decreases by dividing the conductive film into discontinuous segments with a periodic array. The segmental effect has been examined experimentally. Experimental results for the system of periodic conductive segments fixed on soda lime silicate glass agreed very well with the theoretical results when the edge effect of the array was very small. It was found also the power reflection coefficients for aluminium foil segments of 25mm square were less than 0.05 below 200MHz.

The Ionospheric Focused Heating Experiment

The Ionospheric Focused Heating rocket was launched on May 30, 1992. The sounding rocket carried an instrument and chemical payload along a trajectory that crossed the intersection of the beams from the 430‐MHz incoherent scatter radar and the 5.1‐MHz high‐power radio wave facility near Arecibo. The release of 30 kg of CF3Br into the F region at 285 km altitude produced an ionospheric hole that acted like a convergent lens to focus the HF transmissions. The power density inside the radio beam was raised by 12 dB immediately after the release. A wide range of new processes were recorded by in situ and ground‐based instruments. Measurements by instruments flying through the modified ionosphere show small‐scale microcavities (<1 m) and downshifted electron plasma (Langmuir) waves inside the artificial cavity, electron density spikes at the edge of the cavity, and Langmuir waves coincident with ion gyroradius (4 m) cavities near the radio wave reflection altitude. The Arecibo incoherent scatter radar showed 20 dB or greater enhancements in ion acoustic and Langmuir wave turbulence after the 5.1‐MHz radio beam was focused by the artificial lens. Enhancements in airglow from chemical reactions and, possibly, electron acceleration were recorded with optical instruments. The Ionospheric Focused Heating experiment verified some of the preflight predictions and demonstrated the value of active experiments that combine high‐power radio waves with chemical releases.

Reflection of radiowaves from a multilayered medium with rough boundaries and inhomogeneities of the permittivity of the layers

The scattering of radiowaves by a multilayered medium with large-scale roughnesses of the boundaries and inhomogeneities of the permittivity of the layers is theoretically studied. It is shown that there are different mechanisms resulting in the oscillating dependence of the incoherent component of the reflected power Pic on the inverse wavelength λ-1 and the layer thicknesses. It is also shown that for weak reflections the fading effect of scattering for an n-layered medium is at least of orderPic/Pc ∼ (σh1/λ)2n, whereσh12 is the dispersion of roughnesses of the upper boundary and Pc is the coherent component of the reflected power.

Measurement and numerical modeling of radio sounding reflection from a lightning channel

Electric field impulses and radar echo signal from lightning were measured synchronously. The measured electric field was used to calculate currents in return strokes which were used in numerical modeling of lightning discharge. On the basis of this simulation the radiowave reflection from a lightning channel was calculated. From the comparison of measured and computed radar echo signals the validity of the lightning discharge and the radiowave reflection numerical model was stated. Also, a continuous current was found between the return strokes.

Measurement of distances to nearby thunderstorm discharges

A method for measurement of distances to nearby thunderstorm discharges in the earth-ionosphere interval is examined that is based on differences in the frequency dependences of the static induction and radiation components of the electromagnetic field. Radio-wave reflection from waveguide walls is described by a series of imaginary sources of decreasing amplitude, which allows estimation of the nearzone size, which is about 50 km. It is proposed that the wave impedance of the field be measured to eliminate the effect of the source's spectrum. Simple analytic formulas are obtained that relate characteristic points of the wave-impedance spectrum to the distance to a vertical cloud-earth thunderstorm discharge.