Attenuation Of Microwave Radiation Research Articles

Attenuation of Microwave Radiation by Post-Anode Plasma in a Composite Grid Electrode Structure

This paper presents a new composite grid electrode structure for the generation of plasma that is suitable for modeling microwave radiation transmission through atmospheric plasma. The new grid anode-composite cathode (GA-CC) structure is to be contrasted with the ordinary grid anode-cathode plate (GA-CP) structure. It is found that the breakdown voltage of the new GA-CC electrode structure is lower than that of the GA-CP structure and is a bit shifted to the right on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Pd</i> -axis. An advantage of the GA-CC structure over the GA-CP structure is that it requires a lower voltage to generate the same current. It is found that plasma generated by the new GA-CC structure selectively strengthens electromagnetic waves attenuation in the frequency range of 9–12 GHz.

Enhancement of EMI shielding effectiveness of flexible Co2U-type hexaferrite (Ba4Co2Fe36O60)-poly(vinylidene fluoride) heterostructure composite materials: An improved radar absorbing material to combat against electromagnetic pollution

In this report, for the first time, Co2U-type hexaferrite (Ba4Co2Fe36O60) nanomaterials have been considered along with their conjugation with a poly(vinylidene fluoride) (PVDF) matrix for the fabrication of heterostructure composite materials with superior electromagnetic interference shielding effectiveness (SE) that can produce better attenuation of microwave radiation. The high value of magnetizations of ∼7.21 and 10.63 emu/g is significant for the improvement of the shielding effectiveness due to absorption (SEA) of the composite materials. The unique combination of magnetic and dielectric responses of these Co2U-type hexaferrite nanomaterials helps to get significantly high total shielding effectiveness (SET) in their PVDF composite states within the frequency range of 8–18 GHz. A very high value of SET of approximately −83 dB at 14.2 GHz with &amp;gt;99.999 999% of attenuation of incident radiation has been observed for the Co2U-type hexaferrite–PVDF composite materials. The β-phase modified PVDF structure also plays the most important role to generate induced polarization inside the structure of composite materials. Also, this PVDF matrix provides the application friendly laminated structure of Co2U-type hexaferrite–PVDF composite materials. Structural, morphological, and chemical studies reveal the presence of the polycrystalline phase of Co2U-type hexaferrite and β-phase crystallization of polar PVDF in the multi-phase Co2U-type hexaferrite–PVDF composite materials. This study has helped us to understand the importance of magnetic Co2U-type hexaferrite nanomaterials for the fabrication of lightweight, large area, flexible, and thickness-controlled radar absorbing materials to combat against electromagnetic pollution.

Estimating Gravimetric Water Content of a Winter Wheat Field from L-Band Vegetation Optical Depth

A considerable amount of water is stored in vegetation, especially in regions with high precipitation rates. Knowledge of the vegetation water status is essential to monitor changes in ecosystem health and to assess the vegetation influence on the water budget. In this study, we develop and validate an approach to estimate the gravimetric vegetation water content (mg), defined as the amount of water [kg] per wet biomass [kg], based on the attenuation of microwave radiation through vegetation. mg is expected to be more closely related to the actual water status of a plant than the area-based vegetation water content (VWC), which expresses the amount of water [kg] per unit area [m2]. We conducted the study at the field scale over an entire growth cycle of a winter wheat field. Tower-based L-band microwave measurements together with in situ measurements of vegetation properties (i.e., vegetation height, and mg for validation) were performed. The results indicated a strong agreement between the in situ measured and retrieved mg (R2 of 0.89), with mean and standard deviation (STD) values of 0.55 and 0.26 for the in situ measured mg and 0.57 and 0.19 for the retrieved mg, respectively. Phenological changes in crop water content were captured, with the highest values of mg obtained during the growth phase of the vegetation (i.e., when the water content of the plants and the biomass were increasing) and the lowest values when the vegetation turned fully senescent (i.e., when the water content of the plant was the lowest). Comparing in situ measured mg and VWC, we found their highest agreement with an R2 of 0.95 after flowering (i.e., when the vegetation started to lose water) and their main differences with an R2 of 0.21 during the vegetative growth of the wheat vegetation (i.e., where the mg was constant and VWC increased due to structural changes in vegetation). In addition, we performed a sensitivity analysis on the vegetation volume fraction (δ), an input parameter to the proposed approach which represents the volume percentage of solid plant material in air. This δ-parameter is shown to have a distinct impact on the thermal emission at L-band, but keeping δ constant during the growth cycle of the winter wheat appeared to be valid for these mg retrievals.

Attenuation of microwave radiation at 34.09–34.19 GHz in a composite material based on diamond micron powder structured by a nanocarbon binder vapor-deposited at subatmospheric pressure

Microwave attenuation in samples of a diamond micron powder based composite material structured by a nanocarbon binder vapor-deposited at subatmospheric pressure is measured at frequencies of 34.09 to 34.19 GHz in a cylindrical resonator at H111 mode. Return losses are determined from the measurements. Volume resistivity of the samples is measured. The potential of using diamond-containing structured composite materials as microwave radiation absorbers is demonstrated.

Measuring snow liquid water content with low-cost GPS receivers.

The amount of liquid water in snow characterizes the wetness of a snowpack. Its temporal evolution plays an important role for wet-snow avalanche prediction, as well as the onset of meltwater release and water availability estimations within a river basin. However, it is still a challenge and a not yet satisfyingly solved issue to measure the liquid water content (LWC) in snow with conventional in situ and remote sensing techniques. We propose a new approach based on the attenuation of microwave radiation in the L-band emitted by the satellites of the Global Positioning System (GPS). For this purpose, we performed a continuous low-cost GPS measurement experiment at the Weissfluhjoch test site in Switzerland, during the snow melt period in 2013. As a measure of signal strength, we analyzed the carrier-to-noise power density ratio (C/N0) and developed a procedure to normalize these data. The bulk volumetric LWC was determined based on assumptions for attenuation, reflection and refraction of radiation in wet snow. The onset of melt, as well as daily melt-freeze cycles were clearly detected. The temporal evolution of the LWC was closely related to the meteorological and snow-hydrological data. Due to its non-destructive setup, its cost-efficiency and global availability, this approach has the potential to be implemented in distributed sensor networks for avalanche prediction or basin-wide melt onset measurements.

Structural transformations of supercooled water in nanopores, studied by microwave radiation

It is shown that the state of supercooled water in nanoporous materials can be studied by measuring the attenuation of microwave radiation. By analyzing variations in the intensity of radiation transmitted through a moistened medium during the supercooling of water in the range of −37 to −190°C, the temperatures at which structural transformations take place can be determined. Using the example of KSKG silica gel with a mean pore size of 8 nm, it is shown that at a moisture of 3–18%, water is found in the liquid state up to a temperature of −130°C, at which the transition to glass occurs.

The Effect of Intercepted Precipitation on the Microwave Emission of Maize at 1.4 GHz

Terrestrial microwave emission is sensitive to soil moisture. Soil moisture is an important yet unobserved reservoir of the hydrologic cycle linked to precipitation variability. Remote sensing satellites that observe terrestrial microwave emission have the potential to map the spatial and temporal variabilities of soil moisture on a global basis. Unfortunately, terrestrial microwave emission is also sensitive to water within the vegetation canopy, and the effect of free water residing on vegetation, either as intercepted precipitation or dew, is not clear. Current microwave emission models neglect the effect of free water. We found that the precipitation intercepted by a maize (corn) canopy increased its brightness temperature at 1.4 GHz. This effect is opposite that of dew: dew decreases the brightness temperature of maize at 1.4 GHz. The increase in brightness temperature due to the intercepted precipitation was only about 1 K for vertically polarized brightness temperature and about 3 K for horizontally polarized (H-pol) brightness temperature. It may be acceptable to neglect the effect of free water in microwave emission models. A more serious concern, however, is the underestimation, by current microwave emission models, of the sensitivity of the H-pol brightness temperature to soil moisture through maize. Understanding the physics associated with the effect of free water in vegetation on the emission, scattering, and attenuation of microwave radiation will lead to improved emission models, and potentially, models that correctly reproduce the sensitivity of the 1.4-GHz brightness temperature to soil moisture at high levels of biomass when vegetation effects are greatest.

Variable Microwave Transmission Attenuation by Conducting Polymers

AbstractThe microwave properties of polypyrrole, polyaniline and poly(3-hydroquinonylpyrrole) films were examined as a function of doping level, conductivity, counter anion and thickness. The microwave transmittance of the polymer films was varied from less than 10% to greater than 90%; by oxidizing and reducing the polymers. The potential use of conductive polymers in the design of an electromagnetic modulator for the tunable attenuation of microwave radiation is described.