Near-field effects in thermal radio emission

Our research is related with a new idea in the area of subsurface radiometry. This idea is based on a specific character of the quasi-stationary part of a thermal emission field (evanescent modes at interface). Until the present time this problem remained experimentally unsolved. The difficulty of these measurements is related to the strong influence of the media on antenna parameters, because the quasi-stationary field could be measured only at a small distance above the surface and only using electrically-small antennas (much less than the wavelength in a medium). The theoretical analysis shows that the effective depth of the formation of quasi-stationary component depends on the height of antenna above the surface of a medium and on the antenna size. At the surface, this skin-depth could be very small (for small antennas); it increases with the antenna height, and at the height comparable to wavelength in the medium it converges to skin-depth for the wave component of thermal emission. So, it is possible to discover the influence of the quasi-stationary field by measurements of the temperature-stratified medium using for the calibration the same medium at two different constant temperatures. Two near-field effects could be detected: (i) the effective radio brightness dependence on the height of the small antenna and (ii) radio brightness dependence on the aperture size at small distance above the surface. These effects lead to new one-wavelength methods of non-invasive temperature sounding of absorbing media, such as water and living tissue