the neutrons are no longer able to limit the electron kinetic energy (= ’’thermal runaway”). This might cause the electrons to move along the earth’s magnetic field lines and this again could effect interference with radio communications which depend on properties of the natural ionosphere. These thermal effects ^re expected to become serious at power densities above 30 mW/cm (a more recent investigation found a sharp increase of electron temperature already at power-densities higher than 15 mW/cm2). A limiting power density of 23 mW/cm2 in the ionosphere is used in most publications. Series of experiments of Rice University, however, indicate that the level probably could be raised up to 40 or even 50 mW/cm before causing instabilities in the F-layer. Refraction of the beam by the ionosphere is found to be neg- lectible (the displacement at the rectenna will be less than 100 meters). Also Faraday rotation effects are projected to produce insignificant polarisation losses. Self induced or ambient irregularities of the ionospheric electron density will cause phase-fluctuations across the wave front of the retrodirective steering beam. These random phase variations which will be less than 10°, will subside within a few hundred meters and within tens of seconds. Very little data is available on potential impacts of the microwave beam on the ozone-layer, which might effect a higher ultraviolet radiation on earth. As a conclusion to this sub-category, it can be stated, that, although there have been made several investigations considering these areas of concern, the understanding of ionospheric interactions of the microwave beam is very poor. So, to get more confident information, a lot of studies in this field are very necessary. 1.2.13.3 Interactions between the Beam and the Troposphere These interactions will result in some heating, and therefore some risk of affecting weather patterns does exist. The absorption losses , however, are small. Below 10 GHz the attenuation resulting from molecular absorption is approx. 0.1 dB or less for normal atmospheric conditions. The most severe condition is expected in rain or hail clouds, where attenuation may increase to 4 % at 3 GHz. The theoretical losses produced by a 1 km path through wet hail could reach 13 %. Rain or hail will effect a forward scattering of the RF-beam, but it is not expected to significantly increase sidelobe levels or broaden the main microwave beam. Again, understanding of atmospheric interactions with the beam is in its infancy and requires much additional investigations.
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