didac
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There's a million ways to see the things in life
Posts: 247
manresa,spain
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Hi, If I remember correctly an antenna will only seen an equivalent to black body radiation if it's inside a perfect anechoic chamber since it will achive thermal equilibrium with the environment, in reality KTB it's KTaB (Ta it's the antenna equivalent noise temperature). T=290ēK it's obtained through statistical measurements in earth environment in the more general case(or for a more realistic estimation) it's necessary to know the noise sources that "see" the antenna and apply: Ta=nradTb+(1-nrad)Tp where :nrad is the antenna efficiency Tb:equivalent brightness temperature seen by the main beam(secondary lobes pick up noise also but usually are neglected for this calculation) Tp:it's the physical temperature of the antenna(it takes into account the ohmic component of the antenna). So in reality if an antenna picks up through it's main lobe a noisy source you can see a higher level of thermal floor(or viceversa). Hope it helps, PD:in which band are you working?, maybe it could be a problem of fast fading on the received signal so the SNR will degrade(or improve) but not caused by noise, i.e it's not a true AWGN channel, it's a frequency selective-time varying one and the attenuation it's not fixed.
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