Friis formula (Derivation and problems) and Equivalent Noise Temperature

Friis originated the concept and terminology “noise-figure” and “noise-factor” while working at Bell Labs in Holmdel, New Jersey, in 1942. Fundamentally, Friis’s NFgen “noise-figure” is a function of two variables that is a dB metric that answers the question, “If I connect a particular block (like a single transistor amplifier, or an entire complex receiver chain) to a source (like an antenna) that is putting out noise power in addition to the signal, how much worse (lower in dB) will the block’s output SNR be, relative to the SNR going into the block?