Sometimes the noise-factor is given as a “noise-temperature”, or more precisely the equivalent temperature. This value is given in degrees-Kelvin, ie you will never see a negative value!
Conversions between dB and noise-temperatures
To convert a value given in noise-temperature, T, to dB the following formula is used;
were NR is noise ratio, ie not in dB! Converting this expression will give;
It is assumed that the reference temperature is the same as room-temperature. Having about 20 degrees Celsius in a room will give about 290 degrees Kelvin.
A pre-amp with 22 degree Kelvin this corresponds to a noise-factor of;
If a noise-factor of 0,5dB is specified, this will correspond to;
When dealing with “ordinary” systems and amplifiers, ie not very low noise-figures, it’s easiest to use noise-factors in dB. Since the VHF-DX-amateur, or UHF-SHF, is most often dealing with very low signal to noise ratios and very low noise-factors are involved, it’s a very good idea to use noise-temperatures!