SM7OVK
Noise-temperature

Introduction

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!
Calculating with noise-temperatures is very easy since one just add the figures when calculating a total noise-temperature, this could later be converted into a noise-figure.
Noise-temperatures are most often used at very high frequencies, as LNBs for satellite reception or microwave-links. (When it’s LNBs for satellite reception you normally only see these types of figures if you are dealing with C-band reception. Ku-band LNBs are normally given in dB.)
When noise-temperatures are used for VHF or UHF systems it’s most often for very low noise systems were it would be unpractical to use figures in dB – the figures will be to similar in dB when it’s only differing by 1-2 degrees in noise-temperature.

Conversions between dB and noise-temperatures

To convert a value given in noise-temperature, T, to dB the following formula is used;

NR = T/290 + 1

were NR is noise ratio, ie not in dB! Converting this expression will give;

T = 290 ( NR – 1 )

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.

Calculating examples

A pre-amp with 22 degree Kelvin this corresponds to a noise-factor of;

NF = 10*log(T/290 + 1) = 10*log(22/290+1) = 0.31756… = 0.32dB

If a noise-factor of 0,5dB is specified, this will correspond to;

T = 290*( 10(0.5/10) – 1 ) = 35.3853... = 35.4K

Conclusion

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!

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