Calculator to Estimate Radio System Noise Figure
from Quiet Sky and Earth Noise Measurements

Bob Larkin W7PUA

Purpose - This calculator applies to radio receiving systems operating at frequencies above about 100 MHz. The concept is that the Earth and "quiet sky" are noise sources of (approximately) known levels. Relative measurements of the total receiver input power for both of these allows estimation of the receiver noise contribution. The theory behind these measurments, along with ways to apply this information are contained in a 2003 Microwave Update Paper. The calculator below uses the same equations as the paper.

Important: This calculator is a simplified portion of the Sun-Quiet Sky-Earth (SQE) calculator, and only estimates noise figure (and noise temperature). If you can measure Sun noise, you probably want to use the full SQE calculator.

The initial values for this calculator are for the example case in the paper. Those "Data Input" values should be replaced by the applicable values for your station. To calculate the receiver noise and antenna gain, click on the "Calculate" button. The "Clear" button clears all data and the "Reset" button brings back the sample data.

Environmental Data - One might expect that the quiet sky has almost no noise. This is enough not the case to require some consideration of the equipment, the atmosphere and the background noise. The 2003 MUD paper linked above has considerable discussion of these effects all of which add to the apparent noise temperature (power). In addition, when the antenna is pointed at the Earth, the observed noise temperature is less than the physical temperature, primarily because of reflections of the quiet sky. The best answers come from studying these effects and estimating the values. Sometimes however, one just wants answers, and using the preset values below (TQ=30K and TE=250K) will provide a start. An exception is for frequencies above 15 GHz or so, because of water vapor attenuation. For instance, at 24 GHz, it might be best to add to the quiet sky temperature an amount that depends on the angle of the quiet sky measurement. At 30 degrees elevation, add 60K and at 90 degrees, add 30K. This is again assuming that actual data is not available.

Environmental Data Data Input
Quiet Sky Temp, Kelvin
Earth Temp, Kelvin

Measured Data Data Input
Noise, Earth to Quiet Sky, dB

Calculated Performance
Receiver Noise Temperature, Kelvin Calculated  
Receiver Noise Figure, dB Calculated  

Enter only numeric values (no commas), using decimal points where needed. Non-numeric values will cause errors.

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