Calculator to Determine Radio System Performance
from Sun, 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 Sun, Earth and "quiet sky" are noise sources of (approximately) known levels. Relative measurements of the total receiver input power for all three allows estimation of the receiver noise contribution and of the antenna gain. The theory behind these measurements, along with ways to apply this information are contained in a 2003 Microwave Update Paper. The calculator below uses the same equations the paper. 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.

Current Solar Intensity - The information on the noise output from the Sun can be set by either of two different methods. Most internet data sources show Solar Flux Units (SFU) corresponding to particular RF frequencies. This data can be interpolated to find the SFU corresponding to the frequency of interest. For the following two buttons, choose "Solar SFU and Frequency" for this type of input. Solar intensity, i, will be calculated from this SFU and frequency.

Alternatively, if you have already calculated the Solar Intensity, i, this can be entered directly by checking the button, "Solar Intensity."

Solar SFU and Frequency
Solar Intensity


Current Solar Flux Units (SFU)
Receiving Frequency, MHz


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, a t 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

Noise Figure - If one only has measurements of the Quiet Sky and the Earth and no Sun noise measurement, the estimate of Receiver Noise Temperature/Noise Figure can continue. Put any value in for "Noise, Sun to Quiet Sky" measurements and ignore the calculated antenna gain. Alternatively, a separate calculator is available that does only the noise figure/noise temperature calculations.

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

Calculated Performance
Receiver Noise Temperature, Kelvin Calculated  
Receiver Noise Figure, dB Calculated  
Antenna Gain, dBi Calculated  

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

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