Calculator to Interpolate or Extrapolate
Solar Flux Unit (SFU) 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 antena gain. This calculator is a help for finding the Solar Flux Units at the operating frequency. In general the measurements of SFU are not at this RF frequency. A log-log interpolation is applied, assuming that over a limited range of frequencies, the SFU is found by multiplying a constant times the frequency taken to some power.

The data in the boxes is a sample only and gets replaced with real values. The "Clear" button clears all data and the "Reset" button brings back the sample data.

The internet data sources show Solar Flux Units (SFU) corresponding to particular RF frequencies. Select the two frequencies, f1 and f2 that are closest to the operating frequency. It is best to select f1 and f2 to surround the operating frequency. If this is not possible, the operating frequency may be above or below both f1 and f2. Note these frequencies as well as the SFU values for entering into the calculator.

Internet data may be available from multiple measuring facilities. There is no rating of data quality, and thus no best method for determing the "correct value." The data has already been evaluated for obvious errors and thus averaging of the multiple SFU values could be justified. Typically the differences are small and this is of minor importance.

Now, enter the two pairs of measured frequency and SFU values in the boxes below. Then enter the operating frequency, in MHz, and click on "Calculate."

Internet Data Frequency, MHz SFU value
Lower Frequency Data f1   S1  
Higher Frequency Data f2   S2  

Operating Frequency, MHz Data Input, f  

SFU at Operating Frequency Calculated S 

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

The answer for the test case should be SFU = 324.5. This is slightly higher than was used in the 2003 MUD paper (315) which was derived graphically.

Note the SFU value above, and return to the Noise Figure/Ant Gain calculator by your browser back button, or for fresh data, click here.