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 DK8KW Longwave Information Background Noise

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I recorded the background noise at my QTH for 24 hours to see what the best window for a sked might be. I wake up early anyhow, so 04.00 UTC is the best time, but also the time up to 10.00 UTC is fine. After that time, the noise level is too high. I have no idea what causes that high noise, but especially the sharp noise decrease just before 04.00 UTC is a clear evidence for man-made noise.
   
 
I used a simple multimeter with RS-232 output (Voltcraft/METEX M-3850) and measured the s-meter level output of my  MV61 (0-2 Volt). One datapoint every 2 1/2 minutes was recorded, which results in about 600 datapoints over 24 hours. The data where then imported into Excel and the voltage (0 to 2 Volt) converted into dBu (75). The frequency 137.400 kHz was selected, because this is a frequency that is relatively free, bandwidth was 100 Hz.
     
   

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