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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
i have about 70 turns of 0.125mm copper wire wound around a 31cm diameter ring. This is then connected to a battery operated 2 stage op-amp preamp circuit and then to my computers audio line in. Im sampling at 96ksps (0 to 48khz can be received) in speclab and here is the resulting spectrum. I can see our TV at 15.625khz as expected but the signal in question is the ~10khz bandwidth one with a center about 34Khz.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
It is difficult to say. It appears to be composed of harmonics. Any chance I could get some of the raw data, such as a .wav file of the soundcard's input?
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Thanks for the file! Always interesting to hear VLF from some other part of the world.
From listening, the signal sounds very familiar to noise generated by a computer monitor or TV set. Try shutting off your monitor and see if the signal vanishes. You might also try shutting off any nearby TV sets. There is also a repetitive broadband pulsing apparent throughout the sample, probably due to CPU noise. An isolation transformer would help with this a great deal (and with some of the 50Hz harmonics), but constructing one that will work at such low frequencies is no easy task.
I also don't see any sign of the military MSK stations in your spectrum, indicating that received signals are probably being swamped by op-amp noise, and the thermal noise in the loop. The level of received stations is proportional to the square of the cross-sectional area of the loop multiplied by the number of turns. If there's any way you can wind the same amount of wire, around a larger frame, say 21 turns at 100cm diameter, you will see a dramatic improvement in the capabilities of your antenna -- about 10dB). A 200cm frame with 10 turns of wire would be an improvement of about 16dB.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
no i don't think it is a TV or monitor, the TV was off at the time and we are not using any more CRT monitors. The circuit i used was 2 stages of using a TL074CN
here is another really weird one, unfortunately i don't have a recording. some interesting points: the pattern is repeated exactly the same every minute with the offset changing every minute some of the lines appear to have a binary type doubling distance between them on the time domain. the set of offsets is repeated every 10 minutes.
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What is this signal?
