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Dear LF,
today I have slowed down the 8.97 kHz grabber and
reduced the FFT bandwidth down to 3.8 mHz. As expected, the noise level
decreased by 6 dB (even though the detection
threshold for a dash of given duration will probably have improved only by 3 dB,
due to the tradeoff between incoherent and coherent averaging).
Soundcard crystal temperature instability is just starting to become an issue,
with the Alpha lines going down a couple of bins when I open the
window.
The dominating noise source at this location seem
to be railway overhead lines, 400 meters from here. They produce a
dense and audible carpet of variable 16.6 Hz multiples up to about 8 kHz,
with some significant enery up to about 12 kHz. I will attempt to run
the grabber from another location sometime next week, using mobile internet
access.
BTW I was stunned to find a little knot on each of
the Alpha lines, just before 21 UT every evening. This must be the
daily phase adjustment mentioned by Trond Jacobsen:
Best wishes,
MArkus (DF6NM)
----- Original Message -----
Sent: Sunday, March 07, 2010 4:50
PM
Subject: 8.97 kHz Grabber online
Dear LF,
many thanks to the group, for the nice and
positive feedback regarding the recent 9 kHz experiments!
I have now lashed up a temporary VLF grabber,
featuring 15 mHz FFT resolution around 8970 Hz:
Sensitivity still leaves a bit to be desired,
with a number of local noise sources around. I won't be able to leave it on
permanently due to limited computing resources, and I will have to disconnect
it during LF transmit activity.
Best regards
Markus
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