Date: Mon, 25 Oct 2010 23:18:07 +0200
From: Pieter-Tjerk de Boer <ptdeboer@cs.utwente.nl>
To: rsgb_lf_group@blacksheep.org
Message-ID: <20101025211807.GA17136@cs.utwente.nl>
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Subject: Re: VLF: DK7FC's 8th VLF kite experiment
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Hello all,

Very nice that Stefan's VLF signals were received in lots of places!

Unfortunately, his signals were not seen on the grabber that I installed
at the University of Twente's WebSDR (in the Netherlands, JO32KF).

Analysis of the recording made during last Saturday revealed two problems:

1) A software/timing issue that caused some data from the soundcard to
   be dropped every now and then, due to the high CPU load on this
   computer. When listening live (e.g. to SAQ on Sunday), this is notable
   as annoying ticks in the sound, but when trying to integrate the
   signal over a long time, such interruptions of the data are fatal.

2) Much funnier: when listening to the recording, I heard some speech-
   and music-like sounds, especially when tuning to 0 kHz USB, i.e.,
   just listening directly to the signal as delivered by the antenna.
   In the end, the sound turns out to be the Dutch radio station
   'Q-music', broadcasting on 100.7 MHz FM!
   There is an FM broadcast station (with 3 transmitters of several kW
   each) on the chimney of the same university building where our radio
   club (and thus the WebSDR) is, and we often have problems with these
   strong signals getting into our equipment.
   Apparently my VLF active loop antenna also acts as a VHF antenna
   and demodulator. The demodulator presumably is slope detection, with
   the loop resonating near 100 MHz, since its (electrical) circumference
   is about 3 m.
   Some decoupling capacitors should cure this...

Last week we tried to install the loop antenna outdoors on the roof,
hoping to get a lower noise floor there than indoors, but instead found
the noise floor much higher outdoors. I now guess that that was also
due to the strong FM broadcast signals.

Oh well, I'll fix these problems before Stefan's next attempt...

73, Pieter-Tjerk, PA3FWM