----- Original Message -----
Sent: Tuesday, October 12, 2010 8:57
PM
Subject: VLF: Detections of 5
microwatt transmission
Dear LF,
for my VLF test transmissions on
the last weekend (Oct 9: 8969.998 Hz, Oct. 10: 8989.997 Hz), I have
received reports from three receiving stations:
- Walter DJ2LF near Erlangen
(20.2 km) received the carrier again in good quality, using 0.95 mHz
resolution. Radiated power and received SNR were quite similar to our
two-way QSO on June 4th.
- Stefan DK7FC in Heidelberg
(178.5 km) reported about 10 dB SNR, and both dashes are still visible in
the QRN minima on his 47uHz grabber window
This was the intended
purpose of the experiment, and a nice counterpart to the earlier
detection of a 200 uW kite transmission on Aug 29th. Even though both of
us were locking the samplerate to a 10 kHz GPS-derived reference, the
dashes appeared about two pixels low - perhaps due to a very minute
rounding error in SpecLab's frequency scale display.
- To my utter surprise, Paul
Nicholson (Todmorden, 1030.5 km) produced two spectra, taken over the
duration of the transmissions:
After taking a deep breath, we
now have to deal with the question whether this is a significant positive
detection. Except for a known central artifact on 8970, the highest peak
appears in the correct frequency bin in both spectra. Naively, one could
then propose that the probability of this happening at random would simply
be the inverse of the number of displayed bins, ie. around 1:230 for
Saturday and 1:170 for Sunday. Thus the combined probability of a false
positive detection on both days would seem to be only 1 in 40000.
Certainly there is a degree of arbitrariness in the choice of the display
range (8 mHz).
Paul estimated that the signal
was about 3 standard deviations on Saturday (0.3% false detection rate),
and 2 sigma in the higher noise on Sunday (5%), giving a combined false
positive probability of 1 in 6667.
We can also look at the
plausibility of the absolute fieldstrength of the peaks (about 0.2 fT,
equivalent to 0.06 uV/m). If I remember correctly, Paul's first detection
of Stefan's kite signal on March 15 was at about 3 fT, and Stefan was then
radiating approx. 1 mW EMRP. Scaling this down to my estimated 5 uW EMRP,
and taking another dB for the slightly higher distance, would
theoretically result in 24 dB less fieldstrength, or 0.19 fT - almost a
perfect match.
So by these lines, it would seem
at least very likely that Paul has indeed observed my feeble signal! We
intend to do repeat the experiment in the near future for additional
confirmation.
Very many thanks to all involved
in this work!
73, Markus
(DF6NM)
