----- 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)
