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Dear LF,
this morning, June 4th between 6 and 8 UT, Walter
(DJ2LF) and myself (DF6NM) were able to successfully conduct a two-way contact
(aka "QSO") on 8.97 kHz. The distance between us is 20.2 km, well outside the
reactive near field zone (lamda / 2pi = 5.3 km).
Both of us were using our small backyard Marconi
antennas at home, for transmit and receive. Walter's antenna has 260 pF at an
estimated effective height of about 5 m. He achieved about 370 mA current,
resulting in a radiated power of about 5 uW (EMRP). I also got 5 uW from 270 mA
into 240 pF at about 7 m. I resonated the antenna using the same little "yoghurt
coil" described a while ago, having 1.3 henry from 9 x 480 turns of 0.2 mm
magnet wire, and a room-temperature DC resistance of 820 ohms. Walter used a
scaled up version of this coil, about 25 cm diameter and 60 cm long, consisting
of seven polypropylene household buckets inserted into one another. Each bucket
has 460 turns of 0.4 mm wire on it, about 320 ohms total DC resistance. Each of
us applied approximately 100 W of "RF" power from a car-HiFi
amplifier.
Reception was done by soundcard, without
preamplification. We set Spectrum Lab to 0.95 mHz FFT bandwidth, with noise
blanking on, and samplerate tracking based on the German 23.4 kHz MSK signal. We
chose to transmit 30 minute dashes, separated by 15 minute quiet intervals to
allow for FFT processing delay.
We employed a special "micro-QSO" format, which was
designed to exchange the minimum single-bit report and confirmation in only
three turnovers, each containing a single dash. Station identification was by
frequency alone, with preassigned pairs (8969.92 / 8970.02 Hz for DJ2LF, 8970.0
/ 8970.1 Hz for DF6NM). Generally the lower frequency was assigned to a negative
response (ie. "not confirmed"), whereas the upper one is positive ("received
ok").
DF6NM started by transmitting a carrier on his
lower frequency, from 4:00 to 4:30 UT. This was indeed seen by DJ2LF, who
replied on his upper frequency from 4:45 to 5:15, reporting positive reception.
Then DF6NM confirmed reception on the upper frequency from 5:30 to 6:00. The
whole procedure took two hours.
Attached are screenshots from both sides.
Unfortunately the QRN level this morning was much worse than we had hoped for,
with active thunderstorms in South Italy, Poland and Belorussia. To my
surprise, our dashes still made it through and were unambiguously
visible, with an estimated SNR of 6 to 8 dB. Walter's signal can actually
still be seen between my own dashes on my VLF grabber http://www.mydarc.de/df6nm/vlf/vlfgrabber.htm
.
As far as we know, this was probably the first
two-way amateur QSO on VLF, using truely radiated fields. Another dream came
true...
Best regards,
Markus
(DF6NM)
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