Dear VLF,
Today i spent some hours to do the next experiment with my earth
electrode antenna that i have installed in a forest near Heidelberg.
The 2 earth electrodes are about 600 m spaced, with 20 copper earth
rods at each end. Location is 49.44894N_8.724619E.
The todays goal is to reach the RX of Michael Oexner in about 50 km
distance at 8970 Hz. He is one of those who successfully copied my
signal on my 2nd VLF experiment, 15th, March (> 70 km distance).
Today he started running his PC using the soundcard and SpecLab as the
RX. His antenna is a PA0RDT design. He is still at work, so i am
awaiting his report with interest;-) It would be a new distance record
on VLF with earth electrode antennas.
The day appeared reasonably suitable in the morning hours. Although
there was some rain, the QRN was down to -90 dB on my DFCW-600 window
on the VLF grabber
(http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber.html),
just about the level as in my last test. The last test was done with
the 300 m antenna and about 70 W. There, the signal was about 10 dB
above noise on my grabber in 5.0 km distance. A summary of that can be
found on rogers site:
http://g3xbm-qrp.blogspot.com/2010/06/live-897khz-transmission-from-germany.html
. Today i transmitted about 0,2 Hz lower since DF6NM also tried to copy
the signal and he has currently some noise at 8970.0 Hz.
Today i used the SMPS, generator and the small PA (300 W), running at
250 W. Sadly i had forgotten my amperemeter for the antenna current
and so the power was just measurable at the DC side of the PA, taking
about 20 A at 13,8 V DC. The first attempt was arround 8:30...9:00 UTC
(still visible on my grabber) but i didn't get the power into the
antenna (means too high Z), so there was something wrong with it. I
found that the electrode connections were disconnected, most probably
done by some animals. They even tried to grup out some earth rods ;-)
So in the first attempt the antenna was rather a lossy inv L antenna.
When applying about 400 V rms (not kV :-) ) to this inv L antenna, the
signal was about 10 dB above ground anyway (pic1.jpg). After solving
that problem the antenna acted like a loop again.
Sadly a storm and heavy rain came up during the experiment. I couldn't
hear and thunder so i thought it is a normal rain. But there must have
been some charged clouds and charged raindrops falling down since later
i found heavy white vertical lines on the grabber like is is usual
during thunder storms. This kind of "noise" appears rapidly and goes
rapidly, often the noise level increases and decreases by several 10 dB
during just some seconds. So it was a pity that this happend just
during the test. Maybe the situation at Micheal Oexner's QTH is totally
different!
Compared to the last test, the TX power was 6 dB higher and the antenna
has twice the length. The gain due to the increased size was measured
to be 8 dB (RX level difference of DHO-38). But, the signal was 30
dB above noise in 4,5 mHz ( pic2.jpg), (DFCW-600 window) although
the noise was about 5 dB higher than in the last experiment (see Rogers
picture on his site). So, there is another 11 dB gain (35dB
-(10dB+6dB+8dB)). The radiation direction of the antenna and distance
didn't change significantly so where does this gain come from?!
I suppose it is the wet soil that decreases the earth resistance near
the earth electrodes! If this mountain/hill is mostly out of stone, the
inner conductivity does not change too strong but the electrode
resistance seems to do!? So, i mean, a wet soil is an advantage,
not a disadvantege, as long as the overall soil is not wet but just the
upper layer. What do the experts mean? :-)
BTW, i have never seen people looking so astonished, seeing this
generator running in the middle of the forest on a rainy friday morning!
Since it is a first test for a new distance record i transmitted just a
30 minutes carrier, not a message. It would have been easy sending a
message to my grabber since the signal was VERY clearly readable, even
in DFCW-60 (pic3.jpg) the signal was up to 25 dB above noise.
Most unexpected, it was even readable in DFCW-3 (!), 10...15 dB
above noise! (pic4.jpg) :-)
So, hopefully Micheal will tell us some news this evening. If not
positive, we will go on. It was a first attempt at that distance and
the conds were really not perfect. Additionally there is a loss of
about 2 dB if the antenna acts like a loop. Markus/DF6NM received
nothing but if the antenna acts like a loop, it has it's minimum into
Markus' direction, sadly. Marco/DD7PC is in 60km distance and pretty
exact in the radiation maximum of the antenna, so he could become
another RX station? :-)
Lastly, a picture of the impressing foggy forest like it was during
this experiment (pic5.jpg) :-)
73, Stefan/DK7FC
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