|
Since 10:25, Stefan is on air again with his ground
loop. Signals on his own grabber are even better than yesterday, perhaps due to
somewhat lower QRN. I am also looking here but don't have much hope,
as the TX antenna is N-S oriented and Nuernberg is 180 km due
east.
Good luck,
Markus (DF6NM)
----- Original Message -----
Sent: Friday, August 27, 2010 3:25
PM
Subject: VLF: Transmitting 250W at 8970
Hz on a 600m spaced earth antenna
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
|
|
