Essais VLF DK7FC electrodes enterrées

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F4DTL
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Essais VLF DK7FC electrodes enterrées

Message par F4DTL » 27 août 2010, 18:54

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/schaef ... abber.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/l ... rmany.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

F4DTL
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Re: Essais VLF DK7FC electrodes enterrées

Message par F4DTL » 28 août 2010, 12:09

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)

F4DTL
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Re: Essais VLF DK7FC electrodes enterrées

Message par F4DTL » 28 août 2010, 21:45

Hi Michael, Markus, VLF,

Well, time and frequency absolutely corresponds with my transmission so
i would say it is a positive reception. Anyway i think we should do the
test again, sending a message or a some charakters to better confirm the
reception. Maybe DD7PC is QRV in DFCW-600 (abt 4 mHz) again the next time?

The distance of the todays experiment is: 49.6 km.

My antenna bearing is 9/189 deg, the QTH of Michael is in an angle of 54
deg to this antenna. So, not an ideal orientation, if the antenna acts
like a loop, but a weak reception anyway. The picture shows the
locations of TX and RX. The yellow line is the distance, the white line
is the antenna orientation. TX power was 250 W, just like yesterday.
Antenna current will be measured in the next experiment...

Tnx to Michael for the effort to catch that signal (Markus as well).

73, Stefan/DK7FC

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