Der VLF dreamers, LF,
I am glad to present you my new (and first) grabber for the dreamers
band! It is running (in the web) not even one hour!
Take a look at:
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber.html
Next, i will add a ALPHA beacon window as well.
The grabber accuracy is that of the soundcard (one time calibrated with
SpecLab to the ALPHA stns).
Many thanks to Markus/DF6NM and Wolfgang/DL4YHF for the support to
become QRV. Also tnx to my professor who allowed me to host the page
directly on our institutes server (makes it easy and secure to upload
the pictures).
The grabber works with an active E-field antenna (Daniele, i can give
you the circuit if you want) using 2x BF862 and a 500Hz band filter
arround 9 kHz.
BTW: The input had a 1 MOhm Resistor to prevent static overvoltages to
the gate. Therefore the highpass edge frequency of the input C (abt 3pF)
and R= 1MOhm was about 55kHz. This afternoon i changed this resistor to
22MOhm, giving a edge frequency of abt 2,4 kHz. The difference can be
seen now on the upper picture. In the left side you can see the signal
of yesterday, in the middle i shut down the preamp to charge the battery
and in the right part you see the signal with the 22MOhm resistor. I
have not changed the volume level or QTH, just the supply voltage by
<1V. So, using a high R resistor, well above 1MOhm makes sense on VLF!
On LF(137), 1MOhm is fine!
The preamp is fed by a 3,4 Ah battery. This has to be charged each 2
days. So, i will take a solar cell ( i have many 21V/55Wp) to keep the
supply decoupled from the local earth. I use RG174 and a AF decoupling
transformer. Now, the antenna is placed temporarely in the 2nd floor.
Since everything seems to be OK i will place it on the roof of the
building, in the 7th floor. Then, i will probably use the fiber optical
cable to further reduce the local noise and prevent HV problems due to
lightnings!
Distance to my /p QTH is abt 37km, so i think i can write a message in
DFCW60 without any problems.
BTW: This weekend i wanted to do the next experiment but the wind
forecast is not very promising! But we will see... The 600W PA is ready
to use! So, 1000km could be cracked? :-)
Wish you fun and motivation to improve everything in the LF/VLF world :-)
73, Stefan/DK7FC
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