Thank you for the explanation Stefan

Looks really hard to get oriented in such a dense wood !
Good idea to pimpoint the exact location from GPS ;-)

OK to the low impedance input of NPN. And agree the ZDiodes may help there
I had the 70m perimeter horizontal loop installed last summer on top of the 
building
tower. A nearby lighting induced a voltage peak which damaged a few things. The
motor speed regulator for variable capacitor, a USB hub and other controller
card. That explains my concern about big loops when lighting strikes around

What you mentioned about adding capacitors in parallel to the antenna and not by
increasing topload reminds my idea of using the UltraBeam vertical dynamic 
antenna
as a high voltage variable capacitor. I use the antenna fiberglass pole as 
support of the
vertical section of the MF/LF wire antenna. Inside the fiberglass pole, a metal 
element is
deployed (SteppIR style). Actually use this to adjust the MF antenna and works 
very
well. Will be interesting to check how far can i get down resonance with the LF 
coils
using this variable capacitor approach and try to calculate the capacity range 
I can get

Which capacitors do you plan to use to add those 1760pF ?

I plan to add the VLF monitor in my remote (2nd QTH) at 7Km in Albir IM98wx
Not a perfect clean enviroment for low bands, but population is much reduced in 
winter
so the man made noise sources too. I have a 5m mast for the miniwhip and would 
be able
to hang a squared (diamond shaped) loop there oriented to central Europe

Yes, the idea is to keep a permanent grabber and be useful for you, Markus or 
the Italian group
working at VLF and beyond. A remote Rx at 7Km has proved to be a very useful 
tool for me so far
and the plan is to keep it running

73 de Luis
EA5DOM
________________________________________
De: [email protected] [[email protected]] en 
nombre de DK7FC [[email protected]]
Enviado: domingo, 31 de julio de 2016 15:18
Para: [email protected]
Asunto: Re: ULF: Key down signal on the 101 km band, still on the air

Hi Luis,

Am 31.07.2016 13:53, schrieb VIGILANT Luis Fernández:
> Hi Stefan
>
> Very nice experiment down there. Congratulations again :-)
>
Thanks. But so far there is no result. On monday i will drive there
again and hope to find the equipment and a few 10 GB of wav data :-)
But i'm optimistic. I took a short recording before i started the long
recording. It is for finding the equipment again because i didn't
exactly where i have been in the forest :-) The recording contains the
NMEA data and so the exact location :-)
On this recording i saw that the noise background is MUCH lower than on
the tree and in pre-tests in 5 km distance to the city.
Here is a spectrum of the recording with 250 ms scroll rate:
https://dl.dropboxusercontent.com/u/19882028/ULF/JN49KM.jpg
You see the resonance is a bit sharp and not perfectly on the frequency
but i hope it will work well anyway. It even looks like the gain of the
preamp is not sufficient. On the test QTH in 5 km distance to the city
the gain was high enough (the noise background increased 15 dB relative
to a dummy antenna). So this gives an idea how low the background noise
is in this location!
When this test is completed i will do another one in 30 km distance with
6 dB more pwoer :-)

> I have been trying to track your calculations but got lost with your antenna 
> voltage figures
> At 6470Hz with L=1.29H that should be an antena reactance of 52K Ohm, right ?
>
Right.
> Then at 270mA antenna current that should be much more than 3.2KV ?
>
It will be 6.5 kV then.
Currently i am running 3.2 kV with this arrangement:
https://dl.dropboxusercontent.com/u/19882028/ULF/ULF%20loading%20coil%206.1H.jpg

> Another question about your multi turn Rx loop. Any proteccion for nearby 
> lightings ?
>
Ehm, no :-) I will see if there is a problem later. I thought it is a
low impedant input, a NPN transistor, not a N-CH J-FET or OP AMP.
Ah BTW here is a photo of the loop in the forest:
https://dl.dropboxusercontent.com/u/19882028/ULF/20160730_170105.jpg
Not so easy to find between the trees but it is visible. The
circumference is 40m and the height above ground is maybe 4m.
And https://dl.dropboxusercontent.com/u/19882028/ULF/20160730_165911.jpg
shows the recording equipment which will hopefully not be stolen...

> I'm planning to set a VLF Rx here and concerned about this. Would be a good 
> idea to
> short the loop with a relay in case of nearby storm to protect the NPN and Rx 
> chain ?
>
I would rather tend to use anti-parallel diodes instead, maybe 2 or 3
pairs in series. Or maybe Z-Diodes. Must be checked if it has an effect
on the noise figure.

Where do you want to place it and what is the goal? Will you run a
grabber 24/7? Which of the bands is in the focus?

73, Stefan


> 73 de Luis
> EA5DOM
> ________________________________________
> De: [email protected] [[email protected]] 
> en nombre de DK7FC [[email protected]]
> Enviado: domingo, 31 de julio de 2016 0:28
> Para: [email protected]; Renato Romero
> Asunto: Re: ULF: Key down signal on the 101 km band, still on the air
>
> Hi all,
>
> The 100 nW ERP 2970 Hz carrier has been on the air since 26th of June,
> 13 UTC. The transmitter phase has been stable since this time. There
> were only a few very short interruptions, 90 minutes on the 17th of July
> and then about 5 hours (5:45....11 UTC) on the 21st of July.
> So it has been on the air since nearly 5 weeks.
> The signal is still on the air but tomorrow 8 UTC i will do a frequency
> shift of + 2.5 mHz. Later i will do an EbNaut test.
> If someone still want to integrate the overall transmission into one
> peak, tomorrow, 31th July 8 UTC will be the stop time.
>
> In a few days i will then come back with a 6 dB stronger signal:
> Coming from the LF world we assume that using a parallel capacitor to
> the antenna is not a good idea. More C will mean less L, so you need
> less wire and could think that this lowers the losses. But higher
> currents are then needed which will cause even higher losses...
> Thus i didn't consider to use a parallel C on ULF too. BUT on ULF, with
> voltage limited systems it can be a good choice!
> I run the yoghurt cup coil at 270 mA antenna current on my 470 pF
> antenna at 6470 Hz. So it must have 1.29 H. So at 2970 Hz and 270 mA it
> must have 6.5 kV! Currently i have about 3.2 kV. So with this
> alternative matching i will get a 6 dB stronger signal.
> I will then need 1.76 nF parallel to the antenna.
>
> My 17 km distance experiment is running now! I have installed the loop
> antenna somewhere in JN49KM. The Raspy is now recording stereo wav files
> to the 32 GB USB stick... I'm using 2x 7 Ah lead acid gel batteries for
> the supply. 1PPS+NMEA is used on the right channel.
>
> 73, Stefan
>