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On Tuesday, March 12, 2019 10:03:45 PM you wrote:
> From observation: when a charged cloud passess by, you can get about 15mm
> sparks from a dipole for 80m hung between two 7-story buildings. This
> happens more often than once per second. Say 40m horizontal section + 30m
> vertical coax, the capacitance is around:
>
> 40m*6pF/m + 30m*5pF/m = 390pF
>
> 15mm is about 15kV.
>
> (390*10^-12 [F] ) * 15000 [V] / 1[s] = 0.00000585 [A] = 5.85 uA
> (i hope i got the units right :)
>
> The charging current is probably a bit higher than calculated because
> there is some leakage to ground.
>
> But this is an extreme case when a cloud passess by, and the charging is
> probably via corona discharge (so it's "no risk no fun" again).
>
> Before calculating this i thought the current would be a few orders of
> magnitude higher.
>
> VY 73
>
> Jacek / SQ5BPF
I suggest this paper for theory:
https://arxiv.org/pdf/0906.1280
And in the following link a simple measuring instrument:
https://arxiv.org/pdf/physics/0701296.pdf
73 de Claudio, ik2pii
>
> On Tue, 12 Mar 2019, Markus Vester wrote:
> > Date: Tue, 12 Mar 2019 16:33:26 +0000 (UTC)
> > From: Markus Vester <[email protected]>
> > Reply-To: [email protected]
> > To: [email protected]
> > Subject: Re: LF: Re: Re Loomis? & ... 12.47 Hz
> >
> > Hi Stefan,
> >
> > why not drive the antenna in parallel resonance? This would provide a
> > better impedance match to the low current high voltage DC source.
> > You'd preferably want a high voltage transistor, which could also
> > operate in class-C with low duty cycle.
> >
> > From my experience, 100 uA DC seems overly optimistic. Best I got from my
> > ant (40 m of wire) was 1 to 2 uA, which was actually during a
> > precipitation static event (aka Grabber whiteout). In fair weather, the
> > current is normally not measurable with a 10 Mohm millivoltmeter, i.e.
> > less than ~ 0.1 nA.
> >
> > Best 73,
> > Markus
> >
> > -----Ursprüngliche Mitteilung-----
> > Von: DK7FC <[email protected]>
> > An: rsgb_lf_group <[email protected]>
> > Verschickt: Di, 12. Mrz 2019 16:09
> > Betreff: Re: LF: Re: Re Loomis? & ... 12.47 Hz
> >
> > Well, today: The circuit in operation on the INV-L since last night,
> > 100% WSPR @MF. The reports on the local RX in 20m distance were between
> > 0...+7 dB at night and -11...-9 dB in daylight.
> > On the tree grabber in 3.5 km distance the signal was only detectable at
> > night, levels reaching about -24 dB in average. Nothing in daylight.
> >
> > Where does the energy come from? It must be the voltages coming from
> > other transmitters (DC to HF) which leave voltages up to the range of 1
> > V at this antenna. This RX voltage spectrum is modulated by the circuit
> > and radiates a very weak signal. That's why the signal is about 13 dB
> > weaker in daylight. In this experiment this is an unwanted effect. It
> > causes some kind of offset, i.e. it lowers the SNR difference to a
> > signal generated by cloud charges. So the 'sky-current' must be higher
> > to become clearly observable. And it is easier to detect it in daylight.
> > The effect should be less expressed at LF because the external voltages
> > from other transmitters are the same but the expected voltage from a 100
> > uA 'sky-current' will be 475/137 higher, i.e. the system is 10 dB more
> > sensitive for the observation of the expected effect.
> > Unfortunately there is no LF monitor on the tree...
> >
> > Anyway i need to be more patient. There has been no static rain yet
> > anyway...
> > Meanwhile i read a bit about Miller capacity and
> > https://en.wikipedia.org/wiki/Cascode. I built one out of two BS170 FETs
> > somehow it does not work yet.
> >
> > 73, Stefan
> >
> > Am 11.03.2019 17:24, schrieb DK7FC:
> > > ...the circuit is completed (attached).
> > > First i used a simple rectangular input signal coming from the source,
> > > over an ferrite core. The gate voltage was +-10V. But then, obviously
> > > due to the FETs internal reverse transfer capacitance i got a 200 mVpp
> > > output across 470 pF (tested in the lab, later to be replaced by the
> > > antenna). I built a test source for the assumed 100 uA 'sky-current'
> > > by using a 1 kV voltage source and 10 MOhm in series. As assumed, a
> > > linear rising voltage was observed during the off-times of the FET.
> > > However, together with the voltage coming over the Crss, the residual
> > > rms value even dropped! Thus i had to reduce the gate peak-peak
> > > voltage and ended up with the circuit shown in the attachment. There
> > > is a DC offset and the AC component is as low as possible. The reduced
> > > the unwanted voltage 8 mV.
> > > Then, 50 mV rms at 475 kHz at 100 uA DC 'sky-current'.
> > >
> > > How far will a 50 mV rms WSPR signal be detectable? And will the
> > > 'sky-current' reach 100 uA at all? At least the signal strength will
> > > rise with the square of that current, so the report are nearly as good
> > > as a current measurement.
> > >
> > > 73, Stefan
> > >
> > > Am 11.03.2019 13:29, schrieb DK7FC:
> > >> Hello Jim,
> > >>
> > >> Yes, indeed.
> > >> I'm now preparing the circuit and want to set it up in the evening.
> > >> Found an IRF820 in the 'junk box', a good choice. Or, maybe even a
> > >> BS170 will work. It handles 60V only but i had the following thought:
> > >>
> > >> The ERP at a constant voltage across the antenna (say 50 V rms at the
> > >> fundamental frequency) will rise with the sqare of the frequency (for
> > >> electrically short antennas). But that voltage will not be constant
> > >> in practice. It will drop linearly with the frequency ( ~ 1/f)
> > >> because the sky-current is assumed to be constant and the charge up
> > >> time is T/2. And, doubling the voltage at the antenna will also rise
> > >> the ERP by a factor of 4. All in all it means the the ERP is
> > >> constant, or independend of the frequency!?
> > >> That means, it would be wise to select a band where many RX stations
> > >> are watching and propagation is promising. The number of RX stations
> > >> is higher at MF but (groundwave) propagation is better at LF.
> > >> Since the peak voltage will be lower at MF (shorter charge-up time),
> > >> MF is less critical.
> > >>
> > >> And which mode?! I think the best choice is a 100% duty cycle WSPR
> > >> beacon. It also has the advantage that one can check the results in
> > >> the database. And there are many RX stations, even in the groundwave
> > >> distance and even in less than 100 km here. And i have my tree which
> > >> can detect small differences at very low levels.
> > >> At MF, the BS170 is a good choice i think. And it can be driven by a
> > >> Raspi directly, maybe with a ferrite transformer in the gate-source
> > >> path, to decouple the shack from the antenna...
> > >>
> > >> If that works, one could try LF.
> > >>
> > >> 73, Stefan
> > >>
> > >> Am 10.03.2019 20:18, schrieb [email protected]:
> > >>> All great ideas in the preceding messages.
> > >>>
> > >>> A steady signal from the 136 kHz sky-current transmitter could
> > >>> indicate that a data bit might be available later from the 10 Hz
> > >>> sky-current transmitter.
> > >>>
> > >>> Agreed, the triggered gap sounds better than an HV vacuum relay (for
> > >>> jitter and reliability).
> > >>>
> > >>> Sounds like the voltage divider doesn't need to be more than a
> > >>> gigohm, but perhaps for future reference: a reliable 50kV 1 terohm
> > >>> divider can be made very easily: encapsulate ten 100 gigohm
> > >>> resistors in hard epoxy, being sure not to touch them during
> > >>> assembly. A single linearity calibration after curing will be remain
> > >>> valid +/- 10% for years in varying humidity and temperature.
> > >>>
> > >>> 73,
> > >>>
> > >>> Jim AA5BW
> > >>>
> > >>>
> > >>> -----Original Message-----
> > >>> From: [email protected]
> > >>> [mailto:[email protected]] On Behalf Of DK7FC
> > >>> Sent: Sunday, March 10, 2019 5:52 AM
> > >>> To: [email protected]
> > >>> Subject: Re: LF: Re: Re Loomis?& ... 12.47 Hz
> > >>>
> > >>> Hi Jacek, DC,
> > >>>
> > >>> Yes yes, i already see a new project for summer :-)
> > >>>
> > >>> A floating antenna will charge up to a voltage where the E field
> > >>> strength is high enough to start partial discharges, something like
> > >>> 20 kV maybe, depending on the wire diameters and homogenity of the
> > >>> arrangement, also on pressure and humidity. So there will be a
> > >>> voltage limit given by the arrangement. But a higher field strength
> > >>> in summer will help to charge up the antenna faster, so higher
> > >>> switching frequencies are possible.
> > >>>
> > >>> One could build/use a triggered spark gap,
> > >>> https://en.wikipedia.org/wiki/Trigatron, this will hold higher
> > >>> voltages and you can connect the trigger directly to a ublox GPS
> > >>> module running e.g. at 3 Hz :-) I think this will give a lower
> > >>> jitter than when using a vacuum relay.
> > >>> Could this work over my 3.5 km path if i use my inv-L? I don't think
> > >>> so.
> > >>> The time to integrate will be to short and there will be much QRN
> > >>> during such an experiment, so the SNR will be very low. Currently
> > >>> (at 12.47 Hz) i already have 15 kV DC available...
> > >>>
> > >>> Anyway, interesting questions, there is something interesting to
> > >>> learn.
> > >>> Maybe i will do a few experiments in summer. Building a HV divider
> > >>> and measurement should be no problem...
> > >>>
> > >>> 73, Stefan
> > >>>
> > >>> PS: Assuming 100 uA constant current 'coming from the sky', my 470
> > >>> pF antenna would charge up with 213 kV/s, so it would be possible to
> > >>> switch at 10 Hz and get a 21 kV 'square wave'. Maybe the harmonics
> > >>> could be detected easier than the fundamental frequency.
> > >>> At 137 kHz it would be 1.5 V only (0.53 V rms). This can be switched
> > >>> with a normal FET! Imagine we would switch the antenna permanently
> > >>> at, say 136.172 kHz, using a FET and a ublox GPS module. Then, as
> > >>> soon as the field strength is high enough, we would see a carrier on
> > >>> the grabbers? We need to try that out! Amazing! What a great hobby
> > >>> we have!?!
> > >>> PPS: For this experiment, the antenna can be series resonated, which
> > >>> would help to concentrate the energy to the wanted spectrum...
----
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