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| Subject: | Re: LF: RE: ZEVS question |
| From: | Markus Vester <[email protected]> |
| Date: | Wed, 3 Sep 2014 10:48:12 -0400 |
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Eddie, Stefan,
yes I was surprised and exhilarated to find the Schumann humps along with the 82 Hz signal. The quest to see those always seemed like some kind of holy grail regarding low frequency work ;-)
Actually I had tried to see ZEVS a couple of times in the past without success. But that was without any preamplification, suffering from severe mismatch loss between the small antenna capacitance and the 1 kohm soundcard input. Another attempt using the 6 squaremeter magnetic loops in the garden also failed due to low induction voltage and coupling to underground cables.
The source follower has only a couple of resistors at the gate, 2.5 Mohm in series and 10 Mohm shunt. Together with the capacitances of antenna (270 pF) and a piece of coax (200 pF), the shunt makes a cutoff at 27 Hz. There is a 10 uF coupling capacitor to the soundcard which inserts another pole at 10 Hz, and in addition the soundcard internally fades out below about 7 Hz.
Connecting a scope to the source, I am seeing monster signals from the grids, about 1.2 V pp on 16.7 Hz (railway at 400m) plus 0.3 Vpp on 50 Hz (relatively weak due to underground supply lines). The recording gain slider is set low such that the ADC input range is about 2 Vpp. With 75% of clipping levels, the little SpecLab input scope just occasionally becomes red. Note that all software postprocessing (like noiseblanking or notch filtering) is disabled.
Working back through mismatch, frequency response, and antenna height, the interfering fieldstrength at this location is around 1.2 V/m at 16 Hz and 0.15 V/m at 50 Hz. The railway also has a strong fifth harmonic around 83.3 Hz, which is about half a Hz wide but fortunately doesn't seem to leak much into the ZEVS band.
The ZEVS carrier itself reaches -91 dB fullscale, translating to about 30 uV/m or 0.1 picoTesla. From Fig.2 in http://www-ee.stanford.edu/~acfs/82Hz.pdf, the fieldstrength at 2.5 Mm should be -130 dBA/m or 0.4 picotesla in the main lobes (77 and 257 degrees azimuth). At 220° from Murmansk, one would expect a reduction by cos(37°), giving 0.32 pT. Thus my observation is still about 10 dB short of expectation. There also seems to be a bit of diurnal variation both in Schumann and ZEVS levels, exhibiting a shallow minimum at sunrise.
Best 73,
Markus (DF6NM) Von: g3zjo <[email protected]> An: rsgb_lf_group <[email protected]> Verschickt: Mi, 3 Sept 2014 2:44 pm Betreff: Re: LF: RE: ZEVS question Hi Makus, Stefan, All
Very jealous here of Markus' Schumann resonances, the response of my E-Probe filter necessarily cuts off at that end. I did try different responses a while ago and if I let much 50Hz in it overwhelms everything at this location. It is nice to be able to see the 16.7Hz and harmonic which is sort of DX here. 73 Eddie G3ZJO On 02/09/2014 23:45, Markus Vester wrote:
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