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... been scratching my head about that 10 dB
deficit. Errors in the scope measurement, or in the effective height estimate?
There are uncertainties but it seems hard to explain a factor of three. Or
has ZEVS gone to power save mode?
I now suspect there may be a more fundamental
reason behind the discrepancy. When converting from magnetic to electric
field, I had simply assumed that their ratio is the free space wave
impedance Z0, ie. E / H = 377 ohms, or equivalently
E / B = c. However in a waveguide whose
height (60 km) is smaller than the wavelength (3600 km), this may no longer
be the case, but the wave impedance could be less. Then on a
relative scale, magnetic antennas would be more effective than
electric antennas (even though in practice it still
seems much easier to receive the Schumann's on a whip than a loop).
This could also explain why the formula for radiated power on http://en.wikipedia.org/wiki/Ground_dipole predicts an order of magnitude more radiation than my
simple loop-in-free-space approach.
73, Markus
PS. Most likely I'm stumbling upon things
which have been known for decades. For me, it's new and fascinating
anyhow....
Sent: Wednesday, September 03, 2014 4:48 PM
Subject: Re: LF: RE: ZEVS question
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)
-----Ursprüngliche Mitteilung-----
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:
Hi Eddie, LF group,
it looks like despite very strong 50 Hz
and 16.7 Hz harmonics, I can still receive useful ELF signals from my
Marconi (currently only 3 m above the roof) with a simple BF245
source follower. The top panel of my VLF grabber http://www.df6nm.de/vlf/vlfgrabber.htm currently
shows the ZEVS carrier, as well as several Schumann
resonances.
Best 73,
Markus (DF6NM)
Sent: Tuesday, September 02, 2014 11:14 AM
Subject: Re: LF: RE: ZEVS question
As part of my checking that everything is OK and ready for the
next
tests on ELF/ULF/SLF ? I have my Grabber
http://www.g3zjo.talktalk.net/vlfgrabber.htm
Live on Wide
and ZEVS. ZEVS is showing a good carrier at this time in my
urban
environment.
73 Eddie
G3ZJO
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