Hi Markus and group,
Congrats for the nice achievement with such a (relatively) small
antenna. I also did some tests with the Alpha signals in the past days
but don't know how to interpret them, because -like you- I am not sure
about their ERP. 500 kW as seen in a list must be the TX power.
Anyway, for comparison here is my 'Alpha' spectrum:
http://www.qsl.net/dl4yhf/t/alpha_spectrum_zoomed.png
This spectrum was made with an effective RX bandwidth of 13.5 mHz . It
also confirms that the distribution of the peaks depends on the
location. The antenna was an *indoor* loop which explains the high noise
level, but despite that the beeps are easily audible even in a 3 kHz
audio bandwidth.
Cheers,
Wolf .
P.S. 7 liters of Stracciatella yoghurt for the plastic coil 'formers'...
yumm yumm... a very nice idea ! ;-)
Markus Vester schrieb:
Dear LF,
on two evenings this week, I have transmitted an 8.97 kHz signal from
my LF Marconi at home, and attempted to receive it at various
locations. The experiment was very similar to the one in April 2003,
but with a moderate improvement in ERP and FFT bandwidth. Now on both
occasions, the carrier could be detected at a distance of 12.1
km: http://www.mydarc.de/df6nm/vlf/vlf_12km.jpg
My transmit antenna is relatively small, about 220 pF and 9 m
effective height at 137 kHz. Assuming a 20% reduction due to
shielding, radiation resistance would be around 74 microohms at 9 kHz.
The 1.4 henry loading coil is about 30 cm long by 12 cm diameter, and
is split into seven slightly conical sections, partly inserted into
one another (http://www.mydarc.de/df6nm/vlf/9kHz_aircoil.jpg). Each
section has 700 turns of 0.2 mm enameled wire, total DC resistance is
830 ohms. Fine tuning is achieved by shifting a thick block of ferrite
into the last section. Using a 35 W car-radio audio amplifier and a
1:32 ferrite transformer, I now got up to 0.135 A and 11 kV rms at the
antenna. Radiated power was thus approximately 1.3 uW (EMRP).
I used the same 6 m portable receive antenna with series inductor as
before. I tried connecting directly to the microphone input of the
netbook computer, and also inserting a simple bipolar preamplifier,
which was fed from the 2.5 VDC present at the mic jack. Both versions
turned out to have almost the same sensitivity, but resonance peaking
was less critical with the transistor. Postprocessing was now done
using SpecLab, with software noise blanking, and either 15 mHz or 3.8
mHz FFT bin width. SNR at 12.1 km was somewhere around 5 dB in 1.5x
3.8 mHz. With an expected signal of 0.9 uV/m there, this would imply a
noise level on the order of 16 dBuV/m/sqrtHz. However on the last
receive site at 15.4 km, no trace of the signal could be retrieved.
The lowest of the Alpha navigation frequencies was included in the
decimated frequency range to check soundcard drift. Due to the
repeating dashes, the beacon spectrum is split into several lines
1/3.6 Hz apart. The true center frequency (16*15625/21 = 11904.762 Hz)
is one of the weaker lines here. But this depends on the relative
phases of the two strongest stations, and will be different in other
areas.
The reception could possibly be a new amateur VLF distance record.
However with all the ongoing activity, I expect (and actually hope ;-)
it won't last long...
Best wishes,
Markus (DF6NM)
_______________________________________
Von: "Markus Vester" <[email protected] <mailto:[email protected]>>
An: <[email protected] <mailto:[email protected]>>
Betreff: LF: Re: 9kHz noise level
Datum: Sonntag, 28. Februar 2010 14:06
Dear Alexander, LF,
the frequency rulers of the modified Argo are actually correct, and
you can see how I reduced the bandwidth when going further away. The
minimum setting was 90 second dots, giving 0.042 Hz FFT resolution
when running at 4x normal samplerate (ie. 0.063 Hz noise BW) .
The marginal "T" trace at 6 km was probably no more than 0 dB SNR.
Thus the noise level (including spherics) would have been on the order
of 15 dBuV/m/sqrtHz.
Best 73,
Markus, DF6NM
_______________________________________
Von: "Markus Vester" <[email protected] <mailto:[email protected]>>
An: <[email protected] <mailto:[email protected]>>
Betreff: LF: Re: 9kHz noise level
Datum: Samstag, 27. Februar 2010 23:11
Dear Jim, LF,
yes I'm aware of the fact that the shielding from trees etc is more
significant at lower frequency. Their ohmic conductance becomes a
better shunt in comparison with decreasing capacitive admittance,
somewhat similar to a C-R highpass equivalent circuit. There used to
be two beautiful 15 m high fir trees in the vicinity of our house. At
137 kHz, I measured a ~ 15% increase in effective height when the
trees were deeply frozen, but the effect on 9 kHz may have been more
severe. A couple of years ago our neighbours had these trees chopped
down, good for LF but otherwise sad.
In April 2003, I attempted to transmitt an 8.97 kHz carrier, radiating
about 1 microwatt from my normal LF antenna (220 pF at ~ 9m eff.
height). I drove around and stopped in different places, putting up a
6m fishing pole with a wire, connected to a resonant circuit and the
laptop soundcard. Each time I took a short Spectrogram full-band
screenshot, along with a narrowband capture from a special Argo
version, patched for 22 kHz samplerate. An assembly of the screenshots
is at http://freenet-homepage.de/df6nm/8970_ALL.gif. Maximum
detection range was 6 km, just marginally outside the reactive
nearfield. No noiseblanking was attempted at the time.
If you look at the Spectrogram strips, you can see that the first (1.6
km) and third (6.0 km) images have a much lower absolute receive
level. At first I thought something was wrong with the receive
antenna, until I realized that this was purely due to these sites
being in a forested area.
I have now rigged up SpecLab again for VLF reception. The Russian
Alpha beacons seem to be usefiul calibration markers, the nearest one
is currently about 20 dB SNR here in a 42 Hz FFT. Does anybody in the
group have information about their EMRP, or has someone attempted to
measure their fieldstrength in Europe?
Best 73,
Markus (DF6NM)
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