Hello Marcus,
first of all congrats with your achievement.
Based on the numbers you gave I tried to figure out the loss resistance
at 9kHz:
- 35W and 0.135A would give 1920 Ohm.
- with a 32/1 transformer the TX (audio amp) would be loaded with 1.8
Ohm
Assuming that the loss at 9kHz is slightly more than the DC loss (let's
say 900 Ohm) the "ground loss" (including greenery and
buildings) would be about 1 kOhm.
Can you confirm this figures ?
73, Rik ON7YD
At 21:16 6/03/2010, you wrote:
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]>
An:
<
[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]>
An:
<
[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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