----- Original Message -----
Sent: Saturday, March 06, 2010 9:16
PM
Subject: LF: 12 km on Dream(ers)
Band
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)
