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)
