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Date: Sat, 06 Mar 2010 22:16:23 +0100
From: =?ISO-8859-1?Q?Wolfgang_B=FCscher?= <dl4yhf@freenet.de>
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Subject: Re: LF: 12 km on Dream(ers) Band
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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" <markusvester@aol.com <mailto:markusvester@aol.com>>
> An: <rsgb_lf_group@blacksheep.org <mailto:rsgb_lf_group@blacksheep.org>>
> 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" <markusvester@aol.com <mailto:markusvester@aol.com>>
> An: <rsgb_lf_group@blacksheep.org <mailto:rsgb_lf_group@blacksheep.org>>
> 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)
>