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Re: LF: 12 km on Dream(ers) Band - TX loss calculation

To: [email protected]
Subject: Re: LF: 12 km on Dream(ers) Band - TX loss calculation
From: Markus Vester <[email protected]>
Date: Mon, 08 Mar 2010 05:44:14 -0500
In-reply-to: <[email protected]>
References: <2CDE578A1E1B49AF96D61BB5D91FA03D@White> <[email protected]>
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Sender: [email protected]
Dear Rik, LF,
 
yes these numbers are pretty much where I arrived in my calculations. There is some loss in the transformer, about  1024* 0.25 ohm from the primary and 60 ohm in the secondary, adding up to 315 ohm series loss. Thus antenna resistance at 9 kHz would be more like 700 ohm. The same antenna has a much lower resistance on 137 kHz, around 35 ohms normally, and down to 28 ohms in cold and dry weather. This includes the LF coil resistance (RF litz wire, Q ~ 700, Rcoil ~ 7 ohm). The large difference indicates that capacitively coupled losses from high-resistance objects (trees, roof etc.) are probably dominating at 9 kHz. 
 
BTW I just noticed an error in the the 9 kHz coil description, it actually has 10 sections, 500 turns each (back to kindergarten then to learn counting to ten ;-). Skin effect is probably still neglegible with the thin 0.2 mm wire, but there may be some eddy current loss (proximity effect) in the overlapping layers.

Best 73,
Markus


-----Ursprüngliche Mitteilung-----
Von: Rik Strobbe <[email protected]>
An: [email protected] <[email protected]>
Verschickt: Mo., 8. Mrz. 2010, 9:22
Thema: Re: LF: 12 km on Dream(ers) Band

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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