Hello Marcus,
could the loss in the impedance transformer be avoided by tapping the
loading coil ?
I just had a look at the "copper wire tables" and 0.2mm OD wire
(actually 0.15mm copper) has a resistance of 1 Ohm/m. Quite a
challenge to get 830m wire on the coil without breaking the wire
:-).
Would it also work with less (but thicker) wire and a ferrite core ?
The ferrite core will introduce some additional loss, but on the other
hand you will have less (and thicker) wire reducing the copper losses.
But I am not sure what the overall effect will be.
73, Rik ON7YD
At 11:44 8/03/2010, you wrote:
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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