Dear LF,
today around noon, I will attempt
another kite transmission from the same site. If all works well, I
will send a 1 mW carrier on 8970.002 from around 11 to 14 UT.
Best regards,
Markus (DF6NM)
Sent: Tuesday, June 21, 2011 12:00 AM
Subject: VLF: DF6NM kite activity June 19
Dear
Dreamers,
thanks
very much to everyone who commented on my signal yesterday night, and
all who are running their VLF grabbers.
Transmission:
My
signal was on air on 8970.002 Hz from 18:52 to 22:01, with a long break
from 19:26 to 19:52 due to a prolonged rain period, and many shorter
interruptions caused by precipitation, low wind, a coil falling over,
or a breaking antenna connection.
I'm
aware that an advanced notice would have been valuable for
potential receiving stations. But I find it difficult to plan
ahead this type of activity. Yesterday even while setting things up on
the site, I was not at all sure whether I would be able to actually get
on air for any significant time.
Site:
The location (49.3874 N 11.1692 E,
"JN59OJ02HX" for those who prefer cryptic notations ;-) was the
same as in the experiment on Nov 13 last year (see email copy at
bottom). While Google Maps still shows large patches of unused land
around the former US airfield, "my" field is now one of very
few undeveloped plots in the middle of an industrial area, inhabited
mostly by large logistics companies. Though I
had originally not intended to work into darkness, I found there was
still plenty of light from the surrounding truck parking and unloading
areas.
Kite antenna and ground:
This time I used a 4 sqm lifter kite
(HQ Power Sled L3.0, similar to Stefan's smaller kite). It is held by a
150 m line. This may be stretching the limit a bit, but due to the
slope and sag of the line I am sure that the peak altitude always
stayed below 100 m.
The
previously used 0.4 mm magnet wire had a voltage limit of about 12 kV
due to corona appearing along the whole length. It has now been
replaced by insulated wire (0.25 sqmm, outer diameter 1.3 mm). The
total length is 180 m, with the last 30 m dangling down from the kite,
intended to act as a sort of extra top load. Effective height would be
around 50 m, giving 3.5 milliohm radiation resistance, or 1.3 mW EMRP
at 0.6 A.
At the western end of the unused
site, there is a long mesh wire fence running north-south, held by
solid metal posts which served very well both for anchoring and
grounding. Total resistance of the antenna circuit was around 280 ohms
(including 140 ohms for the coil). This is about half of that when I
used the same kite antenna in Bamberg with only a few short ground pegs.
The coil was placed below the kite line at about 3 m from the
anchor point, hoping to keep the wire well above ground. However this
turned out to be problematic due to the horizontal and vertical
movement of the connection point. When the kite was low and due west,
the wire was too long and touched the wet grass, exhibiting pretty
little fireworks. When it went up or drifted aside, the wire became too
short, and either plugged itself off, or tipped over the whole coil.
You guess how much I like having to reconnect the ungrounded wire, with
dark clouds overhead and a slight drizzle going down... I twice found
myself holding down the kite line with my foot, then slowly stepping
forward until the open end at least touched the earth. Next time I
will definitely place the coil much closer to the anchor, and try to
hold the connection up by means of some elastic plastic tube.
Coil:
From the previous experiment I knew
that the wire capacitance was very close to 900 pF. During the
afternoon, I experimented with different coil configurations, and found
that I could either use four buckets (184 ohms) with a large 10
cm pitch (ie. 9 Lego blocks), giving about 8 mm radial clearance
between one coil layer and the inner radius of the next bucket. The
other option was three buckets (138 ohms), stuffed into one another 3
cm pitch and 2 mm clearance (see picture http://df6nm.bplaced.net/VLF/pictures/two_coils_350mH.jpg).
Mainly for mechanical reasons, I
opted for the more compact three-bucket solution, but this turned out
to be the wrong choice. With power applied, I heard
corona sizzles around the coil at about 0.55 A (11 kV). In darkness I
could see violet seams near the bottom end of the upper two windings.
This means that the maximum usable layer-to-layer voltage for this
design is about 3.7 kV. Next time I will definitely go for the longer
version, which also has more surface exposed to the wind for heat
removal.
BTW all seven buckets connected to
900 pF resonate at about 4.6 kHz - Stefan, how about a 65 km band ;-)
Amplifier and power supply:
Like before, I used one channel
of a class-B car-audio amp (Nitro 3000), feeding a 12 : 80 turns
ferrite transformer originally wound by DJ2LF. It delivered 0.6 A
antenna current (~ 100 W) from 12V x 13 A DC input. It was possible to
turn the volume up to 0.7A, but I did not dare to run that level of
corona permanently. The amp was fed by a standalone 35 Ah lead-acid
battery, which was repeatedly recharged via jumper wires from the car
engine during the latter half of the experiment.
Signal generation:
SpecLab's samplerate was locked to
DHO, which is very slightly (3 ppb) above its nominal frequency 23.4
kHz. To keep the average phase steady, I took care to never interrupt
the software generator whenever I had to take the carrier off
air. Anyway the time series recorded by
Paul Nicholson shows two major phase jumps around 19:40 (while the TX
was off) and 21:20, perhaps due to soundcard buffering issues. One
problem I did not see immediately was that the DHO ferrite antenna was
too close to one of the jumper wires, temporarily picking up
interference when the car alternator was running - once found, this one
was easily fixed.
Results:
Though the traces at TF3HZ (2547 km)
and YO/4X1RF (1253 km) are close to the noise, they are at the right
time and frequency, and I do not think that they are random pixels.
Halldór's trace even seems to show the outage period after the first
bright dot.
On my own grabber, I attempted
to compare signal levels from DK7FC (-71 dB, 180 km) and myself (-43
dB, 4.5 km). Correcting this difference (28 dB) by the EMRP ratio (26
mW / 1.2 mW = 13 dB), distance ratio (1/r = 32 dB) plus an
additional nearfield correction (1/r³ instead of 1/r from 4.5 to 5.3
km = 3 dB), we find that Stefan was 28+13-35
dB = 6 dB weaker than we would expect by pure groundwave. This
corresponds very well to the earlier experience that there is a midday
skywave cancellation effect for about 200 km.
Best wishes,
Markus (DF6NM)
Sent: Monday, June 20,
2011 10:48 AM
Subject: Re: VLF: DF6NM
on 8970 Hz!! Fwd: Fwd: VLF: More food for grabbers
Hello Markus,
This was your strongest signal received here so far. Tell us about your
rig! Where did the TX power come from? A battery? Must have been a big
battery or you must have had very low losses.
Transmitting during the night could be an interesting point. I could
try to use the 100m kite antenna on 8970 Hz as well. Then, no special
permit would be needed but my signal would be 10 dB lower. Anyway this
could be interesting. Or i could try night transmissions on the fixed
antenna....
Happy to see more activity here again! :)
73, Stefan/DK7FC
________________________________________
Sent: Sunday, November
14, 2010 12:04 AM
Subject: Re: VLF: TX on
6.47 and 8.97
Dear LF, Paul,
here's a short summary of today's
VLF kite transmission. I'm sorry for the late notice, but
the experiment had not been planned ahead, and I wasn't sure if I could
get on at all. Also, the kite I am using (a 0.8 m^2 delta toy bought
for 4.99 Euro) is a bit small for this sort of activity, and really
requires a good and steady wind.
The QTH was actually at 49.3874 N
11.1692 E, a few 100 meters from the former location reported earlier.
The GPS-controlled carriers were on air
11:15 - 12:15 6470.000 Hz ~ 50 uW,
and
12:30 - 13:10 8970.002 Hz ~ 200 uW
(EMRP).
Getting on 6470 was easier than
expected. I simply used the same "seven-bucket" coil, which resonates
my 240 pF home antenna to 8.97 kHz. Connected to the kite wire, the
resonance was close enough to 6.5 kHz to be tuned by the ferrite plate
variometer. Then the drive for the audio amp was increased until the
onset of corona sizzling at 0.25 A. At that point, the amp drew 10 A DC
from the car battery and probably delivered around 50 W. After
connecting mobile internet and staring at Stefan's grabber, I was
positively surprised to find a faint trace there!
For the frequency change, I removed
the two bottom coil buckets, which brought the resonance to ~ 8.5 kHz.
Then I had to carefully pull the remaining 5 buckets apart, inserting
bits of plastic foam material as spacers. Going through a few
iterations took about 10 minutes, and I got to "full power" (380 mA,
100W) at 12:30. Unfortunately, the wind had started to calm by then,
and there were several dips during this transmission. Finally at 13:10,
the kite landed gently in the grass, and I decided to close down and
pack up early.
Traces from both transmissions were
visible on DK7FC's 4.5 mHz "600" windows, and the second one also
produced a nice dash on the 0.45 mHz window. It is likely that these
signals would be useful for a kite-to-kite QSO on either frequency,
using 10 minute dashes.
The screenshot from F1AFJ has an
interesting dash before 13:00. The indicated frequency seems a bit high
(8970.020 instead of 8970.002), which could perhaps be due to a small
samplerate calibration error. I am also not sure about the time
scale - judging by the width of the noise blobs, the setting could
have been 11 mHz and 10 seconds per pixel. It would be nice
if Jean-Pierre could confirm the frequency calibration and scroll
settings.
I also carefully inspected the
OE3GHB grabber, which runs at 1.4 mHz resolution and seems to be very
sensitive. It showed an unusually dark period of very low noise between
8 and 11 UT today. But no luck for my transmission there yet.
Well, in addition to the technical aspects, being
outdoors in the warm wind was again a rewarding and enjoyable
experience.
Best 73, and thanks again for the interest.
Markus (DF6NM)
...
