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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 It has now been replaced by insulated wire (0.25 sqmm,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
).
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
(xxxx) are close to the noise, they are at the right time and frequency,
and I do not think that they are random pixels. Halldor'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.
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)
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