Sent: Wednesday, November 30, 2011 11:11 PM
Subject: VLF: MFSK-37 kite transmission on Nov 27
This is a somewhat belated report of
my Sunday afternoon VLF kite activity, Nov 27th, 15 to 17 UT. The site
and configuration was the same as on June 19 and July 30 this year (see
emails below). This time I had mostly good kite elevation and an
antenna current up to 0.85 A, radiating about 2 to 3 mW EMRP. I had
originally hoped to engage in a two-way contact with DK7FC, but
unfortunately sufficient wind came up only much later than expected.
Stimulated by discussions with Paul
Nicholson and Stefan about the advantages of visual decoding and the
usefulness of DFCW, I decided to experiment with a simplistic
multifrequency coding approach, with one frequency channel assigned to
each letter in the alphabet, plus the numbers. The concept is somewhat
similar to Alberto's Jason mode, but simpler by using absolute
frequencies (instead of differences), only one dash (not two) per
character, and bit timing based on UT alone.
For a given speed, such MFSK dashes
can be about 5 times longer compared to straight DFCW, indicating a 7
dB advantage. There are other subtle factors involved in the
sensitivity, like the higher probability of a false hit in the much
larger number of channels, versus the necessity to have several dashes
correct to decode one complete DFCW character. But a rigid calculation
based on gaussian noise statistics confirmed that at the same speed and
error rate, 32-channel MFSK is actually 6 dB more sensitive than 5-bit
DFCW. On the other hand, trading bandwidth efficiency for sensitivity
is nice on VLF, but may not be appropriate for densely populated bands.
The first MFSK transmission from 15:00
to 15:25 was targeted at Stefan's 4.5 mHz "600" window; and I sent
"DK7FC" using 5 minute dashes with 10 mHz spacing. One of 37 tones were
generated by manually editing the frequency of SpecLab's signal
generator every five minutes, according to the following scheme:
- base frequency, space: 8970.00 Hz,
- numbers 1 - 9: 8970.01 to 8970.09, zero: 8970.10 Hz,
- letters A - Z: 8970.11 to 8970.36 Hz.
Then Stefan called on the phone and
informed me that I had good SNR in his "60" window. I switched to 60 s
conventional DFCW sending "FC DF6NM" from 15:33 to about 16:05. After
that, I repeated my callsign in 37-MFSK, this time using 1 minute
dashes spaced 0.1 Hz. The last two characters were "73" on 8970.7 and
8970.3 (see attached pic, revealing my typing error).
From 16:24 on, I finally sent a
straight carrier again on 8970.002, until I finally had to shut down at
16:54.
As an aside result, I found that it is
quite easy to receive on the kite antenna without additional T-R
switching hardware, by moving the little 23.4 kHz samplerate-lock
ferrite antenna towards the loading coil, and shorting the TX
transformer to ground.
Many thanks to Stefan, Chris, Mal,
Lubos, Eddie, Victor and Jacek for the valuable reports, and to Paul
for running his very sensitive spectrogram.
Best wishes,
Markus (DF6NM)
_______________________________________________________
Von: Markus Vester <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Sa, 30 Jul 2011 4:24 pm
Betreff: Re: VLF: DF6NM kite attempt
The carrier was on air from 10:32 till 11:22, and again from 11:32 till
12:02 UT. Antenna current peaking up to 0.85 A, which would radiate
about 2 mW. But the wind was not quite strong enough, so the kite came
down several times, and it's altitude varied a lot. I had to quit after
12:02 due to continuous rain and no more wind.
Thanks to all who were looking out, or running their grabbers. It seems
the signal was visible across Europe and also in Iceland.
Best 73,
Markus (DF6NM)
_______________________________________________________
From: Markus Vester
Sent: Saturday, July 30, 2011 9:52 AM
To: [email protected]
Cc: Paul ; Stefan Schäfer
Subject: VLF: DF6NM kite attempt
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)
_______________________________________________________
From: Markus Vester
Sent: Tuesday, June 21, 2011 12:00 AM
To: [email protected]
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)
|
MFSK_60.jpg
Description: JPEG image
Kite_RX.jpg
Description: JPEG image
|
