Hello
Eike,
Thanks for your ideas. That Alu-wire is good for normal E-field
antennas carrying some kV. A good source, thanks!
BUT, here i need an isolated wire because it is laying on the
ground directly. OK, one could say that the wire would have only a
very bad contact to the ground, anyway when laying on dry grass.
But over a length of 900 m i expect the losses will be
significant.
It could be worth a test to compare both solutions but the
isolated cable will work for sure.
My wire is already ordered and payed anyway (
https://www.pollin.de/p/lautsprecherkabel-2x0-75-mm2-100m-transparent-cca-562602
)
73, Stefan
Am 31.07.2018 13:19, schrieb Eike DL3IKE:
Hi Stefan,
instead of expensive and heavy copper-wire take a look at this
fence-wire:
https://www.weidezaun.info/voss-farming-aludraht-alu-draht-400-m-2-0-mm.html#
400m of 2mm diameter aluminium wire for aprox 30eur has a dc
resistance of 4 Ohm an a weight of only 3,5kg.
You´ll find also other usefull things like these:
https://www.weidezaun.info/weidezaun/verbinder-spanner.html and
more for buildig long antennas.
A lot of those things are sold much mor expensive on
ham-radio-markets...
73 de Eike DL3IKE
Am 30.07.2018 um 21:31 schrieb DK7FC:
Hi VLF and ULF friends,
In the attachment you can find 3 spectrograms from
transmission of the last weekend. For the 8270 Hz and 2970 Hz
transmission i took the data of the 900m ground loop, 5170 Hz
is from the 450m ground loop.
The 8270 Hz signal was very strong even in 3.8 mHz
('DFCW-600'). Some real QRSS-60 would have been possible!
The spectrograms also show that there was no trace on the
frequency of interest before and after the transmission (no
traces from the right channel ( 1 PPS) ).
A few minutes ago i built the switchable network for
resonating the antenna properly. And i actually ordered 1000 m
of that 0.75 mm^2 loudspeaker cable. Thus, in the next
experiment i think the Q of the antenna will be much higher
and it will be possible to tune it accurately. Then, the
tuning capacitance will tell us what the ground loop
inductance is! And then, we can calculate the depth where the
current is flowing backwards in the ground, asuming a simple
model in first steps.
Next, we can see whether the inductance or depth stays
constant at lower frequencies (hopefully it will increase!) by
resonating the antenna at 2.97 kHz for example.
BTW i measured the DC resistance of the wire i used last
weekend, it is 116 Ohm, so it agrees well with the calculated
120 Ohm...
The new wire should have 21.4 Ohm only.
The big advantage of this antenna is that no high voltage and
large coils are needed. Usually the voltage is the limiting
factor for a VLF or ULF TX antenna. But not here. Here it is
the power of the PA. The antenna could easily handle 2 kW,
which means 5.16 A, no problem for the wire. And it would be a
20 dB stronger signal than last weekend...
73, Stefan
Am 29.07.2018 23:11, schrieb DK7FC:
As a next result from today's
experiment, here is a spectrum peak of the 2970.005 Hz
transmission. I reached 20 dB SNR in 180 uHz! This far more
than expected, in July anyway.
It is the first ULF transmission (by amateurs) from an earth
electrode TX antenna, detected in the far field.
Spectrograms will be produced soon...
73, Stefan
Am 29.07.2018 16:01, schrieb DK7FC:
Hello Roman, VLF,
I transmitted pure carriers each time. It was just a first
test.
Today i've done a second experiment, this time using a
900m long wire! So the wire should have about 125 Ohm DC
resistance. I measured 220 mA DC at 38.4 V, so the overall
loss is 175 Ohm. Very interesting; the ground loss
resistance stays at 50 Ohm although the distance between
the electrodes has doubled!
If i would use the 0.75 mm loudspeaker cable then i can
reduce the losses by 100 Ohm which is more than 50 %, so i
will gain 3 dB at the same output power! Then with a
switch mode PA having nearly 100% efficiency i may gain 2
dB more (mmy lonear mode PA was not well matched today and
quite warm). So maybe i can reach 1 A antenna current with
just 75 watts??!!!
Today i tuned to 550 mA antenna current again, at 8270 Hz.
With the scope, i measured the phas, it was more inductive
than yesterday. I actually found a 1 uF MKP-10 cap in my
car and switched it in series to the antenna. This
improved the phase slightly. Then i also found a low pass
filter for 137 kHz, the pypical pi configuration, i.e. it
acts as 4*22 nF in parallel here on this frequency.
Switching this in series leads to a low current, so the C
is to small. So for the next experiment i'm preraring a
switchable C network (47n, 100n, 220n, 470n, 1u, 2.2u,
4.7u). It will be even more necessary when i lower the
wire resistance (higher Q).
Well, today the QRN was much lower, especially for the
2970.005 Hz transmission period. Yesterday this was
totally buried in the noise on my RX on the tree. But
today! I transmitted another 90 minutes with much lower
QRN background. I already have a clear spectrum peak but i
like to try to improve it a bit more before presenting it
here.
Todays carrier transmissions:
8270.000 Hz : 08:12...09:33 UTC
2970.005 Hz : 09:42...11:15 UTC
Sorry for the confusing email ;-)
73, Stefan
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