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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