Hello SLF freinds,
Just a note from a recent experiment at 270.1 Hz.
On sunday morning, 2019-05-12_10:34,+150m, i've done a carrier
transmission on my ground loop antenna again. I did not expect more
than, hopefully, a detectable spectrum peak in 57.6 km distance, i.e.
at my tree site. The tree receiver site was listening and recording
data using vlf-rx tools.
One E field antenna and two orthogonal loops were listening. The loops
have been improved recently! They consist out of a single circular turn
of 1.2 m diameter using 10mm diameter copper tube (about 25 mm^2). It
is a closed loop, non-resonated, with an impedance matching
transformer. This transformer previously had 1:100 turns. Now it (they)
has 2:240 turns, i.e. two turns primary (out of 14mm^2, AWG6). This
improved the sensitivity below 2 kHz significantly ( abt. 4...5 dB).
Furthermore the TX antenna length and angle has been improved,
resulting in about +3 dB more signal strength on the RX site!
In a previous experiment at 270.1 Hz, some month ago, there was no
result at all, not the weakest trace, despite excessive tweaking of all
parameters. So the question was, will the improvements result in a
detectable signal now?
Several things went wrong in that experiment. I forgot a bag containing
important equipment such as the power supply for the netbook that
generates the carrier signal. Also the output power was not as high as
planned, just about 380 W, giving 2.2 A antenna current (I
measured 64.7 V at 1 A DC). Anyway i managed to improvise so the
experiment was started, but with some hours of delay which meant i
higher QRN background level. Then, on the WLAN link to the tree, there
were several interruptions of the stream (i'll move to 5 GHz very
soon!). I even got some QRM from my battery charger for some short time
periods (forgot to disable the charger remotely). So there were several
factors that could have been improved or avoided. And the middle of May
is not the ideal time anyway.
Well, 270.1 Hz, that's the 1110 km band! The far field begins
at 177 km distance, i.e. i am clearly in the near field here. Thus,
from a 'magnetic' TX antenna, we would expect that the signal is mainly
detectable on the H field, i.e. the loop antennas.
The first interesting results is that this expection is actually
confirmed. There is nothing detectable on the E field but the carrier
S/N in the H fields is close to 10 dB in the first run. Mixing the H
fields and tweaking the filters rises the carrier S/N to 10.7 dB,
see attachment.
So far not really an undoubtly detection but it is a candidate for
optimism! With a few less problems during the experiment there is a
chance for 14 dB SNR. Also, there is quite much sideband QRM arround
300 Hz which makes 270 Hz a bit harder to work on.
73, Stefan
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