Hello SLF friends,
Once again i want to report about another successful experiment in the
SLF range, where i'm trying to reach lower and lower frequencies.
This time a 5 character EbNaut message was decoded on a new lowest
frequency: 176.5 Hz, the 1700 km band!
As usual, the path between TX and RX is 57.6 km. It is a H field, near
field experiment.
The TX antenna is a 1130m ground loop antenna. The average transmitter
power was 460 W and the antenna current reached nearly 3 A.
Two nightly transmissions had to be stacked to obtain the SNR that is
necessary to decode the message. Each transmission takes 2 hours and 7
minutes.
Despite the lower performance of the RX loops (single turn, abt 1m
diameter, 10mm copper tube, ferrite transformer 2:240 turns, low noise
preamp) at that frequency i even got a higher SNR than in the recent
experiments at 221.5 Hz and 263.55 Hz. Does the TX antenna become
significantly more efficient on such frequencies?
The transmissions have to be done at night. The tram close to the RX
location is producing a lot of QRM and there is just a 3 hour period at
night when it does not drive arround. This is when i can be on the air
without to much interference.
The first transmission was done on January, 19th, the second on the
31th. It is noticable that the level of sferics in the SLF range becomes
stronger already, so the season for such experiments will be over soon!
Due to the presence of such QRN, it turned out that it is helpful to add
a litte E field component, although the E field does not contribute to
rise the signal level. It just helps to lower the noise level by forming
a cardioide antenna steering away from the QRN and thuse increases the
SNR.
Also it was helpful not just to aply the auto notch filter within vlfrx
tools but also to watch the outcoming filtered and blanked spectrum and
then add further discrete band stop filters to disable sidebands
appearing a few Hz arround arround strong QRM peaks, like 150 Hz and 300
Hz.
As usual, the attached screen capture shows the signal post-processing
and message decode and resulting SNR of the stacked transmissions.
The next step is somewhere between 100 Hz and 150 Hz :-)
73, Stefan
