Hi Paul,
Well, i'm on the way to produce the results from the data recorded on
the recent 970 Hz experiment. Meanwhile i managed to actually
decode one of the EbNaut messages but it is quite a struggle. So that's
the first good news, but later more.
Now, There is a problem with the E field reception: The trees were
higher than expected so the active E field antenna was not high enough
above them, i.e. it has a very low effective height. I knew about that
effect but i didn't think that it is so expressed: When looking on a
'wideband' (the word developed during the kite experiments time)
spectrogram you can see that the noise floor is almost completely given
by the preamp noise floor, it almost looks like white noise and some
frequency response. One would not expect that the message can be found
in that kind of noise background but it is in there! So it could have
been much better...
Here is a first image showing the E field in the upper part and the H
field in the lower part:
http://www.iup.uni-heidelberg.de/schaefer_vlf/ULF/E+H_first2days.png
The spectrogram covers about the first 2 days of the recording, without
filters and blankers, just the raw recording.
I learned how to apply a filter and a noise blanker and i played with
the threshold level. For the E field i found it is best at -a25. The H
field is best at -a10.
First i concentrated on the 2 character EbNaut message. This message
was transmitted two times, so there was the chance to stack it. From
the E field i found the message at -5.0 dB and -6.5 dB. E field stacked
gives -3.5 dB! So far so good.
Next i tried to decode the message from the H field. But the H field
has a problem too. It looks like is some lack of sensitivity and i can
also hear a Spanish broadcast station at night, i.e. some AM
demodulation. This also happens on the E field but just slightly.
Anyway, i need to improve the ULF reception system and i need to find a
better location for the next experiment.
The 2char message was found at -5.9 dB and -5.3 dB on the H field.
These two days stacked give -2.8 dB!
Just for fun i stacked E and H now, ignoring the phase. And suddenly i
got a prediction of -0.1 dB!!!! This is quite an unconventional
way to combine E and H i guess :-) But i put this stacked file to the
decoder and tried a few variations with the -a option. A screenshot is
in attachment but a summary of the results follows later. So i decoded
the 2 character message with Eb/N0 = 0.2 dB.
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So far so good. But now i'm trying to combine E and H before applying
the noise blanker. This should help because when building a cardioid
with a good F/B, the sferics from the back side are rejected by say 30
dB, so the noise blanker will not be triggered when the E, H amplitude
weighting is set correctly. I already played a bit arround with vtmix
-c1,1 or vtmix -c1,j or vtmix -c1,-1 (1st channel is E, 2nd channel is
H) and watched how the result changes. But if i want to get the best
results i will first have to remove the mains hum from the H field
channel. Mixing E and H by using vtmix may be easy when both antennas
work well but in this special situation the H field has a high hum
component whereas the E field is almost white noise. When mixing first
and applying a hum filter then, i think the result gets worse.
I would tend to select E or H and apply a filter and noise blanker and
then save this to a file, i.e. one for E, one for H. Next i would need
to combine/mix these files again. Will the phase and timing still be as
in the original file or will it change, depending on the filter poles
and so on?
How can i mix E and H again from separate files? Is it done with
vtjoin? Do i have to write a script to do that? Starting to read both
files and send the output to a buffer. Then i call both buffers, join
them, and save them into a 2-channel file again? Then i can read that
preapred 2 channel file and use vtmix to build the cardioid antenna, as
well as the resampling to SR=1, converting to base-band etc. to
generate the file which can then be moved to the EbNaut decoder?
Sounds quite complex.
Do you have a better idea?
73, Stefan
PS: Still not watched for the carrier, i guess it will be detectable...
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2ch970Hz40km_E+H.png
Description: PNG image
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