Hello Jim, ULF,
Am 12.12.2016 17:16, schrieb [email protected]:
Congratulations on your ~20dB SNR at 65 km, 2970 Hz, that is quite a landmark
supported by hard work.
Many thanks.
You mentioned that the phase changes at 65 km do not seem to be from the
transmitter, and that the phase changes are slow (I could not determine how
slow from your earlier plot).
So far i don't know about the phase stability in 65 km distance. The
observed effect is from 3.5 km distance to the transmitter. In the
attachment you can see changing colours in the trace on 2970 Hz. You can
also see timestamps and the RDF circle, so you can get an impression
about these changes.
Does slow phase change (not from transmitter) lead you to suspect that the
phase changes (and some signal strength variation) may be from modal
interference (multiple rays interfering)? This question derives from modal
interference being more prevalent and greater in magnitude at lower VLF and ULF
frequencies, and distances less than 1,000 km. I didn't ask this before,
because it would be unusual for modal interference to be a substantial issue at
two widely separated locations (i.e at 65 km and at Todmorden). I didn't see
mention of this possibility in Markus' comments so I'm wondering if this
possibility is ruled out by other observations?
From summer recordings i saw that the sferics become very weak below
and arround 1.5 kHz, i daylight this cut off frequency is a bit higher,
arround 2 kHz. You can see it quite clearly on this spectrogram from the
17 km experiment, which was in summer:
https://dl.dropboxusercontent.com/u/19882028/ULF/17kmwide.jpg
So at night the reflection height is higher, as far as i know arround 90
km. Now 2.97 kHz is a factor 2 of this night cut off frequency. So the
path from the transmitter to the ionosphere and back is about 2 lambda
at night and about 1.5 lambda in daylight?
Could this explain why the phase changes are stronger at night? 2 lambda
would be 2x 360 deg, which does not cause interferences and 1.5 lambda
can cause a compensation of groundwave and skywave?
I'm just trying to find an explanation from what i know so far.
Let us try to experiment with this phenomenon. Is it be possible to
modulate (pulse?) the carrier in a special way to see a result proofing
the reflection on the ionosphere, assuming the effect actually exists?
Markus will come up with an idea i bet :-)
BTW i can run a two-tone signal since i'm using a linear PA. However the
freq deviation can only be a few Hz, due to the low bandwidth of the
antenna.
73, Stefan
Your results today are great news, nice work!
73,
ULF phase changes in 3.5 km.jpg
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