Wonder what effects soil conductivity changes has on the propagation at these VLF freqs?? Temperature here this pasted week has been and averaged 4-8 F degrees. Yesterday and last night temp climbed to 40+F! It is a fact that the conductivity in this region changes drastically with temperature. Much higher with freezing temps. Usual conductivity of 3-4 at above freezing and upwards of 10-15 with cold temperatures-the colder the higher the conductivity-Bob
> Date: Sat, 8 Mar 2014 13:37:16 +0000
> From:
[email protected]> To:
[email protected]> Subject: Re: LF: Daytime 29.499 kHz
>
> Here is the background noise diurnal for 2013 at 29.5kHz
>
> http://abelian.org/vlf/tmp/29499_noise_2013.png
>
> The noise is averaged over a 500Hz band and converted to units
> of fT per root Hz.
>
> The noise is before sferic blanking. Divide the noise
> amplitude by 10 to estimate the noise after blanking.
>
> Multiply the noise amplitude by the square root of your
> bandwidth to obtain your RMS noise floor. Or instead, divide
> the noise by the square root of your Fourier transform width
> in seconds.
>
> For example, from the plot the noise during the night in early
> March is in the purple but on poor nights has a hint of red.
> Lets say a bad night then is 15 fT/root(Hz).
>
> After blanking that would be 1.5 fT, and in a 600 second Fourier
> transform the floor would be 1.5/sqrt(600) = 0.06 fT in each
> Fourier bin.
>
> Compare that with the noise level between 19:00 and 21:00 in
>
> http://abelian.org/vlf/tmp/29499_140307a.gif
>
> and we see the estimate from the noise map is in the right
> ballpark.
>
> Markus wrote (in a nearby thread):
>
> > saw practically nothing here on 29.499 last night.
>
> Signal level from Bob is lower. NAA maintains normal level.
>
> > are you continuously saving all raw data from your VLF loops?
>
> Only the lower 24 kHz of 3 channels (2xH + vertical E) are
> saved raw, spectrum data is saved for 0 to 96kHz. Storage of
> the timestamped flac-compressed data is not too onerous, at
> the last count it was heading for 18 Tbytes.
>
> Hard disks are big and cheap these days fortunately.
> Also reliable too, in the last 3 years I've only had one disk
> error (which was revealed by md5 checksum verification) which
> turned out to be a single bit flip in one byte of one file.
>
> I strongly recommend recording signals to disk and doing signal
> analysis by post-processing. Then you don't miss anything
> and you don't have to guess in advance the optimum antenna
> pointing, spectrogram settings, and so on. It is easy to
> keep a cache of 10 days or a month of signal. The fact is,
> almost always you get to hear about interesting things *after*
> they've happened, especially with natural radio events.
> You really are very limited and will miss a lot if you only
> work with real time signals.
>
> --
> Paul Nicholson
> --
>
