On Tue, May 7, 2019 at 4:19 PM Marcus D. Leech <patchvonbr...@gmail.com>
wrote:

> On 05/07/2019 04:05 PM, Ben Hilburn wrote:
>
> Hey Brad - just checking in! This is an interesting experiment, and I
> would love to hear how it went!
>
> Big thanks to Kevin and JMF for providing very helpful guidance, here, too
> =)
>
> Cheers,
> Ben
>
> I should perhaps have entered this discussion earlier, and pointed out one
> of my early applications using a sound-card for VLF work:
>
> https://github.com/patchvonbraun/SIDSuite
>
> It's OLD now--I don't think it was ever converted to GR 3.7
>
> One of the problems with mag-loop antenna is that they're very high Q, and
> thus have very small fractional bandwidths, which means that
>   they're wildly inefficient at all but the resonant frequency.  I made up
> for that using a Behringer microphone pre-amp using the balanced input.
>   That meant I could use a fairly "random" multi-turn mag-loop and not
> worry about efficiency very much.
>
>
Thanks Marcus - I'll see if I can get it to compile again. In the meantime
I have put together an AM receiver flowgraph using recommendations from
this thread, along with what I remembered from the gnuradio tutorials and
Mike Osman's video tutorials.

https://github.com/regulatre/vlfCoilEperiment

Given a 5-minute recording, which I included in the repo, I quickly found
that QRM interference will be a hurdle and as you pointed out Marcus, my
coil (an old VGA degaussing coil) seems to be resonant at undesirable
frequencies. In its current installation it's getting overwhelmed by a
steady interference source that sounds like ripples coming from a 60Hz
half-wave rectifier. There are some gaps in the noise, and as I tuned
around within the baseband using my flowgraph (in the repo above), I was
able to tune to various parts of the baseband, but in all cases I had too
much interference noise.

I have a Focusrite Si2 I could use instead, which would have more gain
potential and a very low noise floor, but first I think I'll need to find a
way to get away from the noise sources.
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