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From: Paul Nicholson <vlf0403@abelian.org>
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Date: Mon, 10 Apr 2017 20:49:12 +0000
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 > Meanwhile the 7 is completed :-)

In the time it took to send 'DK7' you could
have sent 330 characters using a more efficient
mode.

Or sent DK7 with a signal 20dB weaker.

 From a nearby thread:

 > if you could release a new ebnaut-tx.exe for Windows
 > which handles 200 characters and 0 CRC bits, i would
 > send you a 200 character long message.

That's in the next revision, and some other alterations
which I hope I can finish testing soon.

This talk of longer messages makes me wonder about the
possibility of arranging a source encoding for binary
data.   How about sending a .jpg at VLF?   Crazy, but just
about feasible.

Eddie wrote:

 > Loops at this QTH are not very successful, yes lots of
 > mains borne QRM.

Often, H-field is worse.  You have not only the harmonics,
but the sidebands of the harmonics to deal with.

My rural location is very noisy from the mains.  All the
farms around here are supplied by overhead lines.  My loops
are about 200m from the house which is plenty far enough
not to pick up any domestic interference.    I think 100m
would have been good enough.

Moving away from the nearest power line the hum falls off
with distance cubed as you would expect, but by 100m falls
to a background level unrelated to nearby lines and further
distance is no better.  I presume this hum floor comes from
more distant national grid lines.   The H-field hum level
varies a lot but is unrelated to local loads.

I wouldn't worry too much about the mains hum level.  It's
a fact of life and is not going to go away.  But if it is
fairly clean hum (not too many sidebands) and your rx and
A/D are impeccably linear, then there's plenty of space
between the hum harmonics for reception down to the
natural noise floor.

I should perhaps post a short mp3 of my raw unfiltered H-field
...

  http://abelian.org/vlf/tmp/170410a.mp3

You wouldn't think it possible to pick up anything through that.

If I filter out the hum harmonics, you can hear the sidebands.
The filter kicks in at about 5 seconds:

  http://abelian.org/vlf/tmp/170410b.mp3

Here's the same thing from the E-field rx

  http://abelian.org/vlf/tmp/170410c.mp3

Note the much lower level of sidebands on the E-field.

In the UK it is hard to find anywhere clear of foreground hum.
But with a bit of effort you can often reach the natural noise
floor in a suburban location.

--
Paul Nicholson
--