Hi Roger!
Congratulations to your interesting achievements!
I have some questions: What was the antenna current you achieved @ 4 watts on
0,8 kHz? Will you repeat the experiment on 8,97 kHz? It would be interesting if
the signal at a given distance/location is stronger or weaker on 0,8 or 8,97
kHz. Maybe you can create a diagram of the antenna impedance as a function of
the frequency? This could give you further infos about you ground loop.
Could you use a saw tooth modulator (with a constant output voltage) that
FM-modulates from DC to 9 kHz with a very slow rise time of say 1 minute? Thean
you could evaluate the propagation as a function of the frequency (since your
antenna is non resonant, so very broad band)! This would be most interesting!
I will also go on with my tests, reaching the far field on the Dreamers Band,
there on the hill, without any water pipes or so. I have prepared many small
and two big earth rods (steel, 1,5m long and a 2kg hammer ;-) ). Additionally i
have organised further 280m wire... All very interesting stuff and an excellent
hobby for the summer! :-)
GL and go on! Most of us haven't thought that this would work so good over such
distances. Seems the radio amateurs are re-discovering the VLF band, just like
at the beginning of radio...
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Roger Lapthorn
Gesendet: Mo 21.06.2010 18:55
An: [email protected]
Cc: M0BXT
Betreff: LF: Earth mode DXing today - 3.6kms!
MODS TO THE SYSTEM
Some significant steps taken with my local earth mode / induction tests today,
which I'll summarise here. Spectran screen captures will appear on my earth
mode web page <http://sites.google.com/site/g3xbmqrp/Home/earthmode> tomorrow.
* Firstly, having installed the K1EL keyer chip, I decided to use its
internal 800Hz sidetone oscillator, after a bit of LP filtering, to drive my 4W
PA. This arrangement now allows me to run either QRSS3 or 10wpm CW beacons on
800Hz at the flick of a switch. It is more stable than the external twin-tee
oscillator I was using and is perfectly suitable for QRSS3 at audio frequencies.
* Secondly, I modified my loop/preamp to have marginally better sensitivity
at around 700-900Hz.
* A quick "by ear" test with the switch in the 10wpm position and the 0.5km
range I'd managed before was achieved using the 80cm loop and the 20m baseline
earth electrodes.
QRSS3 DX
Now here comes the real fun. At this point I decided to take my loop, preamp
and laptop equipped with Spectran out into the field to see what I could
see/hear.
* My first location was a field 0.75km south of the QTH just out of the
village. Although nothing could be heard, the QRSS trace was solid on the
screen.
* Next step was to proceed further south along the Heath Road out of Burwell
(QTH JO02dg) to a point 2.5km away from the QTH. Again, with Spectran, a good
signal was visible on the screen. At one point when rotating the loop I could
hear the signal too.
* The next test was to go out of the village on a different road (towards
Cambridge) to a car park adjacent to the Devils Dyke. This was 1.4km away from
the TX but about 40 degrees further to the west than the earlier test
locations. Immediately the loop was set up (leaning against the back of the
car) a good clear signal was visible, again no audible signal though.
* At this point I decided to "go for broke" and drove down another rural lane
to a roadside spot 3.6km away from home, but in the direction of fire of the TX
earth electrode loop. Setting up beside the country lane I could see the QRSS
signal on the screen!! Best orientation was with the loop orthogonal to the
village, which by now was too far away to see over the slightly rising ground.
At this point my laptop batteries were dying and I had to get some food for my
wife for tea, so I packed up and came home, very happy.
CONCLUSIONS
* Going from 10wpm CW to QRSS3 has made an amazing difference: signals that
cannot be heard are visible, and at decent distances. This is a new experience
for me, but not others here.
* The free running sidetone oscillator in the K1EL keyer is fine for QRSS3,
certainly for tests of about 1-2 hours long.
* I suspect some "utility coupling" is going on, although at the furtherest
point loop orientation did line up with the village. Is the signal propagating
along the water pipes to some extent?
* With "utility coupling" I suspect that getting a report from G6ALB in the
next village will already be possible on VLF as I've already heard my signal at
a greater distance than he is from me.
* At VLF, I am wondering just how far is possible in semi-rural areas with
manageable 50Hz noise levels (it is well filtered in my RX) and with some
degree of "utility coupling" which is inevitable in today's world where pipes
and wires criss-cross everywhere.
* Even with just 4W and QRSS, remarkable distances can be achieved. Clearly I
could get further than 3.6km already with the same equipment at each end.
NEXT STEPS
* I need a location right out in the wilds (a bit hard to find in SE England)
for further tests, to see what performance is like without any coupling into
water pipes etc..
* Add the screen shots and audio clips to the sub-9kHz webpage
<http://sites.google.com/site/g3xbmqrp/Home/earthmode> .
* Increase the area of the TX loop part that is in the air as this could add
another 3dB to signal level.
* More power? Loops at both ends? ....?
73s
Roger G3XBM
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk <http://www.g3xbm.co.uk/>
http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
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