Hi Joe,
thanks for sharing your experiment details. I have been doing similar
tests on VLF, using about 120 m of wire, about 10 m of the ground. I
have used initially much less power, so signals disappear after a
kilometre or so. The attenuation over distance does seem to be a lot
more pronounced than you would expect and I think this has to do with
the fact that at a kilometre or so distance from the transmitter, we
are still in the near field of the antenna! This paper has some more
explanation and a graph of field strength versus distance (in lambda):
http://robotics.eecs.berkeley.edu/~pister/290Q/Papers/Antennas%20propagation%20interference/near%20field%20path%20loss.pdf
Anyway, how did you go about the 5:1 impedance transformation? I was
trying to radiate at about 10 kHz, so I used a beefy audio line
transformer (100V to 4 ohm/8ohm), but I am not sure how efficient this
is.
73, Dimitris VK2COW/VK1SV
2015-12-21 6:46 GMT+11:00 <[email protected]>:
> It was a nice but windy day today so the VLF TX was put on the air
> for a quick test. About 30 watts were fed to a 5:1 XFMR and a ~0.35H
> tuning coil to the 100m wire at 10m height. CW signals were heard
> clearly at 2 km using a homebuilt DC RX and a PA0RDT miniwhip at 2 km.
> The sigs could not be heard on the DC RX at 2.7 km and were much weaker
> on the XH100/LF10 with 20m of wire strewn in the trees. This is
> a new distance record for me for audible reception at 8.277 kHz. The
> ERP was about 3uW.
>
> 73
> Joe VO1NA
>
>
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