Dear VLF,
Today i tried a first attempt to transmit a 8,97 kHz Signal over a distance of
5km, using an earth antenna.
The antenna was, for a first step at the new QTH, the 120m long wire that i
used in the successful 2km distance reception. TX power was abt 50W, so 10 dB
more than in the 2km test. But there was no reception :-(
One reason is the very strong QRM that makes not even a break for some hours in
the morning within the last 3 days.
Another reason was the TX time: 16:40 UTC.
A third reason is the high earth resistance: 3 kOhm! So i got about 135 mA @
400V! Maybe this is due to a bad earth connection (negative) but it could be
also due to the bad soil conductivity there on that hill, 520m ASL(positive!)
The earth rods were 3 copper rods on each side, 10mm diameter and just 40cm
long, 1m spaced. There are stones that makes it impossible to run a 2m rod into
the bottom so i decided to use many small rods. Those are available out of the
trash of my work so i will take MANY of them in the next tests ;-)
I will check what happens to the earth resistance if i double the earth rods
(well spaced to each other) on each pole. If the losses gets the half by taking
twice the earth rods, the losses are mainly caused by the bad earth connection.
If almost nothing happens, the losses are mainly caused by the soil
conductivity...
Furthermore i will increase the wire length to up to 1000m. But first i want to
see a signal on my grabber with a minimal effort. Maybe this kind of antenna
will be suitable for DX if the dimensions and earth conditions are suitable.
A first test with receiving on this antenna was disappointing: I received the
strong MSK military VLF transmitters well but the S/N of my grabber (out of the
center of the city) is at least 20 dB better. Maybe there is an influence of
that old notebook so i will repeat this RX test with my new netbook that has a
very good internal soundcard! (providing 96 ks/s and -120dB noise and 8 hours
accu load). The test was done by directly connecting the wires to the audio
input. Maybe a isolating transformaer is a better choise here, giving something
like a wide bandpass (DC, ELF, LF rejection).
The antenna will be fixed at this place. If it will be a very successful
arrangement in some days/weeks and you are in holidays here you may use it for
a nice /p QSO ;-)
Will see how it works on 137 but currently it's a little strange for me to use
such high frequencies ;-)
Wish you fun and good luck with all your tests and preparing for the coming
period!!! :-)
73, Stefan/DK7FC
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