Hello Roger,
Am 04.06.2010 21:48, schrieb Roger Lapthorn:
Stefan,
A few observations on your tests using the earth electrode "antenna"
today:
- In my experience, most of the signal transmission is by
induction, so I think it would be worth trying to receive the signal
with a loop antenna when the received signal may be much stronger than
on the E-field probe. In my case I use a 30turn, 80cm square, loop
which can be rotated to minimise 50Hz interference.
Yes, a loop would be a better RX antenna when transmitting with such a
earth antenna, at least in the near field. But i do not have a loop RX
antenna and my goal was to compare by both grabbers that use E field
antennas. It is important to have a signal within the dreamers band to
compare the grabber setups. Here, a weak signal may be even an
advantage since it is easier to compare the S/N (readability) of those
2 systems. If i want to produce a strong signal i think i prefer the
kite ;-)
- On my 10m and 20m baseline earth electrodes I was measuring
resistance of between 40-60 ohms between 1-9kHz, so your figure seems
very high. This may be as a result of the soil and rock type in your
area but this is worth checking. If my theory of the "loop in the
ground" is correct then a poor conductivity soil would result in a
larger effective loop area within the ground.
This hill is very stony and so the losses are high as well. I had
problems to push those earth rods into the earth. They stopped in about
30...40 cm. I had just 2 of them. In the next test i will take 2x3
copper earth rods (already prepared). Probably my next near field test
will be in 3,2km and i will use a 800m wire antenna, also on a hill.
- Did you try matching the PA to this high impedance through a
transformer? I think not in this test, so better results may well be
possible.
My PA has an output transformer where you can get 12V,24V,36V,48V and
96V (about, depending on the battery voltage, of course). These 96V
were not enough. So i just have to add another winding for 96V and
switch it in series to get a 6dB stronger signal or i use a second
ferrite core transformer for e.g. 400V output (no problem).
Additionally the earth losses will be reduced. But my main interest
will be the kite antennas ;-)
Maybe i will do a test with Bernd/DF8ZR in 16km, using a 800m antenna
and 600W. This can be done with rain and without wind ;-) But it
depends on Bernd's interest...
73, Stefan/DK7FC
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