Regarding Dave Gibson's thoughts on
earth electrodes, I'm puzzled. As Stefan says, the current is returned
by a path within the soil/rock and a loop MUST exist. Can the structure
be modelled both as a magnetic loop and a electric dipole?
Hello Roger,
simplified one could state that
current causes a magnetic field and voltage causes a electric field.
A small loop antenna is fed by a
large current and small voltage and is considered to be "magnetic".
A small vertical monopole (Marconi
antenna) is fed by a large voltage and small current and is considered
to be "electric".
But lots of antennas (in fact most
antennas that are mot small in regard with the wavelength) are fed by a
significant current and voltage and can be considered as partly
"magnetic" and partly "electric".
I believe that the same is true for
an earth antenna.
In regard with this a "mind game":
Assume a small loop, fed at the
center of one side but with a switch at the center of the opposite side.
If the swithch is closed the antenna
is small (magnetic) loop to be fed with large current and small
voltage. A series capacitor is needed to bring this structure to
resonance.
If the switch is opened however the
antenna becomes "electrical". It has to be fed with a large voltage and
small current. To bring it to resonance a series inductor (loading
coil) is needed.
What if the switch is replace by an
inductor, let's say with a value of half of the loading coil ?
Now it is neither a magnetic loop
neither a electric antenna, but something inbetween.
It still need to be brought to
resonance with a capacitor, but a much smaller value than needed for
the pure loop.
In regard with the pure loop the
volatge at the feedingpoint will rise and the current will sink.
73, Rik ON7YD - OR7T
On 22 July 2010 09:15, Markus Vester
<[email protected]>
wrote:
Dear VLF enthusiasts,
browsing through saved VLF-grabber
screenshots, I noticed some recent changes of VLF MSK-band usage:
2010-06-24 06:00 20.27 kHz (ICV) QRT
2010-06-30 05:00 16.4 kHz (JXN) QRT
2010-07-05 07:05 18.3 kHz (HWU) QRT
2010-07-08 23:00 21.75 kHz (HWU) break for 6 hours
2010-07-10 06:00 22.1 kHz (GQD) switched from 100 bd to 200 bd
2010-07-10 10:00 19.58 kHz (GBZ) QRT
Thus within a couple of weeks, half of
the European military signals have gone for good.
Perhaps there is less need for submarine
communications in today's world... let's hope that the submarines
themselves (along with all that other military hardware) may become
obsolete in tomorrow's world!
73, Markus (DF6NM)
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