Hi Stefan, my comment was related to the fact that if you support the foil a
few inches above the ground that will be fine but you will get a much smaller
capacitance. If you bury it or place it on the ground you are rather making a
poor quality ohmic connection (lotsa loss!) to the ground and it would be
better to ground the end of the wire to a good copper spike like a "half loop".
It is also quite difficult to make a good connection to the foil, with a big
capacitance there will be a lot of current flowing (hopefully :-)) ).
If you use an HV capacitor you would connect one terminal to the end of the
wire and the other to a ground stake. Rik had some comments on this
configuration at 136, but I dont think he ever tried it in practice.
Alan
G3NYK
--- On Tue, 23/2/10, Stefan Schäfer <[email protected]> wrote:
> From: Stefan Schäfer <[email protected]>
> Subject: AW: AW: LF: VLF_8.79 kHz
> To: [email protected]
> Date: Tuesday, 23 February, 2010, 15:05
> Hi Alan,
>
> Well, i expect the oxide film losses of the foil should be
> neglectible, compared to the ground losses. And one has the
> advantege to reach a high C that is easy to adjust/modify
> and cheep and that has no overvoltage and heating problems.
> Aren't the radials of a LF vertical not almost the same, in
> principal? I mean, one electrode of a HV capacitor with a
> lossy dielectricum? ;-)
>
> Using a HV capacitor or coax cable is also a good choice,
> of course, but it may be difficult to get proper C at those
> voltages. And if we are talking about connecting the end of
> a inv L antenna to ground (here: soil) the next question is:
> Where do you connect the second electrode of your C? A coax,
> say RG213 may be connected to the antenna wire with the
> inner line. But where do you connect the screen? Bringing it
> back to the shack and connecting to ground would mean not to
> use the soil as the conductor any more and throwing the coax
> just on the bottom would be nonsense since then, one could
> just take a standard wire which is principally almost the
> same like aluminium foil. Using such a simple wire would
> mean to require 100s of m of cable to get the same C. And in
> such a configuration a high C to ground is essential since
> one wants to have a practicable small L for resonance (to
> handle the losses).
>
> Therefore i had the idea just to try a real loop that needs
> just a high C with less voltage (probably <1kV).
> If we assume a inv L of 10m hight and 50m horizontal
> section and make a rectangular loop out of it, the XL will
> be in the region of 15 Ohm (?). Then a C of abt 1µF will
> resonate the loop. Applying 100W and assuming 2 Ohm losses
> will produce abt 7A and a voltage of 750V above the C, no
> problem to find such a C :-) Starting with a closed loop is
> much easier since no L and maybe even no R transformation is
> necessary. And with a horizontal section of <50m, the
> increased loop area when using the ground/soil may be not
> really much more, but with the disadvantage to need a L with
> a high R and/or less Q...
>
>
> Markus/DF6NM has also good ideas but i do not want to tell
> his ideas. Markus? Go on! ;-)
>
> Stefan/DK7FC
>
> ________________________________
>
> Von: [email protected]
> im Auftrag von ALAN MELIA
> Gesendet: Di 23.02.2010 13:31
> An: [email protected]
> Betreff: Re: AW: LF: VLF_8.79 kHz
>
>
>
> Hi Stefan, I am not sure about your aluminium foil idea. I
> worry that this will form a lossy capacitor because the
> dielectric a think film of ali oxide will break down in
> places at this voltage and also there is the inknown of the
> contact with the ground. I would rather use some UR67 it can
> be a length unsuitable for real feeder work (I uses coax as
> a capacitor in an HV pulse generator many years ago and
> though the spec is 6kV it certainly survived 15kV with no
> burns though the dielectric. Rik and I discussed this format
> at length many years ago for 136kHz but I dont think either
> of us did any tests.
>
> I would suggest an HV capacitor might be the alternative
> old TV EHT type??
>
> Actually grounding the remote ends is a lot easier and a
> lot less to carry if you are operating "portable"
>
> Alan G3NYK
>
> --- On Mon, 22/2/10, Stefan Schäfer <[email protected]>
> wrote:
>
> > From: Stefan Schäfer <[email protected]>
> > Subject: AW: LF: VLF_8.79 kHz
> > To: [email protected]
> > Date: Monday, 22 February, 2010, 23:36
> > Rik, Alan,
> >
> > I think we think about the same: using the inv-L as a
> > vertical loop by using the ground.
> >
> > Wouldn't it be easier to use a aluminium foil to get
> proper
> > C (to ground), especially when talking about high
> voltages
> > and high currents? Just covered with some soil and
> easy to
> > adjust/modify...
> >
> > Stefan
> >
> > ________________________________
>
>
>
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