Ok Alan, but what is the difference between a ground stake and the alu foil,
when talking about the resistive losses? That would be also a poor quality
ohmic connection, isn't it? And think about how many meters of stake you need
if you want to have the same C that will be achieved by 5m*30cm Alu foil? How
much time and work for a short test.
Perhaps you know my /p radial for 137kHz: It is a simple copper plate of
0,2m*25m, so 5m^2. With that configuration i reach <30 Ohm overall losses. My
coil losses are 13 Ohm, radiation resistance is 0,8 Ohm, wire losses abt 1 Ohm
and soil conductivity abt 6mS. This radial is just rolled out on a stony field
on the hill, 350m ASL (see my picture @ qrz.com). While others bury 100s of m
of cable and run 10s of meters of earth rods into the ground i achieve earth
losses of less than 15 Ohm with just such a plate. In /p i do not have the
nerves and time to roll out 100s of m of radials so i was happy to find, that
this a good solution too. In my theory, it is necessary to get a good
connection to the ground by a high coupling capacity and that means a high area
metallic electrode close above ground. Although the area is so much
concentrated at the feed point, the results are so good. So i expect that this
may be a good solution on VLF as well.
Trying a cheep alu foil for the household, less than 2EUR for 30m*0,3m should
be worth for everyone to try if earth losses could be decreased :-) Almost no
money and/or time is needed :-)
73, Stefan
________________________________
Von: [email protected] im Auftrag von ALAN MELIA
Gesendet: Di 23.02.2010 16:31
An: [email protected]
Betreff: Re: AW: AW: LF: VLF_8.79 kHz
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
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