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grounds, was RE: capacitance of homemade caps

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 04:24 PM 4/7/2005, Tesla list wrote:
Original poster: "Lau, Gary" <gary.lau@xxxxxx>

Well there are crappy grounds, and then there are crappy grounds
(couldn't resist)!   But seriously, most household waste water pipes
these days are PVC.  Not sure what they are under the streets, but it's
not clear that they'd be metallic.  Cold water supply pipes are much
more likely to connect to a ground-worthy buried metal pipe.

Not any more...
That's why cold water pipes aren't approved as a grounding electrode by the electrical code.

Folks, think about what you're trying to accomplish with a TC ground.

Aren't you providing a path for the RF (and spark currents) to return to the secondary?

If so, you want something reasonably low impedance, and reasonably close to where the coil is operating. Running a 200 ft ground wire down the hall and out into the back yard is only making the current loop really, really big, which is really really bad. The RF current will flow from topload, through the capacitor formed by the air, to ground UNDERNEATH the coil (through the floor, etc.), then through the soil out to that ground stake, then up into your 200 ft wire, then back to the coil.

Look at the field plots from any of the programs that do so... all that E field is between the top load and the "grounded" surface under the coil. The flux density once you get a few coil heights away is very small.

For a tabletop coil (say, anything less than 3 ft tall), use some sort of conductive sheet under the coil (chicken wire, aluminum foil, window screening, etc.). Make its radius comparable to the height of the coil.

For a bigger coil, the problem is a bit trickier, although, clearly, laying out a 20 ft diameter circle of chicken wire for a 6 ft high coil will probably do quite nicely. If you're not on the ground floor (i.e. sitting on a slab on ground), this is what you'll need to do. (the coil on the roof idea...)

if you're sitting on the ground, then you can probably get away with a much smaller mesh disk, relying on the capacitance of mesh to the ground to couple you to the soil.

There's also the issue of the current carried by the streamers, which, on a large coil, can extend a fair distance from the coil.

Let's put some numbers to this...

First, let's look at the streamer issue. 6 ft high coil, with top load 8 ft off the ground, 10 ft sparks. Let's say the topload and selfC combined are around 50-100 pF. This implies that there's a capacitor from the top of the secondary to the bottom of the secondary (and ground) of that value. The streamer will add about 10 pF (1pF/ft). This looks like another 10pF in parallel with the the topload and self C... It's about 1/5th or 1/10th the size, so about 10-20% of the RF current will flow through the streamer C, as compared to the "free air C".

So, it's reasonable to assume MOST of the current will flow through the capacitor between topload and base.

In an attempt to quantify the amount of RF current flowing within a certain radius, I looked at some analysis for short vertical antennas. For a 0.1 lambda high antenna about 2% of the total power is dissipated in earth currents for the band from 0.1 lambda to 0.5 lambda away (that's in the area outside the circle with radius = height)...

I'm sure someone can come up with a nice analytical approximation modeling it as a disc above a ground plane.