Re: grounds, was RE: capacitance of homemade caps

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Paul B. Brodie" <pbbrodie@xxxxxxxxxxxxx>

So, after reading your post, I'm a little confused about grounding my small 
coil, 15 kV, 30 mA NST, 4" diameter by 21" tall secondary. I have driven an 
8' by .5" ground rod approximately 10' from where I plan to run the coil, 
the front part of my garage on a concrete slab. The bottom of the secondary 
is connected to this ground rod by 10' of two 15' long auto jumper cables, 
four conductors of 4 AWG.

I asked this question once before with no responses. Should I use a 
counterpoise ground? Can someone please explain how a counterpoise works? 
Is a counterpoise an either or proposition or should I use both? Thanks. I 
really appreciate any help.
Paul Brodie
----- Original Message -----
From: "Tesla list" <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
Sent: Friday, April 08, 2005 10:39 AM
Subject: grounds, was RE: capacitance of homemade caps

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