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Re: RF Ground, House Ground, Ground....



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

But just how low an RF impedance do you need?

A single wire in free space has an inductance of around 1 uH/meter... So,
assume you've got a 30 foot (10 meter) ground wire.  That's 10 uH.

Now, let's say you're running at 300 kHz.  X = 2 * pi * f *L so X = 6.28 *
3E5 * 10E-6 , or, 18-19 ohms...

That's just inductance, so there's no loss involved, by the way, just a bit
of voltage across the inductance, raising the bottom of the secondary above
"ground" potential, probably by a few hundred volts at most.

The loss would come from the AC resistance, which is going to be a tiny
fraction of the AC resistance of the secondary (which is typically wound
with much smaller diameter wire).

Skin effect will aggravate the AC resistance loss over the DC loss, and
Steve's comment about flat strips is relevant.  However, you could also
achieve the same thing with a set of parallel smaller round wires (poor
man's Litz).

And, even if you use #24AWG as your ground wire, the 20-30 feet of ground
wire is pretty small in comparison to the thousands of feet on the
secondary.


This isn't a radio transmitter kind of set up.  The "RF in the shack"
problem (where good grounds help) has several aspects:
1) Safety... if the transmitter floats up, you can get a shock
1a) Equipment damage ..other equipment in the shack might not like floating
at several hundred volts RF relative to the green wire ground.
2) Loss and pattern distortions.. if there's voltage drop across the ground
wire, it will radiate, possibly causing EMI problems, distorting your
pattern, or, just radiating useful RF energy into places it doesn't need to
be (like the side of your house).
3) Ham transmitters are way higher than 300 kHz, so the problem is much
worse... That 18 ohms at 300 kHz turns into 1.8 k at 30 MHz, and with an amp
or so RF current (typical at 100W output), that's 1800 Volts..  The skin
effect problem is also much, much worse at HF.



----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, June 06, 2002 6:08 AM
Subject: Re: RF Ground, House Ground, Ground....


> Original poster: "Steve White by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<slwhite-at-zeus.ia-dot-net>
>
> Hello All,
>
> I have been following this grounding thread and I think that some are
> missing the point with the RF ground. What you want is a low impedance RF
> ground. A low DC resistance is not sufficient. The DC resistance of the
> house ground is typically very low but it not an adequate RF ground
because
> it has a relatively high impedance at typical coil operating frequencies.
> Remember that the operating frequency of the coil is about 100 KHz. This
> relatively high frequency needs a low impedance ground otherwise very
large
> voltage drops will develop along the ground conductor. The best cross
> sectional shape for an RF ground conductor for low impedance at tesla coil
> frequencies is a flat ribbon. The house ground should still be used for
> tesla coil equipment that is expected to opertate at 60 Hz.
>
> Steve: Coiling in Iowa
>
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Wednesday, June 05, 2002 10:28 AM
> Subject: Re: RF Ground, House Ground, Ground....
>
>
> > Original poster: "Daniel Hess by way of Terry Fritz
<twftesla-at-qwest-dot-net>"
> <dhess1-at-us.ibm-dot-com>
> >
> >
> > Matt;
> >
> > Sounds like your ground may be sufficient. You can test it by measuring
> the
> > resistance between the RF ground and the house ground. Ideally, I
believe
> > you want to read no more than 1 ohm. Mine tests out a 1.5 ohms but seems
> to
> > be performing adequately. I'm also running a 4 Ga. cable from the base
of
> > my secondary to the RF ground; even if the resistance between house and
RF
> > ground is low, too small a gauge cable may not handle the current and
it's
> > performance may suffer. If the resistance is too high you may have to
add
> > additional ground rods. I use six x 10 feet x 1/2" copper water pipe.
> >
> > Regards,
> >
> > Daniel
> >
> > "Tesla list" <tesla-at-pupman-dot-com> on 06/05/2002 09:12:53 AM
> >
> > To:    tesla-at-pupman-dot-com
> > cc:
> > Subject:    RF Ground, House Ground, Ground....
> >
> >
> >
> > Original poster: "Matt Woody Meyer by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <meyerml-at-stolaf.edu>
> >
> > Just for clarification sake, I'm curious if there's any major difference
> > between
> > the RF ground that you're all using and mine.
> >
> > My ground is simply a long copper tube driven a good 6-8" in very moist
> > soil.  My
> > protection filter and my secondary are grounded to seperate tubes, and
> then
> > a
> > discharge rod (long wooden stick with a nail in the end of it wired to
> > ground) is
> > grounded to a third (Discharge rod used for discharging coil after
> > operation and
> > also for measurement purposes (never manually held near coil while in
> > operation)).
> >
> > Is this an appropriate RF ground, or should I be doing something else?
> >
> > Thanks,
> > ><>Matt
> >
> > ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>  ><>
> > Matt "Woody" Meyer                  St. Olaf College Physics Major
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
>
>