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Re: RF ground - what if you can't use a ground rod?

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 02:43 PM 6/29/2005, you wrote: 
Original poster: "Willem Bosma" <wbosma@xxxxxxxxxxx>

Hello Tom,

Even a buried 8 ft copper pipe is no decent RF ground!
You don't need an "RF" ground (in the antenna sense) for a Tesla Coil. A decent DC ground is good enough.



It could be enough for line frequencies, but as radio amateurs know, you should have a distributed ground , say at least 4 pipes buried several ft from each other.

For all intents and purposes, you could use standard DC resitivity equations for ground rods, since the frequency is low enough that the reactive component of the soil and the interconnects is negligible.

It's well known that two ground rods must be separated by at least their length to get close to half the ground rod resistance.

Now, let's consider the losses you're trying to reduce. You've got a fair length of wire wound in the secondary (say, 4" diameter, 1000 turns, about 1000 ft. For AWG 24, that's about 25 ohms. Then, there's all the losses in the primary circuit.

Typical driven ground rods have a DC resistivity (referred to an infinite second electrode) of about 30-100 ohms (per IEEE 142-1991).

Now.. how much power do we dissipate in that 50 ohms? We can use the RMS current to approximate. For a NST, the RMS current is going to be a few tens of mA, but lets assume 100mA. The RMS current in the secondary is then about 1 mA, so the power dissipated in the 50 ohm ground rod isn't all that huge (a few milliwatts). Even if we take the peak current in the primary (assume 2 J/bang, 60 uH) of about 200A. Scaled to the secondary, it's around 2A. Now you're up to a peak power dissipation of 100W, but that lasts a VERY short time, and the average power is down in the milliwatts.


This even holds for sea water! So I think you could, superficially, bury chicken wire (a few square yards) which would give you a good enough RF counterpoise for your tc, to let the secondary stay in tune.
Regards,
Willem Bosma, PA0TW 



----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, June 28, 2005 5:59 AM
Subject: RF ground - what if you can't use a ground rod?


Original poster: "Coyle, Thomas M." <tcoyle@xxxxxxxxxxxxxxx>

Our ground here is solid granite, topped by a couple feet of rock-hard
clay, topped by a handful of inches of soil. No way am I going to be
able to punch in an 8' ground rod for the RF ground (wish I'd thought of
this before I bought it). Are there any other alternatives? Can I bury a
couple square feet worth of copper plate under the soil, maybe? Or maybe
take the rod and bury it horizontally? Any suggestions?

Thanks again,

Tom