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Re: [TCML] New Ground Question. Surface area
sparks@xxxxxxxxxxxxxx wrote:
If I put one of those cheap battery type clamps on the end of
the ground wire to clamp onto the ground rod will that hinder
performance? It
sounds like you want alot of surface area in a grounding cable.
The surface area of the cable is important is that the cable extends for
a much longer length. you need the surface area to reduce the RF
resistance.
The inductance is only weakly dependent on surface area. A factor of 10
in surface area might result in an inductance variation of a factor of 2
(I'm shooting from the hip on the numbers, but it's not a big deal).
(I just found an equation attributed to E.B. Rosa:
Lac = 2L[ln(2L/r) - 1.00]
for tubes at frequencies where skin effect is dominant. ln(2L/r) turns
into ln(2L)-ln(r). The reduction in inductance with "r" (which is
proportional to surface area) goes as ln(r). That's a pretty weak
dependence.
The reason folks use strap instead of round cable is that because of
skin effect (which is a factor in metal, as opposed to human bodies,
because the conductivity is MUCH higher), a round cable doesn't
effectively use the mass of metal. A hollow tube and a flat strip with
width equal to the 1/2 the circumference of the tube have similar
inductance (current can flow on both sides of the strip, and only the
outside of the tube). So, if you're looking to save money, for a given
impedance, the cheapest way is a flat strip about 10 skin depths thick.
The contact area of the clamp needs to be large enough so
that it can carry and short circuit currents as well as the RF from the
coil output. Obviously, the larger the contact area, the better, since
that gives you lower contact resistance and less voltage drop across the
ground clamp connection.
Indeed, one does want good clamping area. And auto jumper cables have
several strikes against them:
Bad clamping area, and,
relatively small diameter wires.
You'd do better with an ordinary banana plug or a 1/4" spade connector,
both of which are easy to attach to your secondary wiring (which is
probably something like AWG 20 or smaller). It's not like you need 2/0
welding cable for your ground. The inductance difference between 2/0
and AWG14 or AWG12 isn't all that much (see above), and the currents in
the secondary are small enough that you're not worried about melting the
AWG 14 wire (after all, the secondary itself is wound from smaller wire).
I would be wary of any sort of ground clamp that can be accidentally
disconnected during operation, perhaps by a startled bystander or
operator jumping out of the way of an unexpected streamer. My
preference is a clamp that is attached with a solidly locking clamp
screw, or a split ground clamp that encircles the ground rod.
The standard 1/4" "Fast-On" type connectors work well. So do PowerPole
type connectors. A small "split bolt" would also work.
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