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Re: [TCML] Treating RF feedback
On 3/17/13 6:01 PM, w5als wrote:
hi
RF is RF it make no difference weather I am on 125.210 running a
1000 watts or
on 40 meter 7.210 running a 1000 watt or a tesla coil. The idea is to
get rid of the RF and if you
are trying to ground and you can not get the rod deep enough you use
more than one
rod, this is simple physic's put the RF in the ground.
There isn't anything in physics about "putting RF into the ground" the
ground (meaning soil, here) isn't all that good a conductor, and it's
also not a very good load.
This is only
grounding the bottom
of the secondary coil nothing else.
OK..
But still, if you model the fields around your coil, you will find that
a ground rod doesn't do a very good job providing a return path for the
RF from the topload to the bottom of the coil. A wire mesh counterpoise
has a tiny fraction of the resistance of even very good soil, and so
provides a good path for the RF.
Very good conductivity soil has a conductivity of something like 0.01
Siemens/meter. Iron has a conductivity of 1E7 S/m. 9 orders of
magnitude better than the soil. Sure, you've got more cross sectional
area for the soil than you do for an iron wire. But consider this..
compare a 1 square meter cross section of soil with a 1 meter wide layer
of chicken wire. The soil will have a resistance of 100 ohms per meter.
A really good ground rod will have a resistance of a few ohms, maybe,
with reference to the surrounding soil.
2" chicken wire has a wire every 2", so for a meter wide, it will have
about 20 wires, each 19AWG (1mm diam) for a total cross section of about
16 square mm. That works out to a resistance of about 0.01 ohms
(probably a bit more because of skin effect, etc.)..
But the take home message is that a roll of chicken wire is a MUCH lower
resistance RF path than any amount of soil, no matter how many ground
rods you pound in, and how many pounds of salt water you dump on the soil.
And for folks who run that 50 foot wire outside to the ground rod... All
you're doing is making a not very effective antenna with that wire.
The same is true for a vertical antenna. Radials beat a ground rod
every which way.
There are a fair number of commercial tesla coils that have no RF ground
connection to earth ground (e.g. if you're mounting the coil inverted).
They still connect to the building structure usually, but that's more
for a safety ground.
For ham antennas, the only real reason for a ground rod is for lightning
protection and/or to comply with the electrical code for fault
protection when a power line falls on your antenna. In fact, driving a
rod under your radial field can actually make the antenna less efficient
because it puts power into a lossy medium which would otherwise be
partly shielded by the radials. I suppose if you had no radials, a
ground rod is better than nothing, but more likely your feedline shield
is going to be part of the radiating system acting as one radial.
I will reiterate that the way to avoid EMI/EMC problems is to make sure that
a) the TC currents are kept close to the TC. A counterpoise is the best
way for the fields at 100kHz. Keep the wires short going to/from the
spark gap and primary: that reduces the amount of VHF power you radiate.
And be VERY aware of where the return path from your spark strikes is
going.
b) power line filters to keep power from going back through the power
line, or the power line becoming part of a transmitting loop. The
greenwire safety ground is particularly troublesome from this
standpoint. RF couples through the primary and through the transformer
and capacitively couples into the green wire ground. If you have other
equipment plugged into the branch circuit, then the case of that
equipment (or at least the ground reference) gets RF on it, perhaps at a
different phase than at the coil, so your equipment can act like the
other terminal of a capacitor with your topload and get current flowing
through it. Older equipment which used the now deprecated (unsafe)
scheme of a 0.01 uF cap from line to ground/chassis is going to be
particularly bad.
Note that most "computer" EMI filters are designed for higher
frequencies than the few hundred kHz of the typical TC. They will knock
down the VHF fairly well, though.
and
c) look at your potential victim devices and make sure you don't
inadvertently have a big receiving loop that intercepts a significant
amount of the field from your coil.
I am not talking about your
electrical plug at all.
The closer the ground rods are to your coil the better it is. The ground
rod have to
be far enough apart that they don't effect each other. Brandon if you
don't understand
what I mean email me direct and can talk on the phone and I can explain
it better
thanks
alton
w5als@xxxxxxxxx
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