Re: [TCML] Treating RF feedback

[Brandon Hendershot <brandonhendershot@xxxxxxxxx>]

Now that I think about it, I think I remember doing something incredibly
asenine; I'm pretty sure I left the extra extension cord for my quenching
fan coiled up half way between the variac and the coil, about five feet
from the coil base if I had to guess. Seems like an ideal receiving loop to
pumping emi back into the house, yeah?
I also recall receiving a little shock from the variac case while the coil
was running. I'll let you make of that what you will, I can only assume at
this point.

Brandon H.

On Mar 17, 2013 9:56 PM, "Jim Lux" <jimlux@xxxxxxxxxxxxx> wrote:
>
> 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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