Re: EMI filtration Questions

["Tesla list" <tesla-at-pupman-dot-com>]

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


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, September 03, 2002 5:33 PM
Subject: EMI filtration Questions


> Original poster: "Dave Hartwick by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <ddhartwick-at-earthlink-dot-net>
>
> I'm searching the archives!...and searching and searching......
>
> 1) I must wonder if commercially made filters are up to Tesla duty? An
> enormous amount of RF garbage must be pumped back through the AC mains
> feeding even modest coils. I'm wondering if custom built filters may not
be
> a better way to go for serious attenuation, or possibly running multiple
> commercial units in series.

Do you suspect the commercial filters from an isolation/attenuation
standpoint or from a RF power handling standpoint.  If the former, series
combinations would improve things, but I suspect that most of the problem
will go around the filter (unless you're running in a shielded room). If the
latter, then series combinations wouldn't solve the problem, because the
majority of the  RF power is going to get handled by the first filter in
line.

>
> 2) The effect of other devices: Inherently inductive devices such variacs
> and ballasts (MOT, welder) must have some low pass characteristics, but to
> what extent?

At lowish RF frequencies (hundreds of kHz) the dominant pass mechanism is
probably stray capacitance over and between windings.  You're right, the
huge inductance of the winding would serve as a low pass filter, but the RF
just goes around it.

> Let us say an MOT is used on either side of 240 VAC mains for ballast.
Could
> not suitable caps be used in conjunction with the MOTs to utilize its
> inductance for additional EMI filtration in addition to its main
ballasting
> function?

A commercial EMI filter would probably serve as well, be cheaper (i.e.
surplus), and be better packaged.  Building your own RF stuff and expecting
good isolation performance is quite an art.

>
>
> 4) Let us assume perfect AC mains filtration is achieved. The AC wiring
> through a typical house must act like a terrific antenna and pick up very
> significant RF from an operating coil. So our perfect filtration is at
least
> partially defeated. If part of the goal is to keep as much RF out of the
> neighbors house as possible, would not filtration at the AC Mains house
> (harboring the offending coil) entrance point be desirable?

I think the basic solution is to reduce the conducted fields (i.e. line
filters) and to keep the house wiring from being part of the RF return path
(i.e. use a separate RF ground from the green wire ground).  TC's don't
radiate much of a field (they're terrible antennas), and typical house
wiring won't pick up much power.


> This is the one thing I really do not like about coiling: The profuse
> garbage produced. There can not be a more efficient method for producing
> intensely energetic RF filth. I AM considering a Chicken wire Faraday
cage.
> Even a partial one would help.
> Dave Hartwick
>

In the shielding business, either you have a complete box, or you don't.  No
partial shielding possible (unless your box dimensions are very much greater
than a wavelength). If you have the budget for a multiwavelength partial box
for a TC, then EMI shouldn't be a big problem, because your nearest neighbor
will be far enough away to not be bothered.

That said, a set of chicken wire panels to provide the "other plate" of the
top load capacitance, connected to RF ground with a reasonably low impedance
connection (don't need welding cable, but a number of smaller wires would
probably work well.. skin effect and all that) might do a lot to control
incidental pickup.