Re: EMI filtration Questions

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

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>


>>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.
	Depends on WHICH commercial filter.  One designed for a 200W

	PC PSU is not of much use.  That 'two brick sized' treasure
	from a hamfest might be most useful.


> 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.
	Exactly so.  Filters, unless _in_ a shield wall, are

	commonly 'radiated around'.


>>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.

	Exactly so.  Serious Isolation transformers had a 'shield'
	or Screen between primary and secondary.


>>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?
	Its worth a try, bering in mid the 'radiate around'

	situation.  Also 'suitable cap' may not be obvious:
	largish caps can actually have highish impedance at RF.


> 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.
	Agreed.


>>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.

	'half a loaf is better than no bread'.  8)>>

>> 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?
	In general, filter as close to the source as possible.

	Adding a filter at the entrance (RFI Exits...) may be
	useful as well.


> 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 can think of some....

>> I AM considering a Chicken wire Faraday cage.
>>Even a partial one would help.

	Whole ones are better.  Whole ones make line filters
	MUCH more effective, if installed IN the shield wall.

 
> 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.

	best
	dwp

...the net of a million lies...
	Vernor Vinge
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	-me