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Re: faraday cages (II)
Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <wysock-at-ttr-dot-com>
To Ray, all.
As you may well remember, "RFI" is a term that is sometimes
used "loosely". It really is a combination of two distinct and
related components; the e-field (electric field) and the h-field
(magnetic). An air borne spark discharge will posess both of these
components, which will have some specific relationship to one
another, based on all the complex attributes of that discharge;
such as rise time, duration, repetition rate, peak voltage and
current, and frequency.
What I was referring to in my original post, was some experiences
I've had with several installations, where the Tesla coil was
operating at about 120 KHz, and having detrimental effects on
sensitive (digital) electronics that were (of necessity), placed
fairly close by to the coil's location. I found out, that I could run
the coil at full power (about 5 KW input), and as long as the
discharge from the coil was limited to less then 3 inches, no
harmful effects were observed in the other electronics. As soon
as I allowed the air gap to increase to more then 6 inches,
even at very low input power, the other electroncis began to
react to the coil's operation. At a 3 foot gap, it drove the other
electronics nuts! I brought in a spectrum analyzer with a
calibrated e-field probe antenna for its input. The interference
produced by the spark discharge itself, was very broad-banded
ranging from the KHz range, all the way up to the MHz range,
and showed somewhat of a periodocity to the envelope peaks.
As this situation was to get the problem solved in the most
expedient manner, there was no time to do an extensive
analysis of the phenomena being observed. The result was
to back-engineer all the RS-232 "daisy-chain" ground loops,
add lots of ferrite toroid filters to the cabling, and make sure
all the sensitive electronics were fed in a "star" fashion, from
one centralized bi-polar power supply. It eventually worked
out quite well, but as we say in the trades...."It was HELL!"
Now, as for references to this kind of work, and some published
standards, there is much available in the EMC (Electro-magnetic
Compatability) branch of the IEEE Transactions. Literally,
tons of case-by-case investigations from all manner of
susesseptibility studies to IEEE standards for test and measurement.
In the relm of Tesla coils, I feel that the most critical elements for
study and understanding are the e-field measurements (as in
volts per meter in the near field of a coil), much more so then the
h-field concerns. Power density measurements would be a close
second concern, as in watts/meter sq. or mw/cm. sq.
I hope this helps. But remember, each coil, each installation, each
separate circumstance, usually has to be considered on its own
specific merrits, environment, power level, etc.
> Date: Sun, 25 Feb 2001 10:08:41 -0700
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: faraday cages (II)
> Original poster: "Ray von Postel by way of Terry Fritz
> Glad you joined this thread. You are preceded by your reputation.
> >From what I can find out, there is little SPECIFIC information available
> to the output of a Tesla coil. I hope you can point me
> to a source. There is an old saying: "You don't really know until you
> can write an equation." (that should produce a flame). The equations are
> there for the design of the coil itself, but almost everything else seems
> Tesla list wrote:
> > Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> > To Ray, et. al.
> > (Terry, my apologies for not "snipping" this thread, but I felt
> > it was a necessary addendum to this discussion).
> You speak of attenuation of a "very broad band" of RFI.
> RFI to me means an annoyance rather than a real problem. Stopping
> it is usually as much public relations as technical skill. It is often
> tedious and time consuming and can involve such little things as
> a ground wire touching a rusty tomato can. What I am
> concerned about is sufficient electrical energy at what ever power and
> at what ever frequency and wave shape sufficient to cause physical
> damage such as burning out transistors.
> When I spoke of the "output" spectrum of a Tesla coil, it was not my
> the problem to the SPARK DISCHARGE ITSELF. By that I assuming
> you mean the fixed or rotary gap. I take it as a given that every Tesla coil
> is a very dirty r.f. source. However, I don't know if anyone has taken the
> time to measure it in terms of frequency and amplitude. i.e. How many
> meter, at what frequencies, at some specified distance from the coil and
> how many
> decibels above or below
> some recognized standard power. Developing a field pattern for the
> from a Tesla
> coil would at best be difficult because of the almost infinite number of
> frequencies. Not
> only that, but such a pattern would be unique to that particular
> installation just
> as it
> is for radio transmitters in general. You can engineer a radio
> installation and
> you get the coverage you want, but the result still has to be proven
> If EMP is generated, how much? What, if any thing is the problem?
> Have you or do you know of any one who has attempted these measurements on
> a Tesla
> As you say, it is an on-going problem.
> The lack of design and construction standards for consumer goods has been
> with us
> since the
> first use of radio. It is not so long ago that every ham held their breath
> their neighbor bought a TV. The difference is that transistors, working
> voltages and currents are much more vulnerable. Cases in point:
> Lightning hit a neighbors tree. I lost the mother board in my computer.
> Lightning hit the tower at a broadcast station and produced a fire ball
> the transmitter. Loss one mica capacitor. Transmitter down for 25 minutes.
> This was before transistors.
> Perhaps my interest in this is related to the fact that I would rather design
> on paper first, then build and test it, with some assurance that what I am
> working on
> has a reasonable chance of doing what I want it to. But then, as Dr.
> Terman was
> as saying: "You can design something, but in the end, you often find
> Tinkering is fun but it is cheaper to waste paper.
> Thanks for your input. It is valuable because you are well know and
> for your
> experience with and knowledge of Tesla coils.
> Ray von Postel
Tesla Technology Research