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Re: Tesla Coils and Lamp Tests - (actually a tiny snip from the big post)



> I Like my illustration too :-))  It does simply get the idea across. 
Even
> though it "looks" like nothing is there, there really is SERIOUS energy
in
> the air space around a Tesla coil.  Objects placed in this area will find
> lots of voltage and current buzzing around.  It would be interesting to
> study how various "cheap" electronic devices behave in this area.  It
would
> be an extension of the habit auto companies have of driving their
> electronically controlled cars under high power transmitter antennas to
be
> sure the electronics will not be affected by the extreme fields.  Perhaps
a
> small business opportunity there.  I can't imagine any thing worse than a
> TC for such a test...  Watches, pagers, calculators... would all be
> challenged in this zone.  My boss keeps wanting me to have a pager.  This
> will definitly be its first test :-))

Oddly, that is just what some companies do, although not typically in a
formalized fashion (The TC is just too unpredictable and inconsistent,
every one is different, so you have a hard time standardizing the test).
However, there used to be an amusing story (on the web, even) of how
Modcomp (the originator of the Progammable Logic Controller) used  a TC in
particularly harsh EMC test (back in '60s or '70s).  The test then became a
popular feature of their trade show exhibits and sales demos.

These days, most EMC tests are done in either a chamber, a TEM cell (which
is basically a big waveguide driven by an RF power amp), or on a suitable
calibrated test range. The AF Phillips Lab in Albuquerque has an enormous
test range for testing EMC effects on full sized airplanes. Imagine a huge
grid of wires suspended in the air to make a truly gigantic capacitor into
which you put your "unit under test"


> 
> 
> I would wonder if one were trying to light an LED at a distance, if
another
> tuned coil or just a wire with an equal RF "profile" would give the
> brightest light?

The power you intercept is essentially determined by the physical size of
the antenna, but a tuned coil might help you transform the impedance of the
antenna to that of the load, making the system more efficient.

> 
> >
> 
> I vote "Faraday".  Richard Hull (I believe) has walked around the
> neighborhood with a radio receiver and has shown that the RF dies out at
> about a block.  Considering that he is trying to pick up multi-megawatt
> transmitter with a modern super whatever-dyn receiver.  A propagation
> distance of one block shows that the antenna is really really poor!!
> 
It may have a pulsed power of megawatts, but the average power (which is
what the AM receiver detects) is probably fairly low (several kW), and is
spread out over a wide bandwidth. The receiver only looks at an arbitrary
10-20 kHz chunk of that.  Say your TC puts out signifiant power over the
whole AM band (500 to 1500 kHz). If you put out a kilowatt, the spectral
power density is only 1 milliWatt/Hz, so to your little AM radio with a
bandwidth of 10 kHz, it looks the same as if you had a 10 watt transmitter,
just at your frequency. Add this to a really inefficient antenna, and you
see the reason the EMI dies out. 


By the way, for most antennas (if not all radiating devices) at distances
less than 2*pi*wavelengths, most of the energy is actually stored in the
electromagnetic field and not radiated. This is the difference between
"near field" and "far field".  In an oscillatory system, the energy is
continually transfering back and forth between Electric and Magnetic
fields. If you put a electric load out there (like the "radiation
resistance"), when the energy is in the "electric field" some gets
dissipated in the load.  If you put a magnetic load out there (like a
magnetically lossy ferrous sheet), the energy gets dissipated when the
energy is in the "magnetic field".  Good shielding relies on both, so you
get maximum dissipation: for instance, I use a lot of ferrite and carbon
loaded plastic for microwave shielding and absorbtion. In a waveguide, you
have various places where the field is largely magnetic, and other places
where it is largely electric. The dual absorber covers all the bases.


Note that a "faraday cage" for shielding does not work by absorbing, but by
containing the fields. Any penetration through the cage can carry the
signals through the shield. Fortunately, at low frequencies, a hole can be
pretty big and it will still work, unless you have a conductor protruding
"through" the hole