RFI source tests...

Hi All,

	Today I did more experiments with the RFI (Radio Frequency Interference)

I used my plane wave antenna probe:


This probe can easily reach 100MHz in bandwidth and is not confused by GHZ
noise affecting any electronics.  As I suspected, the initial spike is
composed of very high frequency RF.  The highest I see is at 300MHz but the
probe (100MHz) and the 60MHz scope are probably losing anything higher.
These frequencies were causing saturation of my fiber-optic probe and
causing just a single big positive pulse (the tell-tail sign of this).

I have now way of calibrating the field probe in this case but I can easily
see relative power levels.

Tank current = 14mV (current probe reading)
Zero Crossing spikes = 30mV
1st spike = 750mV

Since the power is probably proportional to these numbers squared and
assuming the primary current is around 100 amps, the "implied" spike
current is around 3000000 amps!!  I am not sure I believe that but...

As I saw the other night, it is not the tank circuit firing that produces
these nasty bursts.  I would see the burst if the gap would just slightly
click but would never go into full conduction.  I removed the whole tank
circuit and just ran the gap across the neon.  Guess what?  The spikes are
still there in all their glory!!!  Removing the wiring cuts down on the
transmitted signal a little but the vast majority of the spike is still
there just as before!!

So it is not the main conduction that causes this RFI.  It is just a little
initial burst of little power that does it!  It is just a little, hard to
see, arc click without the primary circuit, but the RF created in that
instant is enormous!

So.... time for this minute's theory...

I think the gap electrodes form a capacitor that stores capacitive charge
energy between the electrodes.  When the gap fires, this few hundred micro
Joule of energy is shorted with extremely low resistance and low
inductance.  The air molecules must simply spray all their electrons around
like mad in this instant.  The high frequency and high power RF is created
as the free electrons race back into orbits or whatever this plasma does.
Once the air is ionized, the gap acts like a conductor and all the stuff is
far far less of an issue...  Making are big metal low resistance gaps is no
doubt perfect for generating these spikes.

So it is no wonder my previous efforts at RF filters and such we useless...

In a conventional gap, the capacitance between electrodes is pretty much a
given.  I have no idea how you could eliminate it.  If you could add
resistance to the arc, the oscillation would be damped greatly.  Perhaps
some gas or different atmosphere...  ANY inductance would drop the
frequency like a rock and reduce the RFI level.  Perhaps ferrites beads or
something right at the gap tips...

What one really needs to do is SLOOoooW down the initial gap firing.  The
gap needs to start conducting gently rather than in this terrible plasma
explosion.  This system would also help zero current spikes for those that
have trouble with that too.  However, a small inductance in series with the
tank circuit my help that...

So there are three RFI sources form just the tanks circuit.

1.  The initial burst as the gap electrode's capacitance is shorted with
very low impedance.
2.  The zero crossing spikes where the already ionized gases seem to keep
the capacitances from recharging very much on thus making them much smaller
(but the same basic process).
3.  the normal few hundred kHz from the yank circuit's normal operation.

The last source is the only one that does us any good.  The others serve no
purpose and just waste power in the gap.  Even though the zero crossing
spikes contribute far less RFI than the initial spike,  I think they eat up
far more power.  Thus, causing the linear decrement and all that...

Any comments, ideas, and suggestions are very welcome.  This high power
microwave stuff is way out of my area.  I would think one could put the gap
and all in a big faraday cage (with small mess), but Richie tried that and
it didn't seem to help.  The wires leading from such a cage may conduct the
RFI too but "good" filters may stop that.

If these initial and zero crossing spikes could be reduced, perhaps it
would greatly help RFI problems.  Apparently, output arcs have enough
resistance and capacitance that they are not a big RFI problem.  A Tesla
coil may be made very quiet indeed if this could be solved.

BTW - I was wondering why commercial arc welders would not have the very
same problem.  I assume the low voltage they use stores far far less energy
in the capacitance between the electrode and the work (like 1/500000).
This when the arc is established, there isn't the "great" capacitive energy