Re: 8-9 RFI noise thoughts.
There may well be plasma physics effects that contribute to RFI, ie the
negative resitance, but I
assume the major effect is simple the speed that the gap closes. Presumable
this produces a wave with a fast rise time and hence harmonics up to UHF. I
belive someone suggested the SG raise time was a few ns. The effect of the
reflections is to reinforce particular frequencies. The analogy is similar
to plucking a
I can see how your ferrite rings could reduce the initial rise
time and hence some RFI. But given they saturate at a few amps (as others
noted) how effective can they be. Maybe the loss and propagation effects
(waves travel very slow in ferrite) are more significant than the
When I was taking about a quarter in from the ends of the coil I was
referring to the voltage wave not the current wave, just explore the coil
with your voltage probe or an insulated scope probe.
I never did respond to your last comment about the 1/4 wave theory. I
misunderstood the statement "the secondary can be modelled as a helical
antenna" to mean it was finally agreed to be a 1/4 wave effect.
You may be convinced by the following experiment. If you connect a sweep
generator to the base of a Tesla coil vie a piece of terminated coax and
monitor the amplitude you will notice a series of resonance spikes having
the approximate relationship in frequency of 1, 3, 5. etc.. Possibly the
relationship is not accurately 1,3,5 because the propagation velocity is
slower near the grounded end due to the higher self-capacitance and or
because the propagation velocity varies with frequency.
A particularly surreal effect that I discovered was this. I wound a 10in
long coil on an 18in piece of 1.5in diameter plastic water pipe using 0.5mm
wire. I mounted the coil above a sheet of aluminium to act as a ground
plain. I connected the base of the coil and ground plain to a signal
generator that had a maximum output of 20v pk to pk. When the frequency of
the generator was at one of the resonant frequencies of the coil there was
sufficient voltage strength to light a small florescent tube if it was put
close to the coil. At the higher resonate frequencies it was possible to
observe the standing waves along the coil as a series of nodes at which the
tube would light brightly. The Q was about 150 so tuning is tricky.
Presumable the lumped view can not explain this effect.
The same coil could have been used to demonstrate the relatively slow
propagation down the coil directly by applying a square wave to one end and
the delay to the signal at the other. Despite the fact that one end of the
coil is only 4ns away from the other the propagation of the wave along the
coil axis is never the less hundreds of times slow than its free space
velocity. The now old fashioned analogue TVs contained a delay line that was
inserted in the luminance signal path to compensate for the delay in the
colour signal path. It consisted of a long helical wound coil. An other
example is many oscilloscopes contain a delay line in the signal path to
allow observation of the trigger event. In some cases the delay line is a
piece of coax the centre conductor of which is helical wound.
It has been suggested that because the phase of the current flowing in the
base of the coil is in phase with the current in to the top C this
invalidates the above argument. At resonance the voltage at the top of the
coil will lag the bottom by about 90deg, the current in to the top C will
lead the voltage at the top by 90deg, hence no phase shift of current from
bottom to top. If the coil is resonating in its times three mode the voltage
at the top will lag the bottom by 270deg, hence the current into the top C
will lag by 180deg. This can be easily verified.
Perhaps somebody with a big coil would like to modify it so that the primary
operates at approximately three or five time the resonate frequency of the
secondary. It should be possible in a darkened room to observe the rings of
corona round the secondary coil. Or alternatively the hot end can be
connected to the ground plain in which case the frequency need only be
approximately doubled. And it would avoid the effect being swamped by corona
from the top. You can than take a pic and post it.
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 20 August 1999 11:06
Subject: Re: 8-9 RFI noise thoughts.
>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> You bring up a very good point. The UHF gap oscillations are probably
>more based on plasma physics rather than electronics. Although we can
>"simulate" these waveforms with Ls and Cs. That is probably not too
>realistic. However the use of ferrites does seem to significantly affect
>the UHF oscillations.
> The length of the primary and interconnect wiring is 23.2 meters. The 1/2
>wave frequency is about 6.45MHz. This would be in the range of the 5-30MHz
> Just like the old 1/4 wave secondary theories, there is a question if the
>primary is a lumped L and C or if this frequency is related to wave
>propagation along the primary coil. I really cannot say for sure but I
>would think the magnetic coupling in the primary coil would make it more of
>a lumped situation. It may be difficult to probe the right spots (1/4 in
>from each end) in the primary coil. However, the currents at the center
>and spark gap should be 180 out of phase. Perhaps one could tap into and
>get information there? I am not sure at just this moment...
> Hopefully, in a short time, there will be a bunch of people with fiber
>probes to test things like this. I probably won't have time in the next
>few weeks but I will ponder it...
>At 04:58 PM 8/18/99 +0100, you wrote:
>>I think your trying to account for the wave effects with lumped
>>When the gap fires a wave probagates from each side of the SP. they will
>>have oppositer polarities. The waves will be partialy reflected at each
>>impeadance. SP to interconnection, interconnection to coil, coil to
>>If I remember correctly your primary is about 30m long. The wave velocity
>>will be some were between a half c and c. If its half a wave (I now think)
>>then thats a frequency some where between 2.4MHz and 4.8MHZ. The wave
>>should be at a maximum one quater from each end of the primary as the
>>will cancel at the SP and at the center of the primary. Check it out.
>>Regards Alwyn Jones (FL)