Terry's coil if it performs as you suggest is a little more efficient thana spark gap TC.
This is why I kept emailing him.. It depends exactly how much improvement there is.
Your coil however is extremely inefficient. You should get a 38" sparks from 500W with an reasonably efficient spark gap coil.
You are right, however I had to dig out my coil stuff from storage (of maybe 5 years) and throw it together in about a hour as a film crew came to video my coil in action (all that after a long day at the office aswell...) I came across the old coil data pdf on barts site, had one of my very early coils in there, 240watts input 18" spark output, I thin that was pretty good going as it was in my "early days" too.
Loose coupling causes the spark gap to remain firing for a long time *before* the first notch. That is bad because losses are continually occuring during that time. This is where the energy is lost, and is why loose coupling is bad.
I agree that longer conduction time is bad, though does the spark gap conduct less current during loose coupling ? If that is the case then less coupling should reduce spark gap losses due to less current over the gap, though saying that , as said before, it is conduction for longer which is bad. so there has to be some trade off between the two. More to the point, what are the actual factors....
I would also expect loose coupling to reduce the mutual inductance and increase the primary current which should be a good thing to a point. Though if you use a loose coupling and a higher frequency then you come back to what I was saying before...
The secondaries in our coils tend to be high Q. Of course when they start sparking it slashes the Q.
So for example, the spark loads the coil as a capacitor and reduces Q. though the toroid does the same and reduces Q, in anycase I would have thought that using a much higher Q towards the lower MHz range would be better for larger spark outputs.. I don't have any info on spark with Q factors, though have an idea that it is supposed to reduce the Q factor by like 90% ?
Does the secondary only spark when the spark gap turns off ? I am trying to see if the coil could spark while the primary to secondary energy transfer is still active... I was told that it only happens when the spark gap quenches ( turns off) and it is at this point the energy is trapped, has nowhere to go, and must break out... though I actually wonder as the voltage on the secondary is being ramped up and up that it would break out before the spark gap turns off ?
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