From: Greg Leyh
Date: Tue, 10 Sep 2002 08:43:24 -0700
Subject: Re: [TSSP] Quiz, Sec diameter question
Hi Malcolm, At 01:40 PM 9/9/2002, Malcolm wrote: >[snip] > > What effect is it, that limits the performance of a > > secondary as it gets thinner? It's certainly not > > Q degradation due to copper losses. > >I wound a coil with an outrageous h/d ratio a couple of years ago to >see what effects might be present. I never got around to properly >characterising it due to a lack of an electrically quiet enough space >but a quick look showed its Q to be pretty low and the response >rather broadband with a lot of resonant modes present. I agree it >can't be due to copper losses and the only thing I could think of was >a rather poor L/C ratio. In effect, it was degenerating into a >longwire. The diameter from memory was around 2" or less and length >must have been around a metre or so. I think the wire was around >0.8mm. That's a rather extreme aspect ratio! One thing that I have noticed at the simulations level is that with very thin [< 1:10 aspect] coils, it becomes almost impossible to set up a *practical* primary geometry that provides more coupling than k=0.10 or so. > > From a design standpoint it appears that a thinner, > > more densely turn-packed secondary offers both a > > higher resonant freq (Fo) and output impedance (Zo), > > without reducing the Q to unusable levels. > >I never fired it but it would be interesting to see how it would >perform as both 1/4 wave and 1/2 wave. It might also be fun to run it >with a primary much too high in frequency. I would expect looping >arcs of corona to form along portions of it as seen on another >outrageous coil I once built. Your coil there is definitely in the realm of delay lines, as opposed to a lumped element. ^_^ The other part that I was curious about is whether very thin secondary coils might suffer an increased secondary voltage stress factor. After seeing some of Paul's GIF animations that show the voltage profiles whipping up and down the secondary [ala chaotic pendulum] I became far less comfortable with treating the secondary as a lumped element. [Transmission line theory? Aieee!] Even though placing the primary inside the secondary can offer the same overall coupling it seems possible that this might result in a *nasty* stress factor, if most of the coupling action is into a concentrated portion of the lower turns. Awhile back I was toying with the idea of connecting a capacitor across the lower 1/6th or so of the secondary turns, in order to convert the coil into a kind of 'hybrid' coil -- somewher e between a standard and a magnifier configuration. The idea was to minimize the time it took to get the energy out of the primary and cut the losses there. I'm now wondering if this hybrid arrangement might help smooth out the secondary voltage stress factor, as well. Comments?
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.