Yes, I think you may be absolutely right, Don. I measured a maximum effect of 9% in the one example I tested - a random core lying on my bench that had about a dozen turns on it. That agrees with your experience of finding up to a 10% change available.
I saw your suggestion before, and it certainly sounds plausible. I wonder if something else might be at work here too. Or perhaps I'm looking the same effect you are in a different way. Here's my thought. The increased inductance/turn of the toroid compared to ordinary air-wound coils is caused by the very high permeability of the core compared to air. That makes the inductance fairly independent of the spacing between turns since spacing the turns hardly reduces the magnetic flux in the core that is available to each turn. But I wonder if crowding the turns together doesn't slightly increase the inductance by providing a lower permeability path for the magnetic flux, since the distance the flux must travel along the toroid before it passes through all the turns of the coil is shorter. In other words, the same effect one sees by using closer spacing in an air wound coil, only much, much less due to the efficient magnetic path provided by the core. Does the formula you are using account for a reduction of flux density around the distance of the torus due to the losses in the core, or does it assume a constant flux at all points? It's always interesting when experiment fails to support predictions. True, it most often turns out to be an invalid experiment that causes that result, but I can't see the problem here, especially considering that the effect of added distributed capacitance has an inverse effect on the reading on this type of 'meter'. Ron AC7AC -----Original Message----- From: W3FPR - Don Wilhelm [mailto:[EMAIL PROTECTED] Sent: Monday, August 29, 2005 9:22 PM To: Ron D'Eau Claire; [email protected] Subject: RE: [Elecraft] OT: Effect of Compression and ExpansionontheInductanceof Toroids? Ron, I have the same inductance meter, and it does a good job because it does measure using the LR time constant rather than frequency, but 'physics is physics', and with a true toroidal inductor, there is still no dependency on the turns spacing (the math says so). Now, for the practical side of things, I do believe that your results (and others with similar findings) deviate from the 'classic physics' treatment of the ideal (ignoring practical behavior) - and when the toroid turns are irregular (not evenly spaced around the circumference of the core), there is some part of that coil behaving as a solenoidal inductor where turns spacing IS a factor. Remember that a toroid is simply a solenoidal coil formed into a circle with the ends of the coil meeting. This ideal toroid has equal turns spacing all the way around. So my current conclusion is -- IF the toroid is equally spaced around the core, the inductance is not dependent on the spacing of the turns, BUT, if the turns are compressed anywhere around the core, additional factors rear their ugly heads because the inductor is a combination of a solenoidal coil and a toroid coil, and the math becomes quite complex - just how much of each effect depends on just how much deviation from an ideal toroid shape exists in the configuration at hand. In a frequency dependent tuned circuit, how much of the frequency shift is due to the inter-turn capacitance or a change in inductance is (to me) only a matter of curiosity - the plain fact is that some change in the resonant frequency (or inductance) can be achieved by changing the turns spacing, but we all know that the adjustment range is small - the really big determining factor is simply the number of turns. As an example, I often improve the 10/12 meter Low Pass Filter characteristics of a KPA100 by squeezing the toroid turns "just the right way", but I determine what is the 'right way' by monitoring the impedance with my MFJ259B as I do it. Sometimes it is 'this way', while other times it is 'that way' - the batch to batch change in the permeability of the cores likely accounts for more variation than the turns spacing itself. BTW, this change does not really help the KPA100 output, but it improves the base K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed. The overall inductance change that I have experienced is about 10%, so that is within the normal design tolerances using 10% resistors and capacitors - so except for satisfaction of the curiosity factor, I would say just to wind the toroids with the proper number of turns, and 'tweak' them in-circuit as required and as close as is permitted by your measurement capability. 73, Don W3FPR _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
