[Elecraft] RE: OT: Effect of Compression and Expansionon the Inductance of Toroids?
Ron AC7AC wrote: ... I'm confident that the added capacitance by squeezing the turns together is not what is causing my L-meter to show increased inductance. If anything, the capacitance would tend to cause the L-meter to show lower inductance. Inductance is a result of the magnetic field intercepted by each turn. An ideal inductor has each turn intercepting the same (entire) field. In a solenoid much longer than it is wide, however, this does not happen, and the inductance realized is lower than the ideal case, sometimes much lower. While a ferrite core concentrates the field, and a toroid insures almost all of it remains within the core, the winding is after all halfway surrounded by air and some of the field is still not shared by all turns. Compressing it reduces this effect. A Bug Catcher coil comes closer to L changing as the square of turns than a Hamstick(r)! That's my take, anyway. One way to test this would be to cover the winding with ferrite. A babushkoroid! Or try a pot core. If my surmise from High School physics is correct, the more the field is in ferrite, rather than air, the less effect compressing the turns will have. Indirectly, we might also try surrounding a toroid with a solenoid to see if coupling to that winding decreases as toroid turns are compressed, but leakage coupling will depend on the angle each part of the toroid winding has to the solenoid -- and the sum of all the fields on a completely occupied cores at a surrounding solenoid should be zero regardless of leakage, I think. Perhaps instead of a solenoid, an external toroid wider than the inductor under test might be used. But I like the babushkaroid. Remember, you saw it here first! Cortland KA5S ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] RE: OT: Effect of Compression and Expansionon the Inductance of Toroids?
Ron said: I'm confident that the added capacitance by squeezing the turns together is not what is causing my L-meter to show increased inductance. If anything, the capacitance would tend to cause the L-meter to show lower inductance. On Tue, 30 Aug 2005 07:00:35 -0700, Cortland Richmond wrote: Inductance is a result of the magnetic field intercepted by each turn. Having recently completed a LOT of research on how coils wound on ferrites behave, I think I have a handle on what is going on. Several basic principles are at play. 1. When the mu of a ferrite toroid is much much larger than air, it contains virtually all of the flux. This is the case at frequencies where LOSSES in the ferrite are low. 2. Ferrites chosen as cores for resonant coils and transformers are usually chosen to have low losses (high Q) in the frequency range where they are used (for example, in the K2). 3. Conversely, ferrite cores used for RFI suppression should be chosen to have HIGH losses (low Q) in the spectrum where suppression is needed. 4. Many ferrites are semiconductors (that is, between conductors and insulators), so they also have permittivity, and they will act as a dielectric. So even if all we do is pass a wire through a long ferrite core (like a bead or a clamp-on), there will be capacitance through the core between the opposite ends of the wire. This capacitance will be in addition to the capacitance between turns. 5. Ferrite parts also can exhibit a DIMENSIONAL resonance, whereby standing waves are set up in their cross sectional dimension at the half-wave frequency. This mostly happens with LOW frequency ferrite materials (MnZn). 6. The equivalent circuit of a ferrite choke or coil is two parallel resonant circuits in series. One resonance is the DIMENSIONAL resonance, the other is the CIRCUIT resonance between the coil and the stray capacitances (of #4). Both of these resonances have significant R components as well. So the apparent change in L as the turns are expanded or compressed is simply the CIRCUIT resonance moving as the stray capacitance changes. For all practical purposes, L does NOT change. C changes. There's a lot more about this in an applications note on my website, that also includes some references to the literature. http://audiosystemsgroup.com/SAC0305Ferrites.pdf Jim Brown K9YC ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
