Re: Ferrite Sleeve Antenna Permeability
I would like to respond here and clarify some points made by Ben in a more direct manner. The page referenced is an excellent page. Perhaps some should skip past some of the more technical parts, and look at the data published, and go back later when ready to tackle the equivilent descriptions of the inductor. What Ben did was in two parts; the first was to establish the Litz wire as best suitable for the application by trying different bundles of #46 Litz. Smaller and larger bundles were compared in making 250uH ferrite inductors. What he found was that at larger bundles, the length of the coil winding became too great, and small bundles did not occupy enough of the ferrite rod length. What did the best was one particular bundle that had 125 strands, that when wound upon the rod occupied about 1/3 of the ferrite rod length.toggle quoted messageShow quoted text
Part two involved using that "best" configuration directly on the rod, and then placing insulators between the rod, and the winding. Note the Q values encountered (Table 2)in the AM-BCB, especially those at the low (520kHz) and high (1710 kHz). There is a trade-off here, and one's needs might favor one set of results over another, thus, "your methods may vary". Also of note is the "Air-Core Inductance" in table 1. As the spacing between core and coil increases, the "Air Core" inductance increases. BUT NOTE the Ferrite Inductor does not change inductance much at all.
In brief, Ben has demonstrated a reduction of the Electric field (in the wire) that causes losses in the inductor by spacing the coil away from the ferrite rod by using an insulator: plastics, or air. When first written this was a large leap of understanding of ferrite inductors for most of us "ordinary" folks that wanted a High-Q but reletively small package.
Now lets look at the real-world of "what you may have seen". Some of the older pocket transistor radios used to wrap the coil on a paper or cardboard sleeve, and place that assembly on the powdered iron/ferrite strip/rod. This effect that Ben wites and exemplifies is what was to be achieved. Ben's choice of Polypropylene, and Polyehtylene plastics are novel, and quite good for finding a common material "around the house". I also would add that "heat-shrink tubing" is made of the same family of plastic as these other two, and can also work quite well.
So I am quite sure that Ben's theories and experimentaion leads one to believe that a shorter coil length was better than a longer one: keeping the perspective that such coil winding occupies about 1/3 of the ferrite rod length. Thus the aspect of one part of the Ferrite Sleeve Core Inductor has been established: the insulation of ferrite core and wire-wrapping.
Paul S. in CT
--- In ultralightdx@..., Gerald Wolczanski <jerrywolczanski@...> wrote: