Re: Ferrite loop antenna for LW/MW band

On Jan 17, 3:23�am, "Pete KE9OA" <p.gian...@xxxxxxxxxxx> wrote:
Hi,

Is there any kind of program that makes the calculations a bit easier? If I
knew how to do the calculations, I could throw the formula into a
spreadsheet. What I need to do is calculate the inductance with the
parameters of cross sectional area, length, position on the ferrite rod,
number of turns, and permeability of the ferrite material itself.
I would like to throw these parameters into the spreadsheet/program, and let
the software calculate the inductance for me.

Pete


Hello Pete

AFAIK there is no set of formulae which will do the job for you. What
follows comes from E.C. Snelling, "Soft Ferrites: Properties and
Applications", 2nd ed, 1988, p 155. Sadly this book's quality is
exceeded by it's rarity.

Snelling starts off by saying that the inductance ratio mu(coil), or
apparent permeability, of a coil when it has a ferrite core is
diffiult to calculate, "since it depends upon on the geometry of the
winding and the core and to some extent the core permeability".

Snelling goes on to present a simpler approach which "applies only to
centrally placed windings". He starts by assuming that a
"magnetically long cylinder" is covered over its whole length. (I do
not know if these means that the coil is tight wound or not, or even
whether this matters). He then goes on to state that the inductance
of the coil is proportional to mu(rod) times the area times the
number of turns squared divided by the length of the rod.

He then presents curves based on experimental data, based on coils
where the centrally placed winding covered 5-100% of the coil, the
initial permeability ranged from 15 to 1000, and the length/diameter
ratios of the rod varied from 3 to 18. The results are shown on p.
155. Snelling comments that using the mean curve will produce an
error of 12% or less.

The curve plots 10exp09 times L / (A.N.N.mu(rod)), against the
fraction of the rod which is covered. The approximate values
(fraction of rod covered first) are:
0.1 2.75
0.2 2.25
0.3 2.1
0.4 1.85
0.5 1.7
0.6 1.5
0.7 1.35
0.8 1.15
0.9 1.05
1.0 0.95

Note that you have to calcluate mu(rod), which depends upon the
initial permeabilty of the ferrite, and the demagnetization factor.
The demagnetization factor depends up the length /diameter ratio, and
to some extent the intial permeability of the rod. You can read the
demagnetization factor off a chart (Snelling p. 151), or read mu(rod)
directly from a chart based upon the initial permability and l/d radio
(Snelling p. 152). You will see the same chart at, eg,:
http://www.magneticsgroup.com/pdf/erods.pdf
To calculate mu(rod) you would have to able to calculate the
demagnetization factor. I do not know how to do this. The
calculation of mu(rod) is straightforward once the demagnetization
factor is known.

Apparently the whole process above is only approximate. If you slide
the winding away from the center of the core, then the inductance will
decrease.. By how much I do not know. BTW, was this the way in which
the coil inductance was adjusted in LW/MW radio sets during intial
alignment?

I also vaguely recall reading some work done by W.J. Polydoroff on the
inductance of coils with ferrite cores. A paper in 1945, and a book
in 1960 come to mind. He took a slightly different approach. In
those days the permeability of ferrite was relatively low. I have
never compared his approach to Snelling's to see if they give
approximately the same results. If I can find the stuff here, I will
look a little harder.

73 John KC0G
.

Relevant Pages

  • Re: Ferrite loop antenna for LW/MW band
    ... What I need to do is calculate the inductance with the ... and permeability of the ferrite material itself. ... not know if these means that the coil is tight wound or not, ...
    (rec.radio.amateur.homebrew)
  • Re: saturable inductor
    ... The ferrite will saturate pretty soon ... The total voltage on the coil is equal to the IR drop of the wire plus the back-emf on the coil. ... The back-emf of the coil is proportional to the the rate of change of the product of the current and the permeability of the core material -- and while the current is going up, the permeability of the core material is going down. ... So initially your inductor will have high inductance and the current will ramp up at a more or less constant rate commensurate with the zero-current inductance. ...
    (sci.electronics.design)
  • Re: saturable inductor
    ... The ferrite will saturate pretty soon ... The total voltage on the coil is equal to the IR drop of the wire plus the back-emf on the coil. ... The back-emf of the coil is proportional to the the rate of change of the product of the current and the permeability of the core material -- and while the current is going up, the permeability of the core material is going down. ... So initially your inductor will have high inductance and the current will ramp up at a more or less constant rate commensurate with the zero-current inductance. ...
    (sci.electronics.design)
  • Re: Wanted, Diodes
    ... can't magnetize brass. ... I think you're confusing permeability with susceptibility. ... experience, is if I put a brass slug into a solenoid, the inductance ... The same thing happens when you put a coil ...
    (rec.radio.amateur.homebrew)
  • Re: Permeability change of Ferrites
    ... how the permeability change with time. ... magnetic path running through the ferrite has a very limited effect on ... the total inductance. ... through the ferrite by permeability of the ferrite. ...
    (sci.electronics.design)