amdx wrote:
Hi All,

Info below from the following site----
http://www.ee.surrey.ac.uk/Workshop/...x.html#bhcurve

Unlike electrical conductivity, permeability is often a highly non-linear
quantity. Most coil design formulæ, however, pretend that it is a linear
quantity.

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My question is-

If I wind a transformer using the specified A sub L and then use that
transformer in a receive antenna where the voltages are very small, wouldn't
I be low on the curve and cause the transformer to function poorly
especially at the lowest frequency of the design?

Mike

PS Thinking about a Flag antenna, which has a small output signal.

Al is usually the value for low flux density. That is, it's the value
you'll have when the flux level is low. Permeability will drop from
there at high flux levels.

If you're making a broadband (untuned) transformer, you only need to
insure that the winding impedance is high enough. If you design it to
have adequate impedance at the lowest frequency, you should be ok for
frequencies above that. If you're making a tuned transformer, you'll
probably be using either powdered iron core or a ferrite core with a big
air gap in the magnetic path like a ferrite rod. Either will withstand
many orders of magnitude of flux density above what a received signal
will produce before there's any noticeable change in permeability.

The assumption of constant permeability is often a reasonable one.
Change in permeability with flux density is certainly nothing you have
to worry about in a receiving application unless you've got a lot of
turns and a lot of DC current in the winding.

Roy Lewallen, W7EL

I agree with Roy that the linearity of coils wound on powdered iron
cores is unlikely to be a problem for you. A while back, I had
occasion to question if some filters we use, made with small powdered
iron toroid core inductors, were causing distortion, so I built up some
similar filters with air-core coils and very good capacitors that I
knew would not distort. The result was "no change". That corresponds
in this case to third order intercepts in excess of +50dBm, which would
be considered at least pretty good by all but the fanatics for use in
receivers. I don't know how much in excess of +50dB, because that was
about the limit of what I could see in that test. Also, I know that
the broadband transformers used in the best H-mode mixers have allowed
those mixers to perform at similarly high--and higher--third order
intercepts. I suppose those transformers are transmission-line types,
wound on ferrite cores.

Cheers,
Tom

Roy Lewallen wrote:

Al is usually the value for low flux density. That is, it's the value
you'll have when the flux level is low. Permeability will drop from
there at high flux levels.


Not to nit-pick but the permeability of nearly all powdered iron
formulations actually rises with increasing flux levels (AC) and then
falls off. For #26 material (u=75), the effect is very much exagerated
with the permeability increasing nearly 300% at ~5000 Gauss and then
falling very quickly. However the permeability does drop for any value
of DC bias current and larger DC bias currents produce greater
reductions in permeability.

73, Larry Benko, W0QE