TopBand: Beverage transformer core question

[w8ji.tom]

Hi John, 

I certainly respect your opinion on may things, but we'll just have to
agree to disagree on this one.

> this does not mean that other (73 or 43 mix) won't work, but you will
need
> just as many if not more turns to get the same result.
 
73 and 43 materials have nearly twice ui at 2 MHz than the cores Misek
speced. That's true both in catalogs and in measurements I've made. 77
material has about four times the ui at 2 MHz. That means those materials
will require considerably less turns when used as a 2 MHz transformer than
the VLF core specified in some text.
  
There's no reason to debate this, just get out a catalog, measure the right
material the correct way, or call a "real" ferrite design engineer.

The cores Misek spec'ed are specifically designed for VLF transformers.
They are cataloged that way, and the graphs displaying their
characteristics demonstrate that fact. They have a dc permeability of
10,000 but by the time 2 MHz is reached the permeability is about 400 or
less because of eddy currents inside the core. Permeability is NOT linear
or uniform with frequency.

Not only that... if transmission line type coupling between windings is
minimized and energy is transferred through the very low Q core, losses
skyrocket. Eddy currents in resistive materials cause loss.

The loss tangent of the Misek core is astronomical on 2 MHz, since Q
reaches one in the VLF range. Neither the ferrite design engineer at
Ferrite Corporation or I can understand why a core spec'ed for broadband
operation to a few hundred kilohertz is being used at 2 MHz and above.
   
With certain winding styles almost any core will work, but a proper core
will have less loss, require less turns, and be less sensitive to winding
methods and other problems. Low SWR does not mean good performance, because
it is the ratio of unterminated SWR to terminated SWR that determines
efficiency in matching. A resistive L pad can be used to match 450 ohms to
50 ohms, and often is.

While none of this stops the Misek cores from "working", it certainly means
anyone ordering a core should consider ordering a more appropriate
material. As I said before, when going through the bother of locating a
part you might as well get the best one for the job. That's especially true
if the best material is also a more common material.

73 Tom

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