On Mon, 30 Mar 2009 13:50:43 +0300, Alex Eban wrote:
>Hi Jim and all:
>Like I said in the 1st paragraph, most high power HF transformers ARE wound
>on ferrite cores. they employ transmission line based windings in which the
>energy is coupled to the secondary by a different mechanism than flux link
>coupling, which relies on the core. This goes as far as employing semi rigid
>coax for the windings. We did it at Elisra for multi kilowatt transformers,
>using a pair of 3 inches toroids!
>Even the use of tubing as the one turn winding creates a section of coaxial
>line!
Not unless it is driven as coax (that is, with a center conductor).
>There is very little flux leakage into the core, as a percentage.
Coax is a special form of transmission line, in that it has zero leakage flux.
ALL of the differential flux is contained within the dielectric, NONE of it is
outside the line. When you wind coax around a ferrite core you are forming a
common mode choke, NOT a transformer, and ONLY the common mode voltage appears
energizes the ferrite core. AND -- that choke is still a parallel resonant
circuit, determined by the geometry of the conductor and the characteristics of
the ferrite core.
>I mentioned the lower tangent loss of the ferrites. What causes losses in
>cores is primarily Hysteresis; ferrites have the lower values of loss due to
>their squarer hysteresis curves.
It is a HUGE mistake to think of mu as a constant, Mu varies WIDELY with
frequency. Ferrites are typically characterized on data sheets as having u' and
u'', where u' describes the inductive component of impedance and u'' describes
the resistive component. The loss tangent is the ratio of those two components,
which determines the phase angle of the impedance of the choke. BUT -- this
simple series equivalent circuit fails to consider the capacitance, so it is
only valid well below resonance.
>Cores with high u and low bulk resistance get lossy very fast with
>increasing frequency while low u would negate their use at the low
>frequency end.
Yes.
73,
Jim Brown K9YC
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