On Tue, 14 Mar 2000 08:14:38 -0500 "Tom Rauch" <w8ji@contesting.com>
writes:
Hi Tom,
>Resonant or not, the choke current is high and it varies at a RF
>rate. I'd be really careful what ferrite I used around the choke, no
>matter how "loosely coupled" the "choke designer" claims it is.
>Guessing isn't a good idea, but what the heck.
No but, it does leave a lot of latitude...
>61 material has a low flux density rating, and provides a Q of less
>than 2 at 30 MHz. If they had the material in the axis of the coil, no
>matter what the air gap between the rod and the winding, coupling
>would be very high.
>Q of that material, or inductors wound on that material, is ten at 10
>MHz.
My Fair-Rite catalog (13th ed. p. 10) shows about a Q of 85 at 10 MHz,
do you have a different source?
>Whatever value of "inductance" was measured, you can bet the
>parallel resistance was only about ten times higher at 10 MHz, and
>nine times higher at 2 MHz...and of course around twice the
>reactance value at 30 MHz.
>
>All of that loss heats the core material. A 1/2 inch by 5 inch rod in
>free air at normal ambient would reach curie temperature of the
>material (350 C) in about ten minutes with 25 watts dissipation.
Sounds reasonable but, the question is how much energy is actually
being imparted to the core?
>> I don't have any formulas for flux density in rods... do you have a
>> reference?
>
>Flux density is not the problem, since saturation isn't the issue
>unless the duty cycle is so short heating is not a limitation.
I beg to differ, since the flux density mulitplied by the loss factor
is the power loss... or heating.
>Saturation causes the core to suddenly behave like air, because it
>is "full" of magnetic flux. The core simply can't carry any more flux,
I think we both previously noted that saturation is not an issue in
this instance.
>so additional magnetizing forces can only increase flux outside the
>core material, it's like the core is partly air and partly a soft
>iron.
>
>Heating is primarily caused by eddy currents in the resistive core
>material. Loss caused by eddy currents are the primary concern,
>and they vary with frequency. If we know the Q and reactance, and
>the voltage across the winding we can calculate heating.
>I think most chokes in this application should have a Q in the
>hundreds, not single digits. That would require mounting the 61
>ferrite off to one side on the outside of the choke, and a few winding
>diameters away. Maybe on the cabinet wall.
It wouldn't do much over there... for the inductance.
73,
Marv WC6W
*
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