On 7/21/20 4:40 PM, Jim Brown wrote:
Hi Jim,
Choking impedance is driven by both noise and power handling. The
fundamental principle on which my designs are based is that the choke
should have a high value of RESISTIVE impedance at the frequency(ies) of
interest. Reactive impedance can be cancelled by the common mode
impedance of the feedline.
Because the choke is resistive, it dissipates power due to the common
mode current. And because power is proportional to the resistance but to
the square of voltage or current, dissipation falls twice as fast as
choking Z. So Z must be high enough to minimize the dissipation. That's
why some of my designs for higher power consist of two chokes in series.
Aha.. that's the other design consideration, then - not just "no current
on outside of coax" but also "don't melt the choke" And as you say big
R helps.
I have learned, through extensive work in the lab, that Fair-Rite #31 is
the only ferrite material suitable for use at HF. The reason is, that
because it is a very special MnZn mix, the dimensional resonance
component of its impedance combines with the circuit resonance to make
the resonance much broader than with NiZn materials like #43, #52, and
#61. It is the higher Q resonance of the NiZn, combined with the wide
tolerance of ferrite parts, that every single choke must be measured to
find it's resonance, AND that the resonance is too narrow to cover
adjacent bands. In other words, the wide tolerance of the ferrite
material moves the resonance too much for a cookbook like mine to be
published.
So that allows broadband chokes - as I was winding two at lunch time
today to cover 10-80, following your cookbook designs.
73, Jim K9YC
On 7/21/2020 3:30 PM, jimlux wrote:
And that leads to a "choking impedance should be around 5k" which is
100x.
But if it's 4k, it's still good. If it's 3k, it's still good.
I think the value of Jim's cookbook is not so much the choking
impedance, per-se, but that he's
a) bounded the core variability (designed for worst observed case
performance of the core)
b) also taken into account side effects like resonance and leakage
capacitance
The latter is MUCH less affected by core properties (I think, Jim can
correct me if I'm wrong).
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