On 5/21/2013 9:42 AM, Tom W8JI wrote:
31 materials has a Q of about 1 on 80 meters. This means a good part
of the impedance is resistive.
Not quite.. A strong resistive component is present only around
resonance, and the resonance of a single pass through a #31 or #43 core
is in the range of 150 - 300 MHz, depending on the dimensions of the
core. This is clearly shown by Fair-Rite data sheets. While there is
some resistance at HF, it is small compared to the inductive component
until the inductance is resonated lower by the stray capacitance of
multiple turns.
To get effective choking impedance from these materials at HF, the
resonance must be moved down to the HF spectrum by winding multiple
turns through the core. Those multi-turn chokes _do_ show a low Q
resonance (on the order of 0.5 for #31, a bit higher for #43) in the HF
range. Both the inductance and the resistance coupled from the core are
multiplied by the square of the turns, making the choking impedance much
higher. The stray capacitance of the winding increases linearly with the
number of turns, also helping move the resonance down.
The only material I know of whose resistive component strongly dominates
at HF is the #73 material chosen by W2DU for his chokes, but they are
only made to fit rather small coax. It takes a very large number of
these beads (at least several hundred) to achieve enough choking
impedance to handle the high common mode voltages encountered in many
antenna systems at high power. Most so-called W2DU "current baluns" are
made with far fewer beads, and many don't even use the #73 mix.
73, Jim K9YC
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
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