> Interesting Tom. Why not discuss how the coil form (bobin?) affects the
> capacitance turn to turn? I have a ceramic bobin where the wire only
> touches the ceramic only at a few points each turn. There is a lot of air
> beetween the turns. Low self capacitance then?
>
> ??? de Hans SM5KI
Hi Hans,
Anything you do to reduce capacitance from turn-to-turn decreases
frequency of the self-resonance, while anything done to reduce
capacitance from the winding "hot spot" to the outside increases
the "series" resonance.
My point is that it is not actually a true series-resonance, but
rather a series resonant effect caused by the back-to-back L
networks. That's why the voltage at the hot-spot(s) can be
extremely high.
There is so much conflict in the effects, that choke winding is
almost a black-magic art.
What I generally do is close-wind the choke and mount it in the
final location, then check for resonances. If I find something in or
near a Ham band I use a metal wand moved along near the choke
to find the hot spot that moves the resonance the most. Then I
remove turns at that spot to move the resonance above the Ham
band a safe amount.
73, Tom W8JI
w8ji@contesting.com
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