The venerable formula for a solenoidal inductor has been with us for over 60
years, and is really all you need. I have wound hundreds of coils based on
this formula and whenever I have taken the time to verify the result, it was
always within about 10%. You couldn't ask for much more.
The most convenient form of this formula is as follows. Make a copy of it and
hang it up in your shop. All units are in inches.
L(uh)=d * N(squared)/[18+40l/d] d=diameter l=length N=#turns
The formula starts to lose accuracy when the coil becomes too fat or too
skinny, but is fine for average coils with l/d ratios of 2-10. It is relatively
independent of wire size because within the range of wires typically used, the
internal inductance of the wire is such a small fraction of the total
inductance that its variation with diameter doesn't mean much/
In the last amplifier I built, I used this for winding the coils for both
plate and input circuits, using inductance values based upon standard
calculations for Pi-L and LC networks. When I fired the amp up, everything
tuned up
perfectly the first time.
73
Eric von Valtier K8LV
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