Howdy,

That's an excellent point. I expect that the sections
of the coil in question will not have very good coupling.
I hadn't considered it! I calculate the same inductance
values.

Bah! In my previous posts the "(sqrtL*C)" should be "(1/sqrtL*C)"
Heh... with a pencil and paper the formula turned out right.
I noticed this when I calculated the resonance of the two
values 16.32uH and 11.54uH with an arbitrary value of 50pF
just to verify how I thought things should be.

So from this I gather that the extension of the coil
connected to the grid circuit will have even less
effect on the tuning than I expected.

The Easy Teenage NewYork method of solving this would be
to put the problem into SPICE with the estimated coupling
coefficent of 0.2.

I've never liked shorting turns to reduce an inductance.
Seems like an avoidable source of some losses no matter
how you slice it. But for some long coils, like antennas,
it works ok. I guess.



K7ITM wrote in
:

One thing to be a bit careful about is including the coupling between
the coil sections. Since I don't know how those particular coils were
designed, I can't say for sure, but in general the coupling between
pieces of the coil isn't as high as you might think.

You can make a good estimate for typical HF single-layer air-core
solenoid coils just using your favorite coil calculation. For
example, consider a coil that's one inch diameter, two inches long, 20
turns per inch, and tapped at the 30th turn (1.5 inches) up from the
bottom. Then the whole coil is about 16.32uH, the 1.5" part is about
11.54uH, and the top 0.5" is about 2.63uH. If the coupling were
perfect between the sections, I believe the inductance of the whole
would be about 11.54uH + 2.63uH + 2*sqrt(11.54*2.63)uH = 25.19uH. At
16.32uH for the whole coil, the implied coupling coefficient between
those two sections is only about 0.20, and you need to be careful to
not think of the tapped coil as simply a transformer with a 3:1 turns
ratio, with implied close coupling between the sections. (This also
illustrates why you can short out turns of a tank coil without totally
killing the net inductance...)

I trust if I've hosed the calculation too badly, someone will point
out the error of my ways. ;-)

Cheers,
Tom