Query: design equations for tapped coil

[rjtucke]

I'm doing an analysis / simulation of a tube Hartley oscillator, shown
below. (full circuit from Sept. 92 issue of QST, also in _QRP_Power_)
My problem is that I don't know how to treat the coil, mathematically.
I asked some of my professors, but they couldn't give a satisfactory
answer. I was rereading some of my textbooks and found in the
derivation of the usual formula for inductance that we need to assume
the wire was uniformly wrapped about the core. Also, that only assumes
a single uniform current flow in the core, whereas my circuit has taps
that act as current sources... Finally, I am not even sure that the
same value of L should be used for all three secions of the coil! Any
help would be greatly appreciated.

Regards,
Ross Tucker (NS7F)
Arizona State Univ, Physics dept.

circuit schematic: (use mono font)
+---|(----+
| |
IN --)|--+------+--/\/\/--+-- -- -- --
| Ct Rg ________
+---|(--+ |
| _|__ |
|| (_ / / / |
Lt || (_ Cf |
|| (_/---|(--+ |
|| (_ | |
|| (_/-------)-----------+
|| (_ |
| _|__
| / / /
|
_|__
/ / /


coil details:
toroidal core, unknown type
total 19 turns
middle (Cf) tap at turns above ground
bottom (cathode) tap at 1 turn above ground
coils cover ~300deg of circle on core
Wound on a

 

[linnix]

I'm doing an analysis / simulation of a tube Hartley oscillator, shown
below. (full circuit from Sept. 92 issue of QST, also in _QRP_Power_)
My problem is that I don't know how to treat the coil, mathematically.
I asked some of my professors, but they couldn't give a satisfactory
answer. I was rereading some of my textbooks and found in the
derivation of the usual formula for inductance that we need to assume
the wire was uniformly wrapped about the core. Also, that only assumes
a single uniform current flow in the core, whereas my circuit has taps
that act as current sources... Finally, I am not even sure that the
same value of L should be used for all three secions of the coil! Any
help would be greatly appreciated.

My guess is to treat it as 3 separate coils in series, and with
inductive couplings between all of them. The Ls are not constant and
definitely not the same in all three sections.

 

[Phil Hobbs]

My guess is to treat it as 3 separate coils in series, and with
inductive couplings between all of them. The Ls are not constant and
definitely not the same in all three sections.

Nope. Your garden-variety ferrite or powdered-iron toroid will give you
coupling coefficients of at least 98% unless you saturate it. Refer Cf
and the load resistance to the top of the tank by the inverse square of
the turns ratio, and you'll be very close. Failing that, measure the
coupling coefficient with a grid-dip meter--put a cap between the hot
end and the tap, and measure the resonant frequency with the bottom open
and then with the bottom shorted to the tap. You should see a pretty
impressive difference if the coupling is high.

Cheers,

Phil Hobbs