Dave.H said:
I'm winding a coil for a home built regen radio, and I need to know
how much wire I need. The coil form is a 2 inch mailing tube.
Every dimensional change you make to the design will require
a compensating turns change. The trick is to either make
one as directed, measure its inductive properties, and than
adjust the turns count on your modification to have about
the same measured properties, or make calculations or an
educated guess of how much you have to change the turns
count to compensate for the dimensional changes you have
made. At the very least, you should have an idea which way
the turns count much change to be a correction in the right
direction.
If your mailing tube has the same outside diameter as the
author's, that is one variable eliminated.
Main winding is meant to be litz wire, but I'm using 28 gauge magnet wire,
and is 66 turns.
The author says his Litz wire is the same size as 28 AWG, so
this coil should have about the same length as his 66 turn
one. Its inductance will be just a little less, because the
solid wire repels the flux from inside some of the wire
cross section with eddy current, but this is a tiny part of
the total flux, so maybe a single additional turn will
compensate for that. The effect is proportional to
frequency, so there is no count that will work exactly like
the Litz coil would. Your coil will also have a lower Q
because it is absorbing a little of the RF energy into eddy
current losses.
The second winding is meant to be 8 turns of 30 gauge
wire, but I'm using 28 gauge magnet wire again.
Your winding will be quite a bit longer than the author's,
so will have lower inductance (less well coupled turns),
but, again, an extra turn might compensate, or put it just a
bit closer to the tuned winding so that the mid point of
this winding is about the same distance to the midpoint of
the tuned coil as is the case with the author's design. It
won't take much of a correction, and the author may not have
done it the best way possible, so your changes might even
make it better. Too many variables. You might have to make
more than one and compare them in experimental trials.
Third winding is 20
turns of 30 gauge, but again I'm using 28 gauge.
Again, your coil will have more axial length than the
author's, but it is not tuned, nor part of a feedback gain
ratio, so its inductance is not critical. I'm guessing this
one doesn't matter so much. It also might work better if
this coil were closer or further from the tuned one. I
doubt if the author optimized it in any way. It is probably
just something that worked.
