On May 8, 8:50?pm, "Joel Kolstad"
wrote:
I've been thinking of getting some roller inductors to play with next time I
visit a hamfest, and it occurred to me that a generic roller inductor
doesn't have a linear change in inductance with roller position... does it?
E.g., at the half-way point you should get 1/4 the total inductance, right?

Or is the form wound such that the turns are closer together at one end than
nothing, thereby linearizing the inductace vs. roller position to some
degree?

In general, a linearly-wound inductor will follow the cylindrical
inductor equations. There might be some changes from that
from the influence of the "left-over" (unconnected) windings,
including fringing capacitance to ground.

In the few cases of linear-frequency-tuning, notably in Collins
PTO units and the "rack" assemblies in the R-390 family, the
windings were deliberately spaced to handle the powdered-
iron core position effect on inductance. I suspect that Collins
did a lot of cut-and-try to achieve the correct spacing changes
on those; very little quantitative information on it is available
in text or on the web. :-)

73, Len AF6AY