"Joel Kolstad" wrote in message
...
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?

Thanks,
---Joel Kolstad

P.S. -- They never made small (say, size of an old Kodak film
canister)-sized roller inductors meant for QRP work, did they? Most seem
to
be sized for hundreds if not thousands of watts.


Interesting puzzler and as I've a number of these Roller Coasters built in
to various old military radios, I measured one.
This particular inductor is the "Aerial Tuning" inductor fitted inside the
casing of a 1946 vintage, UK "Wireless set number 62", a TR-RX running about
2 to 8MHz. The inductor is a Paxolin former 6.5" long by 2" diameter, wound
with what looks like 1mm dia' Silver wire. There's exactly 100 turns wound
evenly along it's length and the radio front panel has a little window
showing a "00" to "99" turns counter, operated by the tuning knob and a neat
Geneva mechanism. For measurement purposes, it's ideal.

TURNS. INDUCTANCE.
100 132uH
90 120uH
80 103uH
70 89uH
60 73uH
50 58uH
40 43uH
30 29uH
20 15.8uH
10 4.7uH
0 0.12uH

Values look not too non-linear and probably follow the solenoid formulas but
I can't be arsed doing the calcs .












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