Discussion:
rarity of LF and HF adjustable coils, coil forms, etc.
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Big Bad Bob
2021-04-02 07:50:19 UTC
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There seems to be a general LACK of non-N.O.S. and
non-ripped-out-of-gear RF coils for LF and HF band, ESPECIALLY the coil
forms needed to wind your own, particularly ADJUSTABLE ones.

If you're into antique radio restoration this could be a SERIOUS problem!

I _HAVE_ seen sources for ferrite [and most likely powdered iron as
well] core material as ferrite rods, though. There are also toroidal
ferrite beads of an appropriate length that would work.

So getting cor ematerial isn't the problem. It's the COIL FORM that is
the problem!


I think I have a solution involving a 3D printer, nylong screws and
nuts, and those ferrite rods of various dimensions.


Here's the basic plan: Using a ferrite or powdered iron core of a
reasonably correct size, construct a coil form around it that could work
for:

* IF transformer inside of a can
* Peaking or resonant circuit inside or outside of a can
* ratio detector transformers

you'd use a 3D printer to make an appropriate coil form with the right
inside/outside diameter

You'd attach some kind of adjusting yoke, or make it part of the 3D print

You'd use nylon screw and nuts to make the adjustment stem

You'd glue a nut onto the core, and then attach [and glue] the nylong
screw to it


Now you have an adjustable inductor, one that may even fit in the
original can [if there is one], which you can wind yourself as necessary
using standard available coil wire.


This is all DO-able, and might solve the problem of HYPER-EXPENSIVE
N.O.S. parts and (generally) unavailable coil forms.
--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me

'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"
Big Bad Bob
2021-04-14 16:38:31 UTC
Permalink
Post by Big Bad Bob
There seems to be a general LACK of non-N.O.S. and
non-ripped-out-of-gear RF coils for LF and HF band, ESPECIALLY the coil
forms needed to wind your own, particularly ADJUSTABLE ones.
If you're into antique radio restoration this could be a SERIOUS problem!
I _HAVE_ seen sources for ferrite [and most likely powdered iron as
well] core material as ferrite rods, though.  There are also toroidal
ferrite beads of an appropriate length that would work.
I've gone and purchased some ferrite rods. 3 of the 5 big ones (around
6 inches long, 10cm diameter) arrived in one piece; 2 of them shattered
into multiple pieces. But the diameter fits the coil form I printed.

I wanted to cut it into 1 inch pieces anyway. To test the coil form, I
ground down the smallest one to fit the test coil, which adjusts from
20uH to 160uH with 62 turns [one layer only].

Here's the (current) OpenSCAD file for the form:
https://raw.githubusercontent.com/bombasticbob/3DObjects/master/coil_form.scad

I have other cores on order that are 2.8cm in diameter [and shorter].
The coil form for THAT one prints out nicely, but isn't tested yet.
I'll test it when they arrive in about a month [slow shipping].

the OpenSCAD file for the 2nd one:
https://raw.githubusercontent.com/bombasticbob/3DObjects/master/coil_form2.scad
Post by Big Bad Bob
You'd attach some kind of adjusting yoke, or make it part of the 3D print
this works if I use 1/8 inch plus around 0.3mm of clearance for a 4-40
screw. I have 1.5 inch nylon screws on order, and already have 4-40
nuts on hand.
Post by Big Bad Bob
You'd use nylon screw and nuts to make the adjustment stem
You'd glue a nut onto the core, and then attach [and glue] the nylong
screw to it
I actually considered gluing the screw directly to the slug, rather than
using a nut. This is particularly important for the 2.8mm version.

So the basic P.O.C. is good, and the form has plenty of 'things' to wrap
wires around (to anchor them) in order to make custom RF inductors for
whatever you want. The intent is to hand-wind so I figure one layer at
a time and then solder the layers together, to avoid sore fingers.

With the big one having 20uH to 160uH with 62 turns, you should be able
to wind >1mH coils with 4 layers, as needed for peaking coils or
resonant circuits for FM stereo demodulation "the hard way".

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