Making a variable roller inductor

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Once we started to make an air variable capacitor, it became clear that we could use its counterpart... an inductor!
With a variable capacitor and a variable inductor, we can attain a variety of LC combinations for tuned circuits.
Of course, why make one when we can just buy one, certainly more mechanically stable and precise functioning, but making one only requires what we already used for the variable capacitor plus some wire and a coil form.
This is also my first attempt at making a variable inductor!
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As with the previous project, first state the goal:

30 uH, continously variable, capable of handling at least 300 Watts RF, 1.8 to 30 MHz.(Could not make a switchable tapped inductor because it was more difficult to make a good rotary switch).

  1. Locate a suitable non-inductive form to use as solid support for the coil. I used 3 inch O.D. PVC pipe. Based on this size, the calculated number of turn for a 33 uH coil was 30 turns.


  2. Cut the coil form to sufficient length to accommodate the calculated number of turns and smooth out the ends. Use an offset piece of paper to mark the where the coil will be.


  3. According to calculation we needed 25 ft. of copper wire. I used 1 mil diameter wire (14 AWG?).


  4. We also had to make end supports for the coil. I used aluminum sheet leftovers.




  5. Used a triangular file to cut a helical grove on the coil form to insure the copper wire coil would stay in place. This was done by hand in the absence of better tools.


  6. Secured the supports, as they will be the coil electrical contacts.


  7. First attempted to wind the coil on a smaller form to insure the coil would fit tight.


  8. It was very difficult doing it this way.




  9. The best wat was to straighten out the coil wire, tie one end to a solid fence post and the other one to the coil form, then slowly, tensioning the wire, wind the coil walking towards the end of the wire.




  10. Now we used aluminum rod stock to make the roller wheel. First, I filed the grove.




  11. Drilling the hole on the wheel was the most difficult part as it was nearly impossible to get it straight. Had to do a lot of filing to center the hole. At the end it still wobbles.


  12. The exposed copper began to change color from a bright brasslike color to a dull brown. Had to apply a thin layer of deoxidizing oil for protection.


  13. Used leftover Lucite acrylic material and quarter inch threaded stock from the air variable capacitor project for the roller inductor support structure.


  14. Also found brass rod stock good enough for the roller wheel. Springs from discarded ballpoint pens came in handy for mechanical stability at the end of wheel travel. The roller wheel axle and grove are expected to be points of RF loss.


  15. Assembled roller inductor. This was a lot easier then assembling the air variable capacitors.


  16. Since we now had experience fabricating the electrical contacts for the air variable capacitors, we used the same spring loading contact technique here. This is also expected to be a point of RF loss.


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As with any project, at the end, state the conclusion:
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