My first variocoupler
Here are some of the construction details. I used a 3 inch and a 2 inch mailing tube. After cutting the tubes to 4.5 and 1.75 inches respectively, I sprayed some shellac on them. This should keep the moisture out and strengthen the tubes slightly. I then measured 2 inches from one end and drilled a 3/8 inch hole in the side. This is for the bushing. After mounting the bushing, I took a 1/4 inch drill and ran it all the way in by hand. I then twisted the drill to make the hole in the opposite side. Then I was able to drill the 3/8 inch hole in the correct position directly across from the first hole. I then drilled two 1/4 inch holes in the 2 inch form. I centered the hole along the length of the tube. I then placed the small tube inside the big one, and ran the 1/4 inch drill in by hand, through the smaller tube and then cut the opposite end of the small tube. I was able to "eyeball" the placement of the hole by lining up the small tube with the big tube.
After I cut the holes, I tested the whole assembly to make sure nothing would hit. It was perfect! To fasten the ends, I cut a 1/4 inch pheonolic shaft coupler in half, and this gave me a way to fasten the ends of the shaft. You can also use grommets.
Now, it is time to wind the coil. I wanted to wind the two halves without cutting the wire. I took a pointed object and poked a hole about a quarter inch above and a half inch to the left of the bushing. Then I poked another hole a half inch to the left of the first hole. I repeated this procecure, just below and to the right of the other bushing. The wire goes in and out of these holes and allowing for a turn to travel from below the bushings to above the bushings.
Since 30 turns is a little under 50 feet of wire, I measured 25 feet and placed it through the bottom hole closest to the bushing. Then I pulled the wire through the other hole near the hole I passed the wire through. This secures the top of the bottom half of the coil. I then passed the wire through the top left hole and passed it back out of the top right hole. Then I wound the 30 turns, and poked two more holes to secure the winding. Now that the top coil was wound, I then wound the bottom coil, ending it with the two holes punched in the mailing tube. I probably did too much explaining, so forget what you read and look at the pictures.
The fixed winding is wound on a 3-1/8 inch diameter mailng tube. The length is 4-1/2 inches. There are a total of 60 turns of 23 gauge magnet wire. If you use smaller wire, subtract a turn or two, and conversly, if you use large wire, add a few turns. This coil has an inductance of around 280 micro-henries. The coil spacing is just under one inch.
The rotating winding is wound on a 2-1/8 inch mailing tube cut to 1 3/4 inches. This length allows for free rotation inside the coil. There are 20 turns of litz wire. The litz wire is 40 strands of 44 gauge insulated wire. I used the litz so that when the coil is turned, the wire will not break after a while. As in the fixed coil, this coil is split into two sections. There is a half inch spacing between the windings. The inductance is just under 40 micro-henries.
I added two pictures at the bottom to show in detail, the routing of
the rotating coil wires. I drilled a hole about an inch in the end
of the dowel. Then I drilled another small hole in the side of the
dowel to meet with the first hole. I used a thin piece of wire to
fish the litz through the hole. A piece of magnet wire works here.
Tack solder the litz to the end of the magnet wire and pull it through.
This step is actually easier than it looks.
Good luck with your variocoupler project.