Dave's Homemade Variometer Coil
Welcome to my old time homemade variometer page. This is my second rotatable coil that I have built. The first one was a variocoupler. Both of these coils look similar, and they are. The difference is in the wiring and their use. A variocoupler is made up of two coils that aren't wired together. (You might notice that these have 3 coils. For this discussion, disregard the third coil, which is used to couple the antenna to the main coil.) Variocoupler coils are useful when building a regenerative receiver. The movable coil controls the regeneration, or feedback.
A variometer has two coils that are wired in series. When the shaft is rotated, the inductance will go from being additive to subtractive. That is, if the coils are in phase, they are boosting and the inductances add. Conversely, if the coils are out of phase, they are bucking or subtractive. If the fixed coil were 250 micro henries and the movable is 50 micro henries, the inductance might vary from 200 to 300 micro henries. However, this is not completely true as the mutual inductance or coil coupling is somewhat less than the perfect 1.0. Therefore the coil inductance might run from 220 to 280 micro henries.
There is another difference between my variocoupler and variometer coils. The variocoupler movable coil has fewer turns then the variometer. This is because very little inductance is needed in a regenerative radio tickler coil. I wanted large changes in inductance with the variometer, so I wound as many windings as would fit. Let me continue by giving you some construction details of my variometer.
The materials required are a mailing tube 3 inches in diameter
(outside diameters are slightly larger)and cut to 4-1/2 inches in length.
The smaller tube is 2 inches in diameter and cut to 1-3/4 inches long.
Staples sells these.
You will need two outside threaded bushings that have a 1/4 inch hole.
Also a pair of 1/4 inch hole bushings with a set screw.
A 6 to 8 inch long 1/4 inch dowel rod is needed for the shaft.
You will need some wire for the coils. I used my usual 40 strand, 44 gauge litz wire. 26 gauge magnet wire would also work. The small coil requires 22 feet and the large coil 41 feet.
Start by cutting the coil forms to length and gently sanding until the ends are smooth. Then paint with black lacquer or shellac. Next cut two 3/8 inch holes to accept the bushings. I measured up 3 inches from the bottom. I took a small drill bit and punched a hole in the side. I then worked up to a 3/8 inch drill. I then mounted a bushing in the hole. Then I took a 1/4 inch drill bit and ran it through the bushing and started drilling the opposite side. This will insure that your bushings are lined up properly. Finish the hole and tighten the second bushing. Take the smaller coil and hand drill two 1/4 inch hole, at the half way point on the coil form and opposite from each other.
Now comes the fun part! Winding the coils. I didn't want to break the wire between the two halves of each coil, so I had to thread the wire through. You probably will want to tin the end of the litz so it will pass easily through the small holes.
First poke four small holes in the large coil form. The holes are in pairs. Two are below and to the right of one bushing and the other two holes are above and to the left of the other bushing. Each pair of holes are side by side about a half inch from the other. Thread about 21 feet of wire through the bottom hole nearest to the bushing. Be careful not to kink the litz wire. Thread the wire back through the hole near the first one. Then thread the wire up to the top hole that is further from the bushing. Thread the wire in and out of the top holes. You will see that you have nearly a half turn that winds from below one bushing to above the other bushing. I probably used too many words to explain this.
Wind the 24 turns above the bushings and when done, poke another two holes side by side and about a half inch apart. Thread the wire through these holes. The end of the wire should come out the outside of the top of the variometer assembly. Next do the same thing with the wire on the bottom until you have 24 turns there too.
Repeat this procedure for the smaller coil with around 11 feet of wire for each half of the coil. When each half coil is wound, poke only one hole to pass the wire to the inside of the coil form.
Next you have to prepare the dowel rod. First make sure it fits. I had to do a little sanding to make it spin freely. After that, you need to drill a small hole down the center to pass the wires from the movable coil. I have a drill press, so I drilled a 1/4 inch hole in a scrap 2x4. I used that as a holder for drilling. Use a 5/64 inch drill bit and drill an inch and half hole down the center of the dowel rod. Then drill a small hole in the dowel where the first hole ends. Now you can pass your wires through the hole. Two detailed pictures of this are at the bottom of the page.
Now assemble all the parts. The last operation is to pass the two wires from the center coil through the hole to the back of your variometer coil.
Using my grid dip meter and a 33 pf capacitor, the coil resonates at 1.75 mhz. This makes the coil around 250 micro henries! The small coil with the same 33 pf capacitor dipped at just under 2.7 mhz. Thi makes the rotatable coil inductance of about 105 micro henries. I was pretty close to where I wanted to be. With the coils bucking, the reasonant frequency is about 1.9 mhz. With the same 33 pf capacitor, that makes the inductance at 210 microhenries.
Now comes the surprise. The resonant frequency with the 33 pf capacitor and the coil is 1.32 mhz. That makes the coil inductance of 425 micro henries! This appears to be somewhat of an imbalance. I would have expected the inductance change with the coils added to be the same as the coils subtracted.
As it turns out, this really didn't turn out too bad. I built my crystal set that uses this coil. It does tune the high end of the band ok. In the construction plans for the radio, I have recommended that the outside coil be wound with 40 turns instead of 48. This should bring the coil very close to what I wanted it to be. But in my case, if it ain't broke....
I fixed it anyway. The outside coil has 24 turns on the bottom and 15 turns on the top part. If I wound this again, I would probably put 18 turns on the top and bottom. But with my brand new Almost All Digital Electronics LC meter I found this coil goes from 132 uh to 340 uh. I would say this puts the coil in the right ball park. I will go with this one!
I found this link that might be useful. It is a variometer calculator.