Unusual Variable Inductor

Question

I was digging through my electronics graveyard and I found this interesting variable inductor. It has a ceramic drum wound with 14 gauge silver alloy wire. I measure a minimum to max range of 1uH to 12uH. As the drum is turned a small idler wheel rolls over the coil as a variable tap changing the amount of the coil that is included in the circuit. I was hoping that someone could tell me more about its original application. The square plates on the frame are 6.2 cm on a side and the length of the body is 9.5 cm. The front main shaft is coupled to the drum with an insulated universal joint. I see a shadow on the ceramic where there was likely a label at one time but it is gone now. If I can get a little more information about it I will put a tag on it so the next time someone runs across it there will not be a mystery.

Thanks John

D_Hersey · Accepted Answer

To me, it looks like a medium-power 'roller inductor.' The gear makes me think it was intended to be operated by a motor. The most likely application was to tune an HF antenna. Yup, HF autotuner is my guess. Probably tunes a resonant trap to keep the RF load around 50 ohms as the amp sees it. I think the ARRL 'Antenna Handbook' or some such may be for you. You should consider getting a HAM liscence. Or 'license' depending upon which side of the pond you are lurking at. Are you aware of 'fractal antennas?' Neat stuff.

shabaz · Answer

Hi John, I also agree with Don's assessment. You can see the insides of an ATU at this URL, in action: https://www.youtube.com/watch?v=AnztAIWnSxo (it shows motorized variable capacitors instead of variable inductor, but the principle is similar). I think this was an early way to do it, but a later method was to use relays to switch in inductors and capacitors, i.e. no need for motors. It was fun to watch them in action, there would be a buzzing sound as all the relays clicked in an algorithm (i.e. briefly transmitting while measuring the amount of transmitted or reflected signal) until the antenna was best tuned.

Unusual Variable Inductor

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