# Rotary Inductor Replacement

I thought it was time to build a balanced-balanced antenna tuner so I mechanically ganged two B&W 3852 rotary inductors to make a balanced L network.

It appears that the 16 uH inductors I used are not sufficient for the job.

Do you have any ideas how I could increase the variable inductance in this design or how to make it work with the inductors I have?

• Sorry, but finding sources for parts is not considered on-topic here. – Kevin Reid AG6YO Mar 12 '18 at 19:47
• Yes. Your question about where to obtain this rotary inductor would be better asked on one of the forums such as QTH, QRZ, or eHam. We are a Q&A site, and not a forum. However, since Glenn nicely reworded your question to fit the rules, I am reopening your question. – Mike Waters Mar 13 '18 at 2:34

One thing you might try is to add a fixed inductor in series with your rotary inductors to reach the overall required inductance. To maintain balance, I would wind two identical inductors, each on a toroid core with the same wire spacing. By using toroids, the magnetic fields are contained so that they don't couple to each other or to the rotary inductors. I would mount them up off the chassis to minimize winding-to-chassis capacitance, but with identical mounting orientation and distances to minimize imbalance. To operate higher band(s), the fixed inductors might have to be switched out, but again implement any switching arrangement with symmetry to optimize balance.

Since you didn't indicate the power level and band(s) of interest, the exercise of core selection is left to you; you should be able to find this in various ARRL and core manufacturers' publications.

However you solve it, please comment back with your final solution so others can benefit.

Here is a schematic for a simple, wide range balanced to balanced tuner:

simulate this circuit – Schematic created using CircuitLab

It consists of a 32 $\mu$H variable inductor (which you already have) and two double gang 470 pF capacitors. The voltage and current ratings will depend on the power levels you wish to run and the range of impedances the tuner will experience.

If you wish to feed this with a coaxial connection, you can simply put a good current balun with the appropriate transformation ratio on the input of the tuner.

If you are missing any of the parts to build a tuner, swapfests still are a good source. You may also have some success buying parts as replacement parts from some of the commercial tuner manufacturers. MFJ, for example, has their variable capacitors and rotary inductors available for purchase. They published them on their website and in their catalog.