Tuesday, January 26, 2021

Toroidal inductors are highly variable

Subtitle: But Using Dope is Just Fine!

Part of the tune-up procedure for many QRP transceiver kits suggests manipulation of the windings on LP filter toroids in order to maximize RF output.

I was curious just how much of a change in value could be accomplished without changing the number of windings.

The readings below were identical on both the nanoVNA and the DE-5000. The differing values represent a 45% change - quite significant!

The inductor shown is for the LP filter in the 20m QCX-mini, spec'ed to be 0.9 uH, which it is when wound correctly.



VA3RR pointed me to much more info on this topic.



After making the measurements above, I put a light coating of clear fingernail polish on the coil to "lock it in".

I remember stern warnings against doing this from Norcal and others who stated that this would change the inductance of the coil and that there would then be no way to correct it.

So I never bothered to use any kind of dope on any of the kits I built, including a number of them designed for outdoor use (the dope can also be used to fix the coil to the chassis).

My measurements echo those who do use dope (coil dope, that is!) - that it causes no change to a coil's L value once it dries.




  1. I use any of the various UV-cured liquid plastics (Bondic, RapidFix, Loctite UV) to do the same thing. Never had a problem with it.

    1. Hi Gwen,

      I've never heard of the first two items you mentioned but it's nice to know there are several products to do the job and nanoVNA's at affordable prices to verify that no electrical change is made in the process.

      And what's with the two blog entries on your part and then nothing?! Back to the keyboard - we want to hear your perspective!

      73 - John