# Spherical Inductor (Variometer)

Discussion in 'General Technical Questions and Answers' started by KY8D, Nov 6, 2018.

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1. ### KY8DSubscriberQRZ Page

What eventually this will be for, is the tuning element of an antenna for 630 meters.

I saw another ham's project on-line for a homebrew variometer, which is comprised of two inductors, wound as spheres, one inside the other, the outer one stationary, the inner one rotatable for aiding/opposing.

Picture it like you had a globe, and started winding at the south arctic circle in a spiral up to the north arctic circle. So, an inductor, but wound on a ball, not a tube. And with a smaller one inside of that. And the two connected in series. Rotate the inner and it aids or opposes by varying degrees. A variometer.

So then, how to predict in advance the inductance in uH? I'm thinking to calculate it as if the same length of wire were wound an equal number of times for the same length of run on a tube whose diameter is the average of all the windings on the globe.

I have a script, written in Perl, for calculating turns, spacing, and length of wire for any size globe. And it also attempts to make a prediction for inductance based on above. But until I build and measure it, I'll not know for sure.

Anyone else tried something similar?

2. ### NE1UHam MemberQRZ Page

A link would be extraordinarily helpful. Next a cr@ppy drawing on a napkin would a killer.

This stuff can be modeled in 3D electromagnetic field solvers. Not volunteering, but maybe .. .

3. ### KP4SXSubscriberQRZ Page

Yes. I'm having trouble visualizing a "spherical inductor".

Oh maybe he meant cylindrical or solenoidal?

Hey, cool! There really is such a thing as a "spherical inductor" variometer. Here's an example: https://www.qsl.net/dl4yhf/mf/mf.html. Scroll way down.

6. ### KY8DSubscriberQRZ Page

Adding a link to the other ham's homebrew variometer project. Same page as listed by NN4RH but with no need to scroll.

I have found and ordered from Amazon a pair of hemispherical bowls: 8-inch and 6.125-inch, upon which I'm thinking to wind 14 AWG magnet wire spaced apart with 1.75mm filament from 3D printers, those two laced to one another with Size 23 Kevlar twine. Wind the two halves from equator to 'arctic circle' like a basket, but working from equator up, instead of the other way.

Something to putter away at in the winter.

Last edited: Nov 7, 2018
7. ### AC0OBSubscriberQRZ Page

Or modeled in FEKO or COMSOL.

Pheel

8. ### WA7PRCHam MemberQRZ Page

Variometers used to be used quite often. The last ones I saw were in the late 1970s in the 80'x80'x80' copper-clad antenna tuning room at US Navy Radio Station, OSO, WA. There were two, 12' diameter on 2' thick slabs of Steatite. The loops were rotated by geared-down 5hp motors. That might be just a bit larger than you need.

All seriousness aside, take a look at this: http://wg2xka.wordpress.com/the-variometer/

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9. ### KL7AJHam MemberQRZ Page

Greetings!

I just happen to be in the process of writing an article on the tickler coil for CQ magazine, in which I make a passing reference to the Variometer

The spherical structure of the advance variometer is so that the mutual inductance is determined ONLY by the rotation angle, and not the DISTANCE between the rotor and the stator...which changes dramatically in the normal cylindrical configuration. This makes the mutual inductance much more linear with respect to rotation...AND somewhat makes the total inductance and mutual inductance a little more independent. There's some REALLY complicated mathematics to describe the spherical "tickler" however.....and I'm glad someone figgered it out long before I was around.

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Seems to me (scratchpad) if the main inductor is in series with the antenna and you change its inductance via this method, what you achieve is changing inductance but you still have the "loss" of the entire inductor, which may be way more than you'd want to have.

Wouldn't determining the actual inductance required (for such narrow band operation) and just using exactly that amount be considerably more efficient? I'd probably use a roller inductor or something to "field" test what inductance is actually required, then measure that, and replace it with a fixed inductor made of very heavy gauge copper to reduce loss.