Variable inductor

"Grumps"

But do you want to set and forget or have L user adjustable ?

....... Phil

It needs to be 'tweakable' by the user. This won't happen very often. The cap in the tuned circuit is also a variable with a 2:1 range (which are readily available).

For a 2:1 range in resonant tuned frequency you'd need a 4:1 range in variable inductance. That's more than most adjustable pot-core inductors, etc., provide. A quick review of various parts shows about 2:1 maximum.

There are several ways to create variable inductors.

One is to make an air coil and adjust a tuning slug in and out of the coil. This may be your best approach. Coilcraft has a nice selection of coils with tuning slugs, but most, like the Unicoil 10mm series, can't do a 4:1 range.

A second way is the classic variometer. Here the coil is split into two parts, wired in series, where one of the coils is capable of rotating within the other. If they are aligned the inductance is nearly doubled over a single coil. But, if the orientation of one coil is rotated with respect to the other the inductance will be reduced, and if they're reversed it can be very low.

If the coils are wound in spherical shapes, it's easier to nest one inside the other and orient it as desired.

It's easy to get > 10:1 ratios by this technique.

You can also slide one coil in and out of the other.

A third method for obtaining wide variable inductance is with a rolling slider bar making a variable winding tap.

The links and pictures I have provided are for working at low frequencies with large inductors, but I imagine most of the concepts can be miniaturized.

An interesting pot-core tuning technique that may be useful for you is to run a DC current through the coil to saturate the core, dropping its permeability. This scheme could probably be used to exceed a 4:1 ratio.

An issue to evaluate is your required tuning stability.

"Grumps"

What sort ot of dumb wank is this ?

........ Phil

If the tuned circuit is to cover a range 50-100MHz, then the coil, with its ferrite slug, has to cover a FOUR-to-ONE inductance range.

Such coils are not available, not manufactured, no such thing.

Back to the drawing board! Try varying the capacitor.

=========================================

Are you this group's spunk bucket or what?

It's quite simple. Listen. An LC tuned circuit. It needs a 2:1 (or greater) variable I. All I'd like to know is if these are available in small can sizes.

The cap is variable too. Can you get a 2:1 variable I ?

"Grumps"

" I " is reserved for current.

...... Phil

So apart from telling me that, do you actually know the limiting factors that contribute to the max ratio?

"Grumpy F****it "

Go do your own Google searching - f*****ad.

........ Phil

I just did. You seem to be quite a hero according to Google. I'd rather you didn't try to answer questions that are beyond (way beyond) your abilities. It's just wasting everyone's time reading your childish spew.

"Grumpy Asshole F****it "

( snip abuse)

....... Phil

There are several methods to get up to 10:1 inductance ratios in a bit more space, e.g., moving large cores into air coils, rotating concentric series-connected coil sections (variometer), and rolling inductors.

One interesting idea to consider is using DC currents in the winding to magnetically saturate the core, reducing its permeability. This method can easily exceed 4:1 ratios, but it's best done inside a servo loop where stability doesn't matter.

What kind of stability do you need?

Thanks very much for those links. I'll search for miniature variometer, but my gut feeling says that there won't be much available.

You're right that a 4:1 range is required for a 2:1 frequency coverage. However, there is a trimmer cap in this circuit too. I'm not sure the size of those variometers is quite right for this project, but I might just get one to show the boss :)

The stability issue had crossed my mind. Another area to investigate.

Thanks again.

Another way to vary inductance is to put a shorted turn (or a copper disk) in your coil, and vary it's orientation from right angle to parallel with the coil. At your frequencies this may work well, but check the effect on Q.

Another thing that may work is to use an air-core inductor in insert a brass or copper slug to reduce inductance.

I've seen both of these techniques (well, _pictures_ of both of these techniques) used in amateur radio installations.

If you want something you can gang to your cap I think you're going to have to plan on doing some experimenting.

Yes, I've found a couple of manufacturers that have 2:1 (and slightly more), but nothing in the ideal range for me (about 200-400nH).

That's a good question. Are these devices mechanically stable, or should you 'glue' the core?

I'm trying to remember how much variability coils with *copper* slugs had (or were they brass?)

I've haven't seen them since the early days of TV sets (mid '50's).

...Jim Thompson

You can build a 4:1 variable cap by taking a 30:1 variable cap and padding it with an external capacitor.

Bare gapped cores; you do have to wind them yourself.

For example, Ferroxcube's P11/7 with 3D3 ferrite and E16 gap, with yellow adjuster, gives A_L from 16 to 35nH/t^2, which means four turns will give you 256 to 560nH, etc.