powdered ferrite with glue.

spread it over a sheet of rigid plastic such as polystyrene. Why? Because I want to move the sheet over a pancake coil to make a variable inductor. I understand there will be millions of air gaps between the ferrite particl es and also an air gap to the coil. Before I actually go and do that I won der how much change in inductance I will actually get?

inductance as the sheet basically cancels out the AC magnetic field of the wire underneath it. But then of course the AC resistance stays more or les s constant while you are getting less inductance. If you put two pancake co ils face to face and slide a sheet of aluminium between them you can arrang e things to get an even bigger chance in inductance.

Hmmm, how big is the pancake coil? Buy bigger sized ferrite piece and see what happens. Get a bunch of ferrite beads, spread thin and see what it does.

You can buy ferrite powder on Alibaba... min order is only 5 tons :^)

George H.

Depends on the net permeability and thickness and the geometry; nice scientific answer. Ballpark a percent?

There are people who sell rubbery and foamy ferrite loaded stuff with sticky on one side, sort of like refrigerator magnets, for EMI control and supressing RF surface waves inside enclosures. I have sample kits at work. Emerson maybe.

You could also demagnetize a refrig magnet. Or use it as-is.

The Piconics super-wideband conical inductors are filled with ferrite epoxy.

Dump the cheap plastic sheet and use this expensive stuff:

Can you make a solenoid style work? I put together a variable inductor. Varies from 25uh to 242uh. I used 660/46 litz, about 90 turns, on a polystyrene 7/16" tube. I used a 7" ferrite rod and slide that in and out for adjustment.

Q with rod fully inserted is 540 @ 500Khz, 520 @ 1000Khz and

Picture here.

You can see I cut an opening near the end of the litz to apply a finger to the ferrite rod to slide it.

Mikek

You are doing good stuff, Mikek. Keep it up and post results, please.

The magnetic part of ferrite ceramics, is magnetite (Fe3O4); it's also used as a paint pigment, is available in smaller quantities...

Resin/hardener intended for fiberglassing should bond it pretty well.

Am 15.07.2015 um 18:12 schrieb John Larkin:

I once pointed a friend to these Piconics coils, $35 for one and delivery time in months even for samples. He wanted to try it _NOW_. He powdered some RF ferrite in a mortar, added some epoxy and formed a few conical inductors. Ugly, but worked nicely in a bias tee to >15 GHz, flat like a *.

I told that to the MCL guy the next week, and that they might sell one for each ERA-* if they were cheap enough. He was all ears; we may have played a role in the history of their bias tees. :-)

regards, Gerhard

Not strictly true. Low frequency ferrites are nickel-manganese-ferric oxide, and high frequency ferrites are nickel-zinc-ferric oxide.

Its probably pure powdered iron or ground up scrap cars, so not for VHF, but if your in the US....

"JB Kwik Setting Steel Loaded" fast setting epoxy is iron loaded.

Walmart, Home Depot, Autozone, most hardware stores have it...

Steve

The patents have expired, and other people make the conicals now, like Coilcraft. Pricing is more like $8.

Coilcraft has a wideband power inductor, too. We're using it to apply bias to a laser. 4310LC series, 6 GHz. The Piconics things aren't good for much current.

Mini-Circuits has some tiny surfmount bias tees that are cheap, same sort of packaging as their baluns.

Great stuff, but only a few percent steel, as you can tell from the density. The filler seems to be mostly marketing.

Cheers

Phil Hobbs

Well, not a new idea, but I built my first one about 20 years ago when I was measuring piezo transducers. Once I found the proper frequency, I used the "measuring board" in the picture and looked at I and E on the scope. Then I adjusted a series inductor for E and I at 0* phase. When I started the job, the previous tech used pot cores and kept adjusting the gap until E and I were in phase. What a pain. I put the variable inductor together to ease the pain! It worked great. That post in the link was in 2011, not sure I have anything else to report.

Oh, it's your lucky day, I do have more :-)

Page half way down to the post with the scope display. Or read it all! This is the impedance measuring board, schematic, and curly Q's and why Curly Q's are used to hold the scope probes. This is a cheap way to measure impedance up to maybe 1MHz. (low pass filter can be important)

Mikek

On Wed, 15 Jul 2015 13:33:20 -0500, amdx Gave us:

How much is it when it at the beginning of the coil (or beginning of influence) in the tube?

On Wed, 15 Jul 2015 17:15:49 -0400, Phil Hobbs Gave us:

But likely able to influence a previously only air core L, no?

Higher, the ferrite introduces losses, although it's not a bad rod by older standards. The Chinese have some now that will give Q's over 1300.

I'll see if I can get an answer about my inductor for you. Mikek

Thanks for the answers. I had forgotten about iron filled epoxy. The main thing I'm not sure about is the effect of all the air (or glue filled) gaps between the powdered ferrite material. I am trying to get a higher range o f inductance change than I'm currently getting moving a group of ferrite ri ngs glued together through the center of a solenoid coil. One of the posts indicated that you can get a large change of inductance with the solenoid v ersion. I guess that is a indicator that the air gap I have between the tur ns and the ferrite rings to too large and I need to minimize that right dow n. Anyway the interesting thing about pancake coils is that they are very s usceptible to external influence.

I have JB Kwik working as magnetic shielding on a sensor at work. I'm keeping eddy currents out of the sensor's high gain amp. Vlf or lf mainly, but it would influence an air core inductor.

It's strongly influenced by magnets when curing.

It's Not ferrite. But is a low grade powdered metal.

Steve

I'd guess (TM) that an iron-filled epoxy would have a pretty low permeability; the volume will be mostly epoxy, and the iron particles will rarely touch one another. It's like "thermally conductive" filled silicon grease or epoxy; they are mediocre heat conductors because the particle density is low and the particles don't contact one another very well.

Permalloy powder cores are available up to max mu values around 300, pretty dense stuff, and that is roughly 1% of the permeability of metallic permalloy.

Thanks, Mike. I saw all this months ago in your posts here.

I have that stuff but I cannot get any indication of attraction with a very strong magnet.