# Ferrite cores

#### Coucou80

Joined Apr 7, 2018
59
Hello,
I have 1cm (dia.) x 20cm long ferrites rod.
I’d like to identify their caracteristics.
What would be my best approach. I don’t have much info on them. I thought about network analyser to beggin with and meseauring inductance, but not sure if this is the right path.

#### MrChips

Joined Oct 2, 2009
29,187
Make a simple 1:1 transformer by winding two coils of magnet wire around the rod. Wind the same number of turns for each winding, for example, 20 turns each.

Connect one winding to a function generator. Examine the voltage output on the second winding with an oscilloscope.
Try sine wave and square wave at various frequencies.

• atferrari and Coucou80

#### sparky 1

Joined Nov 3, 2018
734
After determining the AL value you can compare materials with those dimensions and have a working formula.
The number of turns N is helpful in winding a specific inductance test value or use an inductance meter. If it is for tuning
there is some satisfaction in determining self resonance however the whole resonant circuit will give the exact resonance.
https://sciencing.com/how-8293803-calculate-inductance-ferrite-inductor.html

So building a resonant circuit will help you the most finding peak resonance therefore finding it's inductance value in Henries for a number of turns.
https://www.qsl.net/o/on7dy//Identificeren Ferriet materiaal AL.pdf

The ferrite material #43 and #61 are common. The chart figure 1 shows AL over coil length using #24 AWG wire.
This pdf shows another method goes back to early radio building and radio instrumentation. The O.D, wire size and length of the coil
so the length of the wire and the self capacitance are involved but the type of material's density related to what part of the spectrum.

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• Coucou80

#### Coucou80

Joined Apr 7, 2018
59
But for thing like relative permeability ou flux density, do I need a network analyser ?

#### sparky 1

Joined Nov 3, 2018
734
A network analyzer would not, the tables showing the closest match will give you a fair approximation to model a simulation.
An analysis can be simple or complex depending on the end goal.

#### Coucou80

Joined Apr 7, 2018
59
What table?
I’m not sure I understand which table you are refering to.

#### Ian0

Joined Aug 7, 2020
8,361
For a ferrite ROD most of the magnetic circuit is in air, completing the loop from one end back to the other.
The characteristics will depend mainly on the geometry and somewhat on the ferrite.
Wind a few turns on it and measure the inductance with an inductance meter.
You can then calculate the reluctance from
$$L=\frac{N^2}{\mathcal{R}}$$
where L is the inductance and N is the number of turns.
You can calculate the magnetomotive force
$$\mathcal{F}=IN$$
where I is the current.
Then you can calculate the flux from Hopkinson's law,
$$\Phi =\frac{\mathcal{F}}{\mathcal{R}}$$
Then you can calculate the flux density from
$$B=\frac{\Phi}{\pi r^2}$$

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• Coucou80 and DickCappels

#### Coucou80

Joined Apr 7, 2018
59
Thank you all for the great answers.
AL value and calculation of flux density through Hopkinson law is definetly something that I’ll calculate. This way I’ll better know what those two rods I have where meant for and what can do with them.

#### Ian0

Joined Aug 7, 2020
8,361
Are ferrite rods used for anything other than AM radio antennae?

Don't forget that you haven't measured any parameters of the ferrite itself - you have just measured the reluctance of the magnetic circuit, which includes the (small) reluctance of the ferrite and a the (large) reluctance of a whole lot of air.

#### Coucou80

Joined Apr 7, 2018
59
I don’t know to be honnest.
They are fairly big. 20cm long by 1cm diam. It was given to me in an old radio shop for 5 bucks. I was planing on maybe using them for high frequency pulse transformer, but no plan as it stands right now.
I agree with what you are saying. Idealy I’d like to know the relative permeability so I can at least find out what type of material this is.

#### Ian0

Joined Aug 7, 2020
8,361
It was given to me in an old radio shop for 5 bucks
Now, there's a clue!
I was planing on maybe using them for high frequency pulse transformer,
Don't do that, unless what you really want is a high frequency pulse transmitter!
If you want to make a transformer, use a core shape that forms a closed loop.

• Coucou80

#### Coucou80

Joined Apr 7, 2018
59
Thank you for the information. No intent in building any kind of transmitter here.
I thought relationship remains the same regardles of the shape.
E1/E2=N1/N2=I2/I1
…I mean if the idea is to wind a primary coil and a secondary coil (with a 2:1 ratio for say) on the rod while powering the primary with an AC (sign wave for say), aren’t we amplifying the signal at the secondary coil wire terminals.

#### Ian0

Joined Aug 7, 2020
8,361
I thought relationship remains the same regardles of the shape
It does - but you effectively have a transformer with a huge air-gap. That means either your magnetising inductance will be small (requiring more current) or you will need a large number of turns (increasing intra-winding capacitance).
Remember than an AM radio with a ferrite rod antenna can pick up radio signals from a thousand miles away, so how far do you think your signal will reach when you launch it into the air out of the end of that ferrite rod?

• Coucou80