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Old 6th Mar 2023, 12:15 pm   #1
Malcolm T
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Default Identifying toroid cores for RF use

I do not posses complex test equipment and am looking for a way of testing toroid cores suitable for HF use.
Has anyone done this with a noise source and a receiver set up before, the receiver being used as a piece of test kit to monitor signal strength as the LC core is rotated between the noise source coil and the Rx pick up coil.
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Old 6th Mar 2023, 12:25 pm   #2
keland_uk
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Default Re: Identifying toroid cores for RF use

I found this article useful
https://alanradiog3nyk.ddns.net/toro...rite%20toroids

"Assessing unknown Ferrite Toroids

Ok so you have had a good day at the Rally and you have come back with a carrier bag full of ferrite toroids that "just might be useful for something". How do you find out what they are? Very few seem to be stamped with any indication of their properties, and some are 'painted' garish colours which presumably means something.

First of all there are basically two types of toroidal core, one is a sintered iron dust and is usefull at higher frequencies, and the other is one of two grades of ferrite material. The ferrite has much higher permiability (more Henries per turn) than the iron dust and is usable at much lower frequencies. Ferrite will thus make fine wide-band transformers, and baluns with bandwidths of at least a decade of frequency (e.g 3 to 30 MHz). There are a range of 'mixes' of the ferrites leading to a wide range of cores with different parameters. Several firms make ferrite material and cores and it is well worth searching for their web sites. This will not help if you do not know the properties of your Bargain cores.

The way to get some insight into the material of your cores is to measure a parameter often given as 'A' which is defined as the inductance per turn (sometimes it has a subscript 10 or 100 in which case it is the inductance of that many turns on the core. It is quite difficult to measure small inductances so wind 8 turns on the core. If you have a handheld DVM with an inductance range just put it across the coil and read the answer. Remember that you cannot have fractional turns on a toroid. every time the wire goes through the central hole it is one turn. Next you need to remember that inductance increases as the SQUARE of the turns. So if you measured 100uH, 4 turns would be 25uH, and 2 turns 6.25uH, so A is 1.56 (uH for 1 turn). You can now compare this with figures quoted for various grades of ferrite in the data sheets, and those used in published designs.

What do you do if you do not have an inductance range on your meter. The next best thing is to put a known value of capacitor with the coil to make a tuned circuit and find the resonant frequency. Lots of people will tell you that you cant 'GDO' a toroid, which is only partly true. If you put a loose wire closed-loop as a single turn through the core you can couple a GDO to that and measure the parallel resonance. You don't need to use the parallel resonance. If you feed a signal generator to the coil and a known capacitor in series you will get a minimum impedance at series resonance. So connect a large resistor 10K to 100K (it depends how sensitive your detector is.) between your signal source and the series LC circuit and connect the other end to the ground of the source. Connect an oscilloscope or an ac milliviltmeter across the LC and swing the generator for a dip. Then use the resonance formula you learned for the RAE, trying not to get confused with the micros, millis and picos. You can even 'ring' a parallel tuned circuit with a square wave and measure the frequency of the ringing on an oscilloscope. I do recommend an inductance meter. Hand held LCR meters can be bought for a very resonable price and help to 'demystify' inductors for you. In fact, like me, you will wonder however you managed without one for so long.

What can you actually use your cores for? Without the full data it is difficult to be sure, but a good guideline is that the very high permiability cores are good for transformers and chokes, but not good for filter inductors. The lower permiability ferrite cores or the iron dust cores will work well as filter inductors. It may be a 'suck-it-and-see' decision, if the filter core gets hot with RF power applied then it is probably not the right material.

Inductors are F
UN, they are one of the last of the "make it yourself" radio components."
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Old 6th Mar 2023, 12:28 pm   #3
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Default Re: Identifying toroid cores for RF use

Malcolm.
Toroids are self shielding, so that won't help you much, (if I understand your method correctly).
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Old 6th Mar 2023, 12:49 pm   #4
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Default Re: Identifying toroid cores for RF use

Thanks for the replies ,
I have some toroids here and want to see what they are good for , not just looking at the colours as a guide.
So if they are self shielding the RF is contained , surly there must be some coupling to another coil possible , i have never done any construction with toroids before, so its a learning curve .
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Old 6th Mar 2023, 1:37 pm   #5
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Default Re: Identifying toroid cores for RF use

Coupling is best done with a tertiary winding on the core itself, or, measure the coil response across the coil itself.

Minimum test equipment is an RF signal generator to excite the coil, (through a resistor) and an oscilloscope to watch the response.

There are several RF gurus on the forum beter qualified to comment (than I!), once you have given some specifics of what you want to achieve.

Edit: the colours don't mean alot unless you know the manufacturer. If you know that then...
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Old 6th Mar 2023, 2:50 pm   #6
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Default Re: Identifying toroid cores for RF use

I have always used the series resonant method where you put a known 1% silver mica in series with the inductor and put that across the line from a sig gen to a scope or speccy analyser.
Then tune for the dip frequency.
Then usually (not always) use software called Mini Ring Core calculator.

Never let me down.
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Old 7th Mar 2023, 10:40 am   #7
Malcolm T
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Default Re: Identifying toroid cores for RF use

I looked at various sites and from the G-QRP pages on toroids the red and yellow colours seem to be the ones for radio use the idea here is to wind a toroid for an antenna noise bridge that spans 2 - 30Mhz
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Old 7th Mar 2023, 10:54 am   #8
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Default Re: Identifying toroid cores for RF use

The red and yellow colours are only meaningful if you know the maker.

The cores popular in the QRP world in those colours are made by Micrometals inc of Anaheim California and QRPers started buying them from a distributor, Bill Amidon. Hence they called them "Amidon cores" They are iron dust cores with different grain size mixtures.

Other manufacturers us red or yellow paint on wildly different materials.

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Old 7th Mar 2023, 2:28 pm   #9
Malcolm T
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Default Re: Identifying toroid cores for RF use

Wrangler
Well i did not know that, so a bit of a jamboree bag then , i dont know the maker !
Looks like its down to winding some cores and experimentation then, .
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Old 7th Mar 2023, 3:50 pm   #10
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Default Re: Identifying toroid cores for RF use

If you have unknown cores to evaluate, the first step is to wind some turns on and measure the inductance.

Here you run into the first difficulty. What frequency do you measure it at? The permeability of most ferrites and dust cores will vary with frequency. So you need an idea of what you're looking at to begin with. The alternative is easy if you have a planned use in mind. Measure at that frequency and work out the "Al value" in nH/(turn squared) If it's too far from your intended use, then that core's eliminated.

If it looks reasonable on inductance/turnsquared then for the second step, you next look at the Q across a range of frequencies.

The frequency range of a core canbe divided into four sections:

Very low where you don't get enough inductance from a reasonable winding to be able to do anything useful. Simply Al is too low.

Good Q -a frequency range where low loss inductors can be made, good for high Q resonant circuits.

Lower Q - not very useful for resonated inductors, but although the loss is climbing, the losses are still OK for RF transformers.

Very low Q - so low that windings look like resistors at these frequencies. Once seen as useless, materials in this region are brilliant for suppressing RFI

Trying Q measurements like this will give you another go/nogo indication but will also suggest what they might be useful for.

The third step is that knowing the happy frequency region of low loss, you can measure the dimensions of the toroid and from your Al, calculate the permeabilityof the material.

Armed with frequency range and permeability, you then have to search through data books looking for a match to identify the material.

I'm afraid it's a lot of work, and needs a fair amount of gear. New cores of known provenance is the escape route.

David
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Old 7th Mar 2023, 4:40 pm   #11
Malcolm T
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Default Re: Identifying toroid cores for RF use

Thanks David for the advice , just what i needed LOL Another learning curve .
I shall have to break out the mini vna now !. Well its all good learning .
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Old 7th Mar 2023, 6:43 pm   #12
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Default Re: Identifying toroid cores for RF use

The book on Ferrites is "Soft Ferrites" by Snelling (pub Iliffe) if you're really going to dive in deep. I don't know of anything comparable covering iron powder/carbonyl iron materials.

Quite seriously, identifying unknown cores is a huge task.

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Old 7th Mar 2023, 7:26 pm   #13
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Default Re: Identifying toroid cores for RF use

This is a classic example of where "traceability" is important.

Who supplied the toroids you are using?

Do they have any sort of supply-chain accreditations?

Or are they eBay best-guess parts?

If the latter then what you get - irrespective of colour-codes - can be anybody's guess [combine your cat's star-sign with the day-of-the-week and multiply by the instantaneous value of the Dow-Jones Index to get a permittivity figure]
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Old 7th Mar 2023, 8:20 pm   #14
Malcolm T
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Default Re: Identifying toroid cores for RF use

Incredible , so no standards / ISO , i have never heard of this before , oh well.
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Old 7th Mar 2023, 8:26 pm   #15
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Default Re: Identifying toroid cores for RF use

https://www.youtube.com/watch?v=Q95Vwk3kZok
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Old 7th Mar 2023, 8:48 pm   #16
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Default Re: Identifying toroid cores for RF use

Quote:
Originally Posted by Radio Wrangler View Post
The book on Ferrites is "Soft Ferrites" by Snelling (pub Iliffe) if you're really going to dive in deep. I don't know of anything comparable covering iron powder/carbonyl iron materials.

Quite seriously, identifying unknown cores is a huge task.

David
Speaking of which:

https://ia903405.us.archive.org/1/it...ITES__1969.pdf
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Old 7th Mar 2023, 11:03 pm   #17
Malcolm T
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Default Re: Identifying toroid cores for RF use

Thanks for those , that is a tome to read , , beats readers digest LOL.
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Old 8th Mar 2023, 2:00 am   #18
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Default Re: Identifying toroid cores for RF use

The ferrite cores Amidon sells to complement the Micrometals dust iron ones are made by Fair-rite inc. of Wallkill, Noo Joisy.

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Old 8th Mar 2023, 8:34 am   #19
Malcolm T
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Default Re: Identifying toroid cores for RF use

Thanks dave , LOL That accent is terrible .
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Old 8th Mar 2023, 5:37 pm   #20
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Default Re: Identifying toroid cores for RF use

Not sure if the following has been discussed yet but I use an antenna analyser to identify the frequency range of an unknown toroid.

Wind a bifilar transformer on a sample of the core i.e. two pieces of thin wire A and B twisted with a drill to about 8 to 12 TPI. Both ends of wire A are the secondary and both ends of wire B are the primary. With very short lead lengths, join a dummy load resistor (50 - 200 ohms) to AA as the secondary; join the primary BB to an antenna analyser (e.g. MFJ259B).

Increase the applied frequency and, at some stage, the analyser will show a reasonable SWR and low reactance - with impedance equal to the dummy load resistance - when the toroid material is operating within its frequency range. When you lose the good SWR and impedance and reactance increases, you know that you are outside the working frequency range of the core you are using.

In my case, I use the VK5JST aerial analyser built from a kit and this method works dependably (found on the Web under the heading "Sexing toroids").

- Peter
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