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Author Topic: Measuring Toroids  (Read 3950 times)
KE7BDI
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« on: March 30, 2010, 07:37:55 AM »

How can you measure, with reasonable accuracy, the frequencies an unknown toroid was designed for with some common test gear?
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K5DVW
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« Reply #1 on: March 30, 2010, 07:51:15 AM »

Do you have a network analyzer? Smiley

How about an MFJ antenna analyzer?

Either way, make an inductor with your toroid core and measure the L vs frequency (or make an LC resonant tank and measure the resonant frequency and work backwards). You will probably want to fill up the core fairly well to make the L as high as possible. Find out where it's resonant and work backwards to figure out what the inductance of the coil is. Once you know that, you can figure out the Al of the toroid and you're done.

The tricky part is picking a frequency where the core works best to measure the L. Start low in frequency. That will take some experimenting and where a network analyzer would be a better tool.
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VK1OD
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« Reply #2 on: March 30, 2010, 12:29:51 PM »

How can you measure, with reasonable accuracy, the frequencies an unknown toroid was designed for with some common test gear?

A good start is to understand the characteristics of the type of thing you are measuring. Inductance of RF cored inductors and transformers may help you.

Then, make an inductor and using the LEAST number of turns (n) that gives a reasonably accurate response from your measuring equipment, measure impedance (R and X) over a range of frequencies. Tabulate and plot the resulting Z/n^2. You may need to change n to make good measurements at some frequencies. Hint: if you instrument does not show the sign of X, do not make the assumption that X is always positive.

If it is ferrite material, a good 'signature' is the frequency at which R=X, compare your measurements with published data for the common mixes.

Owen
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VK1OD
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« Reply #3 on: March 30, 2010, 01:50:30 PM »

... Once you know that, you can figure out the Al of the toroid and you're done.

Al is only useful when Q is very high, µ is constant with frequency, and a long way below self resonance. None are good assumption for ferrites at HF.

Despite its inadequacy, Al is widely published and used for RF inductor design.

A method for estimating the impedance of a ferrite cored toroidal inductor at RF has some discussion about the issue.

Owen
« Last Edit: March 30, 2010, 05:34:03 PM by Owen Duffy » Logged
K5DVW
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« Reply #4 on: March 30, 2010, 04:53:16 PM »

Pardon me for not being overly precise... you'll know the "effective Al" of the core which may or may not be the same as the mfgr listed Al, it probably wont be the same. From that Al that is measured, you can then proceed to make an inductor of suitable value. I've measured and made tons of toroidal inductors this way, so it works well enough for me. You can do it however you want.
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KE7BDI
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« Reply #5 on: April 03, 2010, 05:08:47 PM »

Thank you guys for the input. I should have mentioned that my "basic test equipment" excluded a antenna impedance meter. I have freq. generator, counter, DVM, and oscilloscope.
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VK1OD
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« Reply #6 on: April 03, 2010, 05:36:55 PM »

Thank you guys for the input. I should have mentioned that my "basic test equipment" excluded a antenna impedance meter. I have freq. generator, counter, DVM, and oscilloscope.
One method is to try to measure the impedance of an inductor using the core as mentioned earlier.

You can expect that the impedance is complex, and that the R component may be relatively high... or in other words, a very low Q inductor, especially with ferrite material, and depending on the mix and frequency.

It is possible, depending on the oscilloscope.

Some DMMs can measure inductance, but it is usually at a relatively low frequency and the results might not be too meaningful at HF.

Owen
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KE3WD
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« Reply #7 on: April 03, 2010, 05:39:34 PM »

Impedance? 

If, as the OP stated, all that is needed is the basic frequency info, why not just ring the coil and monitor with the scope for the resonant point area? 

Its ballpark, but that is often enough. 

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VK1OD
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« Reply #8 on: April 04, 2010, 03:58:59 PM »

Impedance? 

If, as the OP stated, all that is needed is the basic frequency info, why not just ring the coil and monitor with the scope for the resonant point area? 

Its ballpark, but that is often enough. 



I think you mean to measure the self resonant frequency of the inductor.

That information alone is of little value in most cases.

If you can't properly characterise a core, then you are better off buying something of known characteristics that suits the application.

Owen
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AA4HA
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« Reply #9 on: April 04, 2010, 04:39:39 PM »

To answer his question isn't he trying to measure the permeability (μ) and permittivity (εr)  of the core material? If he can figure out those values he could do a lookup and find the core material.

This is a good read;   http://lists.contesting.com/_rfi/1999-12/msg00029.html

Tisha Hayes, AA4HA
« Last Edit: April 04, 2010, 04:44:15 PM by Tisha Hayes » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
VK1OD
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Posts: 1697




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« Reply #10 on: April 05, 2010, 12:52:44 AM »

To answer his question isn't he trying to measure the permeability (μ) and permittivity (εr)  of the core material? If he can figure out those values he could do a lookup and find the core material.

This is a good read;   http://lists.contesting.com/_rfi/1999-12/msg00029.html

Tisha Hayes, AA4HA

Tisha,

Permittivity is probably less important thatn permeability, but the complex permeability is what is required. The most useful single graphs in ferrite data for HF designers is the graph of µ' and µ'' vs f. It is a whole nother dimension than knowing µi. You cannot assume that because for instance that µi=800, that you have #43 mix, various manufacturers have materials that are similar, but not identical in all respects. Focussing on µi is focusing the the data point at the very left hand side of the graph I spoke of, and disregarding the rest of the characteristic.

Using Al is a µi focused mistake.

As I mentioned earlier, the frequency at which X=R is not a bad signature for material type, not as good as a full plot of µ' and µ'', but heaps better than µi.

Owen
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N0NZG
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Posts: 127




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« Reply #11 on: February 02, 2013, 02:41:53 PM »

I found a good deal on a batch of toroid cores that look like they were marked with the micro metals color code, but with no labels who knows. What I did was wind a bi-filler transformer on a sample of each core.  What I mean by a bi-filler transformer is 2 pieces of thin wire from #30 to #20 twisted with a drill motor to about 8 to 12 turns per inch. More TPI is better than less and more winding coverage on the core will give more accurate results than less. Both ends of wire A is the secondary and both ends of wire B is the primary. With very short lead lengths I put a BNC on each end. I put a dummy load on the secondary and the primary on the ole MFJ259B. If everything is working good the 259B will show a reasonable SWR and impedance. When you lose the good SWR and impedance you know that you have exceeded the working frequency range of the core you are using. For example I had some red cores and some yellow and white ones. The red one seamd to work good from 2-24 MHz suggesting that they are a #2 mix. The yellow and white cores only seamed to work out to about 1.7 MHz and the function of the transformer wound on this core was seriously degraded above that suggesting that indeed this core is a #26 mix.

#2 core is good for general utility use for antenna couplers, baluns and transformers in the HF range

#26 core is good for common mode chokes , RFI suppression and low frequency work 

   This method will not provide you with any specifics as to the nature of the core you are testing other that it's suitability for the frequency range you want to use it for. This info combined with what you can learn from the color code and other data sources will help you salvage otherwise unusable cores for projects such as antenna couplers, baluns and transformers.
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