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Author Topic: Measuring Complex Impedance of a Toroid  (Read 5693 times)
AD5VS
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« on: April 25, 2012, 06:09:59 AM »

Using an MFJ 259b to measure the complex impedance of what  I believe to be an Ft-240 type 61 material toroid.   I have two turns on the toroid. Resistive impedance stays at 0 ohms from 1.6 through about 13 MHz. The curve for 61 material shows a flat but high u' at low frequencies. Why am I not seeing resistive impedance at low frequencies (1.6 through 13Mhz)? I assume u' is the curve for the real (resistive) component of the complex permeability. Is that correct? Any help understanding what I should expect to see will be much appreciated.
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WB4SPT
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« Reply #1 on: April 25, 2012, 08:34:06 AM »

If I read this correctly, you are measuring the real (resistive) component of a two turn winding on a core.  Well, two turns will have very low R component, not much higher than the ohmic R at DC.  What is the reactive component (L)?
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AD5VS
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« Reply #2 on: April 25, 2012, 10:16:25 AM »

If I read this correctly, you are measuring the real (resistive) component of a two turn winding on a core.  Well, two turns will have very low R component, not much higher than the ohmic R at DC.  What is the reactive component (L)?

Thanks for replying  The reactive impedance ranges from 17 to 35 ohms between 1.6 and 13 Mhz.
What puzzles me is the permeability vs frequency curves indicate resistive impedance should be higher than reactive impedance. Maybe, as you said, the low number of turns on the toroid is causing the discrepancy. Another puzzling factor is I saw a chart on another site that showed resistive impedance only exceeded reactive impedance over a very narrow frequency range for type 61 material. The permeability curves don't seem to indicate this. In fact over the 1.6 through 13Mhz range I am looking at the curves seem to be indicating exactly the opposite. Thanks again for your help. I obviously am missing something.





There is obviously something I don't understand about the relationship of the permeability curves and impedance.
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KF6VB
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« Reply #3 on: April 25, 2012, 12:23:14 PM »

If I read this correctly, you are measuring the real (resistive) component of a two turn winding on a core.  Well, two turns will have very low R component, not much higher than the ohmic R at DC.  What is the reactive component (L)?

*** I wouldn't expect to see much resistance.   Look at it this way - any resistance at all is power that you're putting into the coil, and that it is
absorbing.   Current in phase with voltage.  Such power absorption could only come from:

  * ohmic losses of the wire
  * Low Q or inefficiency of the toroid material
  * radiation resistance - to some extent, your little coil is an *antenna* and power is coupled to the ambient environment.  Of course, this is minimized
    with a toroid core.

                           - Jerry Kaidor, KF6VB
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W0BTU
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« Reply #4 on: April 25, 2012, 01:45:15 PM »

Sounds like you're not sure it's 61 material. Here's a list of materials:
http://www.fair-rite.com/newfair/materials.htm
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VK2TIL
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« Reply #5 on: April 25, 2012, 02:11:37 PM »

The permeability curves are directly related to impedance (R +/-jX); u" produces the resistive component R whilst u' produces the reactive component X.  With suitable scaling, measured R and X curves should correspond with u" and u' curves given by the manufacturer.

If you don't already have it, the Fair-Rite catalogue (16th ed) may be downloaded from the Fair-Rite site; it contains curves of complex permeability for each material.  Note the log scales.

The problem may be in the instrument; measurements of this kind are quite demanding and it might be expecting rather a lot of the analyser to accurately measure quite low values.

I've done a fair bit of this kind of measurement; I use a VNA which must be carefully-calibrated if good results are to be obtained.

I think that the vagaries of the ferrite manufacturing process mean that data is only within about 10% of that given; in the case of 43 material there are at least two mixes, "old" and "new", that give generally-similar but distinctly-different results.
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G3TXQ
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« Reply #6 on: April 25, 2012, 02:43:53 PM »

As VK2TIL said, you have it round the wrong way - μ' determines the inductive component and μ'' the resistive component.

Up to about 15MHz μ' exceeds μ'' by a factor of at least 10:1 for #61 material and the inductive component is dominant. The "cross over" - where they are equal - occurs at around 40MHz.

73,
Steve G3TXQ
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AD5VS
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« Reply #7 on: April 25, 2012, 08:53:39 PM »

The permeability curves are directly related to impedance (R +/-jX); u" produces the resistive component R whilst u' produces the reactive component X. 

As VK2TIL said, you have it round the wrong way - μ' determines the inductive component and μ'' the resistive component.

73,
Steve G3TXQ

Thanks to VK2TIL and G3TXQ. I was off base because u' is called the real component of complex permeability and u'' is called the imaginary component of complex permeability. I assumed the real component defined the resistive characteristics and the imaginary component defined the reactive characteristics. Thanks to Owen (VK1OD) who sent me the complex math which defines Z on wound toroids I now see how the u' factor sets the reactive component and the u'' factor sets the resistive component just as you guys have stated.

Owen also sent me a set of RXZ curves for the FT-240-61 toroid wound with 2 turns which are in reasonable agreement with my MFJ 259b measurements.

Thanks to you all, I've got it straight now, and learned quite a bit in the process.

73s

Dykes
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G8JNJ
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« Reply #8 on: April 26, 2012, 12:38:26 AM »

Hi All,

Here are a few examples of 'signatures' for common toroid cores.

All are measured with four turns of wire. The frequency the lowest frequency at which the value of reactive impedance equal resistive impedance i.e. X=R. There will be some variation between different batches and sizes of materials. But if you can plot the results graphically you can easily identify the  'signature' of each material.
 
Here are my references for some common ferrite materials
 
FT240-31   3.5MHz
FT100-33      7MHz
FT240-43     17MHz
FT240-K      22MHz
FT240-61     58MHz
 
Iron powder has a slightly different 'signature' it usually has a very low resistive component, which peaks to a higher value near self resonance. The more lossy the material the broader and lower value of resistive peak is apparent.
 
T200-52     40MHz   Lime Green (& Blue or Red) colour common in PC switch mode power supplies - moderate loss
T200-26     60MHz   Yellow & White colour common in PC switch mode power supplies - high loss
T200-2       60MHz   Dark Red colour used for HF tuned circuits (& Ruthoff Ununs) - high Q low loss
T200-1       70MHz   Blue colour not common - moderate loss
T200-6     100MHz   Yellow colour used for VHF tuned circuits - high Q low loss

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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AD5VS
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« Reply #9 on: April 26, 2012, 07:18:10 AM »

Hi All,

Here are a few examples of 'signatures' for common toroid cores.


Regards,

Martin - G8JNJ

www.g8jnj.webs.com

Thanks Martin. My MFJ 259b is limited to Z < 650 ohms. It was over ranged near the signature frequency you have listed for type 61 material. Still a useful data set. Thanks for posting it.

73s
Dykes
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W0BTU
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« Reply #10 on: April 26, 2012, 10:34:21 AM »

Here are a few examples of 'signatures' for common toroid cores.

I just learned something! Thank you, Martin!
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G3TXQ
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« Reply #11 on: April 26, 2012, 11:04:47 AM »

My MFJ 259b is limited to Z < 650 ohms. It was over ranged near the signature frequency you have listed for type 61 material. Still a useful data set.
When that happens, try using fewer turns - the cross-over frequency wont change much. I just tried a single turn on a FT240-61 core and measured 230+j230 at 52MHz on my 259B. That's easily good enough to distinguish the #61 from the others in Martin's list of ferrites.

73,
Steve G3TXQ
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K9FV
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« Reply #12 on: April 27, 2012, 07:24:53 AM »

Hey Thanks to the OP for starting this thread, AND the good info posted.  Martin, your info sure looks good.  I'll be testing that with 31, 43, and 61 mix Sat night with an AIM4170 - that should give good graphs to easy ID each time.

73 de Ken H>
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KK7KZ
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« Reply #13 on: May 02, 2012, 08:50:26 AM »

While we're on the subject of identifying cores, does anybody know what material is used in the green cores employed in Balun Designs baluns?

Thanks!
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G8JNJ
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« Reply #14 on: May 02, 2012, 09:17:22 AM »

I believe they use a special mix which is similar to type K.

However, if I lived in North America I wouldn't bother making my own copies.

Balun design prices are not that much more than the material costs - especially if you have to buy the component parts in small quantities.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com

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