EMRFD Message Archive 14180

Message Date From Subject
14180 2017-07-31 22:40:52 mosaicmerc Micrometals Toroid type 26 uses
I have a bunch of these yellow/wht cores (T106) on hand (colored yel/wht but measure out to 63 mu, 95AL) which seem to be type 40 material.

I was wondering if they are suitable for use as RFC for the HF range?

Would performing an S11 on them I could wind a 47uH unit)  be a good idea to assess this?

14181 2017-08-01 21:23:21 kerrypwr Re: Micrometals Toroid type 26 uses
Colour codes for iron-powder materials are a minefield.

Micrometals say that "All Micrometals color codes are protected by US Trademark law. Formal registration numbers have been issued for the -8, -18, -26 and -52 color codes by the United States Patent and Trademark office".

So, unless you are certain that the cores are from Micrometals, there is no certainty in the colours.

But yellow/white is Micrometals' code for #26, which has uo of 75; if you measure 63 that is probably within the tolerances of Micrometals' figure (+/-10%) and the accuracy of your instruments.

Note also that Micrometals" Al values are referred to 10kHz.

Micrometals describe their #40 material (uo = 60) as "... quite similar to our popular #26 material".

But, in any case, the toroids are most unlikely to be suitable as HF (3-30MHz) wideband chokes.

The impedance available in a winding on a metallic core is related to the material permeability; this is often given as a scalar quantity (u) but is really a complex quantity composed of u' & u", ie u = u' + ju".

Note that there are often various subscripts (ui, us etc) attached to u as it is not a fixed quantity but I will just use plain u here.

.Ferrite materials have quite large values of u' & u" over a wide range, eg, for #43 material;

https://s3.postimg.org/4dfz5qbab/image.jpgThe complex permeability is composed of an inductive reactance, related to u', and a resistive loss, related to u"; both properties can be exploited in a wideband choke.  The above diagram shows how the resistive component u" "takes-over" at around the point at which the reactive component u' begins to fall-off.

The high u" (loss) at other than very low frequencies makes this a low-Q material; Q is actually unity at the frequency at which the two curves cross and is less than unity thereafter.

Note also that both u' & u", and therefore u, are frequency-dependent so that care should be taken if, for instance, using Al (which relates to low-frequency/10kHz ui) at high frequencies.

Iron-powder has low-to-moderate u' but very low u", ie it is not "lossy" / has high Q so it makes good inductors but not good chokes.  Diagrams similar to the #43 one above are not usually given for iron-powder materials because the u" is very low; measures such as mW/cm3 dissipation are given instead as that is usually more relevant to the material application.

Iron-powder can be used to make a narrowband choke but that was not your question.

An additional complication is resonance.

Ferrites have high permeability which means that a fairly low number of turns is required to achieve a suitably-high value of complex choking impedance; low-permeability iron-powder requires many turns to achieve a certain inductance/reactive impedance and this creates larger inter-winding capacitance.

Calculate the resonant frequency of your suggested 47uH inductance with, say, 2 or 3pF of // C (this is probably conservative for many turns on a large toroid); it will be mid-HF I think.  Stray C in a real-world installation will lower that frequency.  The choking effectiveness diminishes quite quickly after resonance is reached.

As I noted earlier, very good narrowband chokes exploit this effect.  This is seen in narrowband microwave amplifiers, for instance; it requires good design and good knowledge, obtained by testing, of the characteristics of the choke.

So, speaking generally, iron-powder is good for resonant circuits which usually require high Q, ferrite is good for chokes which usually require low Q; there are "crossovers" though, eg #61 (or equivalent) ferrite is used in the rod antennas of portable receivers as its balance of u' & u" makes it useful for non-critical resonant circuits up to a few MHz and less wire for a given L is required than for iron-powder.

You should certainly test your toroids; you will gain a good understanding of many things.

Wes' wonderful paper The Two Faces Of Q (see his website) is well worth studying.

Kerry VK2TIL.



14182 2017-08-02 23:08:46 mosaicmerc Re: Micrometals Toroid type 26 uses
Thx for the summary as the variety of materials, size, costs and applications can take some digesting.

I have the ARRL handbook to go thru as I spin up on HF applications.

I gather to make a truly broad band choke more than one ferrite material may be required in series.
I understand tank circuit and parasitics effects.

My understanding of impedance matching via ferrite/transmission line transformers is hazy, especially selecting the core material to match the frequency and power requirements.

For impedance matching, I imagine a lower resistive loss is a good thing. but at higher frequencies the lower permeability adds more parasitic C and copper/skin losses. So the core has to balance those constraints. So I would surmise that fixed sub MHz SMPS units utilize Hi Q iron powders to limit heating?

This begins to sound like a Simplex theorem matrix solution approach given all these constraints might be an interesting exercise.

Then there is bifilar, trifilar and quadrifilar to add complexity...

I'll have to make up a DIY PCB to align with the VNA ports as a rigid test rig to evaluate chokes and filters across the frequency of interest.

A pressing question though is how to SIZE a ferrite core for power handling or thermal dissipation? If I have 400W of RF to choke at 5Mhz up, a 5 pole chebyshev looks good.

But which size cores/material would be best?
14183 2017-08-03 16:31:14 kerrypwr Re: Micrometals Toroid type 26 uses
Magnetic materials are complex beasts and it's quite impossible to provide in-depth information in a forum environment.

I've been studying ferrites for some years; I knew nothing about them at the start but I know at least ten times that now.  :)

The main areas in which ferrite or iron materials are used are power supplies, EMI suppressors, RF transformers and inductors; whilst these are different applications, the principles are the same so don't pass-over information on, say, EMI suppression if you are looking for information on RF inductors.

Iron-powder would be impractical in an SMPS; the core would be large and the leakage inductance high.  The tiny power supplies we see now are the result of intensive development of high-u/low loss ferrite materials in recent years.  Snoek's Law, which defines the inverse relationship of permeability and maximum usable frequency, hasn't actually been broken but the materials scientists have certainly bent it substantially in recent years.

I have a considerable number of articles etc on the subject; many are available on-line.

The following are very good; search for;
  • Philips app notes ECO6907 & ECO 7213;
  • Specifying a Ferrite for EMI Suppression - Carole Parker;
  • Soft Ferrites & Accessories Data Handbook 2013 - Ferroxcube;
  • Fair-Rite Catalogue 17th Edition;
  • Designing Wideband RF Impedance Transformers - Ferrera & Ribiero;
  • Measuring Soft Ferrite Core Properties - George Orenchak;
  • Power Considerations - Amidon;
  • Magnetic Core Characteristics - Texas Instruments;
  • Practical Magnetic Design - Louis R Diana (See Sec. IX for power dissipation advice).

I have no experience with high power but 400W will require a fair-size iron-powder toroid to keep flux below about 50G; probably 1.5" to 2" I think.

If you are tempted to use, say, #61 ferrite (which has moderately-low u" at 5 MHz), you should do a design exercise as outlined in some of the above references, notably the final one.  The result of such an exercise will probably be a reversion to iron-powder material. :)

Kerry VK2TIL.

14184 2017-08-03 18:33:57 Nick Kennedy Re: Micrometals Toroid type 26 uses
Yes, I think it would be very complex to calculate the two major things of interest - peak flux and temperature rise. I doubt if I could address all the factors that come into play, but ...

I'd probably start with a model (like LTspice) driving the filter with a source that provides your target power level and a 50 ohm load. The Q of the inductors might be a moving target but maybe you could make a reasonable estimate and incorporate loss resistance to simulate it.

Then modeling on the frequencies of interest, you could get a good idea of the current through and voltage across those inductors from the simulation.

For flux density and your saturation concerns, I think flux density would be directly proportional to your peak current and number of turns and permeability and inversely proportional to the cross-sectional area of the core. So you can probably calculate that once you get all your units right.

Heating would be another thing entirely. Given your estimate of Q you could calculate the power in the core (assuming loss is mostly in the core). If you have the heat capacity of the material you can come up with temperature rise over time assuming no heat "losses", which is conservative. But for steady state you'd need to know the core to ambient thermal resistance. I'd probably tend to just invent my own rule along the line of "for X minutes at full power, I don't want the core temperature rise to exceed Y degrees" to simplify things.

Looking under the hood of a respected commercial design is starting to sound like a good idea. ;^)

Another complicating factor --- if your LPF is connected to the output of a linear amplifier, having the source in your model be a sine wave is probably close enough. But if it were something like a Class-C amplifier, you might want to model the source as a square wave with < 50% duty cycle as the currents and voltages might be substantially different from those with a sinusoidal source.

Nit-picking terminology - I wouldn't generally refer to an inductor used in an LPF as an RF choke.  Maybe that's just me.


Nick, WA5BDU

14185 2017-08-03 19:37:59 AncelB Re: Micrometals Toroid type 26 uses

I think I'll try to do this one step at a time. Sometimes 1 test = 100

Build the CW square wave gen without the finals and do some low wattage
tests that won't blow up anything or give me RF burns. I'll profile the
LPF with the VNA first.

Once I have the LPF parameters inline so that the output is impedance
matched harmonics suppressed I can start work with the finals in place
and ramp up the voltage in stages while monitoring the high wattage
load voltages from an O'scope (differential probe 10:1) to see the
wave forms. I can sniff the harmonics with EMC near field probes with
the DSA or maybe make up a 50dB down sampling tee.

I have an MFJ 874 SWR/wattmeter and a thermal FLIR E8 to monitor core
and other component temperatures- to help get things into the ballpark.

One advantage I do have is I don't need to run the wattage for more than
a minute or so at a time as I am using it for calibration purposes.

Is there a recommended process to assess the output impedance of a
transmitters finals to know what impedance matching is required?

Thanks again.
14186 2017-08-03 21:54:02 kerrypwr Re: Micrometals Toroid type 26 uses
Amplifier output/load impedance is a matter of design; see, for instance, Rick KK7B's "Designing and Building Transistor Linear Power Amplifiers".(Parts 1 & 2), available on-line.

The matter is covered in Helge Granberg's wonderful Motorola application notes, also available on-line.

Also, of course, in EMRFD which is the cornerstone of this Group.

For actual measurement & adjustment of output/load impedance, search "load pull".

Kerry VK2TIL.
14187 2017-08-04 16:35:22 mosaicmerc Re: Micrometals Toroid type 26 uses
Well, I guess I'll have another look thru the EMRFD text, the ARRL hand book and the Art of Electronics.

But, this link has some info specific to the toroids I have on hand with core mix vs frequency & power recommendations.
This as well:

Looks like I should be able to use the mix 26 at a fixed 80 meters with a bit of permeability drop off, but it has the highest permeability of the iron cores to start with.

So I might be able to squeeze by using mix 26 as  a DC choke and for impedance transformer purposes in the low HF.

Based on the dimensions vs power in the palomar link, the 500W peak seems acceptable on the T106 cores I have.