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Re: [Amps] Ferrite (was: how to wind an HF broadband 10:1 transformer)

To: Manfred Mornhinweg <>
Subject: Re: [Amps] Ferrite (was: how to wind an HF broadband 10:1 transformer)
From: Angel Vilaseca <>
Date: Thu, 06 Mar 2008 07:56:34 +0100
List-post: <">>
Another situation where excessive wire length can lead to unwanted 
resonance effects (overheating) is the plate choke.
If the plate choke was wound on a ferrite rod, or toroid, much less wire 
length would be needed.

Why is this never seen in classical designs?

On the other hand, a cathode choke wound on a ferrite rod IS a 
classical, but never a plate choke. Why?

And why is a ferrite rod always used for the cathode choke, but never a 


Angel Vilaseca HB9SLV

Manfred Mornhinweg a écrit :
> Hi Jim,
>>> I didn't know they had so much variation with frequency
>> All of this data has been in Fair-Rite's printed catalog for many 
>> years, and that catalog has been on Fair-Rite's website as a pdf for
>>  at least 5 years.
> Could you give me an exact URL for some page that gives this information
> for the 61 material? I couldn't find it, and after a few hours searching
> I gave up. Living in a rural location, my internet connection is over
> the cellphone network, with dismal performance, so it's not conducive to
> efficient web browsing!
> Also, if you find some place that gives loss curves for ferrite
> materials, that would be very useful too! I could find only very sparse
> single-frequency information.
> Carl,
>> Terms such a "should", "could", "most cost effective", do not give me
>> a very warm feeling Manfred.
> Maybe I misused those words. English is only my third language. When I
> wrote "should be able to work at 1.5kW", I meant that when I set out to
> design this circuit, one of the design goals was 1.5kW operation. Later,
> when the system was ready, I could confirm through years of daily use
> that actually the goal was met.
> And when I wrote "on a transmission line that could have high SWR", what
> I meant is that this thing had to drive an open wire line connected to a
> random antenna, with completely unknown impedance. No design can
> be guaranteed to work over "any" impedance to be found in the real 
> world, so here the goal was to be able to work with good performance at 
> SWR levels up to 5:1 or so, and acceptable performance at somewhat 
> higher SWR. In practice I have operated into antennas that show an SWR 
> so high that the meter reading is undistinguishable from infinite. In 
> those cases of course the efficiency must be lower, but still the 
> transformer has not blown up despite that use. It does get warm though, 
> with some extremely bad loads.
>> However if you can show 1.8 - 30 MHz performance statistics at the
>> 1500W level they would go a long way to put a sense of engineering
>> reality on the subject.
> I have neither the instrumentation nor the time to do a scientifically
> valid investigation of the transformer, so I'm sorry, I can't provide
> precise data. I can only say that in practice the ferrite material has
> worked very much better than powdered iron, and that theory and data
> extracted from the relevant sheets beautifully explains why. So my
> assertation is a relative one: Type 61 ferrite performs much
> better than type 2 powdered iron, in this broadband high power
> application, both in terms of loss and in choking performance, but I'm
> not able to provide precise absolute data on transformers built with 
> each of the two materials.
>> His book went thru a few revisions as he was faced with reality and
>> not a lab test at low level RF.  At one point there were some rather
>> heated on the air discussions that I partook of strictly as a
>> listener.
> That must have been interesting! Unfortunately, I rarely find anyone to
> talk about technical things on the air. That's why I vent here! :-)
> In any case, my ferrite balun has been used for several years, under
> many different conditions, at legal limit power and all bands, and so
> far has worked well.
>> My own experiments with a FT240-61 at 1200W was rather dismal with
>> excessive heating that resulted in tuning drift as well as TVI.
> The three symptoms all sound like you drove the ferrite to a much 
> excessive flux density. What flux density did you use?
>> At that time I did not own a spectrum analyzer to look for non
>> linearities nor a network analyzer. I now own both.
> I'm still at that stage of not owning these instruments! And believe me,
> I would love to get a spectrum analyzer! But it should work at least up
> to UHF, it should have a dynamic range not under 90dB, and it should
> not cost as much as a car. So far this has kept me without one.
>> I then tried T225-2A thru T400-2A powdered iron at various power
>> levels from 500W to "well above" 1500W; the tests used from one to 
>> three cores wrapped with Scotch #27 HV tape and #14, 12 and 10
>> stranded Teflon wire. The result was several balun 4:1 kits for 500
>> to 3500W that were sold for about 10 years thru a part time business 
>> that I owned.
> You must have used many turns to get enough inductance, and as a result 
> there must have been reduced performance at the higher bands.
>> As expected they were not perfect due to winding capacity and trying
>> for a 1.8 to 30 MHz compromise.
> Exactly. On powdered iron cores you can make baluns that work well over 
> a reduced range of bands, but 160 to 10 meters is asking too much from 
> them. There have to be more severe compromises at both ends of the 
> range, than if you used ferrite.
>> However they worked well, did not
>> overheat when used within their stated power ratings and did not
>> cause any TVI/RFI at home or reports from neighbors. I did not
>> attempt to check for feedline radiation as they were only accessible
>> for a very short part of the total runs.
> Then it's OK. Still, a side-by-side comparison with ferrite baluns would 
> have been interesting.
> Peter,
>> I did that when I got my copy of V.O.Stokes book many years ago,
>> where I found his wideband transformer concept. Using the largest
>> ferrit toroids I could get, they were similar to Amidon #61, I
>> sandwiched them between 5mm thick aluminum plates 20x10cm for 
>> cooling, used 2x12 toroids and wound a 50:600 transformer with 3mm
>> teflon covered wire to feed my terminated V-Beam from 160-10m. That
>> worked perfectly well with all the power my modified L4B (3.5KV)
>> could deliver.
> The fact that you needed heat sinks makes me think that you ran those 
> ferrites at pretty high flux density. That was probably necessary to get 
> an acceptable wire length. How much wire did you have to wind on these 
> transformers?
> Jim,
>> Were you building a TRANSFORMER (commonly called a voltage balun) or
>>  a CHOKE (commonly called a current balun)? 
> It was a common mode choke, incorrectly called a current balun. The 
> current is balanced on both sides, so this is really a current balbal! ;-)
>  > They are VERY different.
> I know. Very different in some regards, and very similar in others.
>>  #61 is a good choice for a transformer, but it is a lousy choice for
>>  a choke.
> Why? It works very well for me! I don't see why we should need different 
> kinds of core material for voltage baluns of common mode chokes aka 
> current baluns!
>> See
> I saved that file, to read it in detail when I have time.
> David,
>> To improve the range of choices, could the non-linearity introduced
>> by the core be improved by a feedback system ? 
> Yes, that's possible in many cases when a transformer is used in an 
> amplifier. But with increasing wire length, the phasing problems become 
> harder to manage, and that causes difficulty with feedback.
> My approach is to start from the most linear transformer I can make, and 
> then perhaps use feedback to further linearize the entire amp.
>> Perhaps the goal of 160 to 10 (6=dream?) might have be managed in
>> more than one lump.
> That would make it much easier, but also more expensive. The first prize 
> would go to someone who covers the entire range with one simple, elegant 
> and inexpensive circuit!
> And now for all people interested: I dug up whatever information I could 
> find about Micrometals #2 powdered iron, and 125-permeability ferrites, 
> to make a comparison in the application of a high power broadband 
> transformer used as "current balun".
> First some basics: The best magnetic material for transformers is one 
> that has the highest permeability, and lowest loss. If the permeability 
> is really high, variations of permeability with temperature and other 
> factors become less critical. Also, nonlinearity of the magnetization 
> curve is then less critical, because the magnetization current will be a 
> smaller part of the total. Instead in materials with low permeability, 
> both the instabilities and the nonlinearities become more important.
> Also, a good magnetic material should have a high saturation flux 
> density, but with present day materials used at RF this is irrelevant 
> because anyway the maximum practical flux density is dictated by the 
> losses, and not by saturation.
> Now let's see how the two materials under consideration fare:
> The permeability for the powdered iron material is 10, pretty stable. 
> For the ferrite, it's 125, varying significantly. So, from this point of 
> view the ferrite is 12.5 times better for transformer use, but 
> unsuitable for resonant circuits.
> The volumetric loss depends on flux density and frequency. It is really 
> hard to find enough data at the manufacturer's web sites, to do a 
> meaningful comparison. The ONLY comparison point I could find, with firm 
> data provided for both materials, is at 500kHz and 50mT. The ferrite 
> considered here is the K type, which is Ferronics' equivalent to 
> Fair-Rite's #61, for which I couldn't find the data. At these levels, 
> the loss of the powdered iron is 2100mW/cm^3, while that of the ferrite 
> is only 80mW/cm^3!  Which means that the ferrite from this point of view 
> is a whopping 26 times better that the powdered iron!
> Now, of course, the losses need to be extrapolated to the frequency 
> range of interest, 1.8 to 30Mhz in our case, and then adjusted to the 
> actual level of flux density during design. Amidon gives an equation for 
> their #2 material on their web site. Using it, it turns out that we have 
> to wind the powdered iron cored transformer so that the flux density at 
> 1.8MHz will not exceed 7mT, in order to keep the core loss below 
> 200mW/cm3. This 200mW/cm3 would result in a power loss of about 3W in a 
> T-200-2 core, or about 30W in a T-520-2 core. If the builder thinks he 
> can tolerate a larger loss, then he can apply slightly higher flux 
> density, but SLIGHTLY higher only, because the loss increases very 
> rapidly with flux density.
> The transformer designed by the above guideline would run at only 0.42mT 
> at 30MHz, which would result in 127mW/cm3 of loss. So, designing for the 
> lowest frequency is OK from the core loss point of view, as on the 
> higher frequencies the core loss drops due to the lower flux density.
> Now with the ferrite, I don't have the formula to calculate the loss at 
> arbitrary frequencies and flux densities. If anyone has a link to some 
> site that has it, please let me know! So I can only guess at this 
> moment, which of course is not scientific, but at least might give an idea.
> Power loss in cores comes mainly from two sources: Hysteresis effects, 
> and eddy currents. Hysteresis loss increases mainly in direct proportion 
> to the frequency, while eddy loss increases roughly with the square of 
> frequency. For this reason, the rate at which powdered iron loss 
> increases with frequency is something between linear and quadratic. RF 
> Ferrites instead are essentially nonconductive and thus free from eddy 
> losses, and for this reason I would expect their loss to increase only 
> in direct proportion to the frequency! If this is true, then the loss at 
> 1.8MHz would be 40 times lower for the ferrite than for the powdered 
> iron! And it means that we could drive the ferrite core to about 40mT of 
> flux density at 1.8MHz, for the same core loss of 200mW/cm3! I stress 
> again, this is based on an assumption, and I would gladly to the math 
> again if anybody can provide a link to the relevant ferrite loss data. 
> But my practical experience with ferrite and powdered iron confirms that 
> ferrite does have very much lower loss, at given frequencies and flux 
> densities.
> Now what does this mean in practice? I will do the exercise of designing 
> a plain simple common mode choke, aka current balun, for 50 Ohm, 160 to 
> 10 meters, for 1.5kW, using either #2 iron powder or type K ferrite. I 
> will assume that the choke will see half the total line voltage across 
> it, and that 90% choking action is the minimum acceptable.
> Let's try first with T-200-2 cores. They have a cross section of 1.3cm2, 
> a volume of 17cm3, and a specific inductance of 12nH/turns^2. For the 
> given choking action at 1.8MHz, we need 22uH. That puts us at 42 turns, 
> not very practical. Anyway, the core would run at 3.2mT, lower than 
> necessary. But the loss in the wire would be huge, and the total wire 
> length would be more than a half wavelength on 10 meters, which is not 
> acceptable at all.
> Let's try stacking three of these cores. Now 25 turns would be enough to 
> get the required 22uH. The flux density is now lower, 1.8mT, and the 
> wire length is still much too long for 10 meters, well over a quarter 
> wave!
> I tried a T-300A-2, a T-520-2, and several stacks, but always too many 
> turns are required to give the desired inductance, which results in 
> phasing problems on the higher bands. This material simply has too low 
> permeability!
> It is tempting then to compromise the low end, and use much lower 
> inductance than really required. For example, let's try to wind that 
> stack of three T-200-2 cores for the acceptable flux density. At 7mT at 
> 1.8MHz, we would end up with 6.4 (or rather 7) turns. The inductance 
> would be 1.7uH, and the total wire length would be about three meters... 
> still not good. But you can look on the web and buy baluns and balun 
> kits that are just this! On 160 meters, choking action will be 
> essentially nil. On 80 meters it's very poor, on 40 meters its decent. 
> On 20 meters everything is fine, but above that the performance goes 
> down again due to the excessive wire length. Still, this current-mode 
> balun won't burn up, it won't cause interference, and the typical ham 
> buying it will be happy, never noticing that he is getting decent 
> performance only on the center bands!
> Now let's try ferrite. Let's take a single F-240-K core. It will deliver 
> 22uH with just 11.4 turns on it. Let's make that 12 turns. The flux 
> density will be 9.1mT, at which this core will run stone-cold due to the 
> very low resulting loss. The total wire length will be just slightly 
> over 1 meter, which is fine up to the 10 meter band. The result? A 
> current mode balun, (or more correctly a common mode choke) that will 
> perform beautifully from 160 to 10 meters.
> And this is how it went in practice. All baluns (voltage and current 
> type) I made on iron powder materials were problematic, showing good 
> properties only over a few bands, sometimes only on a single band; 
> instead, the ferrite baluns work very well over the whole MF-HF range, 
> as long as the correct type of ferrite is chosen. I still have my 
> T-200-2 cores here, as I never found a good use for them. In 
> transformers higher permeability is needed, and for resonant inductors 
> such as mobile antenna loading coils, an air-cored coil still performs 
> better.
> Concluding thoughts:
> It seems that ferrite has gotten a bad reputation with some people, 
> because they experimented with the wrong kinds of ferrite. I did those 
> mistakes too in my younger and more desperate years. I remember a case 
> when a balun wound on an old TV flyback core just plain exploded and 
> send shards all over the place, when run at moderately high power on 10 
> meters. I have also seen countless hams complaining about poor balun 
> performance, when the poor guys bought commercial baluns wound on 
> ferrite RODS!!! And there are many of them. Of course, no thinking 
> person would wind a wideband transformer on an open core, such as a rod, 
> when closed cores are available, but still these rod baluns are plentiful...
> As a final remark on the suitability of powdered iron and ferrite for 
> broadband HF high power transformers, I would just like to point out the 
> following thing: Has anyone, and specially Carl, ever seen a 
> professionally built power amplifier using powdered iron for the output 
> transformer? Or has anyone seen a power combiner using powdered iron 
> cores? I haven't. All I have seen use ferrite. There ought to be a 
> reason for that...
> Manfred.
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