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Re: [Amps] how to wind an HF broadband 10:1 transformer

To: Manfred Mornhinweg <>
Subject: Re: [Amps] how to wind an HF broadband 10:1 transformer
From: Dan Sawyer <>
Date: Fri, 29 Feb 2008 20:19:47 -0800
List-post: <">>

How many turns were on the 50 ohm winding?


Manfred Mornhinweg wrote:
> Hi Carl,
>  > Manfred, my reply was assuming use at high power since this is not a
>  >  QRP or receiver forum. In that case iron powder is the only accepted
>  >  material as has been proven time and time.
> A few years ago I needed to make a balun for 160 to 10 meters, that
> should be able to work at 1.5kW on a transmission line that could have
> high SWR. After comparing different powdered iron and ferrite materials,
> I settled for 61-type ferrite for the core. It provided the most
> cost-effective solution, to fulfill the requirements in terms of loss,
> distortion, and bandwidth. I tried to squeeze #2 powdered iron into this
> application, because it's significantly cheaper than #61 ferrite, but I
> found no way to achieve the required performance with this material. So
> I can't agree with iron powder being the only accepted material. At
> least in this particular case, I found ferrite to be much superior.
>  > Even Sevik was forced to back down from his ferrite obsession when he
>  > was challenged by many and it became obvious that he never tested his
>  > designs at power.
> The only thing I know about Sevik is his book about baluns, so I don't 
> know of his trends over time! Maybe I will come back to iron powder 
> someday too, but since having some meltdowns with iron powder, I'm a 
> firm fan of ferrite, for most applications including low and high power 
> broadband transformers! Which does not mean that I would think it 
> impossible to use powdered iron, but just in my experience ferrite 
> allows the more effective solution.
> Jim,
>> I've studied ferrites a LOT with respect to RFI suppression and common 
>> mode chokes, but I'm not an expert on their use in transformers. That said, 
>> there are some ferrite materials that offer VERY low losses at HF, most 
>> notably Fair-Rite #61 and #67. 
> Some ferrite materials have low losses even up into the UHF range. To my 
> knowledge, no powdered iron material achieves that.
>> W8JI has observed that if the transformer is 
>> NOT handling power (for example, a Beverage transformer), a lossy material 
>> may be more suitable, depending on its mu over the frequency range of 
>> interest. 
> Lossy material might be more acceptable there, but never preferrable 
> over low loss material. But there is no need to bother with lossy 
> material, at least not at HF! We have it if we want it, such as the 73 
> material which is great for soaking up HF and thus cleaning up noisy 
> switching power supplies, but we also have low loss materials such as 
> the 61 and 67 you mention.
>> Another VERY important point with respect to mu (permeability). When single 
>> numbers are quoted, they are for the mu at low frequencies. But the mu of 
>> ALL 
>> of these materials varies over a wide range as a function of frequency. Low 
>> loss materials like Fair-Rite #61, #67, and #68 have relatively low values 
>> of 
>> mu, but #61 maintains its value to about 20MHz, and the other two hold 
>> theirs 
>> to nearly 100 MHz. 
> I didn't know they had so much variation with frequency, but also the 
> permeability varies with temperature and with the magnetization (or flux 
> density) level. So, most certainly, ungapped ferrites have no place 
> where a constant, stable permeability is needed. In such applications, 
> either a ferrite core with air gap, or a powdered iron core is needed.
>> By contrast, high loss materials designed for suppression, 
>> like #43 and #31 start out with low frequency mu values of 800 and 1200 
>> respectively, but have dropped to 200 at 10 MHz and 100 at 100 MHz. More 
>> important, #43 is more lossy than inductive above 10 MHz; for #31, that 
>> transition is around 4 MHz. 
> Yes, and for #73, it's even lower. In fact, many designers of switching 
> power supplies liberally install #43 material beads in their products, 
> in an attempt to absorb noise. It's a pretty futile attempt for us hams, 
> because #43 will be effective only in the highest HF range, and at VHF, 
> in absorbing noise! The simple step of using #73 material for noise 
> absorption made my 40 Ampere switcher so RF-quiet that it has since been 
> quoted as an example in several books and magazines!
> Of course, don't ever wind a broadband transformer for HF on #73 
> material! It would make a good heater and not much else! :-)
> Carl,
>> In a broadband low level receiver or transmitter path core losses can easily 
>> be recovered with more gain.
> Only as long as the signal hasn't dropped below the noise floor of the 
> following gain stage!
>> However the smaller you get with the ferrite 
>> the more prone they become to non linearity events, primarily in receivers.
> That's indeed a big problem, and with transmitters it's a problem too. 
> Perhaps not for the user, but sure enough for his neighbors! :-)
> Both iron powder and ferrite cores have to be driven at pretty low 
> levels to keep nonlinearity effects in the green range. But then, at RF 
> anyway they have to be driven pretty low to keep them from overheating! 
> At the flux density levels you can safely put into a core in the upper 
> HF range, be it iron powder or ferrite, without exceeding a manageable 
> loss, the distortion from nonlinearity will be acceptable in almost all 
> cases.
> And now a question for those among you with good ideas: Can anyone come 
> up with a way to make a broadband, high impedance transformer? Say, 
> something to convert from 3 kiloohm to 50 Ohm, covering 160 to 10 
> meters? The application is obvious, a broadband no-tune tube type power 
> amplifier. My problem is, I can't find a way to make such a transformer, 
> because wire length is excessive compared to the wavelength, and 
> interwinding capacitance is a serious problem. I assume that nobody has 
> been able to do this, and that this is the reason for the lack of 
> factory-made no-tune HF power amps using tubes. Anyone who manages to 
> build a suitable transformer would become famous, at least among us HF 
> power freaks! :-)
> Manfred.
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