Hmmm, ferrite has never been recommended for RF tank circuits that Im aware
of. Ferrite is fine in receiver and other low level applications mostly at
low frequencies.
Good amplifier engineering has used powdered iron in various mixes,
generally from Micrometals in North America.The #2 mix is the usual choice
in amps for 160-80M and has been in use since the late 70's.
There are huge differences between the 2 materials in regards to losses and
temperature stability.
Micrometals sells direct and Fairite may be purchased thru Lodestone Pacific
(Micrometals also in smaller quantities) considerably less than Amidon,
Palomar, etc. I also have a fairly large supply of Micrometals and some
Fairite at attractive prices as part of the RadioKit resurrection.
My own studies of ferrite began in the late 60's with pot cores for IF
selectivity filters, and peaked greatly in the late 70's thru the 80's when
I was involved with common mode computer equipment leakage and direct
radiation as part of the US Govt Tempest project at Sanders Associates and
Wang Labs. Maybe I should have written a "tutorial" back then as "Pin 1
grounding" was a part of the process (-;
I still experiment as newer materials become available and am currently
involved with audio and power transformers.
Carl
KM1H
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Sunday, January 30, 2011 5:56 PM
Subject: Re: [Amps] What to buy?
> On 1/30/2011 1:23 PM, Roger (Sub1) wrote:
>> I would think those things would heat if tapped with the unused turns
>> shorted, the wrong mix, insufficient core material, or a mounting that
>> would prevent adequate cooling.
>
> I've been studying ferrites for nearly seven years, most of it looking
> at suppression applications, but in doing so, I've learned quite a bit
> about ferrites in general. In general, we want ferrites to have LOW
> LOSS (high Q) when using them in resonant circuits in transmitters, but
> we want them to have HIGH LOSS (low Q) when using them for suppression.
> And even when using them for suppression, we try to make the choke
> self-resonant at the frequency (ies) where we want suppression (the
> equivalent circuit of a ferrite choke is a parallel resonant circuit).
>
> I recently did some experiments with bifilar common mode chokes wound on
> LOSSY ferrite cores, tuned (by number of turns) to cover the HF bands.
> #31 Fair-Rite material is optimum for that, and yields circuit Qs of 0.2
> - 0.5, so the resonance is VERY broad. I built a choke with 16 bifilar
> turns of #12 THHN, connected as a parallel wire transmission line, added
> SO239 connectors at each end, and patched it in line with the output of
> my Titan 425 (between the amp and the antenna tuner). In this
> configuration, the core sees only leakage flux from the differential
> signal, with no common mode voltage. I then transmitted key-down for
> several minutes, then with key-up, immediately felt the choke for
> heating. What I observed is VERY interesting.
>
> 1) There was virtually NO heating of the ferrite core (a 2.4-in o.d.
> toroid, the form factor called FT-240 so that Amidon and others can
> charge 4X their cost when they resell it to hams).
>
> 2) There WAS modest temperature rise (I'd guess no more than 20-30
> degrees F) in the wire where it was wound around the ferrite core.
>
> 3) There was NO temperature rise in the leads (1-2 inches) going to the
> SO239s.
>
> 4) Doing the math, 1,5kW in 50 ohms is 5.5A at 274 volts, so it's easy
> to understand why there's no observed heating from copper losses.
>
> 5) Because dissipation is small, LEAKAGE FLUX is small.
>
> CONCLUSION: Dissipation occurs IN THE COPPER as a direct result of
> losses COUPLED from the core to the copper, but it does NOT occur IN THE
> CORE ITSELF!
>
> Note that this experiment ONLY looks at dissipation as a result of FLUX
> in the core, and the experiment does not place RF voltage across the
> choke.
>
> How are things different in a transmitter's resonant circuits? First,
> we want to minimize loss, so we (should) use low loss cores (like
> Fair-Rite #61 or #67). These are NiZn cores, and have much lower mu than
> those designed for RFI suppression (like #31 and #43).
>
> Inductors wound on #61 or #67 can have Qs on the order of 10 or more,
> and are typically self-resonant a bit above HF. These resonances CANNOT
> be accurately measured on reflection-based analyzers (that is, S11).
> Rather, we need to measure S21, with the inductor (or choke) wired as
> the series element of a divider.
>
> For more on this, see applications notes and tutorials at
>
> http://audiosystemsgroup.com/publish.htm
>
> Note also that my work has focused on ferrite materials, NOT powdered
> iron.
>
> 73, Jim Brown K9YC
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