What frequency? 16 bifilar turns on a FT-240 size core would be about
8' total of #12 wire. At 20m #12 Cu wire is about 4 ohms per 100' so 8'
would be about 0.32 ohms. Put 5.5A of current through 0.32 ohms will
produce 9.68W which will heat the core modestly without accounting for
any other losses. Just because you can't feel the pigtails at the end
being warm doesn't prove that the wire is not the source of the heat as
the pigtails are in open air compared to the turns wound around the
core. I would take a DC power supply and put enough current into the
#12 wire to produce 9.68W and then decide if the temp rise was less than
the original test. If the 2 temperature rises are about the same then
there is very little loss due to the core. On the other hand if the
temp. rise with DC was significantly lower than with RF then core losses
do come into play.
On 1/30/2011 3:56 PM, Jim Brown wrote:
> 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
> 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
> Note also that my work has focused on ferrite materials, NOT powdered iron.
> 73, Jim Brown K9YC
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