"Barett" wrote in
:

....
I only need about 3 of them I think, so the postage is a killer
unfortunately.

We experience the same problem here in VK. Local resellers charge very
high prices, and many US suppliers charge very high shipping on
magnetics.

Another option is to look on Ebay, Ebay sellers who really want to sell
internationally have reasonable prices... but be prepared for excessive
shipping quotes from some.


I have found some FT140-43's.

You could use these to make a common mode choke, but the physical size
limits the amount of heat that they can dissipate and the power rating.


Do you know if I stacked 3 Ferrite FT140-43's rings and wound 6
winding of RG58cu through, what would the maximum impedance Ohms could
I expect @ 3.5MHz?

Also how much impedance could I expect to gain every time I added
another FT140-43 ferrite onto the stack?

At low frequency, the impedance is close to proportional to the square of
turns, and proportional to the number of cores stacked but for a core of
this type, the inductor will probably be self resonant in the HF range
and stacking cores increases self capacitance and reduces the self
resonant frequency and the above approximation does not apply so well.

I should stress that self resonance isn't necessarily bad for these types
of lossy inductors, you should put aside Rules of Thumb (ROT) that say to
never use an inductor above its self resonant frequency.

I have written a short article entitled "A method for estimating the
impedance of a ferrite cored toroidal inductor at RF" which you may find
interesting, it is at http://vk1od.net/blog/?p=806 .

Is Neosid still selling product in the UK? I believe they stopped
manufacture, but they may be selling product.

In answer to the shipping cost issue, I went looking for a locally
available core that suited a general purpose HF common mode choke, and
Neoside produce a core larger than FT240 with a material that is a good
choice for the purpose, similar to Fair-rite #52. I describe the design
at http://vk1od.net/blog/?p=581 . Moving house has delayed completion of
this, the design is done, the materials are here, I just have to
construct one in a box and measure its common mode impedance to calibrate
the amount of equivalent shunt capacitance. An alternative would be a
stack of two FT240-52 cores.

Everybody, and every manufacturer has a recipe for common mode chokes,
but if you do not provide a plot of common mode impedance, preferably R
and X separately, then you might reasonably question the recipe.

None so more than the commercial purveyors of the magic Caronlina Windom
which calls out aspects many of us might regard as a disadvantages as
positive features (sheer marketing spin) and depends on undescribed
proprietary components.

Owen