On Jun 27, 10:56*pm, m II wrote:
RHF wrote:

Thanks to both of you for the 'square of the turns ratio' bit. I have
found some old computer mother boards and these things are crawling
with ferrite cores suitable for rewinding. Some of them have a
trifiliar (weird word) wrap on them which will be coming off.

I've pulled them out and will be winding them soon.

tertiary mike III

Mike,

These are 'unknown' material Ferrite Cores that
you are getting-off an old PC Mother Board; and
they may or may not work with the specific Number
of Turns that you try on them to cover the Shortwave
Bands 5MHz~30MHz.

You may get 'lucky' with 10T&30T or you may come
up with something weird. So experiment with what
'appear' to be the same Size, Shape and Color Ferrite
Cores. 10T&30T; 6T&18T; 3T&9T; and if you get all
the Turns on one Core 15T&45T.

TIPS:
http://users.catchnet.com.au/~rjandu...ts/tut_5a.html
* Sort these 'unknown' material Ferrite Cores
for Size, Shape and Color {ideally you will have
a few of the same to do similar and dissimilar
things to} in building a Matching Transformer
that covers and does what you want done.
* Use different 'colored' Wires -if- you are using
http://www.w8ji.com/Beverages/Beverage_transformer.jpg
Insulated Wires for the Windings. Or put a
piece of Black Electrical Tape on the Primary
[Coax/Radio] 10T Winding to 'mark' it and easily
identify it from the Secondary [Antenna/Ground]
30T Winding. If you are using Magnet Wire
-aka- 'Enameled Copper Wire'
http://en.wikipedia.org/wiki/Magnet_wire
http://www.radioshack.com/family/ind...goryId=2032240
to make your Windings then again use a piece
of Black Electrical Tape on the Primary [Coax/
Radio] 10T Winding.
*Pre-Cutting your Wires for the Windings
Measure the Cross-sectional Perimeter of
your Ferrite Core Rap by wrapping a String
around it two to three times and getting the
average Distance of One Turn. Multiply this
One Turn Distance by the Number of Turns
and add 3"~6" for each of the two Tag-End
Leads {that's an extra 6"~12" total}
-example- 10T x 1"/T = 10" for the Windings
+ 6"~12" for the Leads : Total 16"~22" Wire
-example- 30T x 1"/T = 30" for the Windings
+ 6"~12" for the Leads : Total 36"~42" Wire
-so- you can see that a Two Winding Matching
Transformer will take about 5-Feet of Wire.
* Space your Wraps all around the Core -or-
http://www.regionalmfg.com/component...nsformer-1.jpg
Make All your Wrap on one-side for the Primary
and across from it for the Secondary
http://www.coilws.com/images/toroidal/CM%20Choke.gif
-but- Keep your Wraps "Tight" :
Tight Wraps = good electrical magnetic 'coupling'
Loose Wraps = poor 'coupling'.

hope this helps - iane ~ RHF