Barett Wrote:



The Fair-Rite part number for the 2.40in #31 toroid is 2631803802. Paste
this number into the search fields at http://gb.mouser.com and
http://uk.farnell.com and search out the best deal.

[...]

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

I have found some FT140-43's.

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?


You're in luck, as I happen to have a few FT-140-43, and the measurement
equipment is out on the bench.

That 3-core choke has its best performance around 15MHz (peaking at 4000
ohms, broadband and predominantly resistive, which is quite a good
result) but at 3.5MHz it's only about 1900 ohms and mostly inductive.

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

I had only one more to add, which moved the peak down to 12MHz, 5000
ohms, but only added a few hundred ohms at 3.5MHz.

Clearly these cores are not the best for your application.

I'm working on a better approach from the new ARRL 2010 handbook, which
uses cores that *are* available in the UK and gives the best "value for
ferrite" of anything I've seen. Publication is due in the May Radcom,
but I may be able to give some previews in the next few weeks.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek