On Dec 31, 2007 12:56 PM, Colman Ahern <Smokey@ziplink.net> wrote:
> Can you give some details about the choke which you added to the
> feedline? Ferrite mix, number of turns, size of core? At my
> receive-only location I have a very high level of RFI/EMI to deal with.
I used 8 turns of RG-6 through a stack of four cores ... two of the
cores were FT-140-77 cores. The other two are probably something like
FT-140-61 or FT-140-43. On 160m the unknown cores have a good bit of
impedance but are almost entirely inductive, while the #77s have a lot
of resistance.
I don't necessarily recommend this design! It's not well thought out,
I just wanted to use some stuff I had in my junkbox to make some
improvement for the contest, measured the impedance of a couple turns
through the stack and decided 8 turns would give enough for me.
You'd be better off checking out the choke cookbook in
http://www.audiosystemsgroup.com/RFI-Ham.pdf
if you're going to buy cores... you can probably do better
impedance-wise with less cost in ferrite.
My 1:1 isolation transformer is a conventional transformer with 3
turns on the primary and 3 turns on the secondary on a Fair-Rite
2873000202 binocular core. It's very much like the main flag
transformer which is the same primary and something like 7 turns on
the secondary (though I'm going to replace that with a 3-core stack
and fewer turns as recommended in Low Band DXing, to see if I can make
it even better)
For RX-only applications, use of conventional isolation transformers
seems like a good idea to me... the little 50 cent core with a few
turns of wire seems like it gives less than 10pF between the primary
and secondary ... so 9000 ohms or better at 1.8MHz
I'd be interested in others' comments on this, but it seems like an
economical way to get lots of common mode isolation. There's probably
a little RX loss but it doesn't seem to matter.
The Fair-Rite 2873000202 binocular cores are available from CWS
Bytemark in small quantities.
http://www.cwsbytemark.com/
73,
Dan
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