On Tue, 28 Dec 2004 11:47:00 -0500, Jeff Maass wrote:
>I've been winding 10-15 turns of the feedline through an
>Amidon FT240-77 (2.4-inch #77 material) core. From Mike's
>data, the large number of turns is providing a large -jX
>component, in addition to the large resistive component.
>
>For a common-mode choke, would I be better off:
> 1) decreasing the number of turns; or
> 2) Using a toroid of a different material; or
> 3) Eliminating the toroid completely.
> 4) Doing something else altogether.
>From Michael's data and mine, I would offer these thoughts.
1) Consider how the X component affects your particular
installation. Would capacitive reactance cause a resonance at the
frequency you are transmitting (or at another frequency that could
be nearby and overload your receiver)? If the answer is no, don't
worry about it.
2) You are basically looking for the max practical R with the
minimum cost and side effects. I haven't measured a #31 toroid of
the same size as the FT-240 (haven't seen any), but the data I
have on #31 materials in other shapes strongly suggests that it is
the most effective suppression material for 160 and 80 meters. 8
turns around a #43 will get you close to 1K ohm R from about 3 MHz
to 10 MHz. I'm guessing that #31 would probably move that down to
1.5MHz to 5 MHz with 6 turns.
3) Increasing the number of turns increases R but also adds stray
C. Eventually you get to the point where the stray C can bite you
by shunting the R and/or creating a resonance where you don't want
it. That's part of why R gets smaller with increasing frequency.
4) Remember that this data is for series R and X, so a large X is
of concern only if R is small -- at a frequency where the antenna
could be excited by you or someone else nearby.
5) If you want to measure this stuff, borrow an AEA CIA, wind some
wire around the material of interest, and measure it. Set the CIA
for minimum sweep width, then use the display that gives you R, X,
Z, and the phase angle. Be aware that the analzyer actually
measures Z plus the phase angle, and has to guess at the sign of
the angle. Also be aware that the analyzer is most accurate close
to 50 ohms and least accurate as the VSWR increases.
6. Michael's measurement setup uses far better instrumentation
than mine, so I trust the accuracy of his data to a lot higher
degree of precision than mine. But alllowing for the relative
precision of our measurement setups, they agree.
7 If what you are doing now works, be happy.
Jim K9YC
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