Topband: FT-240-77 Toroid Impedance R + jX Components

[Jim Brown]

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