I have some FT240-31 (cores and now have a couple of the type 31 big cores - 4"
OD, 3" ID and 1" depth. Looking at Fair-Rite's site, I see the following values
fr single-turn impedance values:
P/N 2631814002 2631803802 4" OD 3"
ID 1" depth 2.4" OD 1.4" ID 0.5" depthFreq 1 turn Z (ohms)
1 turn Z (ohms)1 MHz 14 125
MHz 31 2810 MHz+ 45
4025 MHz+ 76
63100 MHz+ 175 119250 MHz
500 215
The big cores (first column) don't create a lot more impedance per turn until
frequecy exceed 100 MHz. But, I know that one of the limiting charateristics
for CMCs is distributed capacitance between turns (see K9YC's treatise),
expecially when coax is used. But.given the same size of coax, wouldn't we be
able to put the same number of turns on a large core than a small one, or,
alternatively, more turns, before reaching the limits imposed by distributed
capacitance between the turns? Along, of course, with the added benefit of
higher power caoability for the larger cores. For example, if the practical
limit for RG8 coax through a 2.4" core is 5 turns, I'd like to think I'd be
able to get, say, 7 or 8 turns on the 4" core before reaching the limit imposed
by distributed capacitance. Am I missing anything?
Kim N5OP
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