On 8/20/2017 4:06 PM, N1BUG wrote:
Suppose I am trying to use common mode chokes to eliminate noise on a
conductor at 144 MHz. Suppose the conductor diameter is 1/8 inch. This
is VHF so we are not talking about multiple turns - just a wire
passing ONCE through a ferrite core.
One turn will work fine. Although I haven't done any measurements to
confirm it, I wouldn't worry about the loose fit. Check out this data
sheet and note the plot of Z for 1-3 turns at the bottom of the page.
For a 6M choke, I'd use two turns on a core, placing multiple 2-turn
chokes in series along the cable. That's NOT two turns through multiple
cores in series, but two turns through core #1, then a short length (an
inch or less) of coax and another two-turn choke, and another.
http://www.fair-rite.com/product/round-cable-snap-its-431164281/
73, Jim
A Fair-Rite 0431178181 (.161" ID) would be the best fit for this
cable. According to the data sheet its Rs is about 180 at 144 MHz.
The Rs of a Fair-Rite 0431164281 (.250" ID) is about 330 at 144 MHz
but it fits the wire in question very loosely with a larger air gap.
How much, if at all, will this "loose fit" reduce the effective Rs?
Is the answer "it depends"? I've been trying to find an answer to this
for years but I keep coming up empty. Maybe it's buried in the
engineering talk which is over my head.
73,
Paul N1BUG
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