On 8/15/2013 10:09 PM, wyc wrote:
Am I missing something?
Yes. Chokes made from a string of beads are generally ineffective on the
HF bands, especially the lower HF bands, because they are inductive, not
resistive, and because they present relatively little common mode Z.
That's because their natural resonance is in the range of 150 MHz. The
problem with a choke that is inductive is that it cancels the reactance
of a line that is capacitive (by virtue of its length), which increases
common mode current rather than suppressing it. Resistance always
reduces common mode current.
The exception that I know of is the Fair-Rite mix that W2DU used for his
original string of beads (as I recall, it's #73), which IS resonant in
the HF spectrum (and thus resistive). The limitation is that the largest
beads with that mix just fit coax of the size of RG58, and they are
short, so it takes a LOT of beads to create enough choking Z to be
effective.
The easiest way to get very effective common mode choking at 160M is to
wind at least 16 turns through a toroid of #31 material. The
transmission line used for the choke can be coax the size of RG58, or it
can be parallel wire line made by taping together #14 enameled wire.
Tightly spaced enameled wire will yield a Zo close to 50 ohms. Doing the
same with THHN will be in the range of 85-100 ohms. This sort of choke
will yield a choking Z on the order of 5-8K ohms from 160M to at least
40M, and will still be effective at 30M.
Thanks for all the contesting Qs.
73, Jim K9YC
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Topband Reflector
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