Hi Jim,
I agree that strings of ferrite beads don't present a lot of inductive
reactance on the low HF bands, but you lost me with your comment about
the reactance canceling the capacitive reactance of the cable. I've
not thought about this deeply, but it seems to me the cable
capacitance is differential, between the center conductor and the coax
braid. The ferrite beads present a common mode reactance, and so I
don't see how the cable capacitance enters into the picture. I'd think
any common mode inductive reactance would always inhibit common mode
currents, at least far below the series resonant frequency of the
beads.
73,
Jim W8ZR
Sent from my iPad
On Aug 18, 2013, at 10:48, Jim Brown <jim@audiosystemsgroup.com> wrote:
> 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
> _________________
> Topband Reflector
_________________
Topband Reflector
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