On 10/10/2010 5:26 AM, Jim Thomson wrote:
> For 50 MHz, you want two or three turns through a 1-inch long core and
> two turns through a 2-inch core,. and for maximum noise suppression, you
> will want at least three of these multi-turn chokes in series along the
> line, close to the feedpoint. The reason you need multiple chokes in
> series is that you only need 2-3 turns to put the resonance at 50 MHz,
> so you haven't got much benefit from N-squared. Fair-Rite #43 and #31
> are roughly equivalent at 50 MHz.
> 73, Jim K9YC
> ## what do u mean by a 1" long core ? Does this mean 2.4" OD x 1/2"
> torroids used... and 2 of em stacked ? Is a 2 " core consist of 4 x
> torroids ?
No, I mean ferrite cores that are not toroids. See
http://audiosystemsgroup.com/RFI-Ham.pdf and the Fair-Rite catalog on
> ## It can't be ID. You won't fit 9913 through a 1" ID very easily..well
> maybe u can?
The "biggest clamp-on" described in my tutorial is 1-inch i.d. and can
easily take three turns of 0.4-inch o.d. coax (including 9913). See Fig
39 and Fig 46 in the latest version of RFI-Ham.pdf on my website (this
was last updated in April).
> How far apart is each of the 3 x chokes to be spaced from each other ?
They should be as close together as you can easily rig them.
> How do u
> propose to keep this mess away from the boom..and or mast ?? I think I
> would want
> all of them to be at least one outer loop diam away from metal. If they
> are touching
> metal, the C between coax braid and boom/mast/tower will surely screw up
> the chokes ?
> You would end up lowering their resonant freq. The stray C would parallel
> resonate the
> chokes..and probably increase the choking Z... but at a lower freq. It
> might be broad enough
> peak to still cover 6m.
That's good thinking, and it's correct. What matters is the capacitance
between the coax on one side of the choke and the coax on the other side
of the choke. That will not be zero. but it will be small, and the
capacitance between the cable and the boom on one side of the choke will
be in series with the capacitance between the cable and the boom on the
other side of the choke. Remember that the resonance is being tuned by
both the inductance of our choke and the stray C of the choke. That C
has two parts -- the capacitance between turns, and the capacitance
between the coax and the core. For most practical chokes, the
capacitance between turns dominates.
> ## In some cases, you can't hang em below a boom...if the connections are on
> top of the boom.
> Even if hung below a boom, I would not want them flapping in the breeze.
> What's the fix ?
That's the challenge. :) You seem to have a good grasp of the
variables, so do your best with the rigging and call CQ.
73, Jim K9YC
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