On Feb 9, 2:24*pm, Jim Lux wrote:
Dave Platt wrote:
I used off-the-shelf (surplus-store) ferrite tubes - very probably a
43 mix or something close to it, based on the simple inductance
measurements I did with an MFJ analyzer. *They're roughly 1 inch long,
and large enough to allow three through-the-center passes of RG-8X coax..

I glued somewhere around six of them, end-to-end, to create a long
tube, and then ran the RG-8X through... creating a long three-turn
choke. *Added N connectors to the ends of the coax and stuffed the
whole thing into a chunk of PVC tube with end-caps.

You might do better breaking the thing up into multiple cores and
multiple windings. *That is, rather than 3 turns through 6 cores, do 3
turns through 1 core, then 3 turns through another core, then 3 turns, etc.

The logic here is that the impedance scales as the number of cores, but
doing them all in one shot means that the input coax is very close to
the output coax, so you have a lot more capacitance coupling the input
to the output.

I think Jim K9YC actually did some comparisons using #31 2.4" cores
comparing stacking to stringing em out.

Jim's comment is a good one from another standpoint he didn't mention
explicitly too: if you put a single choke right at the antenna
feedpoint, and the feedline is in some sense resonant (e.g. a quarter-
wave long with the far end grounded), the fields from the antenna may
very well excite currents on the outside of the feedline anyway, even
with a "perfect" choke. By putting the separate chokes spaced out
along the line, say a quarter-wave apart at the highest operating
frequency, you break up resonances that might otherwise occur in the
line.

By the way, for a single-band choke on a band (10 meters) that was
extremely troublesome for me in one installation many years ago, I
wound (approx., from memory) 5 turns of the feedline coax into a
solenoid coil about 12cm diameter, secured so they were stable. That
made a coil of roughly 3 microhenries inductance, self-resonant just a
bit above 30MHz. A pure 3uH would only be a little over 500 ohms
reactance, but resonated with a wee bit of capacitance across the
coil, the result was a very high impedance that solved the problem
nicely. I've since found the technique valuable and easy to apply to
RG-58C/U type line used at 147MHz--much smaller coils, secured on
sections of PVC water pipe.

Cheers,
Tom