In article ,
Lostgallifreyan wrote:

I'll pass. I think the reason no-one knows is that as you (and others)
say, it's not important enough. What does seem important is to try to reduce
localised noise, and to break the current link to protect the radio input
from static discharges. Whether I use coax or a balanced loop made from
speaker wire, it loooks like my next step is to get Amidon FT-114-75 ferrite
cores to play with, and in Britain I can't easily do that, but if anyone
knows a local direct equivalent to them I can try that. I'll Google for
things that fit the description (AL about 3000, permeability u=5000, about
1.14 inch outside diameter) but I think it's wise to ask here to try to save
time.

You can probably at least start your experimentation using the common
sort of interference-suppression ferrites that are found in many
computer accessories - e.g. molded onto DC cords, VGA cables, USB
cables, and so forth. Here in the U.S. these are easily available at
electronics surplus stores, ham-fest flea markets, and other such
sources.

In my experience, these tend to be a ferrite mix such as type 43,
which is optimized for use at somewhat higher frequencies than
HF/SWL... so they will probably not be optimal for your purposes.
However, they can be made to work.

A few years ago I constructed a common-mode feedline choke for our
local ARES/RACES ham station, to try to keep 40- and 80-meter signals
from being carried back down the outside of the coax and into the
building (our signal was audible on phones in the city's "911"
emergency dispatch center... *not* good). I took several tubular
computer-interference-suppression ferrite cores (large inner
diameter), glued them end-to-end with cyanoacrylate, let them dry, and
then ran some RG-8X coax through the center and back around the
outside and through the center again. The coax looped through the
tube of ferrites three times.

This resulted in an extremely effective common-mode choke. According
to my MFJ antenna analyzer, the impedance looking up through the coax
in the usual way (standard hookup, into a 50-ohm dummy load) was 50
ohms... the ferrites had no effect at all on the differential-mode
signal in the coax.

But, when I measured the impedance along the braid (i.e. from the
ground shell at one end of the coax, to the ground shell at the other...
a DC short circuit), I couldn't get a reading at any frequency... the
meter just said " 1500 ohms". Even at the lowest frequency of
interest, these non-optimal ferrites added so much inductance to the
common-mode signal path that they were blocking the feedline current
flow very effectively.

[Unfortunately, we determined that the phone interference was caused by
direct RF pickup by the phone wiring, which was in the "near field" of
the antenna above the roof. It occurred even if we completely
disconnected the building feedline, and fed the antenna directly from
a radio located up on the roof. The feedline choke couldn't help us.]

In your situation, I'd guess that you could probably make an efficient
feedline choke by using almost any surplus ferrite toroid which is
sufficiently large to wind your feedline (coax or speaker wire)
through it a few times. Or, use several surplus ferrite cores,
end-to-end, and if they're large enough in diameter, loop the feedline
down through the center more than once.

It won't be perfect (nor as good as if you used a ferrite optimized
for use at lower frequencies) but it will probably help matters, and
will give you some sense as to whether it makes sense to go to the
trouble and expense of buying ferrites that are better for your purpose.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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