In article ,
Rick (W-A-one-R-K-T) wrote:

You mention the "UnUn that comes with the commercial unit"... not sure if
you meant a commercially-available CW antenna, but this one is being built
from scratch using a W2AU-type 4:1 balun and some lengths of insulated
copper-clad steel wire I have here.

For what it's worth - last year, I home-brewed a coaxial RF
isolator/choke, in an attempt to fix an RF-incursion problem involving
our RACES 40/80 trap dipole and the phone system in the building at
which the dipole is installed.

I used some fairly large-diameter ferrite beads - probably 43
material, if I remember the impedance readings I took correctly. They
were an inch or so long... I superglued a bunch of them together
(around 8 or 10, I think) to form a long ferrite cylinder. The ID was
large enough to let me get a length of RG-8X through the cylinder a
total of three times... in effect, making a very long, thin three-turn
choke. The whole thing was installed in a length of PVC pipe with end
caps and N connectors.

The common-mode impedance along the braid of the coax turned out to be
quite high. If I recall correctly, it was at the upper limit of my
MFJ-259's ability to read at 3.5 MHz (probably upwards of 1500 ohms,
mostly reactive) and was off-the-scale at 40 meters and all higher
frequencies. Whatever capacitive coupling occurred between the turns
didn't amount to enough to matter.

[Unfortunately, it didn't fix the RF incursion problem, which later
experiments proved was caused by near-field pickup by the phone lines
of RF from the antenna itself, and not due to feedline RF at all.]

You could probably build a similar unun using the Radio Shack
ferrites. If they aren't big enough to allow multiple turns of RG-8X,
you could use a short length of RG-58 or an even smaller-gauge 50-ohm
coax. Losses in this short a length of thin coax shouldn't amount to
enough at HF to matter, or to cause appreciable heating unless you're
running very high amounts of power.

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