On 7/20/2014 6:41 AM, John Miles wrote:
I often find that when I use coaxial baluns to cut down on ground loop
noise, I end up with more noise and interference than I started with. Not
always, but often enough that I'm leery of them.
Due to skin effect, most signal propagation in a coaxial cable takes place
between the outer surface of the center conductor and the inner surface of
the braid. Ideally, the outer surface of the braid just underneath the
jacket will act like an equipotential shield to keep external EMI away from
the signal path inside the cable.
But that's really only true when the cable connects two devices in
well-shielded enclosures that are themselves at a similar ground potential.
When you "lift the ground" with a coaxial balun such as an FTB-1-1+, you can
no longer pretend that the coax braid is at ground potential along its
length. From an RF perspective the braid is floating at one end, which
makes it an antenna.
Just FWIW, the TADD-1 uses transformers to provide DC isolation, but the
shield side of the coax goes to ground through a 0.1uF cap. The hope is
that this reduces the issue that John's referring to (and which I've
seen plenty of times using baluns).
John
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