On 31 Jan 2006 15:12:40 -0800, "bpnjensen"
wrote:

A choke passes DC unimpeded and blocks higher frequecy AC. Unless
your SWL antenna is mounted on an FM transmitter tower I don't see any
advantage.

I will disagree here. Although a single coax choke of specific design
won't solve all problems across all bands, a choke of the right size
will give you an obvious reduction in common mode noise at certain
minmum requencies and upward.

I installed one on my own antenna at home. I was hearing common mode
noise across all bands from zero to 30 mHz. i fashioned a coil choke
about 7" diameter on a piece of ABS pipe, wrapped neatly enough times
to get about 30 feet of the coax onto the coil, and mounted it just
below the antenna. This has made a noticeable difference for all bands
from 11 mHz and up...the background is considerably quieter than
before. A note here - this thing can get pretty hefty, fast, if you
use substantial coax of the 3/8' variety or larger. Messing with this
thing as an integral part of a relatively stiff run of coax can cause
minor cussing and swearing if you do not have some assistance (and
maybe even if you do ;-)

For lower frequencies, a larger coil able to hold a greater length of
coax would have been necessary; I didn't feel that the difficulty in
fashioning one of these and trying to get it up on top of the mount I
have would have been worth the trouble. I have since reconsidered this
decision many times and one of these days the neighbors just might see
big ugly coils of coax up under my two antennas. Whether my wife goes
along with this, I can't tell you.

Bruce Jensen

Wouldn't a ferrite work better?