> Thanks. I wasn't referring to a magnetic loop that uses the shield for
> pickup. I was referring to the outer shield on the coax that runs from
> any antenna to the shack. If you use an antenna that was chosen for its
> specific directional and/or low-noise properties, and you don't isolate
> that antenna from the outer shield of the feedline, the shield itself
> becomes part of the antenna, spoils the directionality and picks up
> additional noise. Art DelibertKB3FJO
Not necessarily. If the antenna is properly balanced, or properly
unbalanced, that should be all the isolation necessary.
Beverages are sensitive to common mode because people use autotransformers,
or share a common shield and antenna ground, and the antenna almost always
has a poor ground. So the antenna is nether balanced, no unbalanced, but is
somewhere between and the shield has a connection to the antenna's ground.
A small loop, if properly built, should be immune to common mode on the
feeder. It is also more difficult to fix a problem in loop design by
throwing beads on a cable, because loop common mode impedance is so high the
beads are a tiny part of the CM system impedance. Adding a ground rod at the
antenna, where the cable comes vertically to the earth, would probably be
more effective with a small loop than beads.
I never have needed additional isolation on a small loop, but I've never
gapped the loop at the wrong place, or built a poorly balanced loop.
PS Why do my posts take so long to appear on the reflector? Is this normal?
73 Tom
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|