On 9/16/2010 7:41 AM, Bill Ogden wrote:
I have a half-sloper attached to my tower. It is mostly used for 80 meters
(but also for 160 and 40 in rare cases). It is attached to the top of the
middle section of a 55' crankup tower. The tower has a number coax and other
control lines suspended a foot or so outside the tower. I am in the "far
suburbs" but 40/80/160 still have more noise that I hoped and I suspect some
of the noise is from the neighborhood; there is no specific noise generator
that I can detect.

Question: would a coax choke (5 43-material toroids with 5 loops of coax) at
the feedpoint of the half-sloper do any good? I am a little confused since
the tower (and, I assume, the outside of the coax) is an intentional part of
a half-sloper antenna.

At the bottom of the tower, the coax goes underground for about 50' to the
rig.

Bill - W2WO

You're correct that the feedline and/or tower are part of the antenna.
What you have is a dipole (not necessarily symmetrical), with one side
of the dipole being the half-sloper "antenna" and the other side being
the tower and feedline. The feedline has two transmission line
conductors, which are the center conductor and the inside of the shield.
Whatever current flows out of one must flow into the other. The current
going to the half sloper equals the current flowing down the tower and
outside of the feedline shield. If you effectively choke the tower and
feedline currents, you'll also choke the current to the half sloper.

While you can effectively choke the current on the outside of the
feedline (it would probably take two chokes, one at the top and one at
the bottom), it won't help your noise situation. Whatever nearby
vertically polarized noise might have been picked up by the outside of
the coax will still be picked up by the tower. If the problem is
vertically polarized local noise, the only solution is probably to go
with a fully horizontal antenna with a well-decoupled feedline.

Roy Lewallen, W7EL