Chevy454 wrote:

That's unfortunate, because I am looking for a simple arrangement.

What if I just hang a wire dipole vertically from the mast (= bendy
Black Widow crappie pole) with a PL-259 at the feedpoint? How much
would the coax feedline have to stand off from the dipole to keep from
interfering with its operation? I can't pull it too tightly away or
the pole will bend.

Ken KC2JDY

A very long distance I'm afraid. You might be able to live with the
common mode current. Otherwise, a larger diameter air-insulated sleeve
or some other decent isolation method like a J-pole are about all I know
of that would reduce it. Even those methods, though, can result in some
common mode current depending on the feedline length due to mutual
coupling between the feedline and intentionally radiating part of the
antenna.

If you really want to avoid common mode current on a high, elevated
vertical, you need to first effectively choke the conducted current with
a high Z sleeve or J-pole type decoupling stub. Then you need to take
care of the coupled current. One effective way is to use two common mode
chokes ("current baluns") spaced about a quarter wavelength apart along
the feedline. The Isopole took care of both by using two flaring sleeves.

There are a lot of antennas being used out there which have no special
effort taken to suppress common mode current, and they seem to function
well enough to satisfy most users. But it can be a problem in some cases.

Roy Lewallen, W7EL