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

On Fri, 03 Oct 2008 14:47:12 -0700, Roy Lewallen
wrote:
...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.

The large-diameter sleeve is impractical in my app because it make the
antenna non-flexible for storage. Also expensive.

My principal use will be QRP. I will stick some ferrite beads on the
feedline and hope for the best.

The most I would use non-QRP would be 25 watts SSB.

Ken KC2JDY

"Chevy454" wrote in message
...
On Fri, 03 Oct 2008 14:47:12 -0700, Roy Lewallen
The large-diameter sleeve is impractical in my app because it make the
antenna non-flexible for storage. Also expensive.

My principal use will be QRP. I will stick some ferrite beads on the
feedline and hope for the best.

The most I would use non-QRP would be 25 watts SSB.

Ken KC2JDY

One design that I saw that you may try it to just use a 1/4 wavelength of
wire at the end of the coax. Then about 1/4 of a wavelength down the coax
you use a choke. It could be a coil of coax or it could be the beads at
your power level.

I have not tried it, just saw it in one of the ARRL books.