Hello Chad:

Wow good point here you have made.

Yes I agree with you here, the mast would become the counter poise, if
currents are choked at the coax. This is why these none ground plane
antennas will still work when a coax Balun is placed right at the
antennas connector, a Great point made here.

In the 1980's I had a terrible time installing a Sigma 4 antenna. As the
mast was pushed up the SWR changed. So I figured some how the mast and
maybe the coax, was apart of the antenna, and dropped the mast down a
foot at a time to get a decent SWR. I wish I had a Balun then.

Placing the balun down the coax at a 1/4 wavelength might work well on a
fiberglass boat and such.

One of the things while adding such things and testing them, is to keep
an eye on the field strength of the antenna. If something is done and
the field strength increases or decreases you want to see this.

In some cases antenna ranges will monitor the antennas swr as compared
to its field strength.

Jay in the Mojave


Chad Wahls wrote:
"Jay in the Mojave" wrote in message
...

Hello Dr Death:

Ok good deal I saw the impressive photos that Chad had there.

One thing to consider is that those fiberglass none ground plane
antennas use the coax outer shield as the counter poise or other end of
the antenna, so current will be seen on the coax. And in some case you
will see SWR changes from different lengths of coax, because the coax is
actually radiating, acting like a antenna.

I have had customers add in a line coax Balun on these type of antennas.
The Balun uses a Toroid Core, with the coax would around it. But the
trick here is that the Toroid Core is placed at 1/4 wavelength or about
102 inches down from the antennas connector. This allows the coax to
radiate for a 1/4 wavelength, possibility allowing for a low take off
angle, then choking off the currents beyond the 1/4 wavelength. Or the
coil wound coax Balun can be tried.



As you may have guessed the antenna is an Imax 2000. Which mounts to the
mast via a metal base that is directly coupled to the mast/tower. This
being said wouldn't the mast become the counterpoise at this point if the
coax is not? (The outside of the PL259 is directly coupled to the mounting
provisions.)

The balun was made out of spare junk on the premise that I NEEDED a reason
to go out to the shop, drink some beer and relax. What a better project, a
free one I made it removable for obvious reasons and have thought about
Jay's points a couple times when they were brought up after construction of
said device. This spring I hope to have a new (different) tower and at this
time will play with moving the balun from the feed point to right before the
ground rod next to the house. This will allow the coax to act as it should
and hopefully keep my feedline currents down, wadda ya think?