On Oct 13, 12:29*am, "Sal M. Onella" wrote:
On Oct 12, 12:13*pm, Tom Horne wrote:



I am trying to find out how far an open stub, dual band, two meter and
seventy centimeter J-pole needs to be from a parallel metal support
mast on which it is side mounted to avoid detuning the antenna. *I'm
not concerned about the effect on the radiation pattern as I can mount
the antenna on the side of the mast towards the area I'm trying to
cover. *I have to build brackets to accomplish this mounting and I
would like to avoid building multiple sets of them just to test for
clearance. *If anyone has the requisite skills and tools I would
appreciate any information that can be readily found by modeling this
antenna in proximity to a metal support mast consisting of four inch
aluminum tube. *I have two AB-952 masts on which to mount six
antennas. *Three of them will be two meter omnidirectional antennas.
Since we need two limit interaction and interference between the three
two meter stations We will be mounting one of these at the top of one
mast, one a third of the way up the second mast, and the third two
thirds of the way up the first mast. *Both masts will be deployed with
extension kits so they will be fifty feet in total height.

In truth I just do not have the skills or the time to develop them to
do this so any help would be greatly appreciated. *This is for a
public service event that is now only two weeks away.

--
Tom Horne, W3TDH

I'm not a modeler. * However, I did investigate this experimentally a
few years ago, although not with 4-inch aluminum mast. *I used a 10-
foot section of one-inch galvanized mast. *The tested j-pole was a 2m
copper pipe version, hanging vertically in the open, suspended by a
piece of line.

On transmit, I tuned to a simplex channel, keyed up, ID'ed and noted a
small amount of reflected power. * I locked the key and slowly moved
the section of mast toward the antenna. *I had to get within one and
one-half inches (1.5") before the reflected power increased.

On receive, I tuned to a repeater with a ragchew in progress and
opened the squelch. *I inserted a switchable attenuator in line with
the radio and set the attenuator to the crappiest signal I could copy
-- one dB away from unintelligible. *Enter the mast section, and once
again, I had to get within an inch and a half to lose the
conversation.

The direction from which I approached the antenna didn't seem to
matter, but I didn't do any real measurements of pattern distortion.

I hope this helps you.

Sal
(KD6VKW)

I never tried it as close as Sal but It did mount one that stood off
about 2 ft from the tower with no discernible problems. I was also in
on making a J antenna where the tower was part of the antenna.

Jimmie

Jimmie