I've had just about every type of 2 mtr antenna on a side mount, from 3.2
wavelength booms to j-poles and collinear. The only ones that were sensitive to
side mounts were the horizontal ones and they could be tamed with nonconducting
arms and masts as long as the arms were over a meter in length to keep the
antenna clear of tower.
The closest I would mount a vertical would be a 1/4 wavelength from tower.
The farther you go out the more omnidirectional coverage you will get. You will
never get 360 degree coverage with one antenna but you can set phased sets
around tower if complete coverage is required.
There is nothing special about a J-pole. It is just a end fed 1/2 wave
dipole. There were even plans to use a J-pole as the driven element in a beam.
Most of the copper cactus type I have seen mount from the bottom but if you
needed a second attachment point for stability in high wind you can use pvc at
the center of the upper 1/2 wave element.
A simple pvc arm with U-bolt at tower end and a T fitting glued on antenna
end would be plenty stout. For antennas of this size I've made the whole side
mount of pvc T's and SS hose clamps.
If you think you need to tune the antenna you can temporary mount it at about
5 feet on tower and tune away till you get it where you want it. I've found
that the formula antennas are close enough for the transmitters I have and with
low loss coax enough power was getting to antenna to work anything it will hear.
Good luck on you installation,
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