"Robert11" wrote in message
...
Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in
coverage ?

No.
A J-Pole is not an isotropic radiator.

It is "omnidirectional" only if you consider signals that intersect the
antenna in the plane that is at a right angle to the major axis of the
antenna.
The closer you get to having the signal intersect the antenna at the end
the the less sensitive the antenna is. i.e. it is less sensitive to
transmissions that are from below or above the antenna.

You will get an idea of the plot the antenna's free space pattern if you
imagine an orange with two major parabolic dimples 180 degrees apart, the
centers of which are in line with the major axis of the antenna.

In practice the antenna is considered an omni because they are mounted
vertically to communicate over the surface of the earth with other radio
units on the surface of the earth .In day to day operations the surface of
the earth is very close to the geometric defintion of a plane.

Dan
AI8O