J-poles suck.
You NEVER see one in commercial use either. MUCH easier to make a sloping-radial ground plane antenna with wire soldered to a BNC or SO-239 socket AND it has a much cleaner pattern too.
J-poles are often dual band. If you are looking for a simple dual band antenna it works pretty well for line of sight radio.
For the better omni directional gain, a vertical collinear phased array antenna is the solution. They can even be tuned with down tilt for coverage from hilltops. Many are made from UHF coax and or copper tubing sections. There are some excellent commercial antennas made that way.
VHF and UHF are not normally for over the horizon radio.
If you want real directional gain, the Yagi, corner reflector, patch antenna and Parabolic dish are the solution.