You can't mount a j-pole up against a mast. If you don't want to move the existing top mounted antenna then the easiest thing is to add a cross arm or bracket that is at least 1/4 wavelength long and mount a vertical dipole on the end so it is at least 1/4 wavelength from the mast.How can you (or should you not?) mount two vertically polarized antennas on the same mast? I have a specialized 70cm vertically polarized antenna on top of a mast and would like to add a 2m J-pole but am not sure that will work to be so close to the other antenna? Thanks.
So ballpark would be that it would need to be 234/ 140 (MHz) = 1.67 ft away from the mast? Maybe round up to about 2' ?