Mounting FM antennas on towers

[Chris Hall]

There is a real science to the mounting of FM antennas on towers and structures called pattern studies which can cost upwards 
of $30,000 dollars depending on the situation.  Antenna manufacturers build appropriate sized scale tower models and place an element on
an antenna range. They will then send you many horizontal and vertical patterns for you to choose which best suit your needs.
Mounting the antenna on the tower side of the city you want to cover could be a disaster depending on the structure itself, the opposite of what youm might think.
Generally all antennas on a structure will exhibit directional characteristics, that’s why a top pole mount is best for non directional
results, even a pole can change what you would get if the antenna is in free space. Variations can include face mount on three or four sided 
towers, corner mount or on offset brackets to provide space between the tower and the antenna itself  Unless you have a DA license
the antenna manufacturers will not design an antenna with parasitic elements to deliberately direct the energy but will with a  wink and 
a nod show you patterns of mounting that will give you the best signal advantage. 
Directional FM antennas are a whole different mater.