For lightning protection - it must be put under a zone of protection or it will be subject to a direct strike.
If it is under a zone and not subject to a direct strike it still may require bonding to the lightning protection system.
If the steel antenna frame is connected to the steel frame of the building and the steel frame of the buildng is grounded nothing more may be required.
If the steel antenna frame is an isolated (nongrounded) body that is located near a lightning conductor then bonding may be required.
If the steel antenna frame is a grounded metal body bonding may be required.
See NFPA 780 Section 4.21 "Bonding of Metal Bodies" for all the details and formulas.