AM towers are unique, because in many cases, the tower structure itself is radiated with radio frequency (RF). The FCC addresses this in §73.49 which states, in part:
Antenna towers having radio frequency potential at the base (series fed, folded unipole and insulated base antennas) must be enclosed with effective locked fences or other enclosures.
This is because if someone was to touch an AM tower structure, they could be subject to injurious burns.
For FM (including LPFM), the tower itself is not a radiator. Instead, the antenna mounted on the tower is the radiator. Therefore, touching an FM-only tower structure at the base is not likely to give an RF burn.
FM antennas though should be constructed in a manner that prevents easy access to the radiated portion through climbing the tower and such.
Requirements for fencing may come into play if it is determined that radio frequency exposure exceed a certain guideline for uncontrolled (general public) environments.
For LPFM stations, as a part of the services “simple” processes, the FCC has published a worksheet for determining whether additional environmental studies or fencing is required.
The worksheet can be found on Page 17 of this document (Worksheet 3):
https://transition.fcc.gov/Forms/Form318/318.pdf
In that worksheet, the FCC states for a tower with only an LPFM (LP-100) station antenna on it, there should be a minimum of 21 feet (6.5 meters) from the base of the antenna to the lowest part of the antenna. For all intents and purposes, it is good practice that for rooftop antennas, they should be placed at least 21 feet above the roofline. The worksheet also calls for a minimum of 20 feet (6 meters) from the radiating part of the antenna to any person. When an LPFM antenna shares a tower with other FM or TV broadcast stations, there is a more complex process.
For more information on this subject, see REC LPFM Advisory Letter #8:
https://recnet.com/node/3343
