Hopefully this answers the questions regarding FCC/FAA Tower registration. First off any tower can be registered in under the FCC's ASR system. If the tower owner wants to put out the effort they're free to do so. FCC ASR numbers are 7 digits and if the tower has a 6 digit number it's not a current number. The only reason a site would HAVE to be registered would be because of height. The FCC/FAA does NOT require registration based on technologies used, frequencies, color of tower owners hair or anything other than height. Straight from the FCC, no hearsay on this. http://wireless.fcc.gov/antenna FCC registration is required if a tower is: 200ft above ground level (AGL). Within a certain slope (distance/height) from a local airport. If the tower is in an approach area of an airport you'll also need to notify the FAA in addition to FCC registration. If the tower is located on an airport. Now a tower is EXEMPT from registration if it in the following: It's completly shielded by either existing structures of permanent or substantial character or by naturally occuring terrain of equal or greater height AND is located it is located in the congested part of a city where there is NO WAY the tower is shielded from all aircraft. Any tower less than 20ft (except when it increases the height of another structure) Any air navigation device or landing aid type accepted by the FAA and which it's height and location is fixed by it's function. Let's detail that 20ft rule as it affects rooftop locations. If the building and the tower and the height of the tallest antenna does NOT break 200ft, then it DOES NOT need to be registered. If the building is 201ft tall and has a 20ft tower on top of it, but the antennas are below the top of the 20ft tower then it DOES NOT need to be registered. If the building is 201ft tall and has a 20ft tower on top of it and the top of the antennas as above the top of the 20ft tower then it DOES NEED to be registered. If the building is 201ft tall and has a 40ft tower on top of it, regardless of the antenna height it DOES NEED to be registered as the tower breaks the 20ft rule. If the building is 190ft tall and the tallest point on the tower brings the total height to 209 ft if DOES NOT NEED to be registered as it doesn't break the 20ft rule. More detail on the 200ft AGL rule If the total height of the tower including antennas is less than 200ft registration IS NOT REQUIRED. If the tower is less than 200ft but the antennas bring it to MORE than 200ft registration IS REQUIRED. If the tower structure itself regardless of antennas is MORE than 200ft then registration IS REQUIRED. In regards to the slope ration from an airport that is determined via a TOWAIR search. I hope this clears up some of the discrepancies that get posted here.