Total posts : 45366
taybrynn wrote: If you bought an FCC approved transmitter and operated from the window in a 53rd floor of a skyscraper and transmitted over 3/4 mile … thats still not ok or part15 compliant.
Part 15 FM has no restriction on transmit antenna height. The only antenna requirement is that the peak field radiated from it cannot exceed 250 µV/m in any direction 3 meters away (AS INSTALLED). The big issue with this is in accurately knowing what field strength is present there, including reflections from re-radiating conductors and surfaces near the antenna.
Only 11.43 nanowatts of power radiated from a linear, 1/2-wave dipole in free space produces the peak Part 15 FM field, so most “Part 15” FM transmitters are massively overpowered. Even 1 milliwatt of transmitter output power is way too much, unless the antenna radiates almost none of it.
For Part 15 AM systems installed on skyscrapers please see my following comments.
I get tired of reading misleading messages that infer that buying a FCC approved transmitter automatically means you’ll automatically be operating part15 compliant. Yes, probably … if you don’t modify it or use anything besides the included (whip / short wire) antenna … and choose a good fequency and location.
But note that any other conductors leading away from a Part 15 AM transmitter with its stock “whip” antenna also become part of the radiating structure of the antenna system, unless they are de-coupled for r-f current. So a Part 15 certified AM transmitter mounted 15-20 feet above the earth, and using a combined “ground lead” and “massive ground wire” or metal mast or building steel connecting to a buried r-f ground is technically non-compliant, although many seem to operate this way — with only occasional FCC notice/action.