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I searched the FCC database for FM’s operating with less than 1W. I found 3 special temporary authorizations for translators with less than 1W.
They have some high HAAT’s (heights above average terrain), but they are reasonable heights above ground level.
There is a site, rabbitears.info, that has Longley-Rice maps for TV stations. It also has Longley-Rice maps for a lot of radio stations, if the Application ID is put into the address bar for the URL of a map. Since the maps are geared to DTV, there is some stuff that is not relevant to radio. The color coding of the signal level matches the key on the right of the maps.
I pulled the rabbitears maps for the three ERP<1W translator applications above.
With enough height, less than 1 watt can cover a large area. Large enough that convincing the FCC a hypothetical service with more power than part 15 allows but less power than 1W could be hard.
In the above maps, the coverage does not stop at the red, in non-shaded areas reception may still be possible just with more difficulty than in the red shaded areas.
So if someone had a house on a hill taller than the surrounding terrain and they put a 300mW transmitter on a 20’ pole, they could push their signal out 1.5 miles or more (see K275AA’s map), with the potential to cause a signal that could interfere with reception of other stations well beyond that.
Here is a nice discussion of no <1W FM’s from the FCC, and the issues that LPFM 10’s would have faced. http://home.recnet.com/lp-10
I am all for community radio, but with the current regulatory framework nothing smaller than a LPFM-100 or a translator fed from a HD sub channel or AM is going to get licensed on the FM dial.
That leaves 2 FM options:
1. Pirate radio, which is against the spirit of this forum
2. Working with the limited range of 15.239 complaint transmitters on 88.1 to 107.9MHz.
I would love to see something change, but the FCC has been burned by every class of FM station operator so I doubt they would even entertain the notion of letting super low power stations on the air. Full power stations optimize or directionalize non directional antennas; translators are fed with STL’s when they need to be feed by over the air means, LPFM operators don’t meet their community obligations, and so on.
If they made the requirement etched into stone “0.5W TPO into a transmission line with 1dB loss into an antenna installed on an exactly 10’ pole operated exactly at the point of license”, at first the people who used the service would follow the rules. Then some would substitute a feed line with less loss. Then some would use a 10.5’ or 11’ pole. Then some would move their antenna from where licensed to higher ground. Pretty soon someone would come out with an amplifier that turned the 0.5W into 1W. And the FCC would have another enforcement headache on their hands. It is human nature to push the rules and see how much can be gotten away with to get just a little more range.
Also by licensing the service, the FCC would need to make sure stations of the service were being decent. Without recordings of what was aired/record keeping by the station, it would be very hard to prove someone did not drop an F-bomb on the air if someone accused the micro station of indecency.
The micro stations would be bottom of the barrel for users of the FM band. Right now if I use a 15.239 compliant FM transmitter as long as no one complains the big stations have no reason to care I exist. Big broadcasters like their market share. If there was a FCC database of where 500mW was being used and on what channels, some of the broadcasters would go looking for interference.
The micro broadcaster would need to protect co channel, 1st, 2nd, and 3rd adjacent channels, as well as 10.6/10.8 MHz up/down channels. The micro broadcaster would also need to be aware of mixing products from their frequency and any transmissions of comparable strength in their area. Mixing product calculation becomes exponentially harder as the number of signals being considered goes up.