So the question is, co-locating certified FM transmitters would yield in increase in field Stregth? Legal if each transmitter is at or under 250 uV/m? Both fed with same audio source and using the supplied antenna?
The two co-located part 15 FM transmitters are on different frequencies?
There are probably several ways they might interfere with each other because of the respective antennas absorbing some of the energy from each other... haven't heard of it before, but MAYBE the antennas could re-radiate the signal from the co-transmitter...
Could be wrong but I don't think so. If the transmitters are all on the same frequency and close to each other they will all overpower each other or interfere with each other. If they are far apart you would have many transmitting in their area.....not a uniform signal collectively adding up to make a stronger single signal like you are thinking.
Mark
I should have been more clear. They are on the same frequency. So now my next question. If part 15 AM transmitters that are clustered have increased field stregth then why not FM transmitters?
There are several problems with your idea.
The first is no two transmitters will have exactly the same frequency unless they are fed from the same timing source. So the two transmitters would interfere with each other, generate heterodynes, fight to be dominant with the capture effect by the receiver and so on.
On the other hand, if two transmitters were fed by the same timing source and near each other, if inspected, the inspector could call them a single assembly and if they added perfectly the result would be a field strength double the 15.239 limit.
Second, I doubt the transmitters being certified would help if inspected because it is extremely unlikely they were certified to operate in pairs or clusters.
Third, putting multiple transmitters together is going to create a mush of RF junk. My gut feeling is the heterodyning of the signals, even if the transmitters are set for the same frequency, will cause the junk to exceed the bandwidth limit in 15.239.
Field strength will increase as more transmitters are added, but that does not translate into better reception range, it might come across as noise.
Below is a comparison of the fields radiated by a single FM system compilant with FCC §15.239 (at the left) with the total fields radiated by two such system when co-located.
Changing the distances and orientations of the two antennas, and/or adding more antennas to the system can produce many more nulls in the net radiation patterns of such an array.
Thanks Rich. Makes sense.