Perhaps the FCC used the following reason for setting the Part 15 AM power cap at 100 mW to the final stage.
This amount of power with a 10-foot length of wire antenna provides a useful signal to an AM radio located in close proximity to the transmitter. Any more power causes some nearby radios to become overloaded and thus distorted.
The "close proximity" purpose of Part 15 AM has origins in the phonograph oscillator, the original form of Part 15 transmitter, intended to inexpensively input an audio signal to the superior amplifier, loudspeaker and wood cabinet of an upright AM console radio.
I have proven the liklihood of this theory by use of an AMT5000 from SStran.com, which has the convenience of a fine-tuning adjustment for the power output.
When set to the lowest power 0f 36 mW to the RF input, the AMT5000 rides the background noise on the AM dial at night, resulting in other stations creeping in and mixing with the audio.
When set high, 275 mW to the final RF stage, a radio 10-feet away distorts due to overload.
But the mid-setting, 100 mW, presents a solid undistorted signal to radios near and far within the same building.
A further use of Part 15 AM transmitters is a matter of clever invention by users, who have discovered that by placing the transmitter/antenna outdoors with excellent ground radials, a 100 mW signal can cover a distance exceeding a mile, according to many reports.
My Case for 100 mW has not been refuted, proving its truth.
"When set high, 275 mW to the final RF stage, a radio 10-feet away distorts due to overload"
Carl you're a "naughty" boy!
Rob Veld
Rob, my naughtiness is because of my European ancestry, but this test was done in less than 1-minute so I am fast about being naughty.
I suspect the actual reason is that the FCC (and others) wanted to limit range to about 200 feet. They weren't counting on inventive manufacturers and broadcasters to push the envelope.
Welcome to the club Carl. I've been naugthy for the past 10 years. But................. a question, what was first, the power limit or the field strength limit??
Rob V.
The first chicken hatched from its own egg and immediately crossed the road.
A popular "ballpark estimate" for Part 15 broadcasting is the 200-foot rule.
It is generally believed that if a Part 15 AM or FM covers 200-feet in all directions from the antenna, it is most likely in the "safe zone" as far as FCC scrutiny is concerned.
I believe I have discovered an additional "ballpark estimator" in the way described in my Post # 1 above.
If any of your home radios overload because of the strength of your transmitter/antenna, you are probably in the FCC danger zone.
The one exception to this method might be the AMT5000 transmitter from SSTran.com because of its 94% high efficiency design...
Even at 100 mW to the RF final and a proper 3-meter antenna, the AMT5000 is capable of overloading some nearby AM receivers, yet is fully legal.
A few years ago I saw that a new model of a transmitter had come out, and the designer said is had a 99% efficient final stage. I don't know what's up with that claim, and haven't seen it recently, but it was clearly stated at the time.
NC-17