The biggest problem, I think, is frequency stability. Probably your transmitter is detuned easily by body parts or objects nearby. This means that in a given condition your transmitter is broadcasting in a frequency (say, 100.35MHz) and in another conditions it will be broadcasting in another frequency a few MHz away. Probably you can test this with a friend and, preferably, with an "analog tuner" with a conventional dial (rather than the synthesized tuner in your cell phone).
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The biggest problem, I think, is frequency stability. Probably your transmitter is detuned easily by body parts or objects nearby. This means that in a given condition your transmitter is broadcasting in a frequency (say, 100.35MHz) and in another conditions it will be broadcasting in another frequency a few MHz away. Probably you can test this with a friend and, preferably, with an "analog tuner" with a conventional dial (rather than the synthesized tuner in your cell phone).