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In the US the part 15 rules which apply to FM are written in terms of field strength at a distance limits and do not address the power of the transmitter.
Since field strength at a distance is determined by factors including power we can make certain predictions. I predict that it will be impossible to remain legal with a half watt transmitter connected to any antenna. You will most likely exceed the legal limits thousands of times over. So, with this in mind, the answer to your query regarding the legality of a .5 watt transmitter is for all practical purposes NO.
A FM transmitter and antenna operating legally under part 15 rules will yield a typical range of 200 feet. This is not a legal specification, it is just an example of the range to be expected. A system which has a mile of range is most likely not legal.