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I get about 1/4 mile in some directions with a Sosche FM transmitter ($10 from wal-mart) and about 29 inches of wire on the negative battery spring and about a 2 foot headphone extension cable.
I power the transmitter with 2 sets of 3 AA batteries in parallel giving it 3.6 volts and it will run unattend about 4 days. The whole shebang is mounted outside about 8 feet off the ground.
Here is a link to a thread I started about the project, I will try to update the info soon