My first thought is to make a very simple AM transmitter and use an AM radio. There are a few simple ways to create an AM transmitter. One way is to use an oscillator and a transformer. Take a look at this "project.":http://sci-toys.com/scitoys/scitoys/radio/am_xmitter_labeled.jpg
To get 300 hundred yards you will need to use a better antenna, but that should be very easy to do.
