Almost certainly (unless you are using RF modules with in-built MCUs) you haven't read the data sheet on the radio modules and if you had you'd have realized you cannot expect to connect a uart at either end and expect this to work as if it were linking two PCs with an RS232 cable. Look at this: -
When not transmitting data the output from the receiver will be gobbledy-gook and nonsense. Look at left of diagram. When the transmitter starts sending data, it has to "attract" the receiver circuits to lock-in to the transmission - this is what the preamble does and, after a few milli seconds sensible preamble data is received from the receiver and, the micro controlling the uart will know to pretty soon expect the first sensible byte of the payload transmission.
The picture above was drawn with FM modulation in mind but it's pretty much what happens on AM too.