Before deciding outright that the use of the UART on the 89C2051 is a suitable output to drive the transmitter and take in the receiver signal you need to look at the signal requirements of the RF module pair.

Sometimes these things do not work with NRZ data like a UART signal. You may find a requirement to use a biphase modulation such as Manchester or others.

RF Modules may also have a minumum and maximum frequency of modulation that they will work reliably with.

Finally it is not uncommon to use a lead in sync stream of some sort when trying to send an RF modulated packet.

All of these of course depend entirely on the RF module pair design.

If the RF scheme is simply an amplitude modulated type you could do well to design a packet structure that included some type of sync at the beginning so that you can use that at the receiver to validate the beginning of a packet. Then reject all other spurious received junk that does not meet the criteria of being a sync.

In addition to that you should consider adding packet byte count fields and a CRC check field to the packet. These are essential for verifying the integrity of data you have received once a sync has been detected.

Michael Karas