Stevez explained the nitty gritty details above, I would just add that 10mW max radiated power is quite low as transmitters go. So, if you use a single band antenna (and a 'pice of wire' is an example of this), make sure its length is precisely tuned to the frequency to minimise transmission losses. At that frequency, a few millimeteres plus or minus make a big difference.
Also, the connection between the chip output and antenna is critical, odd bits of wire will not do :wink: this job. Use connectors suitable for 433MHz for your coax if you have planned to to include a connector.
You might want to visit your library and look at the ARRL antenna book, its in the electrical/ radio amateur section. Lots of good antenna information in that book.