transmitter antenna desigb

i have bought a fm transmitter that broadcasts to its wireless earphone but it does not have enough output power to be effective at a range of 15 meters. i decided to build up a power amplifier for the output stage but that sounded a little bit hard so the best way i think is designing an antenna. could you give me your advices about this issue? what kind of antenna do you suggest? and do you have any schematics for this antenna?

here is the specifications of the product. And there is a pic at the attachment.

Frequency reaction : 20-20.000 Hz

Impedance: 32 Ohm

sensitivity : 98 dB

thanks aLL...