AF/RF NOTES

The resistive coupling provides a minimal result. The waveform on the collector cannot be greater than the supply and some of the energy will be lost in the load-resistor. The rest of the energy will be delivered into the antenna. The antenna is seen as a very low impedance - resistance (about 50 ohms) and it is very difficult for the transistor to deliver its energy into a low impedance such as this. The actual transfer will be small and this reflects the low output of the design. 
(http://www.emitatoaresystem.go.ro/Bug1/FM-BugsIntro-P2.htm)

This is because there is an impedance mismatch and when there is a impedance mismatch, the power transfer is at the minimum.

A low impedance in the circuit to be fed the signal will result in low power being transferred from the transistor.