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Its output power might be 500mW into a 50 ohm or 75 ohm antenna if all the tuned circuits are carefully adjusted to one frequency.
The BC549 transistor is a bad choice for Q3 because its max peak collector current is 200mA and it performs poorly above 50mA.
A BC549 transistor can easily be destroyed in that circuit.
Transistors have a range of current gain. Some have low gain then the output power of the transmitter will be low. Some transistors have high current gain and they might be destroyed in that circuit.
The power meter uses a very small amount of output power to activate it because its input coupling capacitor value is almost nothing (0.9pF). The power meter will not work because its LED is upside down.