find Power of transmitter

i design s circuit that can propagate wave in enviroment.now i want to know the power of transmitter.
how can i calculate it?

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Estimating Transmitter Distance

There is the technique of attaching an exclusive resistive load and measuring power through it.

In case the power is extreme, I remember an ARRL book had a project called a 'Cantenna' which involved a lot of power resistors immersed in oil, contained in a paintcan.

By the way I once read that a 5W transmitter can broadcast just as far as one much more powerful...

If you use the right antenna (size, height, directionality, grounding, etc.), and if you fine-tune it to the transmitter's output characteristics (lead length, impedance, resonance, standing wave factor, etc.).