Since you want to test at such a wide frequency range, you will typically need two amplifiers and two antennas. Depending on the antennas selected, the power required for an amplifier to generate a given field should be somewhere in the antenna’s datasheet. If this data is not given, then you will need to base your power requirement on the antenna’s gain. Cable losses may be significant and need to be taken into consideration at higher frequencies. Chamber effects (reflections) can be unpredictable and therefore should also be considered by adding margin onto your power requirements.