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There are some good designs of RF power meters by radio amateurs. Check DUBUS magazine, for instance.
Some good professional designs like Krytar company utilizes a calibrated diode detector for very wide band power meter design. Some power detectors cover 10 MHz to 40 GHz, and compared to HP thermistor power meters offer an equal accuracy at much lower cost.
Also R/S company offers calibrated detectors as RF power meters.
We can help, but you would have first to answer the questions asked by other members.
In addition to those questions, if you are planning to build a AC mains power meter, the question would be if you require to be isolated or not? I would advise to be fully isolated.