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Vrms(v) = \frac{Vpp(v)}{ 2 * \sqrt 2} = \frac{1.262}{ 2 * \sqrt 2} = 0.446V

P(w) = \frac{Vrms(v)_{}^{2}}{Rload (\Omega)} = \frac{ 0.446_{}^{2}}{50} = \frac{0.198916}{50} = 0.003981 W

P(dBm) = 10*log^{10}\frac{P(w)}{1mW} = 10*log^{10}\frac{  0.003981}{0.001} = 6 dBm