Once the antenna factor of an antenna is known ( either given by the manufacturer or calculated), the open circuit voltage at the terminals of the antenna can be calculated ( technique presented under the antenna factor post) if the E  field is known ( and the polarization is correct). This post provides a graph that shows the variation of the E-field at a fixed distance using a fixed power. The calculations are such that any of these parameters can be varied and the E – field can be calculated. For more information or custom calculations please contact us using the email contact through the SPG website. Obviously from a curve such as this one can generate a curve fitting expression and use it to calculate the result for other parameters. The parameters used for this calculation was a distance of 1 km and a power of 100mW into the antenna. The input impedance is 50 Ohm.