Figure 7.29 Simplified electrical schematic for an early rectenna element [Brown, 1977] Figure 7.30 Photograph of an early rectenna element [Brown, 1977] Minimum Power Density Estimation Further investigations should be undertaken to provide a relevant minimum value for the power density. Nevertheless, the order of magnitude can be estimated in the following way. Let us assume that each diode is driven by the signal collected by a half-wave dipole. Vo is the voltage drop across the diode, which is the minimum voltage to turn it on. The power collected by the dipole is transmitted to the diode as a voltage wave through the characteristic impedance R of the dipole. So, in order to turn on the diode, the minimum power to be collected is given by
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