# Artículo de Friss/Índice/Ecuación 1

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# Ecuación 1

${\displaystyle {\frac {P_{r}}{P_{t}}}={\frac {A_{r}A_{t}}{d^{2}\lambda ^{2}}}}$
Pt =Energía de alimentación de la antena transmisora en sus terminales de entrada
Pr =Potencia disponible en los terminales de salida de la antena receptora
Ar =Área efectiva de la antena receptora
At =Área efectiva de la antena transmisora
d =Distancia entre las antenas
${\displaystyle \lambda }$ =Longitud de onda

# Equation 1

This note emphasizes the utility of the following simple transmission formula for a radio circuit made up of a transmitting antenna and a receiving antenna in free space:
${\displaystyle {\frac {P_{r}}{P_{t}}}={\frac {A_{r}A_{t}}{d^{2}\lambda ^{2}}}}$
where
Pt= power fed into the transmitting antenna at its input terminals.
Pr=power available at the output terminals of the receiving antenna.
Ar=effective area of the receiving antenna.
At=effective area of the transmitting antenna
d=distance between antennas.
${\displaystyle \lambda }$= wavelength
The effective areas appearing in (1) are discussed in the next section and this is followed by a derivation of the formula and a discussion of its limitations.