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Received Signal Intensity & Power




Let us assume that the communication links are set up between a nearby star at 10 light years distance and Sol. We shall also assume that all three types of transmitting and receiving antenna are located in space, and that the receiving antenna is identical to the transmitting antenna.


The received signal intensity is given by:

Ir = --------  Watts/meter2


EIRP = Effective Isotropic Radiated Power (2.29 x 1018 W),
R = Range (10 light years = 9.461 x 1016 m).


For the 10 meter diameter telescope at 656 nm, the received flux density:


Ir = 2.04 x 10-17 W/m2



The power received by a 100% efficient antenna, be it microwave or optical, is given by:

Pr = Ir.Ar  Watts

Thus, the power collected by the 10 meter diameter telescope:


Pr = 1.60 x 10-15 W (-148 dBW)


This may be expressed in dBm form (decibels with respect to 1 mW), viz


Pr = -118 dBm


This is the value of Pr used to determine the Carrier-To-Noise Ratio (CNR) in the quantum (shot) noise limited optical heterodyne receiver.


The Columbus Optical SETI Observatory
Copyright (c), 1990

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