Received Signal Intensity & Power

9006-004

9006-004a

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:

       EIRP
Ir = --------  Watts/meter2
     4.pi.R2

where:

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:

I_r = 2.04 x 10^-17 W/m²

9006-004b

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:

P_r = 1.60 x 10^-15 W (-148 dBW)

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

P_r = -118 dBm

This is the value of P_r 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