Antenna Gain & EIRP
The gain of a 100% efficient antenna over an isotropic radiating or receiving antenna
is given by:
G = ------
A = At for area of transmitting antenna or telescope (78.5 m2),
A = Ar for area receiving antenna or telescope (78.5 m2),
Wl = wavelength (656 nm).
The gain is normally expressed in dBs by taking 10.log(G).
For the above 10 meter diameter telescope:
G = 2.29 x 1015 = 153.6 dB
The Effective Isotropic Radiated Power (EIRP) is the apparent power transmitted
towards the receiver, if it is assumed that the signal is radiated equally in all
directions, i.e. as a spherical wave emanating from a point source. This power is given
EIRP = Gt.Pt Watts
Gt = gain of transmitter antenna or telescope (153.6 dB),
Pt = power transmitted (1 kW).
For the above 1 kW visible transmitter:
EIRP = 2.29 x 1018 W
This is a little over one million trillion watts! Now the Sun has a radius of 6.96 x
108 m and a luminosity of 3.9 x 1026 W. Thus the output from every
square meter of the Sun's surface is equal to 64 megawatts. Clearly, the spectral radiance
of this 1 kW visible transmitter is enormous.
The Columbus Optical SETI Observatory
Copyright (c), 1990