Page 371 - Satellite Communications, Fourth Edition

Chapter
12

The Space Link

12.1 Introduction
This chapter describes how the link-power budget calculations are made.
These calculations basically relate two quantities, the transmit power
and the receive power, and show in detail how the difference between
these two powers is accounted for.
Link-budget calculations are usually made using decibel or decilog
quantities. These are explained in App. G. In this text [square] brack-
ets are used to denote decibel quantities using the basic power defini-
tion. Where no ambiguity arises regarding the units, the abbreviation
dB is used. For example, Boltzmann’s constant is given as 228.6 dB,
although, strictly speaking, this should be given as 228.6 decilogs rel-
ative to 1 J/K. Where it is desirable to show the reference unit, this is
indicated in the abbreviation, for example, dBHz means decibels rela-
tive to 1 Hz.

12.2 Equivalent Isotropic Radiated Power
A key parameter in link-budget calculations is the equivalent isotropic
radiated power, conventionally denoted as EIRP. From Eqs. (6.4) and
(6.5), the maximum power flux density at some distance r from a trans-
mitting antenna of gain G is

GP S
M (12.1)
4 r 2
An isotropic radiator with an input power equal to GP would produce
S
the same flux density. Hence, this product is referred to as the EIRP, or
(12.2)
EIRP GP S
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