Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 118

Radiation pattern of an isotropic emitter

Radiation pattern of a dipole

Θ

G i

Figure 4.62 Radiation pattern of a dipole antenna in comparison to the radiation pattern of an
isotropic emitter

have to be supplied in order to generate a defined radiation power at distance r .An
antenna with a gain G i may therefore only be supplied with a transmission power P 1
that is lower by this factor so that the specified limit value is not exceeded:

P EIRP
G i

P 1 = ( 4 . 71 )

4.2.5.2 EIRPand ERP

In addition to power figures in EIRP we frequently come across the power figure ERP
(equivalent radiated power) in radio regulations and technical literature. The ERP is
also a reference power figure. However, in contrast to the EIRP, ERP relates to a
dipole antenna rather than a spherical emitter. An ERP power figure thus expresses the
transmission power at which a dipole antenna must be supplied in order to generate
a defined emitted power at a distance of r . Since the gain of the dipole antenna

Table 4.7 In order to emit a constant EIRP in
the main radiation direction less transmission
power must be supplied to the antenna as the
antenna gain G increases

EIRP = 4W Power P 1 fed to

the antenna

Isotropic emitter G i = 1 4 W

Dipole antenna 2.44 W

Antenna G i = 3 1.33 W