RFID-UHF-Antennas
18
The isotropic radiator is a loss-free point radiator with an even spherical radiation
pattern in all directions� Its power density is distributed evenly over a sphere with a
surface of 4π²r, where r is the radius of the sphere.
The gain of the λ/2 – dipole is 1.64 and the power density it produces is 1.64 times
greater than the power density from the isotropic radiator�
An example antenna with a gain of 6 dBi achieves a power density 6 dB greater
than that of the isotropic radiator� If the gain of this antenna is referred to the gain
of the λ/2 dipole, the gain in dBb is obtained:
6 dBi – 2.15 dBi = 3.85 dBd
The circular polarized gain (dBic) is also 3 dB greater than the gain in dBi�
9.2 ERP: Effective radiated power
Product of the power supplied to a transmitter antenna multiplied by its gain rela-
tive to a λ/2 dipole.
EIRP: Equivalent Isotropically Radiated Power
Product of the power supplied to a transmitter antenna and its gain relative to the
isotropic radiator�
With UHF-RFID, the maximum ERP is specified by law� When using antennas with
a high gain, the reader power must be reduced accordingly� This means that the
read range doesn’t increase with the antenna gain – the width of the read range is
merely changed�
ERP [dBm] = P
in
[dBm] + G [dBd]
EIRP [dBm] = P
in
[dBm] + G [dBi]
EIRP = ERP * 1,64
EIRP [dBm] = ERP [dBm] + 2,15 dB
9.3 Examples – antennas – input power
Europe: ERP power is limited according to EN 302208 to 2 W (33 dBm)�
An antenna has a gain of 8�5 dBic
dBic converted to dBi:
8.5 dBic = 8.5 - 3 = 5.5 dBi
dBi converted into dBd:
5.5 dBi = 5.5 - 2.15 = 3.35 dBd
Antenna input power: 33 dBm - 3.35 dBd = 29.65 dBm (0.922 W)
