Page 36 - Antennas for Base Stations in Wireless Communications

Fundamentals of Antennas 9

This technique can sometimes be used to build stable links, even in noisy
radio environments.

1.1.5 Antenna Efficiency
Antenna efficiency is the measure of the antenna’s ability to transmit the
input power into radiation. 1–4 Antenna efficiency is the ratio between
the radiated powers to the input power:
P
e = r (1.10)
P in

Different types of efficiencies contribute to the total antenna effi-
ciency. The total antenna efficiency is the multiplication of all these
efficiencies. Efficiency is affected by the losses within the antenna itself
and the reflection due to the mismatch at the antenna terminal. Based
on the equivalent circuit on Figure 1.1, we can compute the radiation
efficiency of the antenna as the ratio between the radiated powers to
the input power, which is only related to the conduction losses and the
dielectric losses of the antenna structure as

P R R
e = P in r = R in r = R + r R l (1.11)
r
r
Due to the mismatch at the antenna terminal, the reflection efficiency
can be defined as
e = (1 −| | ) (1.12)
Γ
2
ref
Then the total efficiency is defined as
e = e r e ref (1.13)

In this formula, antenna radiation efficiency only includes conduction
efficiency and dielectric efficiency and does not include reflection effi-
ciency as part of the total efficiency factor. Moreover, the IEEE stan-
dards state that “gain does not include losses arising from impedance
mismatches and polarization mismatches.” 5
Efficiency is the ratio of power actually radiated to the power input
into the antenna terminals. A dummy load may have an SWR of 1:1 but
an efficiency of 0, as it absorbs all power and radiates heat but not RF
energy, showing that SWR alone is not an effective measure of an anten-
na’s efficiency. Radiation in an antenna is caused by radiation resistance,
which can only be measured as part of total resistance, including loss
resistance. Loss resistance usually results in heat generation rather than