Design of Antennas for Handheld DVB-H ... - Lunds tekniska högskola

2.3 Antenna theory
This section describes basic antenna theory, to easier understand later discussions.
2.3.1 Efficiency, directivity and gain
Radiation efficiency ηrad is defined as the ratio between the radiated power Prad
and the power accepted by the antenna Pin (see figure 2.8)
P
rad η rad =
(2.1)
Pin
The directivity D and the gain G of an antenna are connected to each other by ηrad
as
G = ηrad
D
(2.2)
Directivity describes the directional property of an antenna and the gain takes into
account the losses in the antenna structure. For an ideal antenna the gain and the
directivity are equal.
Figure 2.8. Voltage reflection from a mismatched load.
Γ is the refection coefficient, ZL and Z0 are the impedances of the load and
transmission line, respectively. Pt, Pin and Prad are the (total) incident power to the
load, the power accepted by the load and the power radiated by the load,
respectively.
2.3.2 Reflection from a mismatched antenna
The antenna impedance, ZL, must be equal to the characteristic impedance for the
transmission line feeding the antenna, Z0, otherwise part of the voltage will be
reflected from the antenna. How large this reflected voltage is can be measured in
a network analyser as the parameter S11. The reflection coefficient can be
calculated with following equation:
Z
Z
− Z
+ Z
L 0
Γ =
(2.3)
L
0
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