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

There will be no reflection only when the impedance of the antenna and the
characteristic impedance of the transmission line are equal, then the antenna is
perfectly matched. To achieve good antenna performance in a wide frequency
range a slight mismatch in the whole band is the best solution. The reflection is
unwanted because a part of the power is not delivered into the antenna, in the
transmission case and vice versa in the receiving case. The impedance matching is
an important task in the antenna design. Some basic methods for matching are
presented later [25].
The voltage at a point on a transmission line is in the general case the sum of two
waves travelling opposite directions. This scenario is known to create a standingwave
pattern along the transmission line. The ratio between the maximum and the
minimum value of this sum is termed the voltage standing-wave ratio and can be
calculated from the reflection coefficient with equation 2.4.
1+
Γ
VSWR = (2.4)
1−
Γ
Power loss due to reflection, called reflection loss Lrefl, can be calculated from
1
L refl = 10 log
(2.5)
2
1 − Γ
Return loss describes the ratio between the propagated and the reflected power
and can be calculated from
1
L retn = 10log
(2.6)
2
Γ
VSWR, Lrefl and Lretn are all calculated from the reflection coefficient and they are
normally used to define the impedance bandwidth. In figure 2.9, the bandwidth
BWabs is defined by the absolute value of the reflection coefficient Γ ≤ 0.
501,
which corresponds to the return loss criterion of Lretn ≥ 6dB
or the VSWR
criterion of S ≤ 3.
01.
This limit has to be set in each case separately. Nowadays
Lretn ≥ 6dB
is an acceptable upper limit on the bandwidth criterion for built-in
antennas [24].
20