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

ohmic losses are small enough. A complex load can always be matched by two
lumped components.
At higher frequencies, when the lumped components not are ideal, it is better to
implement the matching circuit by using distributed elements like tuning stub. It is
possible to use distributed elements even at RF frequencies but the lengths of the
stubs may be to long for printed circuit board.
A resistive attenuator can be placed between a load and a transmission line for
giving resistive matching. If a 3-dB attenuator is used, the signal level and the
total efficiency will be attenuated with 3 dB, while the reflection coefficient
decreases 6 dB. Resistive matching can be used if power consumption is not a
problem, but usually resistive matching is not acceptable, because valuable power
cannot be wasted in the resistive matching circuit in handheld devices.
2.3.6 Bandwidth enhancement
Figure 2.11. Matching circuit scheme.
The matching networks described above give perfect matching at single frequency
points. To achieve matching over certain impedance band the matching criterion
must be loosen. Instead of a criterion with return loss, Lretn = -∞ dB, usually the
return loss is chosen to be less than –6 dB for antennas in handheld devices.
To make the bandwidth as wide as possible, there are two characteristics to
choose to loosen, either to make the volume larger or lower the efficiency.
Impedance, volume and efficiency are three antenna characteristics that depend on
each other, so the best compromise to reach the specified performance must be
found. Changing the volume for internal antennas for DVB-H to make them
wideband is not easy because the small size of the terminal, and users do not want
terminals in non-pocket-size.
One method that makes it possible to enhance the bandwidth is to worsen the
efficiency, which can be made in three ways, accepting mismatching, resistive
matching with attenuator or adding resistive losses. As mentioned before, resistive
24