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

The only thing that could be read out about the antenna performance from the
specification is the power gain requirement for a handheld DVB-T antenna.
Though the requirements of the DVB-H antennas certainly not will be higher than
for DVB-T antennas, these are reasonable limits for our design. However the gain
itself does not say much about the antenna performance for a handheld terminal
since an isotropic radiation pattern is desired. This is why we have chosen to
present both gain, efficiency and radiation pattern. When these three things are
presented it is easier to draw conclusions about the antennas RF performance. The
gain requirement is not very hard to achieve but to get good efficiency is hard.
This is a consequence of electrically small antennas, which all DVB-H antennas
must be to take place in a handheld device. For an electrically small antenna it is
the ground plane that contributes most to the radiation. This means that it is easier
to achieve good efficiency with a large terminal.
Our prototypes fulfilled the gain specification with large margin. Therefore the
antennas could not be separated in RF performance from the gain measurement.
From the efficiency measurement it could be seen that the folded-patch has a bit
higher values than the other prototypes but not so much that any solution can be
excluded. From a gain and efficiency perspective the folded-patch is slightly
better than the other two prototypes.
In a report from EICTA it is claimed that generally no polarisation discrimination
can be expected [31]. However, polarisation discrimination was found during our
DVB-T box measurement, the most sensitive prototype was the loop antenna.
When watching TV with the loop antenna the reception was good when the
antenna was held in vertical led but when the antenna was turned to horizontal led
the reception was bad. After some studies in the subject, the conclusion is that for
these low frequencies the polarisation is not much changed when the signal is
reflected. Therefore antenna designers cannot assume that the income signal has
different polarisations. From the radiation patterns and the DVB-T box
measurement it can be seen that the folded-patch is least sensitive to polarisation
and the loop antenna most sensitive.
Another important thing to discuss is the different control signals the antennas
need. Today it is not common with tunable antennas, but we think that it will be
more common in the near future. More and more antennas are included in the
mobile phones that take up much space. A solution for this problem could be to
use an antenna for more than one application, and then a controllable antenna
could be used. An example is that the switched monopole 2, with one extra
matching network can be used as FM antenna. The control signals available from
the chipsets today are an I2C signal and the VCO voltage. In the future it might be
possible to get more adapted signals to control the antenna. For an example it
would be easier if the DVB-H chipset could deliver the three control signals
needed to control the switches in switched monopole 2.
76