Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 Design-In Page 135 of 182
2.4.2 Antenna radiation
An indication of the antenna’s radiated power can be approximated by measuring the |S
21
| from a
target antenna to the measurement antenna, using a network analyzer with a wideband antenna.
Measurements should be done at a fixed distance and orientation, and results compared to
measurements performed on a known good antenna. Figure 65 through Figure 66 show some example
measurement results. A wideband log periodic-like antenna was used, and the comparison was done
with a half lambda dipole tuned to the 900 MHz frequency. The measurements show both the |S
11
| and
|S
21
| for the penta-band internal antenna and for the wideband antenna.
Figure 65: |S
11
| and |S
21
| comparison between a 900 MHz tuned half wavelength dipole (green/purple) and a penta-band
internal antenna (yellow/cyan)
The half lambda dipole tuned at 900 MHz is known and has good radiation performance (both for gain
and directivity). Then, by comparing the |S
21
| measurement with antenna under investigation for the
frequency where the half dipole is tuned (e.g. marker 3 in Figure 65) it is possible to make a judgment
on the antenna under test: if the performance is similar, then the target antenna is good.
Figure 66: |S
11
| and |S
21
| comparison between a 900 MHz tuned half wavelength dipole (green/purple) and a wideband
commercial antenna (yellow/cyan)
But if |S
21
| values for the tuned dipole are instead much better than the antenna under evaluation (like
for marker 1/2 area of Figure 66, where dipole is 5 dB better), then it can be argued that the radiation
of the target antenna (the wideband dipole in this case) is considerably less.
The same procedure should be repeated on other bands with a half wavelength dipole re-tuned to the
band under investigation.
☞ For good antenna radiation performance, antenna dimensions should be comparable to a quarter
of the wavelength. Different antenna types can be used for the module, many of them (e.g. patch
antennas, monopole) are based on a resonating element that works in combination with a ground
plane. The ground plane, ideally infinite, can be reduced down to a minimum size that must be
similar to one quarter of the wavelength of the minimum frequency that needs to be radiated
(transmitted/received). Numerical sample: frequency = 1 GHz  wavelength = 30 cm  minimum
ground plane (or antenna size) = 7.5 cm. Below this size, the antenna efficiency is reduced.