Installation Instructions

LISA-U2 series - System Integration Manual
UBX-13001118 - R19 Early Production Information Design-In
Page 131 of 175
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 measurement results. A wideband log periodic-like antenna was
used, and the comparison was done with a half lambda dipole tuned at 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)
Instead if |S
21
| values for the tuned dipole are 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 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 has 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.