LISA-U series - System Integration Manual
UBX-13001118 - R17
Advance information
Design-In
Page 144 of 190
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 79) it is possible to make a judgment on the antenna under test:
if the performance is similar then the target antenna is good.
Figure 80: |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 80, 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.
2.4.3
Antenna detection functionality
The internal antenna detect circuit is based on ADC measurement at
ANT
: the RF port is DC coupled to the ADC
unit in the baseband chip which injects a DC current (10 µA for 128 µs) on
ANT
and measures the resulting DC
voltage to evaluate the resistance from
ANT
pad to GND.
The antenna detection is forced by the +UANTR AT command
:
see the
u-blox AT Commands Manual
[3] for
more details on how to access this feature.
To achieve antenna detection functionality, use an RF antenna with built-in resistor from
ANT
signal to GND, or
implement an equivalent solution with a circuit between the antenna cable connection and the radiating
element as shown in Figure 81.
