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SARA-R5 series - System integration manual

UBX-19041356 - R04 Design-in Page 73 of 118

C1-Public

The DC impedance at RF port for some antennas may be a DC open (e.g. linear monopole) or a DC short

to reference GND (e.g. PIFA antenna). For those antennas, without the diagnostic circuit of Figure 48,

the measured DC resistance is always at the limits of the measurement range (respectively open or

short), and there is no mean to distinguish between a defect on antenna path with similar

characteristics (respectively: removal of linear antenna or RF cable shorted to GND for PIFA antenna).

Furthermore, any other DC signal injected to the RF connection from ANT connector to radiating

element will alter the measurement and produce invalid results for antenna detection.

☞ It is recommended to use an antenna with a built-in diagnostic resistor in the range from 5 k to

30 k to assure good antenna detection functionality and avoid a reduction of module RF

performance. The choke inductor should exhibit a parallel Self-Resonance Frequency (SRF) in the

range of 1 GHz to improve the RF isolation of load resistor.

For example:

Consider an antenna with built-in DC load resistor of 15 k. Using the +UANTR AT command, the

module reports the resistance value evaluated from the antenna connector provided on the

application board to GND:

• Reported values close to the used diagnostic resistor nominal value (i.e. values from 13 k to 17 k

if a 15 k diagnostic resistor is used) indicate that the antenna is correctly connected.

• Values close to the measurement range maximum limit or an open-circuit “over range” report (see

the SARA-R5 series AT commands manual [2]) means that the antenna is not connected or the

RF cable is broken.

• Reported values below the measurement range minimum limit highlights a short to GND at

antenna or along the RF cable.

• Measurement inside the valid measurement range and outside the expected range may indicate

an unclean connection, a damaged antenna or incorrect value of the antenna load resistor for

diagnostics.

• Reported value could differ from the real resistance value of the diagnostic resistor mounted

inside the antenna assembly due to antenna cable length, antenna cable capacity and the used

measurement method.

☞ If the antenna detection function is not required by the customer application, the ANT_DET pin

can be left not connected and the ANT pin can be directly connected to the antenna connector by

means of a 50  transmission line as described in Figure 35.

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