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@Mark_M
AT6558 datasheet tells that a passive or active antenna can be connected. Statuses are:
Open - no current
OK - normal current
Short - high current
My understanding it applies to active antennas that consume power.
I am not sure if some significant current can go through a passive antenna. My understanding is that a passive antenna cannot work as a resistor type load, may be it may work as a inductive/capacitive load for high frequency AC current.
The document does not tell if the chip can recognize a passive antenna, is it connected or not.
I hope a passive antenna is connected.
I've found interesting article. https://www.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_%28UBX-15030289%29.pdf
It states that all antennas have 50 Ohm impedance.
But AT6558 does not look like measures impedance, it rather measures DC current in VANT_IN - ANT_BIAS chain. With a passive antenna that current should be 0A.
In AT6558 datasheet Figure 9-2 "passive antenna scheme" shows active antenna with LNA.
Also it says:
10.1 Active Antenna Feed and Detection
The following figure, the chip active antenna detection circuit can detect the state
of the active antenna
So, I think Antenna Open status is normal for a passive antenna.