What could cause the GNSS (GPS) antenna status indicator to show "Open" intermittently?

In TX300s, RXT or MTTplus test platforms, if the GNSS antenna Open indicator shows up intermittently, it may indicate the presence of multiple power sources (or loose connection)

For this, we have to understand how the antenna OPEN/SHORT status-indicator works. GNSS receivers detect open or shorts based on the current provided by the 5 volt feed. That is, if an active antenna is attached, certain amount of expected current will flow (<50 mA), and the circuit declares "OK".

  • If the amount of current is too high, then the status is declared as a "short".
  • If no current flows at all, that means that there is no load at the far-end of the cable, so the circuit is declared "open".

Adding extra loads to the antenna cable, like in-line amplifiers powered by the same coax, may cause the test set to declare "short" or intermittent "short". (e.g., 10mA for the amplifier and 45mA for a multiband quad constellation antenna draw >55mA). Check the antenna and amplifier's datasheets.

Now, let's take a look at the behavior in question: You may see that the antenna status intermittently changes between "Open" and "OK", even though there are no problems with the antenna or its connections, and the signals' quality (C/No) remain good. Yet, the Events table keeps on logging "Antenna Open" alarms.


If you use ordinary T-connectors, to "temporarily" split the RF signal coming from the GNSS antenna, and use them to feed two or more receivers, there may be multiple power sources in parallel, feeding the antenna. In those cases, the receiver in the test set may not need to provide any current to the antenna, so it declares that the far end is "Open". If all the receivers provide similar voltage levels, around 5 volts, then any fluctuation may cause current to flow out of the test set's power feed, marginally declaring that the antenna is sometimes "OK". That causes the intermittent false antenna-open detection.

It is recommended that the antenna is powered by a single power source and that proper RF distribution splitters are used. Many of these splitters provide a port for power injection to the antenna (and any internal amplifiers). In those cases, turn OFF the 5-volt supply in the test set's GNSS receiver settings.

Nonetheless, we understand that when you are out on the field, sometimes you may have to improvise. You just need to understand what you are doing, the compromises being made, the possible side effects, and you have to be extra careful!

Having multiple power sources of the same voltage (e.g., 5 Vdc), in parallel, should not cause damage to the test set or antenna, or affect performance. Nonetheless, it is a good practice to have only one power source. The 5-volt feed in the test set may be the easiest one to turn off.

Never connect two or more devices in parallel, if their power feeds are of different voltages (e.g., 5 Vdc, 3.3 Vdc, 12 Vdc).

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