Speedway Antenna Hub: Antenna Connection Status Report

Summary:

When you use the Impinj Antenna Hub with the Speedway reader, you may see unexpected connect / disconnect status of the antennas. This article describes some example scenarios when you may see this behavior, the reason for it, as well as our recommendations for handling this condition.

Background:

The status of connected antennas may be incorrectly reported in instances where one or more antenna hubs are connected to a Speedway R420 reader and one or more antennas are connected to each hub. This is due to high return loss associated with the hubs and antenna cables. Since the antenna detection algorithm relies on reflected power from an antenna port to determine the connected state of an antenna, it is possible that the antenna disconnect messages may not always be accurate in certain scenarios, such as when operating the reader at lower transmit power and / or when placing the antenna directly facing a highly reflective (e.g. metal) surface. *1

Here are some common ways this behavior presents itself:

1) The antenna on port "X" is physically connected, but the reader reports otherwise (where "X" is one of the antennas connected to the hub).

2) Using the Octane SDK or LTK, you try to retrieve antenna status by calling GET_READER_CONFIG and see an incorrect value is returned.

3) Antenna disconnect event is generated when the antenna cable is disconnected from the reader, but not when the cable is disconnected from the antenna.

We designed the Impinj Speedway Revolution reader to support antennas with poor return loss (10 dBm or above).  Because of this feature, it is more difficult for our reader to provide accurate antenna disconnect information when using the Speedway Revolution reader with Antenna Hub.  When the reader is transmitting at 30 dBm, an open antenna port (no antenna connected) has a large enough reflection such that the reader's carrier cancellation cannot completely suppress it and this triggers the antenna disconnect message. However, the reflection from the open port scales with reader transmit power and as it is reduced, carrier cancellation is good enough to completely cancel the open port reflection and therefore no antenna disconnect message is sent.

*1: For more details, see Impinj_Octane_LLRP.pdf, 3.1.10 LLRP AntennaEvent Parameter

Recommendations:

Use any one or combinations of the recommendations below to minimize the effect of this issue.

1) Place a small near-field tag permanently on or near each antenna. If you can read that marker tag, the antenna is connected. If you cannot, it has been disconnected.

2) Operate the Speedway reader at full or at near full transmit power.

3) Do not place the antenna directly in front of a metal/reflective surface.

4) For the Octane SDK and LTK (or other LLRP programming), our recommendation is to not use the antenna status as a condition for subsequent execution of the code.